Export of internal Abseil changes.

--
4eacae3ff1b14b1d309e8092185bc10e8a6203cf by Derek Mauro <dmauro@google.com>:

Release SwissTable - a fast, efficient, cache-friendly hash table.

https://www.youtube.com/watch?v=ncHmEUmJZf4

PiperOrigin-RevId: 214816527

--
df8c3dfab3cfb2f4365909a84d0683b193cfbb11 by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 214785288

--
1eabd5266bbcebc33eecc91e5309b751856a75c8 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 214722931

--
2ebbfac950f83146b46253038e7dd7dcde9f2951 by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 214701684
GitOrigin-RevId: 4eacae3ff1b14b1d309e8092185bc10e8a6203cf
Change-Id: I9ba64e395b22ad7863213d157b8019b082adc19d

55 files changed, 18696 insertions, 0 deletions

diff --git a/absl/CMakeLists.txt b/absl/CMakeLists.txt
index 689f64e258ba..1d09b1935d84 100644
--- a/absl/CMakeLists.txt
+++ b/absl/CMakeLists.txt
@@ -20,6 +20,7 @@ add_subdirectory(base)
 add_subdirectory(algorithm)
 add_subdirectory(container)
 add_subdirectory(debugging)
+add_subdirectory(hash)
 add_subdirectory(memory)
 add_subdirectory(meta)
 add_subdirectory(numeric)
diff --git a/absl/container/BUILD.bazel b/absl/container/BUILD.bazel
index 7b5f52bc059e..265c5ec9c418 100644
--- a/absl/container/BUILD.bazel
+++ b/absl/container/BUILD.bazel
@@ -185,3 +185,459 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+NOTEST_TAGS_NONMOBILE = [
+    "no_test_darwin_x86_64",
+    "no_test_loonix",
+]
+
+NOTEST_TAGS_MOBILE = [
+    "no_test_android_arm",
+    "no_test_android_arm64",
+    "no_test_android_x86",
+    "no_test_ios_x86_64",
+]
+
+NOTEST_TAGS = NOTEST_TAGS_MOBILE + NOTEST_TAGS_NONMOBILE
+
+cc_library(
+    name = "flat_hash_map",
+    hdrs = ["flat_hash_map.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":container_memory",
+        ":hash_function_defaults",
+        ":raw_hash_map",
+        "//absl/memory",
+    ],
+)
+
+cc_test(
+    name = "flat_hash_map_test",
+    srcs = ["flat_hash_map_test.cc"],
+    copts = ABSL_TEST_COPTS + ["-DUNORDERED_MAP_CXX17"],
+    tags = NOTEST_TAGS_NONMOBILE,
+    deps = [
+        ":flat_hash_map",
+        ":hash_generator_testing",
+        ":unordered_map_constructor_test",
+        ":unordered_map_lookup_test",
+        ":unordered_map_modifiers_test",
+        "//absl/types:any",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "flat_hash_set",
+    hdrs = ["flat_hash_set.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":container_memory",
+        ":hash_function_defaults",
+        ":raw_hash_set",
+        "//absl/base:core_headers",
+        "//absl/memory",
+    ],
+)
+
+cc_test(
+    name = "flat_hash_set_test",
+    srcs = ["flat_hash_set_test.cc"],
+    copts = ABSL_TEST_COPTS + ["-DUNORDERED_SET_CXX17"],
+    tags = NOTEST_TAGS_NONMOBILE,
+    deps = [
+        ":flat_hash_set",
+        ":hash_generator_testing",
+        ":unordered_set_constructor_test",
+        ":unordered_set_lookup_test",
+        ":unordered_set_modifiers_test",
+        "//absl/memory",
+        "//absl/strings",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "node_hash_map",
+    hdrs = ["node_hash_map.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":container_memory",
+        ":hash_function_defaults",
+        ":node_hash_policy",
+        ":raw_hash_map",
+        "//absl/memory",
+    ],
+)
+
+cc_test(
+    name = "node_hash_map_test",
+    srcs = ["node_hash_map_test.cc"],
+    copts = ABSL_TEST_COPTS + ["-DUNORDERED_MAP_CXX17"],
+    tags = NOTEST_TAGS_NONMOBILE,
+    deps = [
+        ":hash_generator_testing",
+        ":node_hash_map",
+        ":tracked",
+        ":unordered_map_constructor_test",
+        ":unordered_map_lookup_test",
+        ":unordered_map_modifiers_test",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "node_hash_set",
+    hdrs = ["node_hash_set.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":container_memory",
+        ":hash_function_defaults",
+        ":node_hash_policy",
+        ":raw_hash_set",
+        "//absl/memory",
+    ],
+)
+
+cc_test(
+    name = "node_hash_set_test",
+    srcs = ["node_hash_set_test.cc"],
+    copts = ABSL_TEST_COPTS + ["-DUNORDERED_SET_CXX17"],
+    tags = NOTEST_TAGS_NONMOBILE,
+    deps = [
+        ":hash_generator_testing",
+        ":node_hash_set",
+        ":unordered_set_constructor_test",
+        ":unordered_set_lookup_test",
+        ":unordered_set_modifiers_test",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "container_memory",
+    hdrs = ["internal/container_memory.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        "//absl/memory",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "container_memory_test",
+    srcs = ["internal/container_memory_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    tags = NOTEST_TAGS_NONMOBILE,
+    deps = [
+        ":container_memory",
+        "//absl/strings",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "hash_function_defaults",
+    hdrs = ["internal/hash_function_defaults.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/hash",
+        "//absl/strings",
+    ],
+)
+
+cc_test(
+    name = "hash_function_defaults_test",
+    srcs = ["internal/hash_function_defaults_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    tags = NOTEST_TAGS,
+    deps = [
+        ":hash_function_defaults",
+        "//absl/hash",
+        "//absl/strings",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "hash_generator_testing",
+    testonly = 1,
+    srcs = ["internal/hash_generator_testing.cc"],
+    hdrs = ["internal/hash_generator_testing.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_policy_testing",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "hash_policy_testing",
+    testonly = 1,
+    hdrs = ["internal/hash_policy_testing.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        "//absl/hash",
+        "//absl/strings",
+    ],
+)
+
+cc_test(
+    name = "hash_policy_testing_test",
+    srcs = ["internal/hash_policy_testing_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_policy_testing",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "hash_policy_traits",
+    hdrs = ["internal/hash_policy_traits.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = ["//absl/meta:type_traits"],
+)
+
+cc_test(
+    name = "hash_policy_traits_test",
+    srcs = ["internal/hash_policy_traits_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_policy_traits",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "hashtable_debug",
+    hdrs = ["internal/hashtable_debug.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":hashtable_debug_hooks",
+    ],
+)
+
+cc_library(
+    name = "hashtable_debug_hooks",
+    hdrs = ["internal/hashtable_debug_hooks.h"],
+    copts = ABSL_DEFAULT_COPTS,
+)
+
+cc_library(
+    name = "node_hash_policy",
+    hdrs = ["internal/node_hash_policy.h"],
+    copts = ABSL_DEFAULT_COPTS,
+)
+
+cc_test(
+    name = "node_hash_policy_test",
+    srcs = ["internal/node_hash_policy_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_policy_traits",
+        ":node_hash_policy",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "raw_hash_map",
+    hdrs = ["internal/raw_hash_map.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":container_memory",
+        ":raw_hash_set",
+    ],
+)
+
+cc_library(
+    name = "raw_hash_set",
+    srcs = ["internal/raw_hash_set.cc"],
+    hdrs = ["internal/raw_hash_set.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":compressed_tuple",
+        ":container_memory",
+        ":hash_policy_traits",
+        ":hashtable_debug_hooks",
+        ":layout",
+        "//absl/base:bits",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/types:optional",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "raw_hash_set_test",
+    srcs = ["internal/raw_hash_set_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkstatic = 1,
+    tags = NOTEST_TAGS,
+    deps = [
+        ":container_memory",
+        ":hash_function_defaults",
+        ":hash_policy_testing",
+        ":hashtable_debug",
+        ":raw_hash_set",
+        "//absl/base",
+        "//absl/base:core_headers",
+        "//absl/strings",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "raw_hash_set_allocator_test",
+    size = "small",
+    srcs = ["internal/raw_hash_set_allocator_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":raw_hash_set",
+        ":tracked",
+        "//absl/base:core_headers",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "layout",
+    hdrs = ["internal/layout.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        "//absl/base:core_headers",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/types:span",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "layout_test",
+    size = "small",
+    srcs = ["internal/layout_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    tags = NOTEST_TAGS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":layout",
+        "//absl/base",
+        "//absl/base:core_headers",
+        "//absl/types:span",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "tracked",
+    testonly = 1,
+    hdrs = ["internal/tracked.h"],
+    copts = ABSL_TEST_COPTS,
+)
+
+cc_library(
+    name = "unordered_map_constructor_test",
+    testonly = 1,
+    hdrs = ["internal/unordered_map_constructor_test.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "unordered_map_lookup_test",
+    testonly = 1,
+    hdrs = ["internal/unordered_map_lookup_test.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "unordered_map_modifiers_test",
+    testonly = 1,
+    hdrs = ["internal/unordered_map_modifiers_test.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "unordered_set_constructor_test",
+    testonly = 1,
+    hdrs = ["internal/unordered_set_constructor_test.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "unordered_set_lookup_test",
+    testonly = 1,
+    hdrs = ["internal/unordered_set_lookup_test.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "unordered_set_modifiers_test",
+    testonly = 1,
+    hdrs = ["internal/unordered_set_modifiers_test.h"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "unordered_set_test",
+    srcs = ["internal/unordered_set_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    tags = NOTEST_TAGS_NONMOBILE,
+    deps = [
+        ":unordered_set_constructor_test",
+        ":unordered_set_lookup_test",
+        ":unordered_set_modifiers_test",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "unordered_map_test",
+    srcs = ["internal/unordered_map_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    tags = NOTEST_TAGS_NONMOBILE,
+    deps = [
+        ":unordered_map_constructor_test",
+        ":unordered_map_lookup_test",
+        ":unordered_map_modifiers_test",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
diff --git a/absl/container/CMakeLists.txt b/absl/container/CMakeLists.txt
index 123e4c4849aa..710bace776a2 100644
--- a/absl/container/CMakeLists.txt
+++ b/absl/container/CMakeLists.txt
@@ -17,12 +17,34 @@
 
 list(APPEND CONTAINER_PUBLIC_HEADERS
   "fixed_array.h"
+  "flat_hash_map.h"
+  "flat_hash_set.h"
   "inlined_vector.h"
+  "node_hash_map.h"
+  "node_hash_set.h"
 )
 
 
 list(APPEND CONTAINER_INTERNAL_HEADERS
+  "internal/compressed_tuple.h"
+  "internal/container_memory.h"
+  "internal/hash_function_defaults.h"
+  "internal/hash_generator_testing.h"
+  "internal/hash_policy_testing.h"
+  "internal/hash_policy_traits.h"
+  "internal/hashtable_debug.h"
+  "internal/layout.h"
+  "internal/node_hash_policy.h"
+  "internal/raw_hash_map.h"
+  "internal/raw_hash_set.h"
   "internal/test_instance_tracker.h"
+  "internal/tracked.h"
+  "internal/unordered_map_constructor_test.h"
+  "internal/unordered_map_lookup_test.h"
+  "internal/unordered_map_modifiers_test.h"
+  "internal/unordered_set_constructor_test.h"
+  "internal/unordered_set_lookup_test.h"
+  "internal/unordered_set_modifiers_test.h"
 )
 
 
diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h
new file mode 100644
index 000000000000..13fbfba51f48
--- /dev/null
+++ b/absl/container/flat_hash_map.h
@@ -0,0 +1,528 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: flat_hash_map.h
+// -----------------------------------------------------------------------------
+//
+// An `absl::flat_hash_map<K, V>` is an unordered associative container of
+// unique keys and associated values designed to be a more efficient replacement
+// for `std::unordered_map`. Like `unordered_map`, search, insertion, and
+// deletion of map elements can be done as an `O(1)` operation. However,
+// `flat_hash_map` (and other unordered associative containers known as the
+// collection of Abseil "Swiss tables") contain other optimizations that result
+// in both memory and computation advantages.
+//
+// In most cases, your default choice for a hash map should be a map of type
+// `flat_hash_map`.
+
+#ifndef ABSL_CONTAINER_FLAT_HASH_MAP_H_
+#define ABSL_CONTAINER_FLAT_HASH_MAP_H_
+
+#include <cstddef>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
+#include "absl/container/internal/raw_hash_map.h"  // IWYU pragma: export
+#include "absl/memory/memory.h"
+
+namespace absl {
+namespace container_internal {
+template <class K, class V>
+struct FlatHashMapPolicy;
+}  // namespace container_internal
+
+// -----------------------------------------------------------------------------
+// absl::flat_hash_map
+// -----------------------------------------------------------------------------
+//
+// An `absl::flat_hash_map<K, V>` is an unordered associative container which
+// has been optimized for both speed and memory footprint in most common use
+// cases. Its interface is similar to that of `std::unordered_map<K, V>` with
+// the following notable differences:
+//
+// * Requires keys that are CopyConstructible
+// * Requires values that are MoveConstructible
+// * Supports heterogeneous lookup, through `find()`, `operator[]()` and
+//   `insert()`, provided that the map is provided a compatible heterogeneous
+//   hashing function and equality operator.
+// * Invalidates any references and pointers to elements within the table after
+//   `rehash()`.
+// * Contains a `capacity()` member function indicating the number of element
+//   slots (open, deleted, and empty) within the hash map.
+// * Returns `void` from the `erase(iterator)` overload.
+//
+// By default, `flat_hash_map` uses the `absl::Hash` hashing framework.
+// All fundamental and Abseil types that support the `absl::Hash` framework have
+// a compatible equality operator for comparing insertions into `flat_hash_map`.
+// If your type is not yet supported by the `asbl::Hash` framework, see
+// absl/hash/hash.h for information on extending Abseil hashing to user-defined
+// types.
+//
+// NOTE: A `flat_hash_map` stores its value types directly inside its
+// implementation array to avoid memory indirection. Because a `flat_hash_map`
+// is designed to move data when rehashed, map values will not retain pointer
+// stability. If you require pointer stability, or your values are large,
+// consider using `absl::flat_hash_map<Key, std::unique_ptr<Value>>` instead.
+// If your types are not moveable or you require pointer stability for keys,
+// consider `absl::node_hash_map`.
+//
+// Example:
+//
+//   // Create a flat hash map of three strings (that map to strings)
+//   absl::flat_hash_map<std::string, std::string> ducks =
+//     {{"a", "huey"}, {"b", "dewey"}, {"c", "louie"}};
+//
+//  // Insert a new element into the flat hash map
+//  ducks.insert({"d", "donald"}};
+//
+//  // Force a rehash of the flat hash map
+//  ducks.rehash(0);
+//
+//  // Find the element with the key "b"
+//  std::string search_key = "b";
+//  auto result = ducks.find(search_key);
+//  if (result != ducks.end()) {
+//    std::cout << "Result: " << result->second << std::endl;
+//  }
+template <class K, class V,
+          class Hash = absl::container_internal::hash_default_hash<K>,
+          class Eq = absl::container_internal::hash_default_eq<K>,
+          class Allocator = std::allocator<std::pair<const K, V>>>
+class flat_hash_map : public absl::container_internal::raw_hash_map<
+                          absl::container_internal::FlatHashMapPolicy<K, V>,
+                          Hash, Eq, Allocator> {
+  using Base = typename flat_hash_map::raw_hash_map;
+
+ public:
+  flat_hash_map() {}
+  using Base::Base;
+
+  // flat_hash_map::begin()
+  //
+  // Returns an iterator to the beginning of the `flat_hash_map`.
+  using Base::begin;
+
+  // flat_hash_map::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `flat_hash_map`.
+  using Base::cbegin;
+
+  // flat_hash_map::cend()
+  //
+  // Returns a const iterator to the end of the `flat_hash_map`.
+  using Base::cend;
+
+  // flat_hash_map::end()
+  //
+  // Returns an iterator to the end of the `flat_hash_map`.
+  using Base::end;
+
+  // flat_hash_map::capacity()
+  //
+  // Returns the number of element slots (assigned, deleted, and empty)
+  // available within the `flat_hash_map`.
+  //
+  // NOTE: this member function is particular to `absl::flat_hash_map` and is
+  // not provided in the `std::unordered_map` API.
+  using Base::capacity;
+
+  // flat_hash_map::empty()
+  //
+  // Returns whether or not the `flat_hash_map` is empty.
+  using Base::empty;
+
+  // flat_hash_map::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `flat_hash_map` under current memory constraints. This value can be thought
+  // of the largest value of `std::distance(begin(), end())` for a
+  // `flat_hash_map<K, V>`.
+  using Base::max_size;
+
+  // flat_hash_map::size()
+  //
+  // Returns the number of elements currently within the `flat_hash_map`.
+  using Base::size;
+
+  // flat_hash_map::clear()
+  //
+  // Removes all elements from the `flat_hash_map`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  //
+  // NOTE: this operation may shrink the underlying buffer. To avoid shrinking
+  // the underlying buffer call `erase(begin(), end())`.
+  using Base::clear;
+
+  // flat_hash_map::erase()
+  //
+  // Erases elements within the `flat_hash_map`. Erasing does not trigger a
+  // rehash. Overloads are listed below.
+  //
+  // void erase(const_iterator pos):
+  //
+  //   Erases the element at `position` of the `flat_hash_map`, returning
+  //   `void`.
+  //
+  //   NOTE: this return behavior is different than that of STL containers in
+  //   general and `std::unordered_map` in particular.
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning an
+  //   iterator pointing to `last`.
+  //
+  // size_type erase(const key_type& key):
+  //
+  //   Erases the element with the matching key, if it exists.
+  using Base::erase;
+
+  // flat_hash_map::insert()
+  //
+  // Inserts an element of the specified value into the `flat_hash_map`,
+  // returning an iterator pointing to the newly inserted element, provided that
+  // an element with the given key does not already exist. If rehashing occurs
+  // due to the insertion, all iterators are invalidated. Overloads are listed
+  // below.
+  //
+  // std::pair<iterator,bool> insert(const init_type& value):
+  //
+  //   Inserts a value into the `flat_hash_map`. Returns a pair consisting of an
+  //   iterator to the inserted element (or to the element that prevented the
+  //   insertion) and a bool denoting whether the insertion took place.
+  //
+  // std::pair<iterator,bool> insert(T&& value):
+  // std::pair<iterator,bool> insert(init_type&& value ):
+  //
+  //   Inserts a moveable value into the `flat_hash_map`. Returns a pair
+  //   consisting of an iterator to the inserted element (or to the element that
+  //   prevented the insertion) and a bool denoting whether the insertion took
+  //   place.
+  //
+  // iterator insert(const_iterator hint, const init_type& value):
+  // iterator insert(const_iterator hint, T&& value):
+  // iterator insert(const_iterator hint, init_type&& value );
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element, or to the existing element that prevented the
+  //   insertion.
+  //
+  // void insert(InputIterator first, InputIterator last ):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently, for `flat_hash_map` we guarantee the
+  //   first match is inserted.
+  //
+  // void insert(std::initializer_list<init_type> ilist ):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently within the initializer list, for
+  //   `flat_hash_map` we guarantee the first match is inserted.
+  using Base::insert;
+
+  // flat_hash_map::insert_or_assign()
+  //
+  // Inserts an element of the specified value into the `flat_hash_map` provided
+  // that a value with the given key does not already exist, or replaces it with
+  // the element value if a key for that value already exists, returning an
+  // iterator pointing to the newly inserted element.  If rehashing occurs due
+  // to the insertion, all existing iterators are invalidated. Overloads are
+  // listed below.
+  //
+  // pair<iterator, bool> insert_or_assign(const init_type& k, T&& obj):
+  // pair<iterator, bool> insert_or_assign(init_type&& k, T&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `flat_hash_map`.
+  //
+  // iterator insert_or_assign(const_iterator hint,
+  //                           const init_type& k, T&& obj):
+  // iterator insert_or_assign(const_iterator hint, init_type&& k, T&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `flat_hash_map` using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search.
+  using Base::insert_or_assign;
+
+  // flat_hash_map::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `flat_hash_map`, provided that no element with the given key
+  // already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace;
+
+  // flat_hash_map::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `flat_hash_map`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search, and only inserts
+  // provided that no element with the given key already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace_hint;
+
+  // flat_hash_map::try_emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `flat_hash_map`, provided that no element with the given key
+  // already exists. Unlike `emplace()`, if an element with the given key
+  // already exists, we guarantee that no element is constructed.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  // Overloads are listed below.
+  //
+  //   pair<iterator, bool> try_emplace(const key_type& k, Args&&... args):
+  //   pair<iterator, bool> try_emplace(key_type&& k, Args&&... args):
+  //
+  // Inserts (via copy or move) the element of the specified key into the
+  // `flat_hash_map`.
+  //
+  //   iterator try_emplace(const_iterator hint,
+  //                        const init_type& k, Args&&... args):
+  //   iterator try_emplace(const_iterator hint, init_type&& k, Args&&... args):
+  //
+  // Inserts (via copy or move) the element of the specified key into the
+  // `flat_hash_map` using the position of `hint` as a non-binding suggestion
+  // for where to begin the insertion search.
+  using Base::try_emplace;
+
+  // flat_hash_map::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the key,value pair of the element at the indicated position and
+  //   returns a node handle owning that extracted data.
+  //
+  // node_type extract(const key_type& x):
+  //
+  //   Extracts the key,value pair of the element with a key matching the passed
+  //   key value and returns a node handle owning that extracted data. If the
+  //   `flat_hash_map` does not contain an element with a matching key, this
+  //   function returns an empty node handle.
+  using Base::extract;
+
+  // flat_hash_map::merge()
+  //
+  // Extracts elements from a given `source` flat hash map into this
+  // `flat_hash_map`. If the destination `flat_hash_map` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // flat_hash_map::swap(flat_hash_map& other)
+  //
+  // Exchanges the contents of this `flat_hash_map` with those of the `other`
+  // flat hash map, avoiding invocation of any move, copy, or swap operations on
+  // individual elements.
+  //
+  // All iterators and references on the `flat_hash_map` remain valid, excepting
+  // for the past-the-end iterator, which is invalidated.
+  //
+  // `swap()` requires that the flat hash map's hashing and key equivalence
+  // functions be Swappable, and are exchaged using unqualified calls to
+  // non-member `swap()`. If the map's allocator has
+  // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
+  // set to `true`, the allocators are also exchanged using an unqualified call
+  // to non-member `swap()`; otherwise, the allocators are not swapped.
+  using Base::swap;
+
+  // flat_hash_map::rehash(count)
+  //
+  // Rehashes the `flat_hash_map`, setting the number of slots to be at least
+  // the passed value. If the new number of slots increases the load factor more
+  // than the current maximum load factor
+  // (`count` < `size()` / `max_load_factor()`), then the new number of slots
+  // will be at least `size()` / `max_load_factor()`.
+  //
+  // To force a rehash, pass rehash(0).
+  //
+  // NOTE: unlike behavior in `std::unordered_map`, references are also
+  // invalidated upon a `rehash()`.
+  using Base::rehash;
+
+  // flat_hash_map::reserve(count)
+  //
+  // Sets the number of slots in the `flat_hash_map` to the number needed to
+  // accommodate at least `count` total elements without exceeding the current
+  // maximum load factor, and may rehash the container if needed.
+  using Base::reserve;
+
+  // flat_hash_map::at()
+  //
+  // Returns a reference to the mapped value of the element with key equivalent
+  // to the passed key.
+  using Base::at;
+
+  // flat_hash_map::contains()
+  //
+  // Determines whether an element with a key comparing equal to the given `key`
+  // exists within the `flat_hash_map`, returning `true` if so or `false`
+  // otherwise.
+  using Base::contains;
+
+  // flat_hash_map::count(const Key& key) const
+  //
+  // Returns the number of elements with a key comparing equal to the given
+  // `key` within the `flat_hash_map`. note that this function will return
+  // either `1` or `0` since duplicate keys are not allowed within a
+  // `flat_hash_map`.
+  using Base::count;
+
+  // flat_hash_map::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `flat_hash_map`.
+  using Base::equal_range;
+
+  // flat_hash_map::find()
+  //
+  // Finds an element with the passed `key` within the `flat_hash_map`.
+  using Base::find;
+
+  // flat_hash_map::operator[]()
+  //
+  // Returns a reference to the value mapped to the passed key within the
+  // `flat_hash_map`, performing an `insert()` if the key does not already
+  // exist.
+  //
+  // If an insertion occurs and results in a rehashing of the container, all
+  // iterators are invalidated. Otherwise iterators are not affected and
+  // references are not invalidated. Overloads are listed below.
+  //
+  // T& operator[](const Key& key ):
+  //
+  //   Inserts an init_type object constructed in-place if the element with the
+  //   given key does not exist.
+  //
+  // T& operator[]( Key&& key ):
+  //
+  //   Inserts an init_type object constructed in-place provided that an element
+  //   with the given key does not exist.
+  using Base::operator[];
+
+  // flat_hash_map::bucket_count()
+  //
+  // Returns the number of "buckets" within the `flat_hash_map`. Note that
+  // because a flat hash map contains all elements within its internal storage,
+  // this value simply equals the current capacity of the `flat_hash_map`.
+  using Base::bucket_count;
+
+  // flat_hash_map::load_factor()
+  //
+  // Returns the current load factor of the `flat_hash_map` (the average number
+  // of slots occupied with a value within the hash map).
+  using Base::load_factor;
+
+  // flat_hash_map::max_load_factor()
+  //
+  // Manages the maximum load factor of the `flat_hash_map`. Overloads are
+  // listed below.
+  //
+  // float flat_hash_map::max_load_factor()
+  //
+  //   Returns the current maximum load factor of the `flat_hash_map`.
+  //
+  // void flat_hash_map::max_load_factor(float ml)
+  //
+  //   Sets the maximum load factor of the `flat_hash_map` to the passed value.
+  //
+  //   NOTE: This overload is provided only for API compatibility with the STL;
+  //   `flat_hash_map` will ignore any set load factor and manage its rehashing
+  //   internally as an implementation detail.
+  using Base::max_load_factor;
+
+  // flat_hash_map::get_allocator()
+  //
+  // Returns the allocator function associated with this `flat_hash_map`.
+  using Base::get_allocator;
+
+  // flat_hash_map::hash_function()
+  //
+  // Returns the hashing function used to hash the keys within this
+  // `flat_hash_map`.
+  using Base::hash_function;
+
+  // flat_hash_map::key_eq()
+  //
+  // Returns the function used for comparing keys equality.
+  using Base::key_eq;
+};
+
+namespace container_internal {
+
+template <class K, class V>
+struct FlatHashMapPolicy {
+  using slot_type = container_internal::slot_type<K, V>;
+  using key_type = K;
+  using mapped_type = V;
+  using init_type = std::pair</*non const*/ key_type, mapped_type>;
+
+  template <class Allocator, class... Args>
+  static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
+    slot_type::construct(alloc, slot, std::forward<Args>(args)...);
+  }
+
+  template <class Allocator>
+  static void destroy(Allocator* alloc, slot_type* slot) {
+    slot_type::destroy(alloc, slot);
+  }
+
+  template <class Allocator>
+  static void transfer(Allocator* alloc, slot_type* new_slot,
+                       slot_type* old_slot) {
+    slot_type::transfer(alloc, new_slot, old_slot);
+  }
+
+  template <class F, class... Args>
+  static decltype(absl::container_internal::DecomposePair(
+      std::declval<F>(), std::declval<Args>()...))
+  apply(F&& f, Args&&... args) {
+    return absl::container_internal::DecomposePair(std::forward<F>(f),
+                                                   std::forward<Args>(args)...);
+  }
+
+  static size_t space_used(const slot_type*) { return 0; }
+
+  static std::pair<const K, V>& element(slot_type* slot) { return slot->value; }
+
+  static V& value(std::pair<const K, V>* kv) { return kv->second; }
+  static const V& value(const std::pair<const K, V>* kv) { return kv->second; }
+};
+
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_FLAT_HASH_MAP_H_
diff --git a/absl/container/flat_hash_map_test.cc b/absl/container/flat_hash_map_test.cc
new file mode 100644
index 000000000000..10a781ffd6c7
--- /dev/null
+++ b/absl/container/flat_hash_map_test.cc
@@ -0,0 +1,241 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/flat_hash_map.h"
+
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/unordered_map_constructor_test.h"
+#include "absl/container/internal/unordered_map_lookup_test.h"
+#include "absl/container/internal/unordered_map_modifiers_test.h"
+#include "absl/types/any.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+using ::absl::container_internal::hash_internal::Enum;
+using ::absl::container_internal::hash_internal::EnumClass;
+using ::testing::_;
+using ::testing::Pair;
+using ::testing::UnorderedElementsAre;
+
+template <class K, class V>
+using Map =
+    flat_hash_map<K, V, StatefulTestingHash, StatefulTestingEqual, Alloc<>>;
+
+static_assert(!std::is_standard_layout<NonStandardLayout>(), "");
+
+using MapTypes =
+    ::testing::Types<Map<int, int>, Map<std::string, int>, Map<Enum, std::string>,
+                     Map<EnumClass, int>, Map<int, NonStandardLayout>,
+                     Map<NonStandardLayout, int>>;
+
+INSTANTIATE_TYPED_TEST_CASE_P(FlatHashMap, ConstructorTest, MapTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(FlatHashMap, LookupTest, MapTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(FlatHashMap, ModifiersTest, MapTypes);
+
+TEST(FlatHashMap, StandardLayout) {
+  struct Int {
+    explicit Int(size_t value) : value(value) {}
+    Int() : value(0) { ADD_FAILURE(); }
+    Int(const Int& other) : value(other.value) { ADD_FAILURE(); }
+    Int(Int&&) = default;
+    bool operator==(const Int& other) const { return value == other.value; }
+    size_t value;
+  };
+  static_assert(std::is_standard_layout<Int>(), "");
+
+  struct Hash {
+    size_t operator()(const Int& obj) const { return obj.value; }
+  };
+
+  // Verify that neither the key nor the value get default-constructed or
+  // copy-constructed.
+  {
+    flat_hash_map<Int, Int, Hash> m;
+    m.try_emplace(Int(1), Int(2));
+    m.try_emplace(Int(3), Int(4));
+    m.erase(Int(1));
+    m.rehash(2 * m.bucket_count());
+  }
+  {
+    flat_hash_map<Int, Int, Hash> m;
+    m.try_emplace(Int(1), Int(2));
+    m.try_emplace(Int(3), Int(4));
+    m.erase(Int(1));
+    m.clear();
+  }
+}
+
+// gcc becomes unhappy if this is inside the method, so pull it out here.
+struct balast {};
+
+TEST(FlatHashMap, IteratesMsan) {
+  // Because SwissTable randomizes on pointer addresses, we keep old tables
+  // around to ensure we don't reuse old memory.
+  std::vector<absl::flat_hash_map<int, balast>> garbage;
+  for (int i = 0; i < 100; ++i) {
+    absl::flat_hash_map<int, balast> t;
+    for (int j = 0; j < 100; ++j) {
+      t[j];
+      for (const auto& p : t) EXPECT_THAT(p, Pair(_, _));
+    }
+    garbage.push_back(std::move(t));
+  }
+}
+
+// Demonstration of the "Lazy Key" pattern.  This uses heterogenous insert to
+// avoid creating expensive key elements when the item is already present in the
+// map.
+struct LazyInt {
+  explicit LazyInt(size_t value, int* tracker)
+      : value(value), tracker(tracker) {}
+
+  explicit operator size_t() const {
+    ++*tracker;
+    return value;
+  }
+
+  size_t value;
+  int* tracker;
+};
+
+struct Hash {
+  using is_transparent = void;
+  int* tracker;
+  size_t operator()(size_t obj) const {
+    ++*tracker;
+    return obj;
+  }
+  size_t operator()(const LazyInt& obj) const {
+    ++*tracker;
+    return obj.value;
+  }
+};
+
+struct Eq {
+  using is_transparent = void;
+  bool operator()(size_t lhs, size_t rhs) const {
+    return lhs == rhs;
+  }
+  bool operator()(size_t lhs, const LazyInt& rhs) const {
+    return lhs == rhs.value;
+  }
+};
+
+TEST(FlatHashMap, LazyKeyPattern) {
+  // hashes are only guaranteed in opt mode, we use assertions to track internal
+  // state that can cause extra calls to hash.
+  int conversions = 0;
+  int hashes = 0;
+  flat_hash_map<size_t, size_t, Hash, Eq> m(0, Hash{&hashes});
+
+  m[LazyInt(1, &conversions)] = 1;
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 1)));
+  EXPECT_EQ(conversions, 1);
+#ifdef NDEBUG
+  EXPECT_EQ(hashes, 1);
+#endif
+
+  m[LazyInt(1, &conversions)] = 2;
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2)));
+  EXPECT_EQ(conversions, 1);
+#ifdef NDEBUG
+  EXPECT_EQ(hashes, 2);
+#endif
+
+  m.try_emplace(LazyInt(2, &conversions), 3);
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(2, 3)));
+  EXPECT_EQ(conversions, 2);
+#ifdef NDEBUG
+  EXPECT_EQ(hashes, 3);
+#endif
+
+  m.try_emplace(LazyInt(2, &conversions), 4);
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(2, 3)));
+  EXPECT_EQ(conversions, 2);
+#ifdef NDEBUG
+  EXPECT_EQ(hashes, 4);
+#endif
+}
+
+TEST(FlatHashMap, BitfieldArgument) {
+  union {
+    int n : 1;
+  };
+  n = 0;
+  flat_hash_map<int, int> m;
+  m.erase(n);
+  m.count(n);
+  m.prefetch(n);
+  m.find(n);
+  m.contains(n);
+  m.equal_range(n);
+  m.insert_or_assign(n, n);
+  m.insert_or_assign(m.end(), n, n);
+  m.try_emplace(n);
+  m.try_emplace(m.end(), n);
+  m.at(n);
+  m[n];
+}
+
+TEST(FlatHashMap, MergeExtractInsert) {
+  // We can't test mutable keys, or non-copyable keys with flat_hash_map.
+  // Test that the nodes have the proper API.
+  absl::flat_hash_map<int, int> m = {{1, 7}, {2, 9}};
+  auto node = m.extract(1);
+  EXPECT_TRUE(node);
+  EXPECT_EQ(node.key(), 1);
+  EXPECT_EQ(node.mapped(), 7);
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(2, 9)));
+
+  node.mapped() = 17;
+  m.insert(std::move(node));
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 17), Pair(2, 9)));
+}
+#if !defined(__ANDROID__) && !defined(__APPLE__) && !defined(__EMSCRIPTEN__)
+TEST(FlatHashMap, Any) {
+  absl::flat_hash_map<int, absl::any> m;
+  m.emplace(1, 7);
+  auto it = m.find(1);
+  ASSERT_NE(it, m.end());
+  EXPECT_EQ(7, absl::any_cast<int>(it->second));
+
+  m.emplace(std::piecewise_construct, std::make_tuple(2), std::make_tuple(8));
+  it = m.find(2);
+  ASSERT_NE(it, m.end());
+  EXPECT_EQ(8, absl::any_cast<int>(it->second));
+
+  m.emplace(std::piecewise_construct, std::make_tuple(3),
+            std::make_tuple(absl::any(9)));
+  it = m.find(3);
+  ASSERT_NE(it, m.end());
+  EXPECT_EQ(9, absl::any_cast<int>(it->second));
+
+  struct H {
+    size_t operator()(const absl::any&) const { return 0; }
+  };
+  struct E {
+    bool operator()(const absl::any&, const absl::any&) const { return true; }
+  };
+  absl::flat_hash_map<absl::any, int, H, E> m2;
+  m2.emplace(1, 7);
+  auto it2 = m2.find(1);
+  ASSERT_NE(it2, m2.end());
+  EXPECT_EQ(7, it2->second);
+}
+#endif  // __ANDROID__
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/flat_hash_set.h b/absl/container/flat_hash_set.h
new file mode 100644
index 000000000000..ccd03a4abb6d
--- /dev/null
+++ b/absl/container/flat_hash_set.h
@@ -0,0 +1,439 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: flat_hash_set.h
+// -----------------------------------------------------------------------------
+//
+// An `absl::flat_hash_set<T>` is an unordered associative container designed to
+// be a more efficient replacement for `std::unordered_set`. Like
+// `unordered_set`, search, insertion, and deletion of set elements can be done
+// as an `O(1)` operation. However, `flat_hash_set` (and other unordered
+// associative containers known as the collection of Abseil "Swiss tables")
+// contain other optimizations that result in both memory and computation
+// advantages.
+//
+// In most cases, your default choice for a hash set should be a set of type
+// `flat_hash_set`.
+#ifndef ABSL_CONTAINER_FLAT_HASH_SET_H_
+#define ABSL_CONTAINER_FLAT_HASH_SET_H_
+
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/macros.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
+#include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
+#include "absl/memory/memory.h"
+
+namespace absl {
+namespace container_internal {
+template <typename T>
+struct FlatHashSetPolicy;
+}  // namespace container_internal
+
+// -----------------------------------------------------------------------------
+// absl::flat_hash_set
+// -----------------------------------------------------------------------------
+//
+// An `absl::flat_hash_set<T>` is an unordered associative container which has
+// been optimized for both speed and memory footprint in most common use cases.
+// Its interface is similar to that of `std::unordered_set<T>` with the
+// following notable differences:
+//
+// * Requires keys that are CopyConstructible
+// * Supports heterogeneous lookup, through `find()`, `operator[]()` and
+//   `insert()`, provided that the set is provided a compatible heterogeneous
+//   hashing function and equality operator.
+// * Invalidates any references and pointers to elements within the table after
+//   `rehash()`.
+// * Contains a `capacity()` member function indicating the number of element
+//   slots (open, deleted, and empty) within the hash set.
+// * Returns `void` from the `erase(iterator)` overload.
+//
+// By default, `flat_hash_set` uses the `absl::Hash` hashing framework. All
+// fundamental and Abseil types that support the `absl::Hash` framework have a
+// compatible equality operator for comparing insertions into `flat_hash_map`.
+// If your type is not yet supported by the `asbl::Hash` framework, see
+// absl/hash/hash.h for information on extending Abseil hashing to user-defined
+// types.
+//
+// NOTE: A `flat_hash_set` stores its keys directly inside its implementation
+// array to avoid memory indirection. Because a `flat_hash_set` is designed to
+// move data when rehashed, set keys will not retain pointer stability. If you
+// require pointer stability, consider using
+// `absl::flat_hash_set<std::unique_ptr<T>>`. If your type is not moveable and
+// you require pointer stability, consider `absl::node_hash_set` instead.
+//
+// Example:
+//
+//   // Create a flat hash set of three strings
+//   absl::flat_hash_set<std::string> ducks =
+//     {"huey", "dewey", "louie"};
+//
+//  // Insert a new element into the flat hash set
+//  ducks.insert("donald"};
+//
+//  // Force a rehash of the flat hash set
+//  ducks.rehash(0);
+//
+//  // See if "dewey" is present
+//  if (ducks.contains("dewey")) {
+//    std::cout << "We found dewey!" << std::endl;
+//  }
+template <class T, class Hash = absl::container_internal::hash_default_hash<T>,
+          class Eq = absl::container_internal::hash_default_eq<T>,
+          class Allocator = std::allocator<T>>
+class flat_hash_set
+    : public absl::container_internal::raw_hash_set<
+          absl::container_internal::FlatHashSetPolicy<T>, Hash, Eq, Allocator> {
+  using Base = typename flat_hash_set::raw_hash_set;
+
+ public:
+  flat_hash_set() {}
+  using Base::Base;
+
+  // flat_hash_set::begin()
+  //
+  // Returns an iterator to the beginning of the `flat_hash_set`.
+  using Base::begin;
+
+  // flat_hash_set::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `flat_hash_set`.
+  using Base::cbegin;
+
+  // flat_hash_set::cend()
+  //
+  // Returns a const iterator to the end of the `flat_hash_set`.
+  using Base::cend;
+
+  // flat_hash_set::end()
+  //
+  // Returns an iterator to the end of the `flat_hash_set`.
+  using Base::end;
+
+  // flat_hash_set::capacity()
+  //
+  // Returns the number of element slots (assigned, deleted, and empty)
+  // available within the `flat_hash_set`.
+  //
+  // NOTE: this member function is particular to `absl::flat_hash_set` and is
+  // not provided in the `std::unordered_map` API.
+  using Base::capacity;
+
+  // flat_hash_set::empty()
+  //
+  // Returns whether or not the `flat_hash_set` is empty.
+  using Base::empty;
+
+  // flat_hash_set::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `flat_hash_set` under current memory constraints. This value can be thought
+  // of the largest value of `std::distance(begin(), end())` for a
+  // `flat_hash_set<T>`.
+  using Base::max_size;
+
+  // flat_hash_set::size()
+  //
+  // Returns the number of elements currently within the `flat_hash_set`.
+  using Base::size;
+
+  // flat_hash_set::clear()
+  //
+  // Removes all elements from the `flat_hash_set`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  //
+  // NOTE: this operation may shrink the underlying buffer. To avoid shrinking
+  // the underlying buffer call `erase(begin(), end())`.
+  using Base::clear;
+
+  // flat_hash_set::erase()
+  //
+  // Erases elements within the `flat_hash_set`. Erasing does not trigger a
+  // rehash. Overloads are listed below.
+  //
+  // void erase(const_iterator pos):
+  //
+  //   Erases the element at `position` of the `flat_hash_set`, returning
+  //   `void`.
+  //
+  //   NOTE: this return behavior is different than that of STL containers in
+  //   general and `std::unordered_map` in particular.
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning an
+  //   iterator pointing to `last`.
+  //
+  // size_type erase(const key_type& key):
+  //
+  //   Erases the element with the matching key, if it exists.
+  using Base::erase;
+
+  // flat_hash_set::insert()
+  //
+  // Inserts an element of the specified value into the `flat_hash_set`,
+  // returning an iterator pointing to the newly inserted element, provided that
+  // an element with the given key does not already exist. If rehashing occurs
+  // due to the insertion, all iterators are invalidated. Overloads are listed
+  // below.
+  //
+  // std::pair<iterator,bool> insert(const T& value):
+  //
+  //   Inserts a value into the `flat_hash_set`. Returns a pair consisting of an
+  //   iterator to the inserted element (or to the element that prevented the
+  //   insertion) and a bool denoting whether the insertion took place.
+  //
+  // std::pair<iterator,bool> insert(T&& value):
+  //
+  //   Inserts a moveable value into the `flat_hash_set`. Returns a pair
+  //   consisting of an iterator to the inserted element (or to the element that
+  //   prevented the insertion) and a bool denoting whether the insertion took
+  //   place.
+  //
+  // iterator insert(const_iterator hint, const T& value):
+  // iterator insert(const_iterator hint, T&& value):
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element, or to the existing element that prevented the
+  //   insertion.
+  //
+  // void insert(InputIterator first, InputIterator last ):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently, for `flat_hash_set` we guarantee the
+  //   first match is inserted.
+  //
+  // void insert(std::initializer_list<T> ilist ):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently within the initializer list, for
+  //   `flat_hash_set` we guarantee the first match is inserted.
+  using Base::insert;
+
+  // flat_hash_set::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `flat_hash_set`, provided that no element with the given key
+  // already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace;
+
+  // flat_hash_set::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `flat_hash_set`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search, and only inserts
+  // provided that no element with the given key already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace_hint;
+
+  // flat_hash_set::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the element at the indicated position and returns a node handle
+  //   owning that extracted data.
+  //
+  // node_type extract(const key_type& x):
+  //
+  //   Extracts the element with the key matching the passed key value and
+  //   returns a node handle owning that extracted data. If the `flat_hash_set`
+  //   does not contain an element with a matching key, this function returns an
+  //   empty node handle.
+  using Base::extract;
+
+  // flat_hash_set::merge()
+  //
+  // Extracts elements from a given `source` flat hash map into this
+  // `flat_hash_set`. If the destination `flat_hash_set` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // flat_hash_set::swap(flat_hash_set& other)
+  //
+  // Exchanges the contents of this `flat_hash_set` with those of the `other`
+  // flat hash map, avoiding invocation of any move, copy, or swap operations on
+  // individual elements.
+  //
+  // All iterators and references on the `flat_hash_set` remain valid, excepting
+  // for the past-the-end iterator, which is invalidated.
+  //
+  // `swap()` requires that the flat hash set's hashing and key equivalence
+  // functions be Swappable, and are exchaged using unqualified calls to
+  // non-member `swap()`. If the map's allocator has
+  // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
+  // set to `true`, the allocators are also exchanged using an unqualified call
+  // to non-member `swap()`; otherwise, the allocators are not swapped.
+  using Base::swap;
+
+  // flat_hash_set::rehash(count)
+  //
+  // Rehashes the `flat_hash_set`, setting the number of slots to be at least
+  // the passed value. If the new number of slots increases the load factor more
+  // than the current maximum load factor
+  // (`count` < `size()` / `max_load_factor()`), then the new number of slots
+  // will be at least `size()` / `max_load_factor()`.
+  //
+  // To force a rehash, pass rehash(0).
+  //
+  // NOTE: unlike behavior in `std::unordered_set`, references are also
+  // invalidated upon a `rehash()`.
+  using Base::rehash;
+
+  // flat_hash_set::reserve(count)
+  //
+  // Sets the number of slots in the `flat_hash_set` to the number needed to
+  // accommodate at least `count` total elements without exceeding the current
+  // maximum load factor, and may rehash the container if needed.
+  using Base::reserve;
+
+  // flat_hash_set::contains()
+  //
+  // Determines whether an element comparing equal to the given `key` exists
+  // within the `flat_hash_set`, returning `true` if so or `false` otherwise.
+  using Base::contains;
+
+  // flat_hash_set::count(const Key& key) const
+  //
+  // Returns the number of elements comparing equal to the given `key` within
+  // the `flat_hash_set`. note that this function will return either `1` or `0`
+  // since duplicate elements are not allowed within a `flat_hash_set`.
+  using Base::count;
+
+  // flat_hash_set::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `flat_hash_set`.
+  using Base::equal_range;
+
+  // flat_hash_set::find()
+  //
+  // Finds an element with the passed `key` within the `flat_hash_set`.
+  using Base::find;
+
+  // flat_hash_set::bucket_count()
+  //
+  // Returns the number of "buckets" within the `flat_hash_set`. Note that
+  // because a flat hash map contains all elements within its internal storage,
+  // this value simply equals the current capacity of the `flat_hash_set`.
+  using Base::bucket_count;
+
+  // flat_hash_set::load_factor()
+  //
+  // Returns the current load factor of the `flat_hash_set` (the average number
+  // of slots occupied with a value within the hash map).
+  using Base::load_factor;
+
+  // flat_hash_set::max_load_factor()
+  //
+  // Manages the maximum load factor of the `flat_hash_set`. Overloads are
+  // listed below.
+  //
+  // float flat_hash_set::max_load_factor()
+  //
+  //   Returns the current maximum load factor of the `flat_hash_set`.
+  //
+  // void flat_hash_set::max_load_factor(float ml)
+  //
+  //   Sets the maximum load factor of the `flat_hash_set` to the passed value.
+  //
+  //   NOTE: This overload is provided only for API compatibility with the STL;
+  //   `flat_hash_set` will ignore any set load factor and manage its rehashing
+  //   internally as an implementation detail.
+  using Base::max_load_factor;
+
+  // flat_hash_set::get_allocator()
+  //
+  // Returns the allocator function associated with this `flat_hash_set`.
+  using Base::get_allocator;
+
+  // flat_hash_set::hash_function()
+  //
+  // Returns the hashing function used to hash the keys within this
+  // `flat_hash_set`.
+  using Base::hash_function;
+
+  // flat_hash_set::key_eq()
+  //
+  // Returns the function used for comparing keys equality.
+  using Base::key_eq;
+};
+
+namespace container_internal {
+
+template <class T>
+struct FlatHashSetPolicy {
+  using slot_type = T;
+  using key_type = T;
+  using init_type = T;
+  using constant_iterators = std::true_type;
+
+  template <class Allocator, class... Args>
+  static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
+    absl::allocator_traits<Allocator>::construct(*alloc, slot,
+                                                 std::forward<Args>(args)...);
+  }
+
+  template <class Allocator>
+  static void destroy(Allocator* alloc, slot_type* slot) {
+    absl::allocator_traits<Allocator>::destroy(*alloc, slot);
+  }
+
+  template <class Allocator>
+  static void transfer(Allocator* alloc, slot_type* new_slot,
+                       slot_type* old_slot) {
+    construct(alloc, new_slot, std::move(*old_slot));
+    destroy(alloc, old_slot);
+  }
+
+  static T& element(slot_type* slot) { return *slot; }
+
+  template <class F, class... Args>
+  static decltype(absl::container_internal::DecomposeValue(
+      std::declval<F>(), std::declval<Args>()...))
+  apply(F&& f, Args&&... args) {
+    return absl::container_internal::DecomposeValue(
+        std::forward<F>(f), std::forward<Args>(args)...);
+  }
+
+  static size_t space_used(const T*) { return 0; }
+};
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_FLAT_HASH_SET_H_
diff --git a/absl/container/flat_hash_set_test.cc b/absl/container/flat_hash_set_test.cc
new file mode 100644
index 000000000000..e52fd532cbee
--- /dev/null
+++ b/absl/container/flat_hash_set_test.cc
@@ -0,0 +1,126 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/flat_hash_set.h"
+
+#include <vector>
+
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/unordered_set_constructor_test.h"
+#include "absl/container/internal/unordered_set_lookup_test.h"
+#include "absl/container/internal/unordered_set_modifiers_test.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using ::absl::container_internal::hash_internal::Enum;
+using ::absl::container_internal::hash_internal::EnumClass;
+using ::testing::Pointee;
+using ::testing::UnorderedElementsAre;
+using ::testing::UnorderedElementsAreArray;
+
+template <class T>
+using Set =
+    absl::flat_hash_set<T, StatefulTestingHash, StatefulTestingEqual, Alloc<T>>;
+
+using SetTypes =
+    ::testing::Types<Set<int>, Set<std::string>, Set<Enum>, Set<EnumClass>>;
+
+INSTANTIATE_TYPED_TEST_CASE_P(FlatHashSet, ConstructorTest, SetTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(FlatHashSet, LookupTest, SetTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(FlatHashSet, ModifiersTest, SetTypes);
+
+TEST(FlatHashSet, EmplaceString) {
+  std::vector<std::string> v = {"a", "b"};
+  absl::flat_hash_set<absl::string_view> hs(v.begin(), v.end());
+  EXPECT_THAT(hs, UnorderedElementsAreArray(v));
+}
+
+TEST(FlatHashSet, BitfieldArgument) {
+  union {
+    int n : 1;
+  };
+  n = 0;
+  absl::flat_hash_set<int> s = {n};
+  s.insert(n);
+  s.insert(s.end(), n);
+  s.insert({n});
+  s.erase(n);
+  s.count(n);
+  s.prefetch(n);
+  s.find(n);
+  s.contains(n);
+  s.equal_range(n);
+}
+
+TEST(FlatHashSet, MergeExtractInsert) {
+  struct Hash {
+    size_t operator()(const std::unique_ptr<int>& p) const { return *p; }
+  };
+  struct Eq {
+    bool operator()(const std::unique_ptr<int>& a,
+                    const std::unique_ptr<int>& b) const {
+      return *a == *b;
+    }
+  };
+  absl::flat_hash_set<std::unique_ptr<int>, Hash, Eq> set1, set2;
+  set1.insert(absl::make_unique<int>(7));
+  set1.insert(absl::make_unique<int>(17));
+
+  set2.insert(absl::make_unique<int>(7));
+  set2.insert(absl::make_unique<int>(19));
+
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17)));
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(19)));
+
+  set1.merge(set2);
+
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17), Pointee(19)));
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
+
+  auto node = set1.extract(absl::make_unique<int>(7));
+  EXPECT_TRUE(node);
+  EXPECT_THAT(node.value(), Pointee(7));
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(17), Pointee(19)));
+
+  auto insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_FALSE(insert_result.inserted);
+  EXPECT_TRUE(insert_result.node);
+  EXPECT_THAT(insert_result.node.value(), Pointee(7));
+  EXPECT_EQ(**insert_result.position, 7);
+  EXPECT_NE(insert_result.position->get(), insert_result.node.value().get());
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
+
+  node = set1.extract(absl::make_unique<int>(17));
+  EXPECT_TRUE(node);
+  EXPECT_THAT(node.value(), Pointee(17));
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(19)));
+
+  node.value() = absl::make_unique<int>(23);
+
+  insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_TRUE(insert_result.inserted);
+  EXPECT_FALSE(insert_result.node);
+  EXPECT_EQ(**insert_result.position, 23);
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(23)));
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
new file mode 100644
index 000000000000..56c5d2df6731
--- /dev/null
+++ b/absl/container/internal/container_memory.h
@@ -0,0 +1,405 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_CONTAINER_MEMORY_H_
+#define ABSL_CONTAINER_INTERNAL_CONTAINER_MEMORY_H_
+
+#ifdef ADDRESS_SANITIZER
+#include <sanitizer/asan_interface.h>
+#endif
+
+#ifdef MEMORY_SANITIZER
+#include <sanitizer/msan_interface.h>
+#endif
+
+#include <cassert>
+#include <cstddef>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/memory/memory.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+namespace container_internal {
+
+// Allocates at least n bytes aligned to the specified alignment.
+// Alignment must be a power of 2. It must be positive.
+//
+// Note that many allocators don't honor alignment requirements above certain
+// threshold (usually either alignof(std::max_align_t) or alignof(void*)).
+// Allocate() doesn't apply alignment corrections. If the underlying allocator
+// returns insufficiently alignment pointer, that's what you are going to get.
+template <size_t Alignment, class Alloc>
+void* Allocate(Alloc* alloc, size_t n) {
+  static_assert(Alignment > 0, "");
+  assert(n && "n must be positive");
+  struct alignas(Alignment) M {};
+  using A = typename absl::allocator_traits<Alloc>::template rebind_alloc<M>;
+  using AT = typename absl::allocator_traits<Alloc>::template rebind_traits<M>;
+  A mem_alloc(*alloc);
+  void* p = AT::allocate(mem_alloc, (n + sizeof(M) - 1) / sizeof(M));
+  assert(reinterpret_cast<uintptr_t>(p) % Alignment == 0 &&
+         "allocator does not respect alignment");
+  return p;
+}
+
+// The pointer must have been previously obtained by calling
+// Allocate<Alignment>(alloc, n).
+template <size_t Alignment, class Alloc>
+void Deallocate(Alloc* alloc, void* p, size_t n) {
+  static_assert(Alignment > 0, "");
+  assert(n && "n must be positive");
+  struct alignas(Alignment) M {};
+  using A = typename absl::allocator_traits<Alloc>::template rebind_alloc<M>;
+  using AT = typename absl::allocator_traits<Alloc>::template rebind_traits<M>;
+  A mem_alloc(*alloc);
+  AT::deallocate(mem_alloc, static_cast<M*>(p),
+                 (n + sizeof(M) - 1) / sizeof(M));
+}
+
+namespace memory_internal {
+
+// Constructs T into uninitialized storage pointed by `ptr` using the args
+// specified in the tuple.
+template <class Alloc, class T, class Tuple, size_t... I>
+void ConstructFromTupleImpl(Alloc* alloc, T* ptr, Tuple&& t,
+                            absl::index_sequence<I...>) {
+  absl::allocator_traits<Alloc>::construct(
+      *alloc, ptr, std::get<I>(std::forward<Tuple>(t))...);
+}
+
+template <class T, class F>
+struct WithConstructedImplF {
+  template <class... Args>
+  decltype(std::declval<F>()(std::declval<T>())) operator()(
+      Args&&... args) const {
+    return std::forward<F>(f)(T(std::forward<Args>(args)...));
+  }
+  F&& f;
+};
+
+template <class T, class Tuple, size_t... Is, class F>
+decltype(std::declval<F>()(std::declval<T>())) WithConstructedImpl(
+    Tuple&& t, absl::index_sequence<Is...>, F&& f) {
+  return WithConstructedImplF<T, F>{std::forward<F>(f)}(
+      std::get<Is>(std::forward<Tuple>(t))...);
+}
+
+template <class T, size_t... Is>
+auto TupleRefImpl(T&& t, absl::index_sequence<Is...>)
+    -> decltype(std::forward_as_tuple(std::get<Is>(std::forward<T>(t))...)) {
+  return std::forward_as_tuple(std::get<Is>(std::forward<T>(t))...);
+}
+
+// Returns a tuple of references to the elements of the input tuple. T must be a
+// tuple.
+template <class T>
+auto TupleRef(T&& t) -> decltype(
+    TupleRefImpl(std::forward<T>(t),
+                 absl::make_index_sequence<
+                     std::tuple_size<typename std::decay<T>::type>::value>())) {
+  return TupleRefImpl(
+      std::forward<T>(t),
+      absl::make_index_sequence<
+          std::tuple_size<typename std::decay<T>::type>::value>());
+}
+
+template <class F, class K, class V>
+decltype(std::declval<F>()(std::declval<const K&>(), std::piecewise_construct,
+                           std::declval<std::tuple<K>>(), std::declval<V>()))
+DecomposePairImpl(F&& f, std::pair<std::tuple<K>, V> p) {
+  const auto& key = std::get<0>(p.first);
+  return std::forward<F>(f)(key, std::piecewise_construct, std::move(p.first),
+                            std::move(p.second));
+}
+
+}  // namespace memory_internal
+
+// Constructs T into uninitialized storage pointed by `ptr` using the args
+// specified in the tuple.
+template <class Alloc, class T, class Tuple>
+void ConstructFromTuple(Alloc* alloc, T* ptr, Tuple&& t) {
+  memory_internal::ConstructFromTupleImpl(
+      alloc, ptr, std::forward<Tuple>(t),
+      absl::make_index_sequence<
+          std::tuple_size<typename std::decay<Tuple>::type>::value>());
+}
+
+// Constructs T using the args specified in the tuple and calls F with the
+// constructed value.
+template <class T, class Tuple, class F>
+decltype(std::declval<F>()(std::declval<T>())) WithConstructed(
+    Tuple&& t, F&& f) {
+  return memory_internal::WithConstructedImpl<T>(
+      std::forward<Tuple>(t),
+      absl::make_index_sequence<
+          std::tuple_size<typename std::decay<Tuple>::type>::value>(),
+      std::forward<F>(f));
+}
+
+// Given arguments of an std::pair's consructor, PairArgs() returns a pair of
+// tuples with references to the passed arguments. The tuples contain
+// constructor arguments for the first and the second elements of the pair.
+//
+// The following two snippets are equivalent.
+//
+// 1. std::pair<F, S> p(args...);
+//
+// 2. auto a = PairArgs(args...);
+//    std::pair<F, S> p(std::piecewise_construct,
+//                      std::move(p.first), std::move(p.second));
+inline std::pair<std::tuple<>, std::tuple<>> PairArgs() { return {}; }
+template <class F, class S>
+std::pair<std::tuple<F&&>, std::tuple<S&&>> PairArgs(F&& f, S&& s) {
+  return {std::piecewise_construct, std::forward_as_tuple(std::forward<F>(f)),
+          std::forward_as_tuple(std::forward<S>(s))};
+}
+template <class F, class S>
+std::pair<std::tuple<const F&>, std::tuple<const S&>> PairArgs(
+    const std::pair<F, S>& p) {
+  return PairArgs(p.first, p.second);
+}
+template <class F, class S>
+std::pair<std::tuple<F&&>, std::tuple<S&&>> PairArgs(std::pair<F, S>&& p) {
+  return PairArgs(std::forward<F>(p.first), std::forward<S>(p.second));
+}
+template <class F, class S>
+auto PairArgs(std::piecewise_construct_t, F&& f, S&& s)
+    -> decltype(std::make_pair(memory_internal::TupleRef(std::forward<F>(f)),
+                               memory_internal::TupleRef(std::forward<S>(s)))) {
+  return std::make_pair(memory_internal::TupleRef(std::forward<F>(f)),
+                        memory_internal::TupleRef(std::forward<S>(s)));
+}
+
+// A helper function for implementing apply() in map policies.
+template <class F, class... Args>
+auto DecomposePair(F&& f, Args&&... args)
+    -> decltype(memory_internal::DecomposePairImpl(
+        std::forward<F>(f), PairArgs(std::forward<Args>(args)...))) {
+  return memory_internal::DecomposePairImpl(
+      std::forward<F>(f), PairArgs(std::forward<Args>(args)...));
+}
+
+// A helper function for implementing apply() in set policies.
+template <class F, class Arg>
+decltype(std::declval<F>()(std::declval<const Arg&>(), std::declval<Arg>()))
+DecomposeValue(F&& f, Arg&& arg) {
+  const auto& key = arg;
+  return std::forward<F>(f)(key, std::forward<Arg>(arg));
+}
+
+// Helper functions for asan and msan.
+inline void SanitizerPoisonMemoryRegion(const void* m, size_t s) {
+#ifdef ADDRESS_SANITIZER
+  ASAN_POISON_MEMORY_REGION(m, s);
+#endif
+#ifdef MEMORY_SANITIZER
+  __msan_poison(m, s);
+#endif
+  (void)m;
+  (void)s;
+}
+
+inline void SanitizerUnpoisonMemoryRegion(const void* m, size_t s) {
+#ifdef ADDRESS_SANITIZER
+  ASAN_UNPOISON_MEMORY_REGION(m, s);
+#endif
+#ifdef MEMORY_SANITIZER
+  __msan_unpoison(m, s);
+#endif
+  (void)m;
+  (void)s;
+}
+
+template <typename T>
+inline void SanitizerPoisonObject(const T* object) {
+  SanitizerPoisonMemoryRegion(object, sizeof(T));
+}
+
+template <typename T>
+inline void SanitizerUnpoisonObject(const T* object) {
+  SanitizerUnpoisonMemoryRegion(object, sizeof(T));
+}
+
+namespace memory_internal {
+
+// If Pair is a standard-layout type, OffsetOf<Pair>::kFirst and
+// OffsetOf<Pair>::kSecond are equivalent to offsetof(Pair, first) and
+// offsetof(Pair, second) respectively. Otherwise they are -1.
+//
+// The purpose of OffsetOf is to avoid calling offsetof() on non-standard-layout
+// type, which is non-portable.
+template <class Pair, class = std::true_type>
+struct OffsetOf {
+  static constexpr size_t kFirst = -1;
+  static constexpr size_t kSecond = -1;
+};
+
+template <class Pair>
+struct OffsetOf<Pair, typename std::is_standard_layout<Pair>::type> {
+  static constexpr size_t kFirst = offsetof(Pair, first);
+  static constexpr size_t kSecond = offsetof(Pair, second);
+};
+
+template <class K, class V>
+struct IsLayoutCompatible {
+ private:
+  struct Pair {
+    K first;
+    V second;
+  };
+
+  // Is P layout-compatible with Pair?
+  template <class P>
+  static constexpr bool LayoutCompatible() {
+    return std::is_standard_layout<P>() && sizeof(P) == sizeof(Pair) &&
+           alignof(P) == alignof(Pair) &&
+           memory_internal::OffsetOf<P>::kFirst ==
+               memory_internal::OffsetOf<Pair>::kFirst &&
+           memory_internal::OffsetOf<P>::kSecond ==
+               memory_internal::OffsetOf<Pair>::kSecond;
+  }
+
+ public:
+  // Whether pair<const K, V> and pair<K, V> are layout-compatible. If they are,
+  // then it is safe to store them in a union and read from either.
+  static constexpr bool value = std::is_standard_layout<K>() &&
+                                std::is_standard_layout<Pair>() &&
+                                memory_internal::OffsetOf<Pair>::kFirst == 0 &&
+                                LayoutCompatible<std::pair<K, V>>() &&
+                                LayoutCompatible<std::pair<const K, V>>();
+};
+
+}  // namespace memory_internal
+
+// If kMutableKeys is false, only the value member is accessed.
+//
+// If kMutableKeys is true, key is accessed through all slots while value and
+// mutable_value are accessed only via INITIALIZED slots. Slots are created and
+// destroyed via mutable_value so that the key can be moved later.
+template <class K, class V>
+union slot_type {
+ private:
+  static void emplace(slot_type* slot) {
+    // The construction of union doesn't do anything at runtime but it allows us
+    // to access its members without violating aliasing rules.
+    new (slot) slot_type;
+  }
+  // If pair<const K, V> and pair<K, V> are layout-compatible, we can accept one
+  // or the other via slot_type. We are also free to access the key via
+  // slot_type::key in this case.
+  using kMutableKeys =
+      std::integral_constant<bool,
+                             memory_internal::IsLayoutCompatible<K, V>::value>;
+
+ public:
+  slot_type() {}
+  ~slot_type() = delete;
+  using value_type = std::pair<const K, V>;
+  using mutable_value_type = std::pair<K, V>;
+
+  value_type value;
+  mutable_value_type mutable_value;
+  K key;
+
+  template <class Allocator, class... Args>
+  static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
+    emplace(slot);
+    if (kMutableKeys::value) {
+      absl::allocator_traits<Allocator>::construct(*alloc, &slot->mutable_value,
+                                                   std::forward<Args>(args)...);
+    } else {
+      absl::allocator_traits<Allocator>::construct(*alloc, &slot->value,
+                                                   std::forward<Args>(args)...);
+    }
+  }
+
+  // Construct this slot by moving from another slot.
+  template <class Allocator>
+  static void construct(Allocator* alloc, slot_type* slot, slot_type* other) {
+    emplace(slot);
+    if (kMutableKeys::value) {
+      absl::allocator_traits<Allocator>::construct(
+          *alloc, &slot->mutable_value, std::move(other->mutable_value));
+    } else {
+      absl::allocator_traits<Allocator>::construct(*alloc, &slot->value,
+                                                   std::move(other->value));
+    }
+  }
+
+  template <class Allocator>
+  static void destroy(Allocator* alloc, slot_type* slot) {
+    if (kMutableKeys::value) {
+      absl::allocator_traits<Allocator>::destroy(*alloc, &slot->mutable_value);
+    } else {
+      absl::allocator_traits<Allocator>::destroy(*alloc, &slot->value);
+    }
+  }
+
+  template <class Allocator>
+  static void transfer(Allocator* alloc, slot_type* new_slot,
+                       slot_type* old_slot) {
+    emplace(new_slot);
+    if (kMutableKeys::value) {
+      absl::allocator_traits<Allocator>::construct(
+          *alloc, &new_slot->mutable_value, std::move(old_slot->mutable_value));
+    } else {
+      absl::allocator_traits<Allocator>::construct(*alloc, &new_slot->value,
+                                                   std::move(old_slot->value));
+    }
+    destroy(alloc, old_slot);
+  }
+
+  template <class Allocator>
+  static void swap(Allocator* alloc, slot_type* a, slot_type* b) {
+    if (kMutableKeys::value) {
+      using std::swap;
+      swap(a->mutable_value, b->mutable_value);
+    } else {
+      value_type tmp = std::move(a->value);
+      absl::allocator_traits<Allocator>::destroy(*alloc, &a->value);
+      absl::allocator_traits<Allocator>::construct(*alloc, &a->value,
+                                                   std::move(b->value));
+      absl::allocator_traits<Allocator>::destroy(*alloc, &b->value);
+      absl::allocator_traits<Allocator>::construct(*alloc, &b->value,
+                                                   std::move(tmp));
+    }
+  }
+
+  template <class Allocator>
+  static void move(Allocator* alloc, slot_type* src, slot_type* dest) {
+    if (kMutableKeys::value) {
+      dest->mutable_value = std::move(src->mutable_value);
+    } else {
+      absl::allocator_traits<Allocator>::destroy(*alloc, &dest->value);
+      absl::allocator_traits<Allocator>::construct(*alloc, &dest->value,
+                                                   std::move(src->value));
+    }
+  }
+
+  template <class Allocator>
+  static void move(Allocator* alloc, slot_type* first, slot_type* last,
+                   slot_type* result) {
+    for (slot_type *src = first, *dest = result; src != last; ++src, ++dest)
+      move(alloc, src, dest);
+  }
+};
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_CONTAINER_MEMORY_H_
diff --git a/absl/container/internal/container_memory_test.cc b/absl/container/internal/container_memory_test.cc
new file mode 100644
index 000000000000..f1c4058298c1
--- /dev/null
+++ b/absl/container/internal/container_memory_test.cc
@@ -0,0 +1,188 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/container_memory.h"
+
+#include <cstdint>
+#include <tuple>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using ::testing::Pair;
+
+TEST(Memory, AlignmentLargerThanBase) {
+  std::allocator<int8_t> alloc;
+  void* mem = Allocate<2>(&alloc, 3);
+  EXPECT_EQ(0, reinterpret_cast<uintptr_t>(mem) % 2);
+  memcpy(mem, "abc", 3);
+  Deallocate<2>(&alloc, mem, 3);
+}
+
+TEST(Memory, AlignmentSmallerThanBase) {
+  std::allocator<int64_t> alloc;
+  void* mem = Allocate<2>(&alloc, 3);
+  EXPECT_EQ(0, reinterpret_cast<uintptr_t>(mem) % 2);
+  memcpy(mem, "abc", 3);
+  Deallocate<2>(&alloc, mem, 3);
+}
+
+class Fixture : public ::testing::Test {
+  using Alloc = std::allocator<std::string>;
+
+ public:
+  Fixture() { ptr_ = std::allocator_traits<Alloc>::allocate(*alloc(), 1); }
+  ~Fixture() override {
+    std::allocator_traits<Alloc>::destroy(*alloc(), ptr_);
+    std::allocator_traits<Alloc>::deallocate(*alloc(), ptr_, 1);
+  }
+  std::string* ptr() { return ptr_; }
+  Alloc* alloc() { return &alloc_; }
+
+ private:
+  Alloc alloc_;
+  std::string* ptr_;
+};
+
+TEST_F(Fixture, ConstructNoArgs) {
+  ConstructFromTuple(alloc(), ptr(), std::forward_as_tuple());
+  EXPECT_EQ(*ptr(), "");
+}
+
+TEST_F(Fixture, ConstructOneArg) {
+  ConstructFromTuple(alloc(), ptr(), std::forward_as_tuple("abcde"));
+  EXPECT_EQ(*ptr(), "abcde");
+}
+
+TEST_F(Fixture, ConstructTwoArg) {
+  ConstructFromTuple(alloc(), ptr(), std::forward_as_tuple(5, 'a'));
+  EXPECT_EQ(*ptr(), "aaaaa");
+}
+
+TEST(PairArgs, NoArgs) {
+  EXPECT_THAT(PairArgs(),
+              Pair(std::forward_as_tuple(), std::forward_as_tuple()));
+}
+
+TEST(PairArgs, TwoArgs) {
+  EXPECT_EQ(
+      std::make_pair(std::forward_as_tuple(1), std::forward_as_tuple('A')),
+      PairArgs(1, 'A'));
+}
+
+TEST(PairArgs, Pair) {
+  EXPECT_EQ(
+      std::make_pair(std::forward_as_tuple(1), std::forward_as_tuple('A')),
+      PairArgs(std::make_pair(1, 'A')));
+}
+
+TEST(PairArgs, Piecewise) {
+  EXPECT_EQ(
+      std::make_pair(std::forward_as_tuple(1), std::forward_as_tuple('A')),
+      PairArgs(std::piecewise_construct, std::forward_as_tuple(1),
+               std::forward_as_tuple('A')));
+}
+
+TEST(WithConstructed, Simple) {
+  EXPECT_EQ(1, WithConstructed<absl::string_view>(
+                   std::make_tuple(std::string("a")),
+                   [](absl::string_view str) { return str.size(); }));
+}
+
+template <class F, class Arg>
+decltype(DecomposeValue(std::declval<F>(), std::declval<Arg>()))
+DecomposeValueImpl(int, F&& f, Arg&& arg) {
+  return DecomposeValue(std::forward<F>(f), std::forward<Arg>(arg));
+}
+
+template <class F, class Arg>
+const char* DecomposeValueImpl(char, F&& f, Arg&& arg) {
+  return "not decomposable";
+}
+
+template <class F, class Arg>
+decltype(DecomposeValueImpl(0, std::declval<F>(), std::declval<Arg>()))
+TryDecomposeValue(F&& f, Arg&& arg) {
+  return DecomposeValueImpl(0, std::forward<F>(f), std::forward<Arg>(arg));
+}
+
+TEST(DecomposeValue, Decomposable) {
+  auto f = [](const int& x, int&& y) {
+    EXPECT_EQ(&x, &y);
+    EXPECT_EQ(42, x);
+    return 'A';
+  };
+  EXPECT_EQ('A', TryDecomposeValue(f, 42));
+}
+
+TEST(DecomposeValue, NotDecomposable) {
+  auto f = [](void*) {
+    ADD_FAILURE() << "Must not be called";
+    return 'A';
+  };
+  EXPECT_STREQ("not decomposable", TryDecomposeValue(f, 42));
+}
+
+template <class F, class... Args>
+decltype(DecomposePair(std::declval<F>(), std::declval<Args>()...))
+DecomposePairImpl(int, F&& f, Args&&... args) {
+  return DecomposePair(std::forward<F>(f), std::forward<Args>(args)...);
+}
+
+template <class F, class... Args>
+const char* DecomposePairImpl(char, F&& f, Args&&... args) {
+  return "not decomposable";
+}
+
+template <class F, class... Args>
+decltype(DecomposePairImpl(0, std::declval<F>(), std::declval<Args>()...))
+TryDecomposePair(F&& f, Args&&... args) {
+  return DecomposePairImpl(0, std::forward<F>(f), std::forward<Args>(args)...);
+}
+
+TEST(DecomposePair, Decomposable) {
+  auto f = [](const int& x, std::piecewise_construct_t, std::tuple<int&&> k,
+              std::tuple<double>&& v) {
+    EXPECT_EQ(&x, &std::get<0>(k));
+    EXPECT_EQ(42, x);
+    EXPECT_EQ(0.5, std::get<0>(v));
+    return 'A';
+  };
+  EXPECT_EQ('A', TryDecomposePair(f, 42, 0.5));
+  EXPECT_EQ('A', TryDecomposePair(f, std::make_pair(42, 0.5)));
+  EXPECT_EQ('A', TryDecomposePair(f, std::piecewise_construct,
+                                  std::make_tuple(42), std::make_tuple(0.5)));
+}
+
+TEST(DecomposePair, NotDecomposable) {
+  auto f = [](...) {
+    ADD_FAILURE() << "Must not be called";
+    return 'A';
+  };
+  EXPECT_STREQ("not decomposable",
+               TryDecomposePair(f));
+  EXPECT_STREQ("not decomposable",
+               TryDecomposePair(f, std::piecewise_construct, std::make_tuple(),
+                                std::make_tuple(0.5)));
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/hash_function_defaults.h b/absl/container/internal/hash_function_defaults.h
new file mode 100644
index 000000000000..dd6cd8f537fd
--- /dev/null
+++ b/absl/container/internal/hash_function_defaults.h
@@ -0,0 +1,148 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Define the default Hash and Eq functions for SwissTable containers.
+//
+// std::hash<T> and std::equal_to<T> are not appropriate hash and equal
+// functions for SwissTable containers. There are two reasons for this.
+//
+// SwissTable containers are power of 2 sized containers:
+//
+// This means they use the lower bits of the hash value to find the slot for
+// each entry. The typical hash function for integral types is the identity.
+// This is a very weak hash function for SwissTable and any power of 2 sized
+// hashtable implementation which will lead to excessive collisions. For
+// SwissTable we use murmur3 style mixing to reduce collisions to a minimum.
+//
+// SwissTable containers support heterogeneous lookup:
+//
+// In order to make heterogeneous lookup work, hash and equal functions must be
+// polymorphic. At the same time they have to satisfy the same requirements the
+// C++ standard imposes on hash functions and equality operators. That is:
+//
+//   if hash_default_eq<T>(a, b) returns true for any a and b of type T, then
+//   hash_default_hash<T>(a) must equal hash_default_hash<T>(b)
+//
+// For SwissTable containers this requirement is relaxed to allow a and b of
+// any and possibly different types. Note that like the standard the hash and
+// equal functions are still bound to T. This is important because some type U
+// can be hashed by/tested for equality differently depending on T. A notable
+// example is `const char*`. `const char*` is treated as a c-style string when
+// the hash function is hash<string> but as a pointer when the hash function is
+// hash<void*>.
+//
+#ifndef ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
+#define ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
+
+#include <stdint.h>
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/hash/hash.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+
+// The hash of an object of type T is computed by using absl::Hash.
+template <class T, class E = void>
+struct HashEq {
+  using Hash = absl::Hash<T>;
+  using Eq = std::equal_to<T>;
+};
+
+struct StringHash {
+  using is_transparent = void;
+
+  size_t operator()(absl::string_view v) const {
+    return absl::Hash<absl::string_view>{}(v);
+  }
+};
+
+// Supports heterogeneous lookup for string-like elements.
+struct StringHashEq {
+  using Hash = StringHash;
+  struct Eq {
+    using is_transparent = void;
+    bool operator()(absl::string_view lhs, absl::string_view rhs) const {
+      return lhs == rhs;
+    }
+  };
+};
+
+#if defined(HAS_GLOBAL_STRING)
+template <>
+struct HashEq<std::string> : StringHashEq {};
+#endif
+template <>
+struct HashEq<std::string> : StringHashEq {};
+template <>
+struct HashEq<absl::string_view> : StringHashEq {};
+
+// Supports heterogeneous lookup for pointers and smart pointers.
+template <class T>
+struct HashEq<T*> {
+  struct Hash {
+    using is_transparent = void;
+    template <class U>
+    size_t operator()(const U& ptr) const {
+      return absl::Hash<const T*>{}(HashEq::ToPtr(ptr));
+    }
+  };
+  struct Eq {
+    using is_transparent = void;
+    template <class A, class B>
+    bool operator()(const A& a, const B& b) const {
+      return HashEq::ToPtr(a) == HashEq::ToPtr(b);
+    }
+  };
+
+ private:
+  static const T* ToPtr(const T* ptr) { return ptr; }
+  template <class U, class D>
+  static const T* ToPtr(const std::unique_ptr<U, D>& ptr) {
+    return ptr.get();
+  }
+  template <class U>
+  static const T* ToPtr(const std::shared_ptr<U>& ptr) {
+    return ptr.get();
+  }
+};
+
+template <class T, class D>
+struct HashEq<std::unique_ptr<T, D>> : HashEq<T*> {};
+template <class T>
+struct HashEq<std::shared_ptr<T>> : HashEq<T*> {};
+
+// This header's visibility is restricted.  If you need to access the default
+// hasher please use the container's ::hasher alias instead.
+//
+// Example: typename Hash = typename absl::flat_hash_map<K, V>::hasher
+template <class T>
+using hash_default_hash = typename container_internal::HashEq<T>::Hash;
+
+// This header's visibility is restricted.  If you need to access the default
+// key equal please use the container's ::key_equal alias instead.
+//
+// Example: typename Eq = typename absl::flat_hash_map<K, V, Hash>::key_equal
+template <class T>
+using hash_default_eq = typename container_internal::HashEq<T>::Eq;
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
diff --git a/absl/container/internal/hash_function_defaults_test.cc b/absl/container/internal/hash_function_defaults_test.cc
new file mode 100644
index 000000000000..464baae02cdd
--- /dev/null
+++ b/absl/container/internal/hash_function_defaults_test.cc
@@ -0,0 +1,299 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/hash_function_defaults.h"
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using ::testing::Types;
+
+TEST(Eq, Int32) {
+  hash_default_eq<int32_t> eq;
+  EXPECT_TRUE(eq(1, 1u));
+  EXPECT_TRUE(eq(1, char{1}));
+  EXPECT_TRUE(eq(1, true));
+  EXPECT_TRUE(eq(1, double{1.1}));
+  EXPECT_FALSE(eq(1, char{2}));
+  EXPECT_FALSE(eq(1, 2u));
+  EXPECT_FALSE(eq(1, false));
+  EXPECT_FALSE(eq(1, 2.));
+}
+
+TEST(Hash, Int32) {
+  hash_default_hash<int32_t> hash;
+  auto h = hash(1);
+  EXPECT_EQ(h, hash(1u));
+  EXPECT_EQ(h, hash(char{1}));
+  EXPECT_EQ(h, hash(true));
+  EXPECT_EQ(h, hash(double{1.1}));
+  EXPECT_NE(h, hash(2u));
+  EXPECT_NE(h, hash(char{2}));
+  EXPECT_NE(h, hash(false));
+  EXPECT_NE(h, hash(2.));
+}
+
+enum class MyEnum { A, B, C, D };
+
+TEST(Eq, Enum) {
+  hash_default_eq<MyEnum> eq;
+  EXPECT_TRUE(eq(MyEnum::A, MyEnum::A));
+  EXPECT_FALSE(eq(MyEnum::A, MyEnum::B));
+}
+
+TEST(Hash, Enum) {
+  hash_default_hash<MyEnum> hash;
+
+  for (MyEnum e : {MyEnum::A, MyEnum::B, MyEnum::C}) {
+    auto h = hash(e);
+    EXPECT_EQ(h, hash_default_hash<int>{}(static_cast<int>(e)));
+    EXPECT_NE(h, hash(MyEnum::D));
+  }
+}
+
+using StringTypes = ::testing::Types<std::string, absl::string_view>;
+
+template <class T>
+struct EqString : ::testing::Test {
+  hash_default_eq<T> key_eq;
+};
+
+TYPED_TEST_CASE(EqString, StringTypes);
+
+template <class T>
+struct HashString : ::testing::Test {
+  hash_default_hash<T> hasher;
+};
+
+TYPED_TEST_CASE(HashString, StringTypes);
+
+TYPED_TEST(EqString, Works) {
+  auto eq = this->key_eq;
+  EXPECT_TRUE(eq("a", "a"));
+  EXPECT_TRUE(eq("a", absl::string_view("a")));
+  EXPECT_TRUE(eq("a", std::string("a")));
+  EXPECT_FALSE(eq("a", "b"));
+  EXPECT_FALSE(eq("a", absl::string_view("b")));
+  EXPECT_FALSE(eq("a", std::string("b")));
+}
+
+TYPED_TEST(HashString, Works) {
+  auto hash = this->hasher;
+  auto h = hash("a");
+  EXPECT_EQ(h, hash(absl::string_view("a")));
+  EXPECT_EQ(h, hash(std::string("a")));
+  EXPECT_NE(h, hash(absl::string_view("b")));
+  EXPECT_NE(h, hash(std::string("b")));
+}
+
+struct NoDeleter {
+  template <class T>
+  void operator()(const T* ptr) const {}
+};
+
+using PointerTypes =
+    ::testing::Types<const int*, int*, std::unique_ptr<const int>,
+                     std::unique_ptr<const int, NoDeleter>,
+                     std::unique_ptr<int>, std::unique_ptr<int, NoDeleter>,
+                     std::shared_ptr<const int>, std::shared_ptr<int>>;
+
+template <class T>
+struct EqPointer : ::testing::Test {
+  hash_default_eq<T> key_eq;
+};
+
+TYPED_TEST_CASE(EqPointer, PointerTypes);
+
+template <class T>
+struct HashPointer : ::testing::Test {
+  hash_default_hash<T> hasher;
+};
+
+TYPED_TEST_CASE(HashPointer, PointerTypes);
+
+TYPED_TEST(EqPointer, Works) {
+  int dummy;
+  auto eq = this->key_eq;
+  auto sptr = std::make_shared<int>();
+  std::shared_ptr<const int> csptr = sptr;
+  int* ptr = sptr.get();
+  const int* cptr = ptr;
+  std::unique_ptr<int, NoDeleter> uptr(ptr);
+  std::unique_ptr<const int, NoDeleter> cuptr(ptr);
+
+  EXPECT_TRUE(eq(ptr, cptr));
+  EXPECT_TRUE(eq(ptr, sptr));
+  EXPECT_TRUE(eq(ptr, uptr));
+  EXPECT_TRUE(eq(ptr, csptr));
+  EXPECT_TRUE(eq(ptr, cuptr));
+  EXPECT_FALSE(eq(&dummy, cptr));
+  EXPECT_FALSE(eq(&dummy, sptr));
+  EXPECT_FALSE(eq(&dummy, uptr));
+  EXPECT_FALSE(eq(&dummy, csptr));
+  EXPECT_FALSE(eq(&dummy, cuptr));
+}
+
+TEST(Hash, DerivedAndBase) {
+  struct Base {};
+  struct Derived : Base {};
+
+  hash_default_hash<Base*> hasher;
+
+  Base base;
+  Derived derived;
+  EXPECT_NE(hasher(&base), hasher(&derived));
+  EXPECT_EQ(hasher(static_cast<Base*>(&derived)), hasher(&derived));
+
+  auto dp = std::make_shared<Derived>();
+  EXPECT_EQ(hasher(static_cast<Base*>(dp.get())), hasher(dp));
+}
+
+TEST(Hash, FunctionPointer) {
+  using Func = int (*)();
+  hash_default_hash<Func> hasher;
+  hash_default_eq<Func> eq;
+
+  Func p1 = [] { return 1; }, p2 = [] { return 2; };
+  EXPECT_EQ(hasher(p1), hasher(p1));
+  EXPECT_TRUE(eq(p1, p1));
+
+  EXPECT_NE(hasher(p1), hasher(p2));
+  EXPECT_FALSE(eq(p1, p2));
+}
+
+TYPED_TEST(HashPointer, Works) {
+  int dummy;
+  auto hash = this->hasher;
+  auto sptr = std::make_shared<int>();
+  std::shared_ptr<const int> csptr = sptr;
+  int* ptr = sptr.get();
+  const int* cptr = ptr;
+  std::unique_ptr<int, NoDeleter> uptr(ptr);
+  std::unique_ptr<const int, NoDeleter> cuptr(ptr);
+
+  EXPECT_EQ(hash(ptr), hash(cptr));
+  EXPECT_EQ(hash(ptr), hash(sptr));
+  EXPECT_EQ(hash(ptr), hash(uptr));
+  EXPECT_EQ(hash(ptr), hash(csptr));
+  EXPECT_EQ(hash(ptr), hash(cuptr));
+  EXPECT_NE(hash(&dummy), hash(cptr));
+  EXPECT_NE(hash(&dummy), hash(sptr));
+  EXPECT_NE(hash(&dummy), hash(uptr));
+  EXPECT_NE(hash(&dummy), hash(csptr));
+  EXPECT_NE(hash(&dummy), hash(cuptr));
+}
+
+// Cartesian product of (string, std::string, absl::string_view)
+// with (string, std::string, absl::string_view, const char*).
+using StringTypesCartesianProduct = Types<
+    // clang-format off
+
+    std::pair<std::string, std::string>,
+    std::pair<std::string, absl::string_view>,
+    std::pair<std::string, const char*>,
+
+    std::pair<absl::string_view, std::string>,
+    std::pair<absl::string_view, absl::string_view>,
+    std::pair<absl::string_view, const char*>>;
+// clang-format on
+
+constexpr char kFirstString[] = "abc123";
+constexpr char kSecondString[] = "ijk456";
+
+template <typename T>
+struct StringLikeTest : public ::testing::Test {
+  typename T::first_type a1{kFirstString};
+  typename T::second_type b1{kFirstString};
+  typename T::first_type a2{kSecondString};
+  typename T::second_type b2{kSecondString};
+  hash_default_eq<typename T::first_type> eq;
+  hash_default_hash<typename T::first_type> hash;
+};
+
+TYPED_TEST_CASE_P(StringLikeTest);
+
+TYPED_TEST_P(StringLikeTest, Eq) {
+  EXPECT_TRUE(this->eq(this->a1, this->b1));
+  EXPECT_TRUE(this->eq(this->b1, this->a1));
+}
+
+TYPED_TEST_P(StringLikeTest, NotEq) {
+  EXPECT_FALSE(this->eq(this->a1, this->b2));
+  EXPECT_FALSE(this->eq(this->b2, this->a1));
+}
+
+TYPED_TEST_P(StringLikeTest, HashEq) {
+  EXPECT_EQ(this->hash(this->a1), this->hash(this->b1));
+  EXPECT_EQ(this->hash(this->a2), this->hash(this->b2));
+  // It would be a poor hash function which collides on these strings.
+  EXPECT_NE(this->hash(this->a1), this->hash(this->b2));
+}
+
+TYPED_TEST_CASE(StringLikeTest, StringTypesCartesianProduct);
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
+
+enum Hash : size_t {
+  kStd = 0x2,       // std::hash
+#ifdef _MSC_VER
+  kExtension = kStd,  // In MSVC, std::hash == ::hash
+#else                 // _MSC_VER
+  kExtension = 0x4,  // ::hash (GCC extension)
+#endif                // _MSC_VER
+};
+
+// H is a bitmask of Hash enumerations.
+// Hashable<H> is hashable via all means specified in H.
+template <int H>
+struct Hashable {
+  static constexpr bool HashableBy(Hash h) { return h & H; }
+};
+
+namespace std {
+template <int H>
+struct hash<Hashable<H>> {
+  template <class E = Hashable<H>,
+            class = typename std::enable_if<E::HashableBy(kStd)>::type>
+  size_t operator()(E) const {
+    return kStd;
+  }
+};
+}  // namespace std
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+template <class T>
+size_t Hash(const T& v) {
+  return hash_default_hash<T>()(v);
+}
+
+TEST(Delegate, HashDispatch) {
+  EXPECT_EQ(Hash(kStd), Hash(Hashable<kStd>()));
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/hash_generator_testing.cc b/absl/container/internal/hash_generator_testing.cc
new file mode 100644
index 000000000000..0d6a9df16f8a
--- /dev/null
+++ b/absl/container/internal/hash_generator_testing.cc
@@ -0,0 +1,72 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/hash_generator_testing.h"
+
+#include <deque>
+
+namespace absl {
+namespace container_internal {
+namespace hash_internal {
+namespace {
+
+class RandomDeviceSeedSeq {
+ public:
+  using result_type = typename std::random_device::result_type;
+
+  template <class Iterator>
+  void generate(Iterator start, Iterator end) {
+    while (start != end) {
+      *start = gen_();
+      ++start;
+    }
+  }
+
+ private:
+  std::random_device gen_;
+};
+
+}  // namespace
+
+std::mt19937_64* GetThreadLocalRng() {
+  RandomDeviceSeedSeq seed_seq;
+  thread_local auto* rng = new std::mt19937_64(seed_seq);
+  return rng;
+}
+
+std::string Generator<std::string>::operator()() const {
+  // NOLINTNEXTLINE(runtime/int)
+  std::uniform_int_distribution<short> chars(0x20, 0x7E);
+  std::string res;
+  res.resize(32);
+  std::generate(res.begin(), res.end(),
+                [&]() { return chars(*GetThreadLocalRng()); });
+  return res;
+}
+
+absl::string_view Generator<absl::string_view>::operator()() const {
+  static auto* arena = new std::deque<std::string>();
+  // NOLINTNEXTLINE(runtime/int)
+  std::uniform_int_distribution<short> chars(0x20, 0x7E);
+  arena->emplace_back();
+  auto& res = arena->back();
+  res.resize(32);
+  std::generate(res.begin(), res.end(),
+                [&]() { return chars(*GetThreadLocalRng()); });
+  return res;
+}
+
+}  // namespace hash_internal
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/hash_generator_testing.h b/absl/container/internal/hash_generator_testing.h
new file mode 100644
index 000000000000..50d771026c7b
--- /dev/null
+++ b/absl/container/internal/hash_generator_testing.h
@@ -0,0 +1,150 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Generates random values for testing. Specialized only for the few types we
+// care about.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HASH_GENERATOR_TESTING_H_
+#define ABSL_CONTAINER_INTERNAL_HASH_GENERATOR_TESTING_H_
+
+#include <stdint.h>
+#include <algorithm>
+#include <iosfwd>
+#include <random>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/container/internal/hash_policy_testing.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+namespace hash_internal {
+namespace generator_internal {
+
+template <class Container, class = void>
+struct IsMap : std::false_type {};
+
+template <class Map>
+struct IsMap<Map, absl::void_t<typename Map::mapped_type>> : std::true_type {};
+
+}  // namespace generator_internal
+
+std::mt19937_64* GetThreadLocalRng();
+
+enum Enum {
+  kEnumEmpty,
+  kEnumDeleted,
+};
+
+enum class EnumClass : uint64_t {
+  kEmpty,
+  kDeleted,
+};
+
+inline std::ostream& operator<<(std::ostream& o, const EnumClass& ec) {
+  return o << static_cast<uint64_t>(ec);
+}
+
+template <class T, class E = void>
+struct Generator;
+
+template <class T>
+struct Generator<T, typename std::enable_if<std::is_integral<T>::value>::type> {
+  T operator()() const {
+    std::uniform_int_distribution<T> dist;
+    return dist(*GetThreadLocalRng());
+  }
+};
+
+template <>
+struct Generator<Enum> {
+  Enum operator()() const {
+    std::uniform_int_distribution<typename std::underlying_type<Enum>::type>
+        dist;
+    while (true) {
+      auto variate = dist(*GetThreadLocalRng());
+      if (variate != kEnumEmpty && variate != kEnumDeleted)
+        return static_cast<Enum>(variate);
+    }
+  }
+};
+
+template <>
+struct Generator<EnumClass> {
+  EnumClass operator()() const {
+    std::uniform_int_distribution<
+        typename std::underlying_type<EnumClass>::type>
+        dist;
+    while (true) {
+      EnumClass variate = static_cast<EnumClass>(dist(*GetThreadLocalRng()));
+      if (variate != EnumClass::kEmpty && variate != EnumClass::kDeleted)
+        return static_cast<EnumClass>(variate);
+    }
+  }
+};
+
+template <>
+struct Generator<std::string> {
+  std::string operator()() const;
+};
+
+template <>
+struct Generator<absl::string_view> {
+  absl::string_view operator()() const;
+};
+
+template <>
+struct Generator<NonStandardLayout> {
+  NonStandardLayout operator()() const {
+    return NonStandardLayout(Generator<std::string>()());
+  }
+};
+
+template <class K, class V>
+struct Generator<std::pair<K, V>> {
+  std::pair<K, V> operator()() const {
+    return std::pair<K, V>(Generator<typename std::decay<K>::type>()(),
+                           Generator<typename std::decay<V>::type>()());
+  }
+};
+
+template <class... Ts>
+struct Generator<std::tuple<Ts...>> {
+  std::tuple<Ts...> operator()() const {
+    return std::tuple<Ts...>(Generator<typename std::decay<Ts>::type>()()...);
+  }
+};
+
+template <class U>
+struct Generator<U, absl::void_t<decltype(std::declval<U&>().key()),
+                                decltype(std::declval<U&>().value())>>
+    : Generator<std::pair<
+          typename std::decay<decltype(std::declval<U&>().key())>::type,
+          typename std::decay<decltype(std::declval<U&>().value())>::type>> {};
+
+template <class Container>
+using GeneratedType = decltype(
+    std::declval<const Generator<
+        typename std::conditional<generator_internal::IsMap<Container>::value,
+                                  typename Container::value_type,
+                                  typename Container::key_type>::type>&>()());
+
+}  // namespace hash_internal
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASH_GENERATOR_TESTING_H_
diff --git a/absl/container/internal/hash_policy_testing.h b/absl/container/internal/hash_policy_testing.h
new file mode 100644
index 000000000000..ffc76ead7a68
--- /dev/null
+++ b/absl/container/internal/hash_policy_testing.h
@@ -0,0 +1,178 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Utilities to help tests verify that hash tables properly handle stateful
+// allocators and hash functions.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HASH_POLICY_TESTING_H_
+#define ABSL_CONTAINER_INTERNAL_HASH_POLICY_TESTING_H_
+
+#include <cstdlib>
+#include <limits>
+#include <memory>
+#include <ostream>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "absl/hash/hash.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+namespace hash_testing_internal {
+
+template <class Derived>
+struct WithId {
+  WithId() : id_(next_id<Derived>()) {}
+  WithId(const WithId& that) : id_(that.id_) {}
+  WithId(WithId&& that) : id_(that.id_) { that.id_ = 0; }
+  WithId& operator=(const WithId& that) {
+    id_ = that.id_;
+    return *this;
+  }
+  WithId& operator=(WithId&& that) {
+    id_ = that.id_;
+    that.id_ = 0;
+    return *this;
+  }
+
+  size_t id() const { return id_; }
+
+  friend bool operator==(const WithId& a, const WithId& b) {
+    return a.id_ == b.id_;
+  }
+  friend bool operator!=(const WithId& a, const WithId& b) { return !(a == b); }
+
+ protected:
+  explicit WithId(size_t id) : id_(id) {}
+
+ private:
+  size_t id_;
+
+  template <class T>
+  static size_t next_id() {
+    // 0 is reserved for moved from state.
+    static size_t gId = 1;
+    return gId++;
+  }
+};
+
+}  // namespace hash_testing_internal
+
+struct NonStandardLayout {
+  NonStandardLayout() {}
+  explicit NonStandardLayout(std::string s) : value(std::move(s)) {}
+  virtual ~NonStandardLayout() {}
+
+  friend bool operator==(const NonStandardLayout& a,
+                         const NonStandardLayout& b) {
+    return a.value == b.value;
+  }
+  friend bool operator!=(const NonStandardLayout& a,
+                         const NonStandardLayout& b) {
+    return a.value != b.value;
+  }
+
+  template <typename H>
+  friend H AbslHashValue(H h, const NonStandardLayout& v) {
+    return H::combine(std::move(h), v.value);
+  }
+
+  std::string value;
+};
+
+struct StatefulTestingHash
+    : absl::container_internal::hash_testing_internal::WithId<
+          StatefulTestingHash> {
+  template <class T>
+  size_t operator()(const T& t) const {
+    return absl::Hash<T>{}(t);
+  }
+};
+
+struct StatefulTestingEqual
+    : absl::container_internal::hash_testing_internal::WithId<
+          StatefulTestingEqual> {
+  template <class T, class U>
+  bool operator()(const T& t, const U& u) const {
+    return t == u;
+  }
+};
+
+// It is expected that Alloc() == Alloc() for all allocators so we cannot use
+// WithId base. We need to explicitly assign ids.
+template <class T = int>
+struct Alloc : std::allocator<T> {
+  using propagate_on_container_swap = std::true_type;
+
+  // Using old paradigm for this to ensure compatibility.
+  explicit Alloc(size_t id = 0) : id_(id) {}
+
+  Alloc(const Alloc&) = default;
+  Alloc& operator=(const Alloc&) = default;
+
+  template <class U>
+  Alloc(const Alloc<U>& that) : std::allocator<T>(that), id_(that.id()) {}
+
+  template <class U>
+  struct rebind {
+    using other = Alloc<U>;
+  };
+
+  size_t id() const { return id_; }
+
+  friend bool operator==(const Alloc& a, const Alloc& b) {
+    return a.id_ == b.id_;
+  }
+  friend bool operator!=(const Alloc& a, const Alloc& b) { return !(a == b); }
+
+ private:
+  size_t id_ = std::numeric_limits<size_t>::max();
+};
+
+template <class Map>
+auto items(const Map& m) -> std::vector<
+    std::pair<typename Map::key_type, typename Map::mapped_type>> {
+  using std::get;
+  std::vector<std::pair<typename Map::key_type, typename Map::mapped_type>> res;
+  res.reserve(m.size());
+  for (const auto& v : m) res.emplace_back(get<0>(v), get<1>(v));
+  return res;
+}
+
+template <class Set>
+auto keys(const Set& s)
+    -> std::vector<typename std::decay<typename Set::key_type>::type> {
+  std::vector<typename std::decay<typename Set::key_type>::type> res;
+  res.reserve(s.size());
+  for (const auto& v : s) res.emplace_back(v);
+  return res;
+}
+
+}  // namespace container_internal
+}  // namespace absl
+
+// ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS is false for glibcxx versions
+// where the unordered containers are missing certain constructors that
+// take allocator arguments. This test is defined ad-hoc for the platforms
+// we care about (notably Crosstool 17) because libstdcxx's useless
+// versioning scheme precludes a more principled solution.
+#if defined(__GLIBCXX__) && __GLIBCXX__ <= 20140425
+#define ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS 0
+#else
+#define ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS 1
+#endif
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASH_POLICY_TESTING_H_
diff --git a/absl/container/internal/hash_policy_testing_test.cc b/absl/container/internal/hash_policy_testing_test.cc
new file mode 100644
index 000000000000..c215c4237ae5
--- /dev/null
+++ b/absl/container/internal/hash_policy_testing_test.cc
@@ -0,0 +1,43 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/hash_policy_testing.h"
+
+#include "gtest/gtest.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+TEST(_, Hash) {
+  StatefulTestingHash h1;
+  EXPECT_EQ(1, h1.id());
+  StatefulTestingHash h2;
+  EXPECT_EQ(2, h2.id());
+  StatefulTestingHash h1c(h1);
+  EXPECT_EQ(1, h1c.id());
+  StatefulTestingHash h2m(std::move(h2));
+  EXPECT_EQ(2, h2m.id());
+  EXPECT_EQ(0, h2.id());
+  StatefulTestingHash h3;
+  EXPECT_EQ(3, h3.id());
+  h3 = StatefulTestingHash();
+  EXPECT_EQ(4, h3.id());
+  h3 = std::move(h1);
+  EXPECT_EQ(1, h3.id());
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/hash_policy_traits.h b/absl/container/internal/hash_policy_traits.h
new file mode 100644
index 000000000000..029e47e175c9
--- /dev/null
+++ b/absl/container/internal/hash_policy_traits.h
@@ -0,0 +1,189 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_
+#define ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_
+
+#include <cstddef>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+namespace container_internal {
+
+// Defines how slots are initialized/destroyed/moved.
+template <class Policy, class = void>
+struct hash_policy_traits {
+ private:
+  struct ReturnKey {
+    // We return `Key` here.
+    // When Key=T&, we forward the lvalue reference.
+    // When Key=T, we return by value to avoid a dangling reference.
+    // eg, for string_hash_map.
+    template <class Key, class... Args>
+    Key operator()(Key&& k, const Args&...) const {
+      return std::forward<Key>(k);
+    }
+  };
+
+  template <class P = Policy, class = void>
+  struct ConstantIteratorsImpl : std::false_type {};
+
+  template <class P>
+  struct ConstantIteratorsImpl<P, absl::void_t<typename P::constant_iterators>>
+      : P::constant_iterators {};
+
+ public:
+  // The actual object stored in the hash table.
+  using slot_type = typename Policy::slot_type;
+
+  // The type of the keys stored in the hashtable.
+  using key_type = typename Policy::key_type;
+
+  // The argument type for insertions into the hashtable. This is different
+  // from value_type for increased performance. See initializer_list constructor
+  // and insert() member functions for more details.
+  using init_type = typename Policy::init_type;
+
+  using reference = decltype(Policy::element(std::declval<slot_type*>()));
+  using pointer = typename std::remove_reference<reference>::type*;
+  using value_type = typename std::remove_reference<reference>::type;
+
+  // Policies can set this variable to tell raw_hash_set that all iterators
+  // should be constant, even `iterator`. This is useful for set-like
+  // containers.
+  // Defaults to false if not provided by the policy.
+  using constant_iterators = ConstantIteratorsImpl<>;
+
+  // PRECONDITION: `slot` is UNINITIALIZED
+  // POSTCONDITION: `slot` is INITIALIZED
+  template <class Alloc, class... Args>
+  static void construct(Alloc* alloc, slot_type* slot, Args&&... args) {
+    Policy::construct(alloc, slot, std::forward<Args>(args)...);
+  }
+
+  // PRECONDITION: `slot` is INITIALIZED
+  // POSTCONDITION: `slot` is UNINITIALIZED
+  template <class Alloc>
+  static void destroy(Alloc* alloc, slot_type* slot) {
+    Policy::destroy(alloc, slot);
+  }
+
+  // Transfers the `old_slot` to `new_slot`. Any memory allocated by the
+  // allocator inside `old_slot` to `new_slot` can be transfered.
+  //
+  // OPTIONAL: defaults to:
+  //
+  //     clone(new_slot, std::move(*old_slot));
+  //     destroy(old_slot);
+  //
+  // PRECONDITION: `new_slot` is UNINITIALIZED and `old_slot` is INITIALIZED
+  // POSTCONDITION: `new_slot` is INITIALIZED and `old_slot` is
+  //                UNINITIALIZED
+  template <class Alloc>
+  static void transfer(Alloc* alloc, slot_type* new_slot, slot_type* old_slot) {
+    transfer_impl(alloc, new_slot, old_slot, 0);
+  }
+
+  // PRECONDITION: `slot` is INITIALIZED
+  // POSTCONDITION: `slot` is INITIALIZED
+  template <class P = Policy>
+  static auto element(slot_type* slot) -> decltype(P::element(slot)) {
+    return P::element(slot);
+  }
+
+  // Returns the amount of memory owned by `slot`, exclusive of `sizeof(*slot)`.
+  //
+  // If `slot` is nullptr, returns the constant amount of memory owned by any
+  // full slot or -1 if slots own variable amounts of memory.
+  //
+  // PRECONDITION: `slot` is INITIALIZED or nullptr
+  template <class P = Policy>
+  static size_t space_used(const slot_type* slot) {
+    return P::space_used(slot);
+  }
+
+  // Provides generalized access to the key for elements, both for elements in
+  // the table and for elements that have not yet been inserted (or even
+  // constructed).  We would like an API that allows us to say: `key(args...)`
+  // but we cannot do that for all cases, so we use this more general API that
+  // can be used for many things, including the following:
+  //
+  //   - Given an element in a table, get its key.
+  //   - Given an element initializer, get its key.
+  //   - Given `emplace()` arguments, get the element key.
+  //
+  // Implementations of this must adhere to a very strict technical
+  // specification around aliasing and consuming arguments:
+  //
+  // Let `value_type` be the result type of `element()` without ref- and
+  // cv-qualifiers. The first argument is a functor, the rest are constructor
+  // arguments for `value_type`. Returns `std::forward<F>(f)(k, xs...)`, where
+  // `k` is the element key, and `xs...` are the new constructor arguments for
+  // `value_type`. It's allowed for `k` to alias `xs...`, and for both to alias
+  // `ts...`. The key won't be touched once `xs...` are used to construct an
+  // element; `ts...` won't be touched at all, which allows `apply()` to consume
+  // any rvalues among them.
+  //
+  // If `value_type` is constructible from `Ts&&...`, `Policy::apply()` must not
+  // trigger a hard compile error unless it originates from `f`. In other words,
+  // `Policy::apply()` must be SFINAE-friendly. If `value_type` is not
+  // constructible from `Ts&&...`, either SFINAE or a hard compile error is OK.
+  //
+  // If `Ts...` is `[cv] value_type[&]` or `[cv] init_type[&]`,
+  // `Policy::apply()` must work. A compile error is not allowed, SFINAE or not.
+  template <class F, class... Ts, class P = Policy>
+  static auto apply(F&& f, Ts&&... ts)
+      -> decltype(P::apply(std::forward<F>(f), std::forward<Ts>(ts)...)) {
+    return P::apply(std::forward<F>(f), std::forward<Ts>(ts)...);
+  }
+
+  // Returns the "key" portion of the slot.
+  // Used for node handle manipulation.
+  template <class P = Policy>
+  static auto key(slot_type* slot)
+      -> decltype(P::apply(ReturnKey(), element(slot))) {
+    return P::apply(ReturnKey(), element(slot));
+  }
+
+  // Returns the "value" (as opposed to the "key") portion of the element. Used
+  // by maps to implement `operator[]`, `at()` and `insert_or_assign()`.
+  template <class T, class P = Policy>
+  static auto value(T* elem) -> decltype(P::value(elem)) {
+    return P::value(elem);
+  }
+
+ private:
+  // Use auto -> decltype as an enabler.
+  template <class Alloc, class P = Policy>
+  static auto transfer_impl(Alloc* alloc, slot_type* new_slot,
+                            slot_type* old_slot, int)
+      -> decltype((void)P::transfer(alloc, new_slot, old_slot)) {
+    P::transfer(alloc, new_slot, old_slot);
+  }
+  template <class Alloc>
+  static void transfer_impl(Alloc* alloc, slot_type* new_slot,
+                            slot_type* old_slot, char) {
+    construct(alloc, new_slot, std::move(element(old_slot)));
+    destroy(alloc, old_slot);
+  }
+};
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_
diff --git a/absl/container/internal/hash_policy_traits_test.cc b/absl/container/internal/hash_policy_traits_test.cc
new file mode 100644
index 000000000000..423f1548e7ca
--- /dev/null
+++ b/absl/container/internal/hash_policy_traits_test.cc
@@ -0,0 +1,142 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/hash_policy_traits.h"
+
+#include <functional>
+#include <memory>
+#include <new>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using ::testing::MockFunction;
+using ::testing::Return;
+using ::testing::ReturnRef;
+
+using Alloc = std::allocator<int>;
+using Slot = int;
+
+struct PolicyWithoutOptionalOps {
+  using slot_type = Slot;
+  using key_type = Slot;
+  using init_type = Slot;
+
+  static std::function<void(void*, Slot*, Slot)> construct;
+  static std::function<void(void*, Slot*)> destroy;
+
+  static std::function<Slot&(Slot*)> element;
+  static int apply(int v) { return apply_impl(v); }
+  static std::function<int(int)> apply_impl;
+  static std::function<Slot&(Slot*)> value;
+};
+
+std::function<void(void*, Slot*, Slot)> PolicyWithoutOptionalOps::construct;
+std::function<void(void*, Slot*)> PolicyWithoutOptionalOps::destroy;
+
+std::function<Slot&(Slot*)> PolicyWithoutOptionalOps::element;
+std::function<int(int)> PolicyWithoutOptionalOps::apply_impl;
+std::function<Slot&(Slot*)> PolicyWithoutOptionalOps::value;
+
+struct PolicyWithOptionalOps : PolicyWithoutOptionalOps {
+  static std::function<void(void*, Slot*, Slot*)> transfer;
+};
+
+std::function<void(void*, Slot*, Slot*)> PolicyWithOptionalOps::transfer;
+
+struct Test : ::testing::Test {
+  Test() {
+    PolicyWithoutOptionalOps::construct = [&](void* a1, Slot* a2, Slot a3) {
+      construct.Call(a1, a2, std::move(a3));
+    };
+    PolicyWithoutOptionalOps::destroy = [&](void* a1, Slot* a2) {
+      destroy.Call(a1, a2);
+    };
+
+    PolicyWithoutOptionalOps::element = [&](Slot* a1) -> Slot& {
+      return element.Call(a1);
+    };
+    PolicyWithoutOptionalOps::apply_impl = [&](int a1) -> int {
+      return apply.Call(a1);
+    };
+    PolicyWithoutOptionalOps::value = [&](Slot* a1) -> Slot& {
+      return value.Call(a1);
+    };
+
+    PolicyWithOptionalOps::transfer = [&](void* a1, Slot* a2, Slot* a3) {
+      return transfer.Call(a1, a2, a3);
+    };
+  }
+
+  std::allocator<int> alloc;
+  int a = 53;
+
+  MockFunction<void(void*, Slot*, Slot)> construct;
+  MockFunction<void(void*, Slot*)> destroy;
+
+  MockFunction<Slot&(Slot*)> element;
+  MockFunction<int(int)> apply;
+  MockFunction<Slot&(Slot*)> value;
+
+  MockFunction<void(void*, Slot*, Slot*)> transfer;
+};
+
+TEST_F(Test, construct) {
+  EXPECT_CALL(construct, Call(&alloc, &a, 53));
+  hash_policy_traits<PolicyWithoutOptionalOps>::construct(&alloc, &a, 53);
+}
+
+TEST_F(Test, destroy) {
+  EXPECT_CALL(destroy, Call(&alloc, &a));
+  hash_policy_traits<PolicyWithoutOptionalOps>::destroy(&alloc, &a);
+}
+
+TEST_F(Test, element) {
+  int b = 0;
+  EXPECT_CALL(element, Call(&a)).WillOnce(ReturnRef(b));
+  EXPECT_EQ(&b, &hash_policy_traits<PolicyWithoutOptionalOps>::element(&a));
+}
+
+TEST_F(Test, apply) {
+  EXPECT_CALL(apply, Call(42)).WillOnce(Return(1337));
+  EXPECT_EQ(1337, (hash_policy_traits<PolicyWithoutOptionalOps>::apply(42)));
+}
+
+TEST_F(Test, value) {
+  int b = 0;
+  EXPECT_CALL(value, Call(&a)).WillOnce(ReturnRef(b));
+  EXPECT_EQ(&b, &hash_policy_traits<PolicyWithoutOptionalOps>::value(&a));
+}
+
+TEST_F(Test, without_transfer) {
+  int b = 42;
+  EXPECT_CALL(element, Call(&b)).WillOnce(::testing::ReturnRef(b));
+  EXPECT_CALL(construct, Call(&alloc, &a, b));
+  EXPECT_CALL(destroy, Call(&alloc, &b));
+  hash_policy_traits<PolicyWithoutOptionalOps>::transfer(&alloc, &a, &b);
+}
+
+TEST_F(Test, with_transfer) {
+  int b = 42;
+  EXPECT_CALL(transfer, Call(&alloc, &a, &b));
+  hash_policy_traits<PolicyWithOptionalOps>::transfer(&alloc, &a, &b);
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/hashtable_debug.h b/absl/container/internal/hashtable_debug.h
new file mode 100644
index 000000000000..c3bd65c9c4ec
--- /dev/null
+++ b/absl/container/internal/hashtable_debug.h
@@ -0,0 +1,108 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This library provides APIs to debug the probing behavior of hash tables.
+//
+// In general, the probing behavior is a black box for users and only the
+// side effects can be measured in the form of performance differences.
+// These APIs give a glimpse on the actual behavior of the probing algorithms in
+// these hashtables given a specified hash function and a set of elements.
+//
+// The probe count distribution can be used to assess the quality of the hash
+// function for that particular hash table. Note that a hash function that
+// performs well in one hash table implementation does not necessarily performs
+// well in a different one.
+//
+// This library supports std::unordered_{set,map}, dense_hash_{set,map} and
+// absl::{flat,node,string}_hash_{set,map}.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
+#define ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
+
+#include <cstddef>
+#include <algorithm>
+#include <type_traits>
+#include <vector>
+
+#include "absl/container/internal/hashtable_debug_hooks.h"
+
+namespace absl {
+namespace container_internal {
+
+// Returns the number of probes required to lookup `key`.  Returns 0 for a
+// search with no collisions.  Higher values mean more hash collisions occurred;
+// however, the exact meaning of this number varies according to the container
+// type.
+template <typename C>
+size_t GetHashtableDebugNumProbes(
+    const C& c, const typename C::key_type& key) {
+  return absl::container_internal::hashtable_debug_internal::
+      HashtableDebugAccess<C>::GetNumProbes(c, key);
+}
+
+// Gets a histogram of the number of probes for each elements in the container.
+// The sum of all the values in the vector is equal to container.size().
+template <typename C>
+std::vector<size_t> GetHashtableDebugNumProbesHistogram(const C& container) {
+  std::vector<size_t> v;
+  for (auto it = container.begin(); it != container.end(); ++it) {
+    size_t num_probes = GetHashtableDebugNumProbes(
+        container,
+        absl::container_internal::hashtable_debug_internal::GetKey<C>(*it, 0));
+    v.resize(std::max(v.size(), num_probes + 1));
+    v[num_probes]++;
+  }
+  return v;
+}
+
+struct HashtableDebugProbeSummary {
+  size_t total_elements;
+  size_t total_num_probes;
+  double mean;
+};
+
+// Gets a summary of the probe count distribution for the elements in the
+// container.
+template <typename C>
+HashtableDebugProbeSummary GetHashtableDebugProbeSummary(const C& container) {
+  auto probes = GetHashtableDebugNumProbesHistogram(container);
+  HashtableDebugProbeSummary summary = {};
+  for (size_t i = 0; i < probes.size(); ++i) {
+    summary.total_elements += probes[i];
+    summary.total_num_probes += probes[i] * i;
+  }
+  summary.mean = 1.0 * summary.total_num_probes / summary.total_elements;
+  return summary;
+}
+
+// Returns the number of bytes requested from the allocator by the container
+// and not freed.
+template <typename C>
+size_t AllocatedByteSize(const C& c) {
+  return absl::container_internal::hashtable_debug_internal::
+      HashtableDebugAccess<C>::AllocatedByteSize(c);
+}
+
+// Returns a tight lower bound for AllocatedByteSize(c) where `c` is of type `C`
+// and `c.size()` is equal to `num_elements`.
+template <typename C>
+size_t LowerBoundAllocatedByteSize(size_t num_elements) {
+  return absl::container_internal::hashtable_debug_internal::
+      HashtableDebugAccess<C>::LowerBoundAllocatedByteSize(num_elements);
+}
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
diff --git a/absl/container/internal/hashtable_debug_hooks.h b/absl/container/internal/hashtable_debug_hooks.h
new file mode 100644
index 000000000000..8f219726bee0
--- /dev/null
+++ b/absl/container/internal/hashtable_debug_hooks.h
@@ -0,0 +1,81 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Provides the internal API for hashtable_debug.h.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_HOOKS_H_
+#define ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_HOOKS_H_
+
+#include <cstddef>
+
+#include <algorithm>
+#include <type_traits>
+#include <vector>
+
+namespace absl {
+namespace container_internal {
+namespace hashtable_debug_internal {
+
+// If it is a map, call get<0>().
+using std::get;
+template <typename T, typename = typename T::mapped_type>
+auto GetKey(const typename T::value_type& pair, int) -> decltype(get<0>(pair)) {
+  return get<0>(pair);
+}
+
+// If it is not a map, return the value directly.
+template <typename T>
+const typename T::key_type& GetKey(const typename T::key_type& key, char) {
+  return key;
+}
+
+// Containers should specialize this to provide debug information for that
+// container.
+template <class Container, typename Enabler = void>
+struct HashtableDebugAccess {
+  // Returns the number of probes required to find `key` in `c`.  The "number of
+  // probes" is a concept that can vary by container.  Implementations should
+  // return 0 when `key` was found in the minimum number of operations and
+  // should increment the result for each non-trivial operation required to find
+  // `key`.
+  //
+  // The default implementation uses the bucket api from the standard and thus
+  // works for `std::unordered_*` containers.
+  static size_t GetNumProbes(const Container& c,
+                             const typename Container::key_type& key) {
+    if (!c.bucket_count()) return {};
+    size_t num_probes = 0;
+    size_t bucket = c.bucket(key);
+    for (auto it = c.begin(bucket), e = c.end(bucket);; ++it, ++num_probes) {
+      if (it == e) return num_probes;
+      if (c.key_eq()(key, GetKey<Container>(*it, 0))) return num_probes;
+    }
+  }
+
+  // Returns the number of bytes requested from the allocator by the container
+  // and not freed.
+  //
+  // static size_t AllocatedByteSize(const Container& c);
+
+  // Returns a tight lower bound for AllocatedByteSize(c) where `c` is of type
+  // `Container` and `c.size()` is equal to `num_elements`.
+  //
+  // static size_t LowerBoundAllocatedByteSize(size_t num_elements);
+};
+
+}  // namespace hashtable_debug_internal
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_HOOKS_H_
diff --git a/absl/container/internal/layout.h b/absl/container/internal/layout.h
new file mode 100644
index 000000000000..0c239fe876c5
--- /dev/null
+++ b/absl/container/internal/layout.h
@@ -0,0 +1,732 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//                           MOTIVATION AND TUTORIAL
+//
+// If you want to put in a single heap allocation N doubles followed by M ints,
+// it's easy if N and M are known at compile time.
+//
+//   struct S {
+//     double a[N];
+//     int b[M];
+//   };
+//
+//   S* p = new S;
+//
+// But what if N and M are known only in run time? Class template Layout to the
+// rescue! It's a portable generalization of the technique known as struct hack.
+//
+//   // This object will tell us everything we need to know about the memory
+//   // layout of double[N] followed by int[M]. It's structurally identical to
+//   // size_t[2] that stores N and M. It's very cheap to create.
+//   const Layout<double, int> layout(N, M);
+//
+//   // Allocate enough memory for both arrays. `AllocSize()` tells us how much
+//   // memory is needed. We are free to use any allocation function we want as
+//   // long as it returns aligned memory.
+//   std::unique_ptr<unsigned char[]> p(new unsigned char[layout.AllocSize()]);
+//
+//   // Obtain the pointer to the array of doubles.
+//   // Equivalent to `reinterpret_cast<double*>(p.get())`.
+//   //
+//   // We could have written layout.Pointer<0>(p) instead. If all the types are
+//   // unique you can use either form, but if some types are repeated you must
+//   // use the index form.
+//   double* a = layout.Pointer<double>(p.get());
+//
+//   // Obtain the pointer to the array of ints.
+//   // Equivalent to `reinterpret_cast<int*>(p.get() + N * 8)`.
+//   int* b = layout.Pointer<int>(p);
+//
+// If we are unable to specify sizes of all fields, we can pass as many sizes as
+// we can to `Partial()`. In return, it'll allow us to access the fields whose
+// locations and sizes can be computed from the provided information.
+// `Partial()` comes in handy when the array sizes are embedded into the
+// allocation.
+//
+//   // size_t[1] containing N, size_t[1] containing M, double[N], int[M].
+//   using L = Layout<size_t, size_t, double, int>;
+//
+//   unsigned char* Allocate(size_t n, size_t m) {
+//     const L layout(1, 1, n, m);
+//     unsigned char* p = new unsigned char[layout.AllocSize()];
+//     *layout.Pointer<0>(p) = n;
+//     *layout.Pointer<1>(p) = m;
+//     return p;
+//   }
+//
+//   void Use(unsigned char* p) {
+//     // First, extract N and M.
+//     // Specify that the first array has only one element. Using `prefix` we
+//     // can access the first two arrays but not more.
+//     constexpr auto prefix = L::Partial(1);
+//     size_t n = *prefix.Pointer<0>(p);
+//     size_t m = *prefix.Pointer<1>(p);
+//
+//     // Now we can get pointers to the payload.
+//     const L layout(1, 1, n, m);
+//     double* a = layout.Pointer<double>(p);
+//     int* b = layout.Pointer<int>(p);
+//   }
+//
+// The layout we used above combines fixed-size with dynamically-sized fields.
+// This is quite common. Layout is optimized for this use case and generates
+// optimal code. All computations that can be performed at compile time are
+// indeed performed at compile time.
+//
+// Efficiency tip: The order of fields matters. In `Layout<T1, ..., TN>` try to
+// ensure that `alignof(T1) >= ... >= alignof(TN)`. This way you'll have no
+// padding in between arrays.
+//
+// You can manually override the alignment of an array by wrapping the type in
+// `Aligned<T, N>`. `Layout<..., Aligned<T, N>, ...>` has exactly the same API
+// and behavior as `Layout<..., T, ...>` except that the first element of the
+// array of `T` is aligned to `N` (the rest of the elements follow without
+// padding). `N` cannot be less than `alignof(T)`.
+//
+// `AllocSize()` and `Pointer()` are the most basic methods for dealing with
+// memory layouts. Check out the reference or code below to discover more.
+//
+//                            EXAMPLE
+//
+//   // Immutable move-only string with sizeof equal to sizeof(void*). The
+//   // string size and the characters are kept in the same heap allocation.
+//   class CompactString {
+//    public:
+//     CompactString(const char* s = "") {
+//       const size_t size = strlen(s);
+//       // size_t[1] followed by char[size + 1].
+//       const L layout(1, size + 1);
+//       p_.reset(new unsigned char[layout.AllocSize()]);
+//       // If running under ASAN, mark the padding bytes, if any, to catch
+//       // memory errors.
+//       layout.PoisonPadding(p_.get());
+//       // Store the size in the allocation.
+//       *layout.Pointer<size_t>(p_.get()) = size;
+//       // Store the characters in the allocation.
+//       memcpy(layout.Pointer<char>(p_.get()), s, size + 1);
+//     }
+//
+//     size_t size() const {
+//       // Equivalent to reinterpret_cast<size_t&>(*p).
+//       return *L::Partial().Pointer<size_t>(p_.get());
+//     }
+//
+//     const char* c_str() const {
+//       // Equivalent to reinterpret_cast<char*>(p.get() + sizeof(size_t)).
+//       // The argument in Partial(1) specifies that we have size_t[1] in front
+//       // of the characters.
+//       return L::Partial(1).Pointer<char>(p_.get());
+//     }
+//
+//    private:
+//     // Our heap allocation contains a size_t followed by an array of chars.
+//     using L = Layout<size_t, char>;
+//     std::unique_ptr<unsigned char[]> p_;
+//   };
+//
+//   int main() {
+//     CompactString s = "hello";
+//     assert(s.size() == 5);
+//     assert(strcmp(s.c_str(), "hello") == 0);
+//   }
+//
+//                               DOCUMENTATION
+//
+// The interface exported by this file consists of:
+// - class `Layout<>` and its public members.
+// - The public members of class `internal_layout::LayoutImpl<>`. That class
+//   isn't intended to be used directly, and its name and template parameter
+//   list are internal implementation details, but the class itself provides
+//   most of the functionality in this file. See comments on its members for
+//   detailed documentation.
+//
+// `Layout<T1,... Tn>::Partial(count1,..., countm)` (where `m` <= `n`) returns a
+// `LayoutImpl<>` object. `Layout<T1,..., Tn> layout(count1,..., countn)`
+// creates a `Layout` object, which exposes the same functionality by inheriting
+// from `LayoutImpl<>`.
+
+#ifndef ABSL_CONTAINER_INTERNAL_LAYOUT_H_
+#define ABSL_CONTAINER_INTERNAL_LAYOUT_H_
+
+#include <assert.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <ostream>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <typeinfo>
+#include <utility>
+
+#ifdef ADDRESS_SANITIZER
+#include <sanitizer/asan_interface.h>
+#endif
+
+#include "absl/meta/type_traits.h"
+#include "absl/strings/str_cat.h"
+#include "absl/types/span.h"
+#include "absl/utility/utility.h"
+
+#if defined(__GXX_RTTI)
+#define ABSL_INTERNAL_HAS_CXA_DEMANGLE
+#endif
+
+#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+#include <cxxabi.h>
+#endif
+
+namespace absl {
+namespace container_internal {
+
+// A type wrapper that instructs `Layout` to use the specific alignment for the
+// array. `Layout<..., Aligned<T, N>, ...>` has exactly the same API
+// and behavior as `Layout<..., T, ...>` except that the first element of the
+// array of `T` is aligned to `N` (the rest of the elements follow without
+// padding).
+//
+// Requires: `N >= alignof(T)` and `N` is a power of 2.
+template <class T, size_t N>
+struct Aligned;
+
+namespace internal_layout {
+
+template <class T>
+struct NotAligned {};
+
+template <class T, size_t N>
+struct NotAligned<const Aligned<T, N>> {
+  static_assert(sizeof(T) == 0, "Aligned<T, N> cannot be const-qualified");
+};
+
+template <size_t>
+using IntToSize = size_t;
+
+template <class>
+using TypeToSize = size_t;
+
+template <class T>
+struct Type : NotAligned<T> {
+  using type = T;
+};
+
+template <class T, size_t N>
+struct Type<Aligned<T, N>> {
+  using type = T;
+};
+
+template <class T>
+struct SizeOf : NotAligned<T>, std::integral_constant<size_t, sizeof(T)> {};
+
+template <class T, size_t N>
+struct SizeOf<Aligned<T, N>> : std::integral_constant<size_t, sizeof(T)> {};
+
+template <class T>
+struct AlignOf : NotAligned<T>, std::integral_constant<size_t, alignof(T)> {};
+
+template <class T, size_t N>
+struct AlignOf<Aligned<T, N>> : std::integral_constant<size_t, N> {
+  static_assert(N % alignof(T) == 0,
+                "Custom alignment can't be lower than the type's alignment");
+};
+
+// Does `Ts...` contain `T`?
+template <class T, class... Ts>
+using Contains = absl::disjunction<std::is_same<T, Ts>...>;
+
+template <class From, class To>
+using CopyConst =
+    typename std::conditional<std::is_const<From>::value, const To, To>::type;
+
+template <class T>
+using SliceType = absl::Span<T>;
+
+// This namespace contains no types. It prevents functions defined in it from
+// being found by ADL.
+namespace adl_barrier {
+
+template <class Needle, class... Ts>
+constexpr size_t Find(Needle, Needle, Ts...) {
+  static_assert(!Contains<Needle, Ts...>(), "Duplicate element type");
+  return 0;
+}
+
+template <class Needle, class T, class... Ts>
+constexpr size_t Find(Needle, T, Ts...) {
+  return adl_barrier::Find(Needle(), Ts()...) + 1;
+}
+
+constexpr bool IsPow2(size_t n) { return !(n & (n - 1)); }
+
+// Returns `q * m` for the smallest `q` such that `q * m >= n`.
+// Requires: `m` is a power of two. It's enforced by IsLegalElementType below.
+constexpr size_t Align(size_t n, size_t m) { return (n + m - 1) & ~(m - 1); }
+
+constexpr size_t Min(size_t a, size_t b) { return b < a ? b : a; }
+
+constexpr size_t Max(size_t a) { return a; }
+
+template <class... Ts>
+constexpr size_t Max(size_t a, size_t b, Ts... rest) {
+  return adl_barrier::Max(b < a ? a : b, rest...);
+}
+
+template <class T>
+std::string TypeName() {
+  std::string out;
+  int status = 0;
+  char* demangled = nullptr;
+#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+  demangled = abi::__cxa_demangle(typeid(T).name(), nullptr, nullptr, &status);
+#endif
+  if (status == 0 && demangled != nullptr) {  // Demangling succeeeded.
+    absl::StrAppend(&out, "<", demangled, ">");
+    free(demangled);
+  } else {
+#if defined(__GXX_RTTI) || defined(_CPPRTTI)
+    absl::StrAppend(&out, "<", typeid(T).name(), ">");
+#endif
+  }
+  return out;
+}
+
+}  // namespace adl_barrier
+
+template <bool C>
+using EnableIf = typename std::enable_if<C, int>::type;
+
+// Can `T` be a template argument of `Layout`?
+template <class T>
+using IsLegalElementType = std::integral_constant<
+    bool, !std::is_reference<T>::value && !std::is_volatile<T>::value &&
+              !std::is_reference<typename Type<T>::type>::value &&
+              !std::is_volatile<typename Type<T>::type>::value &&
+              adl_barrier::IsPow2(AlignOf<T>::value)>;
+
+template <class Elements, class SizeSeq, class OffsetSeq>
+class LayoutImpl;
+
+// Public base class of `Layout` and the result type of `Layout::Partial()`.
+//
+// `Elements...` contains all template arguments of `Layout` that created this
+// instance.
+//
+// `SizeSeq...` is `[0, NumSizes)` where `NumSizes` is the number of arguments
+// passed to `Layout::Partial()` or `Layout::Layout()`.
+//
+// `OffsetSeq...` is `[0, NumOffsets)` where `NumOffsets` is
+// `Min(sizeof...(Elements), NumSizes + 1)` (the number of arrays for which we
+// can compute offsets).
+template <class... Elements, size_t... SizeSeq, size_t... OffsetSeq>
+class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
+                 absl::index_sequence<OffsetSeq...>> {
+ private:
+  static_assert(sizeof...(Elements) > 0, "At least one field is required");
+  static_assert(absl::conjunction<IsLegalElementType<Elements>...>::value,
+                "Invalid element type (see IsLegalElementType)");
+
+  enum {
+    NumTypes = sizeof...(Elements),
+    NumSizes = sizeof...(SizeSeq),
+    NumOffsets = sizeof...(OffsetSeq),
+  };
+
+  // These are guaranteed by `Layout`.
+  static_assert(NumOffsets == adl_barrier::Min(NumTypes, NumSizes + 1),
+                "Internal error");
+  static_assert(NumTypes > 0, "Internal error");
+
+  // Returns the index of `T` in `Elements...`. Results in a compilation error
+  // if `Elements...` doesn't contain exactly one instance of `T`.
+  template <class T>
+  static constexpr size_t ElementIndex() {
+    static_assert(Contains<Type<T>, Type<typename Type<Elements>::type>...>(),
+                  "Type not found");
+    return adl_barrier::Find(Type<T>(),
+                             Type<typename Type<Elements>::type>()...);
+  }
+
+  template <size_t N>
+  using ElementAlignment =
+      AlignOf<typename std::tuple_element<N, std::tuple<Elements...>>::type>;
+
+ public:
+  // Element types of all arrays packed in a tuple.
+  using ElementTypes = std::tuple<typename Type<Elements>::type...>;
+
+  // Element type of the Nth array.
+  template <size_t N>
+  using ElementType = typename std::tuple_element<N, ElementTypes>::type;
+
+  constexpr explicit LayoutImpl(IntToSize<SizeSeq>... sizes)
+      : size_{sizes...} {}
+
+  // Alignment of the layout, equal to the strictest alignment of all elements.
+  // All pointers passed to the methods of layout must be aligned to this value.
+  static constexpr size_t Alignment() {
+    return adl_barrier::Max(AlignOf<Elements>::value...);
+  }
+
+  // Offset in bytes of the Nth array.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   assert(x.Offset<0>() == 0);   // The ints starts from 0.
+  //   assert(x.Offset<1>() == 16);  // The doubles starts from 16.
+  //
+  // Requires: `N <= NumSizes && N < sizeof...(Ts)`.
+  template <size_t N, EnableIf<N == 0> = 0>
+  constexpr size_t Offset() const {
+    return 0;
+  }
+
+  template <size_t N, EnableIf<N != 0> = 0>
+  constexpr size_t Offset() const {
+    static_assert(N < NumOffsets, "Index out of bounds");
+    return adl_barrier::Align(
+        Offset<N - 1>() + SizeOf<ElementType<N - 1>>() * size_[N - 1],
+        ElementAlignment<N>());
+  }
+
+  // Offset in bytes of the array with the specified element type. There must
+  // be exactly one such array and its zero-based index must be at most
+  // `NumSizes`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   assert(x.Offset<int>() == 0);      // The ints starts from 0.
+  //   assert(x.Offset<double>() == 16);  // The doubles starts from 16.
+  template <class T>
+  constexpr size_t Offset() const {
+    return Offset<ElementIndex<T>()>();
+  }
+
+  // Offsets in bytes of all arrays for which the offsets are known.
+  constexpr std::array<size_t, NumOffsets> Offsets() const {
+    return {{Offset<OffsetSeq>()...}};
+  }
+
+  // The number of elements in the Nth array. This is the Nth argument of
+  // `Layout::Partial()` or `Layout::Layout()` (zero-based).
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   assert(x.Size<0>() == 3);
+  //   assert(x.Size<1>() == 4);
+  //
+  // Requires: `N < NumSizes`.
+  template <size_t N>
+  constexpr size_t Size() const {
+    static_assert(N < NumSizes, "Index out of bounds");
+    return size_[N];
+  }
+
+  // The number of elements in the array with the specified element type.
+  // There must be exactly one such array and its zero-based index must be
+  // at most `NumSizes`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   assert(x.Size<int>() == 3);
+  //   assert(x.Size<double>() == 4);
+  template <class T>
+  constexpr size_t Size() const {
+    return Size<ElementIndex<T>()>();
+  }
+
+    // The number of elements of all arrays for which they are known.
+  constexpr std::array<size_t, NumSizes> Sizes() const {
+    return {{Size<SizeSeq>()...}};
+  }
+
+  // Pointer to the beginning of the Nth array.
+  //
+  // `Char` must be `[const] [signed|unsigned] char`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   unsigned char* p = unsigned char[x.AllocSize()];
+  //   int* ints = x.Pointer<0>(p);
+  //   double* doubles = x.Pointer<1>(p);
+  //
+  // Requires: `N <= NumSizes && N < sizeof...(Ts)`.
+  // Requires: `p` is aligned to `Alignment()`.
+  template <size_t N, class Char>
+  CopyConst<Char, ElementType<N>>* Pointer(Char* p) const {
+    using C = typename std::remove_const<Char>::type;
+    static_assert(
+        std::is_same<C, char>() || std::is_same<C, unsigned char>() ||
+            std::is_same<C, signed char>(),
+        "The argument must be a pointer to [const] [signed|unsigned] char");
+    constexpr size_t alignment = Alignment();
+    (void)alignment;
+    assert(reinterpret_cast<uintptr_t>(p) % alignment == 0);
+    return reinterpret_cast<CopyConst<Char, ElementType<N>>*>(p + Offset<N>());
+  }
+
+  // Pointer to the beginning of the array with the specified element type.
+  // There must be exactly one such array and its zero-based index must be at
+  // most `NumSizes`.
+  //
+  // `Char` must be `[const] [signed|unsigned] char`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   unsigned char* p = new unsigned char[x.AllocSize()];
+  //   int* ints = x.Pointer<int>(p);
+  //   double* doubles = x.Pointer<double>(p);
+  //
+  // Requires: `p` is aligned to `Alignment()`.
+  template <class T, class Char>
+  CopyConst<Char, T>* Pointer(Char* p) const {
+    return Pointer<ElementIndex<T>()>(p);
+  }
+
+  // Pointers to all arrays for which pointers are known.
+  //
+  // `Char` must be `[const] [signed|unsigned] char`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   unsigned char* p = new unsigned char[x.AllocSize()];
+  //
+  //   int* ints;
+  //   double* doubles;
+  //   std::tie(ints, doubles) = x.Pointers(p);
+  //
+  // Requires: `p` is aligned to `Alignment()`.
+  //
+  // Note: We're not using ElementType alias here because it does not compile
+  // under MSVC.
+  template <class Char>
+  std::tuple<CopyConst<
+      Char, typename std::tuple_element<OffsetSeq, ElementTypes>::type>*...>
+  Pointers(Char* p) const {
+    return std::tuple<CopyConst<Char, ElementType<OffsetSeq>>*...>(
+        Pointer<OffsetSeq>(p)...);
+  }
+
+  // The Nth array.
+  //
+  // `Char` must be `[const] [signed|unsigned] char`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   unsigned char* p = new unsigned char[x.AllocSize()];
+  //   Span<int> ints = x.Slice<0>(p);
+  //   Span<double> doubles = x.Slice<1>(p);
+  //
+  // Requires: `N < NumSizes`.
+  // Requires: `p` is aligned to `Alignment()`.
+  template <size_t N, class Char>
+  SliceType<CopyConst<Char, ElementType<N>>> Slice(Char* p) const {
+    return SliceType<CopyConst<Char, ElementType<N>>>(Pointer<N>(p), Size<N>());
+  }
+
+  // The array with the specified element type. There must be exactly one
+  // such array and its zero-based index must be less than `NumSizes`.
+  //
+  // `Char` must be `[const] [signed|unsigned] char`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   unsigned char* p = new unsigned char[x.AllocSize()];
+  //   Span<int> ints = x.Slice<int>(p);
+  //   Span<double> doubles = x.Slice<double>(p);
+  //
+  // Requires: `p` is aligned to `Alignment()`.
+  template <class T, class Char>
+  SliceType<CopyConst<Char, T>> Slice(Char* p) const {
+    return Slice<ElementIndex<T>()>(p);
+  }
+
+  // All arrays with known sizes.
+  //
+  // `Char` must be `[const] [signed|unsigned] char`.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   unsigned char* p = new unsigned char[x.AllocSize()];
+  //
+  //   Span<int> ints;
+  //   Span<double> doubles;
+  //   std::tie(ints, doubles) = x.Slices(p);
+  //
+  // Requires: `p` is aligned to `Alignment()`.
+  //
+  // Note: We're not using ElementType alias here because it does not compile
+  // under MSVC.
+  template <class Char>
+  std::tuple<SliceType<CopyConst<
+      Char, typename std::tuple_element<SizeSeq, ElementTypes>::type>>...>
+  Slices(Char* p) const {
+    // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63875 (fixed
+    // in 6.1).
+    (void)p;
+    return std::tuple<SliceType<CopyConst<Char, ElementType<SizeSeq>>>...>(
+        Slice<SizeSeq>(p)...);
+  }
+
+  // The size of the allocation that fits all arrays.
+  //
+  //   // int[3], 4 bytes of padding, double[4].
+  //   Layout<int, double> x(3, 4);
+  //   unsigned char* p = new unsigned char[x.AllocSize()];  // 48 bytes
+  //
+  // Requires: `NumSizes == sizeof...(Ts)`.
+  constexpr size_t AllocSize() const {
+    static_assert(NumTypes == NumSizes, "You must specify sizes of all fields");
+    return Offset<NumTypes - 1>() +
+           SizeOf<ElementType<NumTypes - 1>>() * size_[NumTypes - 1];
+  }
+
+  // If built with --config=asan, poisons padding bytes (if any) in the
+  // allocation. The pointer must point to a memory block at least
+  // `AllocSize()` bytes in length.
+  //
+  // `Char` must be `[const] [signed|unsigned] char`.
+  //
+  // Requires: `p` is aligned to `Alignment()`.
+  template <class Char, size_t N = NumOffsets - 1, EnableIf<N == 0> = 0>
+  void PoisonPadding(const Char* p) const {
+    Pointer<0>(p);  // verify the requirements on `Char` and `p`
+  }
+
+  template <class Char, size_t N = NumOffsets - 1, EnableIf<N != 0> = 0>
+  void PoisonPadding(const Char* p) const {
+    static_assert(N < NumOffsets, "Index out of bounds");
+    (void)p;
+#ifdef ADDRESS_SANITIZER
+    PoisonPadding<Char, N - 1>(p);
+    // The `if` is an optimization. It doesn't affect the observable behaviour.
+    if (ElementAlignment<N - 1>() % ElementAlignment<N>()) {
+      size_t start =
+          Offset<N - 1>() + SizeOf<ElementType<N - 1>>() * size_[N - 1];
+      ASAN_POISON_MEMORY_REGION(p + start, Offset<N>() - start);
+    }
+#endif
+  }
+
+  // Human-readable description of the memory layout. Useful for debugging.
+  // Slow.
+  //
+  //   // char[5], 3 bytes of padding, int[3], 4 bytes of padding, followed
+  //   // by an unknown number of doubles.
+  //   auto x = Layout<char, int, double>::Partial(5, 3);
+  //   assert(x.DebugString() ==
+  //          "@0<char>(1)[5]; @8<int>(4)[3]; @24<double>(8)");
+  //
+  // Each field is in the following format: @offset<type>(sizeof)[size] (<type>
+  // may be missing depending on the target platform). For example,
+  // @8<int>(4)[3] means that at offset 8 we have an array of ints, where each
+  // int is 4 bytes, and we have 3 of those ints. The size of the last field may
+  // be missing (as in the example above). Only fields with known offsets are
+  // described. Type names may differ across platforms: one compiler might
+  // produce "unsigned*" where another produces "unsigned int *".
+  std::string DebugString() const {
+    const auto offsets = Offsets();
+    const size_t sizes[] = {SizeOf<ElementType<OffsetSeq>>()...};
+    const std::string types[] = {adl_barrier::TypeName<ElementType<OffsetSeq>>()...};
+    std::string res = absl::StrCat("@0", types[0], "(", sizes[0], ")");
+    for (size_t i = 0; i != NumOffsets - 1; ++i) {
+      absl::StrAppend(&res, "[", size_[i], "]; @", offsets[i + 1], types[i + 1],
+                      "(", sizes[i + 1], ")");
+    }
+    // NumSizes is a constant that may be zero. Some compilers cannot see that
+    // inside the if statement "size_[NumSizes - 1]" must be valid.
+    int last = static_cast<int>(NumSizes) - 1;
+    if (NumTypes == NumSizes && last >= 0) {
+      absl::StrAppend(&res, "[", size_[last], "]");
+    }
+    return res;
+  }
+
+ private:
+  // Arguments of `Layout::Partial()` or `Layout::Layout()`.
+  size_t size_[NumSizes > 0 ? NumSizes : 1];
+};
+
+template <size_t NumSizes, class... Ts>
+using LayoutType = LayoutImpl<
+    std::tuple<Ts...>, absl::make_index_sequence<NumSizes>,
+    absl::make_index_sequence<adl_barrier::Min(sizeof...(Ts), NumSizes + 1)>>;
+
+}  // namespace internal_layout
+
+// Descriptor of arrays of various types and sizes laid out in memory one after
+// another. See the top of the file for documentation.
+//
+// Check out the public API of internal_layout::LayoutImpl above. The type is
+// internal to the library but its methods are public, and they are inherited
+// by `Layout`.
+template <class... Ts>
+class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> {
+ public:
+  static_assert(sizeof...(Ts) > 0, "At least one field is required");
+  static_assert(
+      absl::conjunction<internal_layout::IsLegalElementType<Ts>...>::value,
+      "Invalid element type (see IsLegalElementType)");
+
+  // The result type of `Partial()` with `NumSizes` arguments.
+  template <size_t NumSizes>
+  using PartialType = internal_layout::LayoutType<NumSizes, Ts...>;
+
+  // `Layout` knows the element types of the arrays we want to lay out in
+  // memory but not the number of elements in each array.
+  // `Partial(size1, ..., sizeN)` allows us to specify the latter. The
+  // resulting immutable object can be used to obtain pointers to the
+  // individual arrays.
+  //
+  // It's allowed to pass fewer array sizes than the number of arrays. E.g.,
+  // if all you need is to the offset of the second array, you only need to
+  // pass one argument -- the number of elements in the first arrays.
+  //
+  //   // int[3] followed by 4 bytes of padding and an unknown number of
+  //   // doubles.
+  //   auto x = Layout<int, double>::Partial(3);
+  //   // doubles start at byte 16.
+  //   assert(x.Offset<1>() == 16);
+  //
+  // If you know the number of elements in all arrays, you can still call
+  // `Partial()` but it's more convenient to use the constructor of `Layout`.
+  //
+  //   Layout<int, double> x(3, 5);
+  //
+  // Note: The sizes of the arrays must be specified in number of elements,
+  // not in bytes.
+  //
+  // Requires: `sizeof...(Sizes) <= sizeof...(Ts)`.
+  // Requires: all arguments are convertible to `size_t`.
+  template <class... Sizes>
+  static constexpr PartialType<sizeof...(Sizes)> Partial(Sizes&&... sizes) {
+    static_assert(sizeof...(Sizes) <= sizeof...(Ts), "");
+    return PartialType<sizeof...(Sizes)>(absl::forward<Sizes>(sizes)...);
+  }
+
+  // Creates a layout with the sizes of all arrays specified. If you know
+  // only the sizes of the first N arrays (where N can be zero), you can use
+  // `Partial()` defined above. The constructor is essentially equivalent to
+  // calling `Partial()` and passing in all array sizes; the constructor is
+  // provided as a convenient abbreviation.
+  //
+  // Note: The sizes of the arrays must be specified in number of elements,
+  // not in bytes.
+  constexpr explicit Layout(internal_layout::TypeToSize<Ts>... sizes)
+      : internal_layout::LayoutType<sizeof...(Ts), Ts...>(sizes...) {}
+};
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_LAYOUT_H_
diff --git a/absl/container/internal/layout_test.cc b/absl/container/internal/layout_test.cc
new file mode 100644
index 000000000000..f35157a3bd85
--- /dev/null
+++ b/absl/container/internal/layout_test.cc
@@ -0,0 +1,1552 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/layout.h"
+
+// We need ::max_align_t because some libstdc++ versions don't provide
+// std::max_align_t
+#include <stddef.h>
+#include <cstdint>
+#include <memory>
+#include <sstream>
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/types/span.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using ::absl::Span;
+using ::testing::ElementsAre;
+
+size_t Distance(const void* from, const void* to) {
+  ABSL_RAW_CHECK(from <= to, "Distance must be non-negative");
+  return static_cast<const char*>(to) - static_cast<const char*>(from);
+}
+
+template <class Expected, class Actual>
+Expected Type(Actual val) {
+  static_assert(std::is_same<Expected, Actual>(), "");
+  return val;
+}
+
+using Int128 = int64_t[2];
+
+// Properties of types that this test relies on.
+static_assert(sizeof(int8_t) == 1, "");
+static_assert(alignof(int8_t) == 1, "");
+static_assert(sizeof(int16_t) == 2, "");
+static_assert(alignof(int16_t) == 2, "");
+static_assert(sizeof(int32_t) == 4, "");
+static_assert(alignof(int32_t) == 4, "");
+static_assert(sizeof(Int128) == 16, "");
+static_assert(alignof(Int128) == 8, "");
+
+template <class Expected, class Actual>
+void SameType() {
+  static_assert(std::is_same<Expected, Actual>(), "");
+}
+
+TEST(Layout, ElementType) {
+  {
+    using L = Layout<int32_t>;
+    SameType<int32_t, L::ElementType<0>>();
+    SameType<int32_t, decltype(L::Partial())::ElementType<0>>();
+    SameType<int32_t, decltype(L::Partial(0))::ElementType<0>>();
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    SameType<int32_t, L::ElementType<0>>();
+    SameType<int32_t, L::ElementType<1>>();
+    SameType<int32_t, decltype(L::Partial())::ElementType<0>>();
+    SameType<int32_t, decltype(L::Partial())::ElementType<1>>();
+    SameType<int32_t, decltype(L::Partial(0))::ElementType<0>>();
+    SameType<int32_t, decltype(L::Partial(0))::ElementType<1>>();
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    SameType<int8_t, L::ElementType<0>>();
+    SameType<int32_t, L::ElementType<1>>();
+    SameType<Int128, L::ElementType<2>>();
+    SameType<int8_t, decltype(L::Partial())::ElementType<0>>();
+    SameType<int8_t, decltype(L::Partial(0))::ElementType<0>>();
+    SameType<int32_t, decltype(L::Partial(0))::ElementType<1>>();
+    SameType<int8_t, decltype(L::Partial(0, 0))::ElementType<0>>();
+    SameType<int32_t, decltype(L::Partial(0, 0))::ElementType<1>>();
+    SameType<Int128, decltype(L::Partial(0, 0))::ElementType<2>>();
+    SameType<int8_t, decltype(L::Partial(0, 0, 0))::ElementType<0>>();
+    SameType<int32_t, decltype(L::Partial(0, 0, 0))::ElementType<1>>();
+    SameType<Int128, decltype(L::Partial(0, 0, 0))::ElementType<2>>();
+  }
+}
+
+TEST(Layout, ElementTypes) {
+  {
+    using L = Layout<int32_t>;
+    SameType<std::tuple<int32_t>, L::ElementTypes>();
+    SameType<std::tuple<int32_t>, decltype(L::Partial())::ElementTypes>();
+    SameType<std::tuple<int32_t>, decltype(L::Partial(0))::ElementTypes>();
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    SameType<std::tuple<int32_t, int32_t>, L::ElementTypes>();
+    SameType<std::tuple<int32_t, int32_t>, decltype(L::Partial())::ElementTypes>();
+    SameType<std::tuple<int32_t, int32_t>, decltype(L::Partial(0))::ElementTypes>();
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    SameType<std::tuple<int8_t, int32_t, Int128>, L::ElementTypes>();
+    SameType<std::tuple<int8_t, int32_t, Int128>,
+             decltype(L::Partial())::ElementTypes>();
+    SameType<std::tuple<int8_t, int32_t, Int128>,
+             decltype(L::Partial(0))::ElementTypes>();
+    SameType<std::tuple<int8_t, int32_t, Int128>,
+             decltype(L::Partial(0, 0))::ElementTypes>();
+    SameType<std::tuple<int8_t, int32_t, Int128>,
+             decltype(L::Partial(0, 0, 0))::ElementTypes>();
+  }
+}
+
+TEST(Layout, OffsetByIndex) {
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial().Offset<0>());
+    EXPECT_EQ(0, L::Partial(3).Offset<0>());
+    EXPECT_EQ(0, L(3).Offset<0>());
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(0, L::Partial().Offset<0>());
+    EXPECT_EQ(0, L::Partial(3).Offset<0>());
+    EXPECT_EQ(12, L::Partial(3).Offset<1>());
+    EXPECT_EQ(0, L::Partial(3, 5).Offset<0>());
+    EXPECT_EQ(12, L::Partial(3, 5).Offset<1>());
+    EXPECT_EQ(0, L(3, 5).Offset<0>());
+    EXPECT_EQ(12, L(3, 5).Offset<1>());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0, L::Partial().Offset<0>());
+    EXPECT_EQ(0, L::Partial(0).Offset<0>());
+    EXPECT_EQ(0, L::Partial(0).Offset<1>());
+    EXPECT_EQ(0, L::Partial(1).Offset<0>());
+    EXPECT_EQ(4, L::Partial(1).Offset<1>());
+    EXPECT_EQ(0, L::Partial(5).Offset<0>());
+    EXPECT_EQ(8, L::Partial(5).Offset<1>());
+    EXPECT_EQ(0, L::Partial(0, 0).Offset<0>());
+    EXPECT_EQ(0, L::Partial(0, 0).Offset<1>());
+    EXPECT_EQ(0, L::Partial(0, 0).Offset<2>());
+    EXPECT_EQ(0, L::Partial(1, 0).Offset<0>());
+    EXPECT_EQ(4, L::Partial(1, 0).Offset<1>());
+    EXPECT_EQ(8, L::Partial(1, 0).Offset<2>());
+    EXPECT_EQ(0, L::Partial(5, 3).Offset<0>());
+    EXPECT_EQ(8, L::Partial(5, 3).Offset<1>());
+    EXPECT_EQ(24, L::Partial(5, 3).Offset<2>());
+    EXPECT_EQ(0, L::Partial(0, 0, 0).Offset<0>());
+    EXPECT_EQ(0, L::Partial(0, 0, 0).Offset<1>());
+    EXPECT_EQ(0, L::Partial(0, 0, 0).Offset<2>());
+    EXPECT_EQ(0, L::Partial(1, 0, 0).Offset<0>());
+    EXPECT_EQ(4, L::Partial(1, 0, 0).Offset<1>());
+    EXPECT_EQ(8, L::Partial(1, 0, 0).Offset<2>());
+    EXPECT_EQ(0, L::Partial(5, 3, 1).Offset<0>());
+    EXPECT_EQ(24, L::Partial(5, 3, 1).Offset<2>());
+    EXPECT_EQ(8, L::Partial(5, 3, 1).Offset<1>());
+    EXPECT_EQ(0, L(5, 3, 1).Offset<0>());
+    EXPECT_EQ(24, L(5, 3, 1).Offset<2>());
+    EXPECT_EQ(8, L(5, 3, 1).Offset<1>());
+  }
+}
+
+TEST(Layout, OffsetByType) {
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial().Offset<int32_t>());
+    EXPECT_EQ(0, L::Partial(3).Offset<int32_t>());
+    EXPECT_EQ(0, L(3).Offset<int32_t>());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0, L::Partial().Offset<int8_t>());
+    EXPECT_EQ(0, L::Partial(0).Offset<int8_t>());
+    EXPECT_EQ(0, L::Partial(0).Offset<int32_t>());
+    EXPECT_EQ(0, L::Partial(1).Offset<int8_t>());
+    EXPECT_EQ(4, L::Partial(1).Offset<int32_t>());
+    EXPECT_EQ(0, L::Partial(5).Offset<int8_t>());
+    EXPECT_EQ(8, L::Partial(5).Offset<int32_t>());
+    EXPECT_EQ(0, L::Partial(0, 0).Offset<int8_t>());
+    EXPECT_EQ(0, L::Partial(0, 0).Offset<int32_t>());
+    EXPECT_EQ(0, L::Partial(0, 0).Offset<Int128>());
+    EXPECT_EQ(0, L::Partial(1, 0).Offset<int8_t>());
+    EXPECT_EQ(4, L::Partial(1, 0).Offset<int32_t>());
+    EXPECT_EQ(8, L::Partial(1, 0).Offset<Int128>());
+    EXPECT_EQ(0, L::Partial(5, 3).Offset<int8_t>());
+    EXPECT_EQ(8, L::Partial(5, 3).Offset<int32_t>());
+    EXPECT_EQ(24, L::Partial(5, 3).Offset<Int128>());
+    EXPECT_EQ(0, L::Partial(0, 0, 0).Offset<int8_t>());
+    EXPECT_EQ(0, L::Partial(0, 0, 0).Offset<int32_t>());
+    EXPECT_EQ(0, L::Partial(0, 0, 0).Offset<Int128>());
+    EXPECT_EQ(0, L::Partial(1, 0, 0).Offset<int8_t>());
+    EXPECT_EQ(4, L::Partial(1, 0, 0).Offset<int32_t>());
+    EXPECT_EQ(8, L::Partial(1, 0, 0).Offset<Int128>());
+    EXPECT_EQ(0, L::Partial(5, 3, 1).Offset<int8_t>());
+    EXPECT_EQ(24, L::Partial(5, 3, 1).Offset<Int128>());
+    EXPECT_EQ(8, L::Partial(5, 3, 1).Offset<int32_t>());
+    EXPECT_EQ(0, L(5, 3, 1).Offset<int8_t>());
+    EXPECT_EQ(24, L(5, 3, 1).Offset<Int128>());
+    EXPECT_EQ(8, L(5, 3, 1).Offset<int32_t>());
+  }
+}
+
+TEST(Layout, Offsets) {
+  {
+    using L = Layout<int32_t>;
+    EXPECT_THAT(L::Partial().Offsets(), ElementsAre(0));
+    EXPECT_THAT(L::Partial(3).Offsets(), ElementsAre(0));
+    EXPECT_THAT(L(3).Offsets(), ElementsAre(0));
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_THAT(L::Partial().Offsets(), ElementsAre(0));
+    EXPECT_THAT(L::Partial(3).Offsets(), ElementsAre(0, 12));
+    EXPECT_THAT(L::Partial(3, 5).Offsets(), ElementsAre(0, 12));
+    EXPECT_THAT(L(3, 5).Offsets(), ElementsAre(0, 12));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_THAT(L::Partial().Offsets(), ElementsAre(0));
+    EXPECT_THAT(L::Partial(1).Offsets(), ElementsAre(0, 4));
+    EXPECT_THAT(L::Partial(5).Offsets(), ElementsAre(0, 8));
+    EXPECT_THAT(L::Partial(0, 0).Offsets(), ElementsAre(0, 0, 0));
+    EXPECT_THAT(L::Partial(1, 0).Offsets(), ElementsAre(0, 4, 8));
+    EXPECT_THAT(L::Partial(5, 3).Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(L::Partial(0, 0, 0).Offsets(), ElementsAre(0, 0, 0));
+    EXPECT_THAT(L::Partial(1, 0, 0).Offsets(), ElementsAre(0, 4, 8));
+    EXPECT_THAT(L::Partial(5, 3, 1).Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(L(5, 3, 1).Offsets(), ElementsAre(0, 8, 24));
+  }
+}
+
+TEST(Layout, AllocSize) {
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial(0).AllocSize());
+    EXPECT_EQ(12, L::Partial(3).AllocSize());
+    EXPECT_EQ(12, L(3).AllocSize());
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(32, L::Partial(3, 5).AllocSize());
+    EXPECT_EQ(32, L(3, 5).AllocSize());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0, L::Partial(0, 0, 0).AllocSize());
+    EXPECT_EQ(8, L::Partial(1, 0, 0).AllocSize());
+    EXPECT_EQ(8, L::Partial(0, 1, 0).AllocSize());
+    EXPECT_EQ(16, L::Partial(0, 0, 1).AllocSize());
+    EXPECT_EQ(24, L::Partial(1, 1, 1).AllocSize());
+    EXPECT_EQ(136, L::Partial(3, 5, 7).AllocSize());
+    EXPECT_EQ(136, L(3, 5, 7).AllocSize());
+  }
+}
+
+TEST(Layout, SizeByIndex) {
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial(0).Size<0>());
+    EXPECT_EQ(3, L::Partial(3).Size<0>());
+    EXPECT_EQ(3, L(3).Size<0>());
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(0, L::Partial(0).Size<0>());
+    EXPECT_EQ(3, L::Partial(3).Size<0>());
+    EXPECT_EQ(3, L::Partial(3, 5).Size<0>());
+    EXPECT_EQ(5, L::Partial(3, 5).Size<1>());
+    EXPECT_EQ(3, L(3, 5).Size<0>());
+    EXPECT_EQ(5, L(3, 5).Size<1>());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(3, L::Partial(3).Size<0>());
+    EXPECT_EQ(3, L::Partial(3, 5).Size<0>());
+    EXPECT_EQ(5, L::Partial(3, 5).Size<1>());
+    EXPECT_EQ(3, L::Partial(3, 5, 7).Size<0>());
+    EXPECT_EQ(5, L::Partial(3, 5, 7).Size<1>());
+    EXPECT_EQ(7, L::Partial(3, 5, 7).Size<2>());
+    EXPECT_EQ(3, L(3, 5, 7).Size<0>());
+    EXPECT_EQ(5, L(3, 5, 7).Size<1>());
+    EXPECT_EQ(7, L(3, 5, 7).Size<2>());
+  }
+}
+
+TEST(Layout, SizeByType) {
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial(0).Size<int32_t>());
+    EXPECT_EQ(3, L::Partial(3).Size<int32_t>());
+    EXPECT_EQ(3, L(3).Size<int32_t>());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(3, L::Partial(3).Size<int8_t>());
+    EXPECT_EQ(3, L::Partial(3, 5).Size<int8_t>());
+    EXPECT_EQ(5, L::Partial(3, 5).Size<int32_t>());
+    EXPECT_EQ(3, L::Partial(3, 5, 7).Size<int8_t>());
+    EXPECT_EQ(5, L::Partial(3, 5, 7).Size<int32_t>());
+    EXPECT_EQ(7, L::Partial(3, 5, 7).Size<Int128>());
+    EXPECT_EQ(3, L(3, 5, 7).Size<int8_t>());
+    EXPECT_EQ(5, L(3, 5, 7).Size<int32_t>());
+    EXPECT_EQ(7, L(3, 5, 7).Size<Int128>());
+  }
+}
+
+TEST(Layout, Sizes) {
+  {
+    using L = Layout<int32_t>;
+    EXPECT_THAT(L::Partial().Sizes(), ElementsAre());
+    EXPECT_THAT(L::Partial(3).Sizes(), ElementsAre(3));
+    EXPECT_THAT(L(3).Sizes(), ElementsAre(3));
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_THAT(L::Partial().Sizes(), ElementsAre());
+    EXPECT_THAT(L::Partial(3).Sizes(), ElementsAre(3));
+    EXPECT_THAT(L::Partial(3, 5).Sizes(), ElementsAre(3, 5));
+    EXPECT_THAT(L(3, 5).Sizes(), ElementsAre(3, 5));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_THAT(L::Partial().Sizes(), ElementsAre());
+    EXPECT_THAT(L::Partial(3).Sizes(), ElementsAre(3));
+    EXPECT_THAT(L::Partial(3, 5).Sizes(), ElementsAre(3, 5));
+    EXPECT_THAT(L::Partial(3, 5, 7).Sizes(), ElementsAre(3, 5, 7));
+    EXPECT_THAT(L(3, 5, 7).Sizes(), ElementsAre(3, 5, 7));
+  }
+}
+
+TEST(Layout, PointerByIndex) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial().Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L(3).Pointer<0>(p))));
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial().Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<0>(p))));
+    EXPECT_EQ(12, Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<1>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int32_t*>(L::Partial(3, 5).Pointer<0>(p))));
+    EXPECT_EQ(12,
+              Distance(p, Type<const int32_t*>(L::Partial(3, 5).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L(3, 5).Pointer<0>(p))));
+    EXPECT_EQ(12, Distance(p, Type<const int32_t*>(L(3, 5).Pointer<1>(p))));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial().Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial(0).Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial(0).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial(1).Pointer<0>(p))));
+    EXPECT_EQ(4, Distance(p, Type<const int32_t*>(L::Partial(1).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial(5).Pointer<0>(p))));
+    EXPECT_EQ(8, Distance(p, Type<const int32_t*>(L::Partial(5).Pointer<1>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int8_t*>(L::Partial(0, 0).Pointer<0>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int32_t*>(L::Partial(0, 0).Pointer<1>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const Int128*>(L::Partial(0, 0).Pointer<2>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int8_t*>(L::Partial(1, 0).Pointer<0>(p))));
+    EXPECT_EQ(4,
+              Distance(p, Type<const int32_t*>(L::Partial(1, 0).Pointer<1>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<const Int128*>(L::Partial(1, 0).Pointer<2>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int8_t*>(L::Partial(5, 3).Pointer<0>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<const int32_t*>(L::Partial(5, 3).Pointer<1>(p))));
+    EXPECT_EQ(24,
+              Distance(p, Type<const Int128*>(L::Partial(5, 3).Pointer<2>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int8_t*>(L::Partial(0, 0, 0).Pointer<0>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int32_t*>(L::Partial(0, 0, 0).Pointer<1>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const Int128*>(L::Partial(0, 0, 0).Pointer<2>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int8_t*>(L::Partial(1, 0, 0).Pointer<0>(p))));
+    EXPECT_EQ(
+        4, Distance(p, Type<const int32_t*>(L::Partial(1, 0, 0).Pointer<1>(p))));
+    EXPECT_EQ(
+        8, Distance(p, Type<const Int128*>(L::Partial(1, 0, 0).Pointer<2>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int8_t*>(L::Partial(5, 3, 1).Pointer<0>(p))));
+    EXPECT_EQ(
+        24,
+        Distance(p, Type<const Int128*>(L::Partial(5, 3, 1).Pointer<2>(p))));
+    EXPECT_EQ(
+        8, Distance(p, Type<const int32_t*>(L::Partial(5, 3, 1).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L(5, 3, 1).Pointer<0>(p))));
+    EXPECT_EQ(24, Distance(p, Type<const Int128*>(L(5, 3, 1).Pointer<2>(p))));
+    EXPECT_EQ(8, Distance(p, Type<const int32_t*>(L(5, 3, 1).Pointer<1>(p))));
+  }
+}
+
+TEST(Layout, PointerByType) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0,
+              Distance(p, Type<const int32_t*>(L::Partial().Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L(3).Pointer<int32_t>(p))));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial().Pointer<int8_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int8_t*>(L::Partial(0).Pointer<int8_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int32_t*>(L::Partial(0).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int8_t*>(L::Partial(1).Pointer<int8_t>(p))));
+    EXPECT_EQ(4,
+              Distance(p, Type<const int32_t*>(L::Partial(1).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int8_t*>(L::Partial(5).Pointer<int8_t>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<const int32_t*>(L::Partial(5).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int8_t*>(L::Partial(0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int32_t*>(L::Partial(0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<const Int128*>(L::Partial(0, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int8_t*>(L::Partial(1, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        4, Distance(p, Type<const int32_t*>(L::Partial(1, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        8,
+        Distance(p, Type<const Int128*>(L::Partial(1, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int8_t*>(L::Partial(5, 3).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        8, Distance(p, Type<const int32_t*>(L::Partial(5, 3).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        24,
+        Distance(p, Type<const Int128*>(L::Partial(5, 3).Pointer<Int128>(p))));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<const int8_t*>(L::Partial(0, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<const int32_t*>(L::Partial(0, 0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const Int128*>(
+                                 L::Partial(0, 0, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<const int8_t*>(L::Partial(1, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        4,
+        Distance(p, Type<const int32_t*>(L::Partial(1, 0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(8, Distance(p, Type<const Int128*>(
+                                 L::Partial(1, 0, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<const int8_t*>(L::Partial(5, 3, 1).Pointer<int8_t>(p))));
+    EXPECT_EQ(24, Distance(p, Type<const Int128*>(
+                                  L::Partial(5, 3, 1).Pointer<Int128>(p))));
+    EXPECT_EQ(
+        8,
+        Distance(p, Type<const int32_t*>(L::Partial(5, 3, 1).Pointer<int32_t>(p))));
+    EXPECT_EQ(24,
+              Distance(p, Type<const Int128*>(L(5, 3, 1).Pointer<Int128>(p))));
+    EXPECT_EQ(8, Distance(p, Type<const int32_t*>(L(5, 3, 1).Pointer<int32_t>(p))));
+  }
+}
+
+TEST(Layout, MutablePointerByIndex) {
+  alignas(max_align_t) unsigned char p[100];
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial().Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(3).Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L(3).Pointer<0>(p))));
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial().Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(3).Pointer<0>(p))));
+    EXPECT_EQ(12, Distance(p, Type<int32_t*>(L::Partial(3).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(3, 5).Pointer<0>(p))));
+    EXPECT_EQ(12, Distance(p, Type<int32_t*>(L::Partial(3, 5).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L(3, 5).Pointer<0>(p))));
+    EXPECT_EQ(12, Distance(p, Type<int32_t*>(L(3, 5).Pointer<1>(p))));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial().Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0).Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1).Pointer<0>(p))));
+    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5).Pointer<0>(p))));
+    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0, 0).Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0, 0).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<Int128*>(L::Partial(0, 0).Pointer<2>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1, 0).Pointer<0>(p))));
+    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1, 0).Pointer<1>(p))));
+    EXPECT_EQ(8, Distance(p, Type<Int128*>(L::Partial(1, 0).Pointer<2>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5, 3).Pointer<0>(p))));
+    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5, 3).Pointer<1>(p))));
+    EXPECT_EQ(24, Distance(p, Type<Int128*>(L::Partial(5, 3).Pointer<2>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0, 0, 0).Pointer<0>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0, 0, 0).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<Int128*>(L::Partial(0, 0, 0).Pointer<2>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1, 0, 0).Pointer<0>(p))));
+    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1, 0, 0).Pointer<1>(p))));
+    EXPECT_EQ(8, Distance(p, Type<Int128*>(L::Partial(1, 0, 0).Pointer<2>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5, 3, 1).Pointer<0>(p))));
+    EXPECT_EQ(24,
+              Distance(p, Type<Int128*>(L::Partial(5, 3, 1).Pointer<2>(p))));
+    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5, 3, 1).Pointer<1>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L(5, 3, 1).Pointer<0>(p))));
+    EXPECT_EQ(24, Distance(p, Type<Int128*>(L(5, 3, 1).Pointer<2>(p))));
+    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L(5, 3, 1).Pointer<1>(p))));
+  }
+}
+
+TEST(Layout, MutablePointerByType) {
+  alignas(max_align_t) unsigned char p[100];
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial().Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(3).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L(3).Pointer<int32_t>(p))));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial().Pointer<int8_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0).Pointer<int8_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1).Pointer<int8_t>(p))));
+    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5).Pointer<int8_t>(p))));
+    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<Int128*>(L::Partial(0, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<Int128*>(L::Partial(1, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5, 3).Pointer<int8_t>(p))));
+    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5, 3).Pointer<int32_t>(p))));
+    EXPECT_EQ(24,
+              Distance(p, Type<Int128*>(L::Partial(5, 3).Pointer<Int128>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int8_t*>(L::Partial(0, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int32_t*>(L::Partial(0, 0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<Int128*>(L::Partial(0, 0, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int8_t*>(L::Partial(1, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(4,
+              Distance(p, Type<int32_t*>(L::Partial(1, 0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        8, Distance(p, Type<Int128*>(L::Partial(1, 0, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int8_t*>(L::Partial(5, 3, 1).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        24, Distance(p, Type<Int128*>(L::Partial(5, 3, 1).Pointer<Int128>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<int32_t*>(L::Partial(5, 3, 1).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L(5, 3, 1).Pointer<int8_t>(p))));
+    EXPECT_EQ(24, Distance(p, Type<Int128*>(L(5, 3, 1).Pointer<Int128>(p))));
+    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L(5, 3, 1).Pointer<int32_t>(p))));
+  }
+}
+
+TEST(Layout, Pointers) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  using L = Layout<int8_t, int8_t, Int128>;
+  {
+    const auto x = L::Partial();
+    EXPECT_EQ(std::make_tuple(x.Pointer<0>(p)),
+              Type<std::tuple<const int8_t*>>(x.Pointers(p)));
+  }
+  {
+    const auto x = L::Partial(1);
+    EXPECT_EQ(std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p)),
+              (Type<std::tuple<const int8_t*, const int8_t*>>(x.Pointers(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2, 3);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+  {
+    const L x(1, 2, 3);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+}
+
+TEST(Layout, MutablePointers) {
+  alignas(max_align_t) unsigned char p[100];
+  using L = Layout<int8_t, int8_t, Int128>;
+  {
+    const auto x = L::Partial();
+    EXPECT_EQ(std::make_tuple(x.Pointer<0>(p)),
+              Type<std::tuple<int8_t*>>(x.Pointers(p)));
+  }
+  {
+    const auto x = L::Partial(1);
+    EXPECT_EQ(std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p)),
+              (Type<std::tuple<int8_t*, int8_t*>>(x.Pointers(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<int8_t*, int8_t*, Int128*>>(x.Pointers(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2, 3);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<int8_t*, int8_t*, Int128*>>(x.Pointers(p))));
+  }
+  {
+    const L x(1, 2, 3);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<int8_t*, int8_t*, Int128*>>(x.Pointers(p))));
+  }
+}
+
+TEST(Layout, SliceByIndexSize) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial(0).Slice<0>(p).size());
+    EXPECT_EQ(3, L::Partial(3).Slice<0>(p).size());
+    EXPECT_EQ(3, L(3).Slice<0>(p).size());
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(3, L::Partial(3).Slice<0>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5).Slice<1>(p).size());
+    EXPECT_EQ(5, L(3, 5).Slice<1>(p).size());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(3, L::Partial(3).Slice<0>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5).Slice<0>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5).Slice<1>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5, 7).Slice<0>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5, 7).Slice<1>(p).size());
+    EXPECT_EQ(7, L::Partial(3, 5, 7).Slice<2>(p).size());
+    EXPECT_EQ(3, L(3, 5, 7).Slice<0>(p).size());
+    EXPECT_EQ(5, L(3, 5, 7).Slice<1>(p).size());
+    EXPECT_EQ(7, L(3, 5, 7).Slice<2>(p).size());
+  }
+}
+
+TEST(Layout, SliceByTypeSize) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial(0).Slice<int32_t>(p).size());
+    EXPECT_EQ(3, L::Partial(3).Slice<int32_t>(p).size());
+    EXPECT_EQ(3, L(3).Slice<int32_t>(p).size());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(3, L::Partial(3).Slice<int8_t>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5).Slice<int8_t>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5).Slice<int32_t>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5, 7).Slice<int8_t>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5, 7).Slice<int32_t>(p).size());
+    EXPECT_EQ(7, L::Partial(3, 5, 7).Slice<Int128>(p).size());
+    EXPECT_EQ(3, L(3, 5, 7).Slice<int8_t>(p).size());
+    EXPECT_EQ(5, L(3, 5, 7).Slice<int32_t>(p).size());
+    EXPECT_EQ(7, L(3, 5, 7).Slice<Int128>(p).size());
+  }
+}
+
+TEST(Layout, MutableSliceByIndexSize) {
+  alignas(max_align_t) unsigned char p[100];
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial(0).Slice<0>(p).size());
+    EXPECT_EQ(3, L::Partial(3).Slice<0>(p).size());
+    EXPECT_EQ(3, L(3).Slice<0>(p).size());
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(3, L::Partial(3).Slice<0>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5).Slice<1>(p).size());
+    EXPECT_EQ(5, L(3, 5).Slice<1>(p).size());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(3, L::Partial(3).Slice<0>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5).Slice<0>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5).Slice<1>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5, 7).Slice<0>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5, 7).Slice<1>(p).size());
+    EXPECT_EQ(7, L::Partial(3, 5, 7).Slice<2>(p).size());
+    EXPECT_EQ(3, L(3, 5, 7).Slice<0>(p).size());
+    EXPECT_EQ(5, L(3, 5, 7).Slice<1>(p).size());
+    EXPECT_EQ(7, L(3, 5, 7).Slice<2>(p).size());
+  }
+}
+
+TEST(Layout, MutableSliceByTypeSize) {
+  alignas(max_align_t) unsigned char p[100];
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0, L::Partial(0).Slice<int32_t>(p).size());
+    EXPECT_EQ(3, L::Partial(3).Slice<int32_t>(p).size());
+    EXPECT_EQ(3, L(3).Slice<int32_t>(p).size());
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(3, L::Partial(3).Slice<int8_t>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5).Slice<int8_t>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5).Slice<int32_t>(p).size());
+    EXPECT_EQ(3, L::Partial(3, 5, 7).Slice<int8_t>(p).size());
+    EXPECT_EQ(5, L::Partial(3, 5, 7).Slice<int32_t>(p).size());
+    EXPECT_EQ(7, L::Partial(3, 5, 7).Slice<Int128>(p).size());
+    EXPECT_EQ(3, L(3, 5, 7).Slice<int8_t>(p).size());
+    EXPECT_EQ(5, L(3, 5, 7).Slice<int32_t>(p).size());
+    EXPECT_EQ(7, L(3, 5, 7).Slice<Int128>(p).size());
+  }
+}
+
+TEST(Layout, SliceByIndexData) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int32_t>>(L::Partial(0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const int32_t>>(L(3).Slice<0>(p)).data()));
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p,
+                 Type<Span<const int32_t>>(L::Partial(3, 5).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        12,
+        Distance(p,
+                 Type<Span<const int32_t>>(L::Partial(3, 5).Slice<1>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<const int32_t>>(L(3, 5).Slice<0>(p)).data()));
+    EXPECT_EQ(12,
+              Distance(p, Type<Span<const int32_t>>(L(3, 5).Slice<1>(p)).data()));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int8_t>>(L::Partial(0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int8_t>>(L::Partial(1).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int8_t>>(L::Partial(5).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(0, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p,
+                 Type<Span<const int32_t>>(L::Partial(0, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(1, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        4,
+        Distance(p,
+                 Type<Span<const int32_t>>(L::Partial(1, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(5, 3).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(p,
+                 Type<Span<const int32_t>>(L::Partial(5, 3).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(0, 0, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p,
+            Type<Span<const int32_t>>(L::Partial(0, 0, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p,
+            Type<Span<const Int128>>(L::Partial(0, 0, 0).Slice<2>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(1, 0, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        4,
+        Distance(
+            p,
+            Type<Span<const int32_t>>(L::Partial(1, 0, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(
+            p,
+            Type<Span<const Int128>>(L::Partial(1, 0, 0).Slice<2>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(5, 3, 1).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        24,
+        Distance(
+            p,
+            Type<Span<const Int128>>(L::Partial(5, 3, 1).Slice<2>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(
+            p,
+            Type<Span<const int32_t>>(L::Partial(5, 3, 1).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<const int8_t>>(L(5, 3, 1).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        24,
+        Distance(p, Type<Span<const Int128>>(L(5, 3, 1).Slice<2>(p)).data()));
+    EXPECT_EQ(
+        8, Distance(p, Type<Span<const int32_t>>(L(5, 3, 1).Slice<1>(p)).data()));
+  }
+}
+
+TEST(Layout, SliceByTypeData) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int32_t>>(L::Partial(0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int32_t>>(L::Partial(3).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<const int32_t>>(L(3).Slice<int32_t>(p)).data()));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(1).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(5).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(0, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p,
+            Type<Span<const int32_t>>(L::Partial(0, 0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(1, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        4,
+        Distance(
+            p,
+            Type<Span<const int32_t>>(L::Partial(1, 0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(5, 3).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(
+            p,
+            Type<Span<const int32_t>>(L::Partial(5, 3).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p,
+            Type<Span<const int8_t>>(L::Partial(0, 0, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int32_t>>(L::Partial(0, 0, 0).Slice<int32_t>(p))
+                        .data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const Int128>>(
+                                 L::Partial(0, 0, 0).Slice<Int128>(p))
+                                 .data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p,
+            Type<Span<const int8_t>>(L::Partial(1, 0, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        4,
+        Distance(p, Type<Span<const int32_t>>(L::Partial(1, 0, 0).Slice<int32_t>(p))
+                        .data()));
+    EXPECT_EQ(8, Distance(p, Type<Span<const Int128>>(
+                                 L::Partial(1, 0, 0).Slice<Int128>(p))
+                                 .data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p,
+            Type<Span<const int8_t>>(L::Partial(5, 3, 1).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(24, Distance(p, Type<Span<const Int128>>(
+                                  L::Partial(5, 3, 1).Slice<Int128>(p))
+                                  .data()));
+    EXPECT_EQ(
+        8,
+        Distance(p, Type<Span<const int32_t>>(L::Partial(5, 3, 1).Slice<int32_t>(p))
+                        .data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<const int8_t>>(L(5, 3, 1).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        24,
+        Distance(p,
+                 Type<Span<const Int128>>(L(5, 3, 1).Slice<Int128>(p)).data()));
+    EXPECT_EQ(
+        8, Distance(
+               p, Type<Span<const int32_t>>(L(5, 3, 1).Slice<int32_t>(p)).data()));
+  }
+}
+
+TEST(Layout, MutableSliceByIndexData) {
+  alignas(max_align_t) unsigned char p[100];
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int32_t>>(L::Partial(0).Slice<0>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<int32_t>>(L(3).Slice<0>(p)).data()));
+  }
+  {
+    using L = Layout<int32_t, int32_t>;
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int32_t>>(L::Partial(3, 5).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        12,
+        Distance(p, Type<Span<int32_t>>(L::Partial(3, 5).Slice<1>(p)).data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<int32_t>>(L(3, 5).Slice<0>(p)).data()));
+    EXPECT_EQ(12, Distance(p, Type<Span<int32_t>>(L(3, 5).Slice<1>(p)).data()));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int8_t>>(L::Partial(0).Slice<0>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int8_t>>(L::Partial(1).Slice<0>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int8_t>>(L::Partial(5).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(0, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int32_t>>(L::Partial(0, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(1, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        4, Distance(p, Type<Span<int32_t>>(L::Partial(1, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(5, 3).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        8, Distance(p, Type<Span<int32_t>>(L::Partial(5, 3).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(0, 0, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int32_t>>(L::Partial(0, 0, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<Int128>>(L::Partial(0, 0, 0).Slice<2>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(1, 0, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        4,
+        Distance(p, Type<Span<int32_t>>(L::Partial(1, 0, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        8, Distance(
+               p, Type<Span<Int128>>(L::Partial(1, 0, 0).Slice<2>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(5, 3, 1).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        24, Distance(
+                p, Type<Span<Int128>>(L::Partial(5, 3, 1).Slice<2>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(p, Type<Span<int32_t>>(L::Partial(5, 3, 1).Slice<1>(p)).data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<int8_t>>(L(5, 3, 1).Slice<0>(p)).data()));
+    EXPECT_EQ(24,
+              Distance(p, Type<Span<Int128>>(L(5, 3, 1).Slice<2>(p)).data()));
+    EXPECT_EQ(8, Distance(p, Type<Span<int32_t>>(L(5, 3, 1).Slice<1>(p)).data()));
+  }
+}
+
+TEST(Layout, MutableSliceByTypeData) {
+  alignas(max_align_t) unsigned char p[100];
+  {
+    using L = Layout<int32_t>;
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int32_t>>(L::Partial(0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<int32_t>>(L(3).Slice<int32_t>(p)).data()));
+  }
+  {
+    using L = Layout<int8_t, int32_t, Int128>;
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(1).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(5).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(0, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<int32_t>>(L::Partial(0, 0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(1, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        4, Distance(
+               p, Type<Span<int32_t>>(L::Partial(1, 0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(5, 3).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        8, Distance(
+               p, Type<Span<int32_t>>(L::Partial(5, 3).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<int8_t>>(L::Partial(0, 0, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<int32_t>>(L::Partial(0, 0, 0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p,
+            Type<Span<Int128>>(L::Partial(0, 0, 0).Slice<Int128>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<int8_t>>(L::Partial(1, 0, 0).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        4,
+        Distance(
+            p, Type<Span<int32_t>>(L::Partial(1, 0, 0).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(
+            p,
+            Type<Span<Int128>>(L::Partial(1, 0, 0).Slice<Int128>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<int8_t>>(L::Partial(5, 3, 1).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        24,
+        Distance(
+            p,
+            Type<Span<Int128>>(L::Partial(5, 3, 1).Slice<Int128>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(
+            p, Type<Span<int32_t>>(L::Partial(5, 3, 1).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int8_t>>(L(5, 3, 1).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(
+        24,
+        Distance(p, Type<Span<Int128>>(L(5, 3, 1).Slice<Int128>(p)).data()));
+    EXPECT_EQ(
+        8, Distance(p, Type<Span<int32_t>>(L(5, 3, 1).Slice<int32_t>(p)).data()));
+  }
+}
+
+MATCHER_P(IsSameSlice, slice, "") {
+  return arg.size() == slice.size() && arg.data() == slice.data();
+}
+
+template <typename... M>
+class TupleMatcher {
+ public:
+  explicit TupleMatcher(M... matchers) : matchers_(std::move(matchers)...) {}
+
+  template <typename Tuple>
+  bool MatchAndExplain(const Tuple& p,
+                       testing::MatchResultListener* /* listener */) const {
+    static_assert(std::tuple_size<Tuple>::value == sizeof...(M), "");
+    return MatchAndExplainImpl(
+        p, absl::make_index_sequence<std::tuple_size<Tuple>::value>{});
+  }
+
+  // For the matcher concept. Left empty as we don't really need the diagnostics
+  // right now.
+  void DescribeTo(::std::ostream* os) const {}
+  void DescribeNegationTo(::std::ostream* os) const {}
+
+ private:
+  template <typename Tuple, size_t... Is>
+  bool MatchAndExplainImpl(const Tuple& p, absl::index_sequence<Is...>) const {
+    // Using std::min as a simple variadic "and".
+    return std::min(
+        {true, testing::SafeMatcherCast<
+                   const typename std::tuple_element<Is, Tuple>::type&>(
+                   std::get<Is>(matchers_))
+                   .Matches(std::get<Is>(p))...});
+  }
+
+  std::tuple<M...> matchers_;
+};
+
+template <typename... M>
+testing::PolymorphicMatcher<TupleMatcher<M...>> Tuple(M... matchers) {
+  return testing::MakePolymorphicMatcher(
+      TupleMatcher<M...>(std::move(matchers)...));
+}
+
+TEST(Layout, Slices) {
+  alignas(max_align_t) const unsigned char p[100] = {};
+  using L = Layout<int8_t, int8_t, Int128>;
+  {
+    const auto x = L::Partial();
+    EXPECT_THAT(Type<std::tuple<>>(x.Slices(p)), Tuple());
+  }
+  {
+    const auto x = L::Partial(1);
+    EXPECT_THAT(Type<std::tuple<Span<const int8_t>>>(x.Slices(p)),
+                Tuple(IsSameSlice(x.Slice<0>(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2);
+    EXPECT_THAT(
+        (Type<std::tuple<Span<const int8_t>, Span<const int8_t>>>(x.Slices(p))),
+        Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2, 3);
+    EXPECT_THAT((Type<std::tuple<Span<const int8_t>, Span<const int8_t>,
+                                 Span<const Int128>>>(x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+                      IsSameSlice(x.Slice<2>(p))));
+  }
+  {
+    const L x(1, 2, 3);
+    EXPECT_THAT((Type<std::tuple<Span<const int8_t>, Span<const int8_t>,
+                                 Span<const Int128>>>(x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+                      IsSameSlice(x.Slice<2>(p))));
+  }
+}
+
+TEST(Layout, MutableSlices) {
+  alignas(max_align_t) unsigned char p[100] = {};
+  using L = Layout<int8_t, int8_t, Int128>;
+  {
+    const auto x = L::Partial();
+    EXPECT_THAT(Type<std::tuple<>>(x.Slices(p)), Tuple());
+  }
+  {
+    const auto x = L::Partial(1);
+    EXPECT_THAT(Type<std::tuple<Span<int8_t>>>(x.Slices(p)),
+                Tuple(IsSameSlice(x.Slice<0>(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2);
+    EXPECT_THAT((Type<std::tuple<Span<int8_t>, Span<int8_t>>>(x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p))));
+  }
+  {
+    const auto x = L::Partial(1, 2, 3);
+    EXPECT_THAT(
+        (Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(x.Slices(p))),
+        Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+              IsSameSlice(x.Slice<2>(p))));
+  }
+  {
+    const L x(1, 2, 3);
+    EXPECT_THAT(
+        (Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(x.Slices(p))),
+        Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+              IsSameSlice(x.Slice<2>(p))));
+  }
+}
+
+TEST(Layout, UnalignedTypes) {
+  constexpr Layout<unsigned char, unsigned char, unsigned char> x(1, 2, 3);
+  alignas(max_align_t) unsigned char p[x.AllocSize() + 1];
+  EXPECT_THAT(x.Pointers(p + 1), Tuple(p + 1, p + 2, p + 4));
+}
+
+TEST(Layout, CustomAlignment) {
+  constexpr Layout<unsigned char, Aligned<unsigned char, 8>> x(1, 2);
+  alignas(max_align_t) unsigned char p[x.AllocSize()];
+  EXPECT_EQ(10, x.AllocSize());
+  EXPECT_THAT(x.Pointers(p), Tuple(p + 0, p + 8));
+}
+
+TEST(Layout, OverAligned) {
+  constexpr size_t M = alignof(max_align_t);
+  constexpr Layout<unsigned char, Aligned<unsigned char, 2 * M>> x(1, 3);
+  alignas(2 * M) unsigned char p[x.AllocSize()];
+  EXPECT_EQ(2 * M + 3, x.AllocSize());
+  EXPECT_THAT(x.Pointers(p), Tuple(p + 0, p + 2 * M));
+}
+
+TEST(Layout, Alignment) {
+  static_assert(Layout<int8_t>::Alignment() == 1, "");
+  static_assert(Layout<int32_t>::Alignment() == 4, "");
+  static_assert(Layout<int64_t>::Alignment() == 8, "");
+  static_assert(Layout<Aligned<int8_t, 64>>::Alignment() == 64, "");
+  static_assert(Layout<int8_t, int32_t, int64_t>::Alignment() == 8, "");
+  static_assert(Layout<int8_t, int64_t, int32_t>::Alignment() == 8, "");
+  static_assert(Layout<int32_t, int8_t, int64_t>::Alignment() == 8, "");
+  static_assert(Layout<int32_t, int64_t, int8_t>::Alignment() == 8, "");
+  static_assert(Layout<int64_t, int8_t, int32_t>::Alignment() == 8, "");
+  static_assert(Layout<int64_t, int32_t, int8_t>::Alignment() == 8, "");
+}
+
+TEST(Layout, ConstexprPartial) {
+  constexpr size_t M = alignof(max_align_t);
+  constexpr Layout<unsigned char, Aligned<unsigned char, 2 * M>> x(1, 3);
+  static_assert(x.Partial(1).template Offset<1>() == 2 * M, "");
+}
+// [from, to)
+struct Region {
+  size_t from;
+  size_t to;
+};
+
+void ExpectRegionPoisoned(const unsigned char* p, size_t n, bool poisoned) {
+#ifdef ADDRESS_SANITIZER
+  for (size_t i = 0; i != n; ++i) {
+    EXPECT_EQ(poisoned, __asan_address_is_poisoned(p + i));
+  }
+#endif
+}
+
+template <size_t N>
+void ExpectPoisoned(const unsigned char (&buf)[N],
+                    std::initializer_list<Region> reg) {
+  size_t prev = 0;
+  for (const Region& r : reg) {
+    ExpectRegionPoisoned(buf + prev, r.from - prev, false);
+    ExpectRegionPoisoned(buf + r.from, r.to - r.from, true);
+    prev = r.to;
+  }
+  ExpectRegionPoisoned(buf + prev, N - prev, false);
+}
+
+TEST(Layout, PoisonPadding) {
+  using L = Layout<int8_t, int64_t, int32_t, Int128>;
+
+  constexpr size_t n = L::Partial(1, 2, 3, 4).AllocSize();
+  {
+    constexpr auto x = L::Partial();
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {});
+  }
+  {
+    constexpr auto x = L::Partial(1);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}});
+  }
+  {
+    constexpr auto x = L::Partial(1, 2);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}});
+  }
+  {
+    constexpr auto x = L::Partial(1, 2, 3);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}, {36, 40}});
+  }
+  {
+    constexpr auto x = L::Partial(1, 2, 3, 4);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}, {36, 40}});
+  }
+  {
+    constexpr L x(1, 2, 3, 4);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}, {36, 40}});
+  }
+}
+
+TEST(Layout, DebugString) {
+  const std::string int64_type =
+#ifdef _MSC_VER
+  "__int64";
+#else   // _MSC_VER
+  std::is_same<int64_t, long long>::value ? "long long" : "long";  // NOLINT
+#endif  // _MSC_VER
+  {
+    constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial();
+    EXPECT_EQ("@0<signed char>(1)", x.DebugString());
+  }
+  {
+    constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial(1);
+    EXPECT_EQ("@0<signed char>(1)[1]; @4<int>(4)", x.DebugString());
+  }
+  {
+    constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial(1, 2);
+    EXPECT_EQ("@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)",
+              x.DebugString());
+  }
+  {
+    constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial(1, 2, 3);
+    EXPECT_EQ(
+        "@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)[3]; "
+        "@16<" +
+            int64_type + " [2]>(16)",
+        x.DebugString());
+  }
+  {
+    constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial(1, 2, 3, 4);
+    EXPECT_EQ(
+        "@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)[3]; "
+        "@16<" +
+            int64_type + " [2]>(16)[4]",
+        x.DebugString());
+  }
+  {
+    constexpr Layout<int8_t, int32_t, int8_t, Int128> x(1, 2, 3, 4);
+    EXPECT_EQ(
+        "@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)[3]; "
+        "@16<" +
+            int64_type + " [2]>(16)[4]",
+        x.DebugString());
+  }
+}
+
+TEST(Layout, CharTypes) {
+  constexpr Layout<int32_t> x(1);
+  alignas(max_align_t) char c[x.AllocSize()] = {};
+  alignas(max_align_t) unsigned char uc[x.AllocSize()] = {};
+  alignas(max_align_t) signed char sc[x.AllocSize()] = {};
+  alignas(max_align_t) const char cc[x.AllocSize()] = {};
+  alignas(max_align_t) const unsigned char cuc[x.AllocSize()] = {};
+  alignas(max_align_t) const signed char csc[x.AllocSize()] = {};
+
+  Type<int32_t*>(x.Pointer<0>(c));
+  Type<int32_t*>(x.Pointer<0>(uc));
+  Type<int32_t*>(x.Pointer<0>(sc));
+  Type<const int32_t*>(x.Pointer<0>(cc));
+  Type<const int32_t*>(x.Pointer<0>(cuc));
+  Type<const int32_t*>(x.Pointer<0>(csc));
+
+  Type<int32_t*>(x.Pointer<int32_t>(c));
+  Type<int32_t*>(x.Pointer<int32_t>(uc));
+  Type<int32_t*>(x.Pointer<int32_t>(sc));
+  Type<const int32_t*>(x.Pointer<int32_t>(cc));
+  Type<const int32_t*>(x.Pointer<int32_t>(cuc));
+  Type<const int32_t*>(x.Pointer<int32_t>(csc));
+
+  Type<std::tuple<int32_t*>>(x.Pointers(c));
+  Type<std::tuple<int32_t*>>(x.Pointers(uc));
+  Type<std::tuple<int32_t*>>(x.Pointers(sc));
+  Type<std::tuple<const int32_t*>>(x.Pointers(cc));
+  Type<std::tuple<const int32_t*>>(x.Pointers(cuc));
+  Type<std::tuple<const int32_t*>>(x.Pointers(csc));
+
+  Type<Span<int32_t>>(x.Slice<0>(c));
+  Type<Span<int32_t>>(x.Slice<0>(uc));
+  Type<Span<int32_t>>(x.Slice<0>(sc));
+  Type<Span<const int32_t>>(x.Slice<0>(cc));
+  Type<Span<const int32_t>>(x.Slice<0>(cuc));
+  Type<Span<const int32_t>>(x.Slice<0>(csc));
+
+  Type<std::tuple<Span<int32_t>>>(x.Slices(c));
+  Type<std::tuple<Span<int32_t>>>(x.Slices(uc));
+  Type<std::tuple<Span<int32_t>>>(x.Slices(sc));
+  Type<std::tuple<Span<const int32_t>>>(x.Slices(cc));
+  Type<std::tuple<Span<const int32_t>>>(x.Slices(cuc));
+  Type<std::tuple<Span<const int32_t>>>(x.Slices(csc));
+}
+
+TEST(Layout, ConstElementType) {
+  constexpr Layout<const int32_t> x(1);
+  alignas(int32_t) char c[x.AllocSize()] = {};
+  const char* cc = c;
+  const int32_t* p = reinterpret_cast<const int32_t*>(cc);
+
+  EXPECT_EQ(alignof(int32_t), x.Alignment());
+
+  EXPECT_EQ(0, x.Offset<0>());
+  EXPECT_EQ(0, x.Offset<const int32_t>());
+
+  EXPECT_THAT(x.Offsets(), ElementsAre(0));
+
+  EXPECT_EQ(1, x.Size<0>());
+  EXPECT_EQ(1, x.Size<const int32_t>());
+
+  EXPECT_THAT(x.Sizes(), ElementsAre(1));
+
+  EXPECT_EQ(sizeof(int32_t), x.AllocSize());
+
+  EXPECT_EQ(p, Type<const int32_t*>(x.Pointer<0>(c)));
+  EXPECT_EQ(p, Type<const int32_t*>(x.Pointer<0>(cc)));
+
+  EXPECT_EQ(p, Type<const int32_t*>(x.Pointer<const int32_t>(c)));
+  EXPECT_EQ(p, Type<const int32_t*>(x.Pointer<const int32_t>(cc)));
+
+  EXPECT_THAT(Type<std::tuple<const int32_t*>>(x.Pointers(c)), Tuple(p));
+  EXPECT_THAT(Type<std::tuple<const int32_t*>>(x.Pointers(cc)), Tuple(p));
+
+  EXPECT_THAT(Type<Span<const int32_t>>(x.Slice<0>(c)),
+              IsSameSlice(Span<const int32_t>(p, 1)));
+  EXPECT_THAT(Type<Span<const int32_t>>(x.Slice<0>(cc)),
+              IsSameSlice(Span<const int32_t>(p, 1)));
+
+  EXPECT_THAT(Type<Span<const int32_t>>(x.Slice<const int32_t>(c)),
+              IsSameSlice(Span<const int32_t>(p, 1)));
+  EXPECT_THAT(Type<Span<const int32_t>>(x.Slice<const int32_t>(cc)),
+              IsSameSlice(Span<const int32_t>(p, 1)));
+
+  EXPECT_THAT(Type<std::tuple<Span<const int32_t>>>(x.Slices(c)),
+              Tuple(IsSameSlice(Span<const int32_t>(p, 1))));
+  EXPECT_THAT(Type<std::tuple<Span<const int32_t>>>(x.Slices(cc)),
+              Tuple(IsSameSlice(Span<const int32_t>(p, 1))));
+}
+
+namespace example {
+
+// Immutable move-only string with sizeof equal to sizeof(void*). The string
+// size and the characters are kept in the same heap allocation.
+class CompactString {
+ public:
+  CompactString(const char* s = "") {  // NOLINT
+    const size_t size = strlen(s);
+    // size_t[1], followed by char[size + 1].
+    // This statement doesn't allocate memory.
+    const L layout(1, size + 1);
+    // AllocSize() tells us how much memory we need to allocate for all our
+    // data.
+    p_.reset(new unsigned char[layout.AllocSize()]);
+    // If running under ASAN, mark the padding bytes, if any, to catch memory
+    // errors.
+    layout.PoisonPadding(p_.get());
+    // Store the size in the allocation.
+    // Pointer<size_t>() is a synonym for Pointer<0>().
+    *layout.Pointer<size_t>(p_.get()) = size;
+    // Store the characters in the allocation.
+    memcpy(layout.Pointer<char>(p_.get()), s, size + 1);
+  }
+
+  size_t size() const {
+    // Equivalent to reinterpret_cast<size_t&>(*p).
+    return *L::Partial().Pointer<size_t>(p_.get());
+  }
+
+  const char* c_str() const {
+    // Equivalent to reinterpret_cast<char*>(p.get() + sizeof(size_t)).
+    // The argument in Partial(1) specifies that we have size_t[1] in front of
+    // the
+    // characters.
+    return L::Partial(1).Pointer<char>(p_.get());
+  }
+
+ private:
+  // Our heap allocation contains a size_t followed by an array of chars.
+  using L = Layout<size_t, char>;
+  std::unique_ptr<unsigned char[]> p_;
+};
+
+TEST(CompactString, Works) {
+  CompactString s = "hello";
+  EXPECT_EQ(5, s.size());
+  EXPECT_STREQ("hello", s.c_str());
+}
+
+}  // namespace example
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/node_hash_policy.h b/absl/container/internal/node_hash_policy.h
new file mode 100644
index 000000000000..065e7009e7e7
--- /dev/null
+++ b/absl/container/internal/node_hash_policy.h
@@ -0,0 +1,88 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Adapts a policy for nodes.
+//
+// The node policy should model:
+//
+// struct Policy {
+//   // Returns a new node allocated and constructed using the allocator, using
+//   // the specified arguments.
+//   template <class Alloc, class... Args>
+//   value_type* new_element(Alloc* alloc, Args&&... args) const;
+//
+//   // Destroys and deallocates node using the allocator.
+//   template <class Alloc>
+//   void delete_element(Alloc* alloc, value_type* node) const;
+// };
+//
+// It may also optionally define `value()` and `apply()`. For documentation on
+// these, see hash_policy_traits.h.
+
+#ifndef ABSL_CONTAINER_INTERNAL_NODE_HASH_POLICY_H_
+#define ABSL_CONTAINER_INTERNAL_NODE_HASH_POLICY_H_
+
+#include <cassert>
+#include <cstddef>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+namespace absl {
+namespace container_internal {
+
+template <class Reference, class Policy>
+struct node_hash_policy {
+  static_assert(std::is_lvalue_reference<Reference>::value, "");
+
+  using slot_type = typename std::remove_cv<
+      typename std::remove_reference<Reference>::type>::type*;
+
+  template <class Alloc, class... Args>
+  static void construct(Alloc* alloc, slot_type* slot, Args&&... args) {
+    *slot = Policy::new_element(alloc, std::forward<Args>(args)...);
+  }
+
+  template <class Alloc>
+  static void destroy(Alloc* alloc, slot_type* slot) {
+    Policy::delete_element(alloc, *slot);
+  }
+
+  template <class Alloc>
+  static void transfer(Alloc*, slot_type* new_slot, slot_type* old_slot) {
+    *new_slot = *old_slot;
+  }
+
+  static size_t space_used(const slot_type* slot) {
+    if (slot == nullptr) return Policy::element_space_used(nullptr);
+    return Policy::element_space_used(*slot);
+  }
+
+  static Reference element(slot_type* slot) { return **slot; }
+
+  template <class T, class P = Policy>
+  static auto value(T* elem) -> decltype(P::value(elem)) {
+    return P::value(elem);
+  }
+
+  template <class... Ts, class P = Policy>
+  static auto apply(Ts&&... ts) -> decltype(P::apply(std::forward<Ts>(ts)...)) {
+    return P::apply(std::forward<Ts>(ts)...);
+  }
+};
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_NODE_HASH_POLICY_H_
diff --git a/absl/container/internal/node_hash_policy_test.cc b/absl/container/internal/node_hash_policy_test.cc
new file mode 100644
index 000000000000..43d287e3c4a0
--- /dev/null
+++ b/absl/container/internal/node_hash_policy_test.cc
@@ -0,0 +1,67 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/node_hash_policy.h"
+
+#include <memory>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/hash_policy_traits.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using ::testing::Pointee;
+
+struct Policy : node_hash_policy<int&, Policy> {
+  using key_type = int;
+  using init_type = int;
+
+  template <class Alloc>
+  static int* new_element(Alloc* alloc, int value) {
+    return new int(value);
+  }
+
+  template <class Alloc>
+  static void delete_element(Alloc* alloc, int* elem) {
+    delete elem;
+  }
+};
+
+using NodePolicy = hash_policy_traits<Policy>;
+
+struct NodeTest : ::testing::Test {
+  std::allocator<int> alloc;
+  int n = 53;
+  int* a = &n;
+};
+
+TEST_F(NodeTest, ConstructDestroy) {
+  NodePolicy::construct(&alloc, &a, 42);
+  EXPECT_THAT(a, Pointee(42));
+  NodePolicy::destroy(&alloc, &a);
+}
+
+TEST_F(NodeTest, transfer) {
+  int s = 42;
+  int* b = &s;
+  NodePolicy::transfer(&alloc, &a, &b);
+  EXPECT_EQ(&s, a);
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/raw_hash_map.h b/absl/container/internal/raw_hash_map.h
new file mode 100644
index 000000000000..1edc0071e7de
--- /dev/null
+++ b/absl/container/internal/raw_hash_map.h
@@ -0,0 +1,182 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_RAW_HASH_MAP_H_
+#define ABSL_CONTAINER_INTERNAL_RAW_HASH_MAP_H_
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
+
+namespace absl {
+namespace container_internal {
+
+template <class Policy, class Hash, class Eq, class Alloc>
+class raw_hash_map : public raw_hash_set<Policy, Hash, Eq, Alloc> {
+  // P is Policy. It's passed as a template argument to support maps that have
+  // incomplete types as values, as in unordered_map<K, IncompleteType>.
+  // MappedReference<> may be a non-reference type.
+  template <class P>
+  using MappedReference = decltype(P::value(
+      std::addressof(std::declval<typename raw_hash_map::reference>())));
+
+  // MappedConstReference<> may be a non-reference type.
+  template <class P>
+  using MappedConstReference = decltype(P::value(
+      std::addressof(std::declval<typename raw_hash_map::const_reference>())));
+
+ public:
+  using key_type = typename Policy::key_type;
+  using mapped_type = typename Policy::mapped_type;
+  template <typename K>
+  using key_arg = typename raw_hash_map::raw_hash_set::template key_arg<K>;
+
+  static_assert(!std::is_reference<key_type>::value, "");
+  // TODO(alkis): remove this assertion and verify that reference mapped_type is
+  // supported.
+  static_assert(!std::is_reference<mapped_type>::value, "");
+
+  using iterator = typename raw_hash_map::raw_hash_set::iterator;
+  using const_iterator = typename raw_hash_map::raw_hash_set::const_iterator;
+
+  raw_hash_map() {}
+  using raw_hash_map::raw_hash_set::raw_hash_set;
+
+  // The last two template parameters ensure that both arguments are rvalues
+  // (lvalue arguments are handled by the overloads below). This is necessary
+  // for supporting bitfield arguments.
+  //
+  //   union { int n : 1; };
+  //   flat_hash_map<int, int> m;
+  //   m.insert_or_assign(n, n);
+  template <class K = key_type, class V = mapped_type, K* = nullptr,
+            V* = nullptr>
+  std::pair<iterator, bool> insert_or_assign(key_arg<K>&& k, V&& v) {
+    return insert_or_assign_impl(std::forward<K>(k), std::forward<V>(v));
+  }
+
+  template <class K = key_type, class V = mapped_type, K* = nullptr>
+  std::pair<iterator, bool> insert_or_assign(key_arg<K>&& k, const V& v) {
+    return insert_or_assign_impl(std::forward<K>(k), v);
+  }
+
+  template <class K = key_type, class V = mapped_type, V* = nullptr>
+  std::pair<iterator, bool> insert_or_assign(const key_arg<K>& k, V&& v) {
+    return insert_or_assign_impl(k, std::forward<V>(v));
+  }
+
+  template <class K = key_type, class V = mapped_type>
+  std::pair<iterator, bool> insert_or_assign(const key_arg<K>& k, const V& v) {
+    return insert_or_assign_impl(k, v);
+  }
+
+  template <class K = key_type, class V = mapped_type, K* = nullptr,
+            V* = nullptr>
+  iterator insert_or_assign(const_iterator, key_arg<K>&& k, V&& v) {
+    return insert_or_assign(std::forward<K>(k), std::forward<V>(v)).first;
+  }
+
+  template <class K = key_type, class V = mapped_type, K* = nullptr>
+  iterator insert_or_assign(const_iterator, key_arg<K>&& k, const V& v) {
+    return insert_or_assign(std::forward<K>(k), v).first;
+  }
+
+  template <class K = key_type, class V = mapped_type, V* = nullptr>
+  iterator insert_or_assign(const_iterator, const key_arg<K>& k, V&& v) {
+    return insert_or_assign(k, std::forward<V>(v)).first;
+  }
+
+  template <class K = key_type, class V = mapped_type>
+  iterator insert_or_assign(const_iterator, const key_arg<K>& k, const V& v) {
+    return insert_or_assign(k, v).first;
+  }
+
+  template <class K = key_type, class... Args,
+            typename std::enable_if<
+                !std::is_convertible<K, const_iterator>::value, int>::type = 0,
+            K* = nullptr>
+  std::pair<iterator, bool> try_emplace(key_arg<K>&& k, Args&&... args) {
+    return try_emplace_impl(std::forward<K>(k), std::forward<Args>(args)...);
+  }
+
+  template <class K = key_type, class... Args,
+            typename std::enable_if<
+                !std::is_convertible<K, const_iterator>::value, int>::type = 0>
+  std::pair<iterator, bool> try_emplace(const key_arg<K>& k, Args&&... args) {
+    return try_emplace_impl(k, std::forward<Args>(args)...);
+  }
+
+  template <class K = key_type, class... Args, K* = nullptr>
+  iterator try_emplace(const_iterator, key_arg<K>&& k, Args&&... args) {
+    return try_emplace(std::forward<K>(k), std::forward<Args>(args)...).first;
+  }
+
+  template <class K = key_type, class... Args>
+  iterator try_emplace(const_iterator, const key_arg<K>& k, Args&&... args) {
+    return try_emplace(k, std::forward<Args>(args)...).first;
+  }
+
+  template <class K = key_type, class P = Policy>
+  MappedReference<P> at(const key_arg<K>& key) {
+    auto it = this->find(key);
+    if (it == this->end()) std::abort();
+    return Policy::value(&*it);
+  }
+
+  template <class K = key_type, class P = Policy>
+  MappedConstReference<P> at(const key_arg<K>& key) const {
+    auto it = this->find(key);
+    if (it == this->end()) std::abort();
+    return Policy::value(&*it);
+  }
+
+  template <class K = key_type, class P = Policy, K* = nullptr>
+  MappedReference<P> operator[](key_arg<K>&& key) {
+    return Policy::value(&*try_emplace(std::forward<K>(key)).first);
+  }
+
+  template <class K = key_type, class P = Policy>
+  MappedReference<P> operator[](const key_arg<K>& key) {
+    return Policy::value(&*try_emplace(key).first);
+  }
+
+ private:
+  template <class K, class V>
+  std::pair<iterator, bool> insert_or_assign_impl(K&& k, V&& v) {
+    auto res = this->find_or_prepare_insert(k);
+    if (res.second)
+      this->emplace_at(res.first, std::forward<K>(k), std::forward<V>(v));
+    else
+      Policy::value(&*this->iterator_at(res.first)) = std::forward<V>(v);
+    return {this->iterator_at(res.first), res.second};
+  }
+
+  template <class K = key_type, class... Args>
+  std::pair<iterator, bool> try_emplace_impl(K&& k, Args&&... args) {
+    auto res = this->find_or_prepare_insert(k);
+    if (res.second)
+      this->emplace_at(res.first, std::piecewise_construct,
+                       std::forward_as_tuple(std::forward<K>(k)),
+                       std::forward_as_tuple(std::forward<Args>(args)...));
+    return {this->iterator_at(res.first), res.second};
+  }
+};
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_RAW_HASH_MAP_H_
diff --git a/absl/container/internal/raw_hash_set.cc b/absl/container/internal/raw_hash_set.cc
new file mode 100644
index 000000000000..10153129fd1e
--- /dev/null
+++ b/absl/container/internal/raw_hash_set.cc
@@ -0,0 +1,45 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/raw_hash_set.h"
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+
+namespace absl {
+namespace container_internal {
+
+constexpr size_t Group::kWidth;
+
+// Returns "random" seed.
+inline size_t RandomSeed() {
+#if ABSL_HAVE_THREAD_LOCAL
+  static thread_local size_t counter = 0;
+  size_t value = ++counter;
+#else   // ABSL_HAVE_THREAD_LOCAL
+  static std::atomic<size_t> counter;
+  size_t value = counter.fetch_add(1, std::memory_order_relaxed);
+#endif  // ABSL_HAVE_THREAD_LOCAL
+  return value ^ static_cast<size_t>(reinterpret_cast<uintptr_t>(&counter));
+}
+
+bool ShouldInsertBackwards(size_t hash, ctrl_t* ctrl) {
+  // To avoid problems with weak hashes and single bit tests, we use % 13.
+  // TODO(kfm,sbenza): revisit after we do unconditional mixing
+  return (H1(hash, ctrl) ^ RandomSeed()) % 13 > 6;
+}
+
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
new file mode 100644
index 000000000000..0c0e5906d206
--- /dev/null
+++ b/absl/container/internal/raw_hash_set.h
@@ -0,0 +1,1906 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// An open-addressing
+// hashtable with quadratic probing.
+//
+// This is a low level hashtable on top of which different interfaces can be
+// implemented, like flat_hash_set, node_hash_set, string_hash_set, etc.
+//
+// The table interface is similar to that of std::unordered_set. Notable
+// differences are that most member functions support heterogeneous keys when
+// BOTH the hash and eq functions are marked as transparent. They do so by
+// providing a typedef called `is_transparent`.
+//
+// When heterogeneous lookup is enabled, functions that take key_type act as if
+// they have an overload set like:
+//
+//   iterator find(const key_type& key);
+//   template <class K>
+//   iterator find(const K& key);
+//
+//   size_type erase(const key_type& key);
+//   template <class K>
+//   size_type erase(const K& key);
+//
+//   std::pair<iterator, iterator> equal_range(const key_type& key);
+//   template <class K>
+//   std::pair<iterator, iterator> equal_range(const K& key);
+//
+// When heterogeneous lookup is disabled, only the explicit `key_type` overloads
+// exist.
+//
+// find() also supports passing the hash explicitly:
+//
+//   iterator find(const key_type& key, size_t hash);
+//   template <class U>
+//   iterator find(const U& key, size_t hash);
+//
+// In addition the pointer to element and iterator stability guarantees are
+// weaker: all iterators and pointers are invalidated after a new element is
+// inserted.
+//
+// IMPLEMENTATION DETAILS
+//
+// The table stores elements inline in a slot array. In addition to the slot
+// array the table maintains some control state per slot. The extra state is one
+// byte per slot and stores empty or deleted marks, or alternatively 7 bits from
+// the hash of an occupied slot. The table is split into logical groups of
+// slots, like so:
+//
+//      Group 1         Group 2        Group 3
+// +---------------+---------------+---------------+
+// | | | | | | | | | | | | | | | | | | | | | | | | |
+// +---------------+---------------+---------------+
+//
+// On lookup the hash is split into two parts:
+// - H2: 7 bits (those stored in the control bytes)
+// - H1: the rest of the bits
+// The groups are probed using H1. For each group the slots are matched to H2 in
+// parallel. Because H2 is 7 bits (128 states) and the number of slots per group
+// is low (8 or 16) in almost all cases a match in H2 is also a lookup hit.
+//
+// On insert, once the right group is found (as in lookup), its slots are
+// filled in order.
+//
+// On erase a slot is cleared. In case the group did not have any empty slots
+// before the erase, the erased slot is marked as deleted.
+//
+// Groups without empty slots (but maybe with deleted slots) extend the probe
+// sequence. The probing algorithm is quadratic. Given N the number of groups,
+// the probing function for the i'th probe is:
+//
+//   P(0) = H1 % N
+//
+//   P(i) = (P(i - 1) + i) % N
+//
+// This probing function guarantees that after N probes, all the groups of the
+// table will be probed exactly once.
+
+#ifndef ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_H_
+#define ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_H_
+
+#ifndef SWISSTABLE_HAVE_SSE2
+#ifdef __SSE2__
+#define SWISSTABLE_HAVE_SSE2 1
+#else
+#define SWISSTABLE_HAVE_SSE2 0
+#endif
+#endif
+
+#ifndef SWISSTABLE_HAVE_SSSE3
+#ifdef __SSSE3__
+#define SWISSTABLE_HAVE_SSSE3 1
+#else
+#define SWISSTABLE_HAVE_SSSE3 0
+#endif
+#endif
+
+#if SWISSTABLE_HAVE_SSSE3 && !SWISSTABLE_HAVE_SSE2
+#error "Bad configuration!"
+#endif
+
+#if SWISSTABLE_HAVE_SSE2
+#include <x86intrin.h>
+#endif
+
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+#include <cstring>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/internal/bits.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/port.h"
+#include "absl/container/internal/compressed_tuple.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hash_policy_traits.h"
+#include "absl/container/internal/hashtable_debug_hooks.h"
+#include "absl/container/internal/layout.h"
+#include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
+#include "absl/types/optional.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+namespace container_internal {
+
+template <size_t Width>
+class probe_seq {
+ public:
+  probe_seq(size_t hash, size_t mask) {
+    assert(((mask + 1) & mask) == 0 && "not a mask");
+    mask_ = mask;
+    offset_ = hash & mask_;
+  }
+  size_t offset() const { return offset_; }
+  size_t offset(size_t i) const { return (offset_ + i) & mask_; }
+
+  void next() {
+    index_ += Width;
+    offset_ += index_;
+    offset_ &= mask_;
+  }
+  // 0-based probe index. The i-th probe in the probe sequence.
+  size_t index() const { return index_; }
+
+ private:
+  size_t mask_;
+  size_t offset_;
+  size_t index_ = 0;
+};
+
+template <class ContainerKey, class Hash, class Eq>
+struct RequireUsableKey {
+  template <class PassedKey, class... Args>
+  std::pair<
+      decltype(std::declval<const Hash&>()(std::declval<const PassedKey&>())),
+      decltype(std::declval<const Eq&>()(std::declval<const ContainerKey&>(),
+                                         std::declval<const PassedKey&>()))>*
+  operator()(const PassedKey&, const Args&...) const;
+};
+
+template <class E, class Policy, class Hash, class Eq, class... Ts>
+struct IsDecomposable : std::false_type {};
+
+template <class Policy, class Hash, class Eq, class... Ts>
+struct IsDecomposable<
+    absl::void_t<decltype(
+        Policy::apply(RequireUsableKey<typename Policy::key_type, Hash, Eq>(),
+                      std::declval<Ts>()...))>,
+    Policy, Hash, Eq, Ts...> : std::true_type {};
+
+template <class, class = void>
+struct IsTransparent : std::false_type {};
+template <class T>
+struct IsTransparent<T, absl::void_t<typename T::is_transparent>>
+    : std::true_type {};
+
+// TODO(alkis): Switch to std::is_nothrow_swappable when gcc/clang supports it.
+template <class T>
+constexpr bool IsNoThrowSwappable() {
+  using std::swap;
+  return noexcept(swap(std::declval<T&>(), std::declval<T&>()));
+}
+
+template <typename T>
+int TrailingZeros(T x) {
+  return sizeof(T) == 8 ? base_internal::CountTrailingZerosNonZero64(x)
+                        : base_internal::CountTrailingZerosNonZero32(x);
+}
+
+template <typename T>
+int LeadingZeros(T x) {
+  return sizeof(T) == 8 ? base_internal::CountLeadingZeros64(x)
+                        : base_internal::CountLeadingZeros32(x);
+}
+
+// An abstraction over a bitmask. It provides an easy way to iterate through the
+// indexes of the set bits of a bitmask.  When Shift=0 (platforms with SSE),
+// this is a true bitmask.  On non-SSE, platforms the arithematic used to
+// emulate the SSE behavior works in bytes (Shift=3) and leaves each bytes as
+// either 0x00 or 0x80.
+//
+// For example:
+//   for (int i : BitMask<uint32_t, 16>(0x5)) -> yields 0, 2
+//   for (int i : BitMask<uint64_t, 8, 3>(0x0000000080800000)) -> yields 2, 3
+template <class T, int SignificantBits, int Shift = 0>
+class BitMask {
+  static_assert(std::is_unsigned<T>::value, "");
+  static_assert(Shift == 0 || Shift == 3, "");
+
+ public:
+  // These are useful for unit tests (gunit).
+  using value_type = int;
+  using iterator = BitMask;
+  using const_iterator = BitMask;
+
+  explicit BitMask(T mask) : mask_(mask) {}
+  BitMask& operator++() {
+    mask_ &= (mask_ - 1);
+    return *this;
+  }
+  explicit operator bool() const { return mask_ != 0; }
+  int operator*() const { return LowestBitSet(); }
+  int LowestBitSet() const {
+    return container_internal::TrailingZeros(mask_) >> Shift;
+  }
+  int HighestBitSet() const {
+    return (sizeof(T) * CHAR_BIT - container_internal::LeadingZeros(mask_) -
+            1) >>
+           Shift;
+  }
+
+  BitMask begin() const { return *this; }
+  BitMask end() const { return BitMask(0); }
+
+  int TrailingZeros() const {
+    return container_internal::TrailingZeros(mask_) >> Shift;
+  }
+
+  int LeadingZeros() const {
+    constexpr int total_significant_bits = SignificantBits << Shift;
+    constexpr int extra_bits = sizeof(T) * 8 - total_significant_bits;
+    return container_internal::LeadingZeros(mask_ << extra_bits) >> Shift;
+  }
+
+ private:
+  friend bool operator==(const BitMask& a, const BitMask& b) {
+    return a.mask_ == b.mask_;
+  }
+  friend bool operator!=(const BitMask& a, const BitMask& b) {
+    return a.mask_ != b.mask_;
+  }
+
+  T mask_;
+};
+
+using ctrl_t = signed char;
+using h2_t = uint8_t;
+
+// The values here are selected for maximum performance. See the static asserts
+// below for details.
+enum Ctrl : ctrl_t {
+  kEmpty = -128,   // 0b10000000
+  kDeleted = -2,   // 0b11111110
+  kSentinel = -1,  // 0b11111111
+};
+static_assert(
+    kEmpty & kDeleted & kSentinel & 0x80,
+    "Special markers need to have the MSB to make checking for them efficient");
+static_assert(kEmpty < kSentinel && kDeleted < kSentinel,
+              "kEmpty and kDeleted must be smaller than kSentinel to make the "
+              "SIMD test of IsEmptyOrDeleted() efficient");
+static_assert(kSentinel == -1,
+              "kSentinel must be -1 to elide loading it from memory into SIMD "
+              "registers (pcmpeqd xmm, xmm)");
+static_assert(kEmpty == -128,
+              "kEmpty must be -128 to make the SIMD check for its "
+              "existence efficient (psignb xmm, xmm)");
+static_assert(~kEmpty & ~kDeleted & kSentinel & 0x7F,
+              "kEmpty and kDeleted must share an unset bit that is not shared "
+              "by kSentinel to make the scalar test for MatchEmptyOrDeleted() "
+              "efficient");
+static_assert(kDeleted == -2,
+              "kDeleted must be -2 to make the implementation of "
+              "ConvertSpecialToEmptyAndFullToDeleted efficient");
+
+// A single block of empty control bytes for tables without any slots allocated.
+// This enables removing a branch in the hot path of find().
+inline ctrl_t* EmptyGroup() {
+  alignas(16) static constexpr ctrl_t empty_group[] = {
+      kSentinel, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty,
+      kEmpty,    kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty};
+  return const_cast<ctrl_t*>(empty_group);
+}
+
+// Mixes a randomly generated per-process seed with `hash` and `ctrl` to
+// randomize insertion order within groups.
+bool ShouldInsertBackwards(size_t hash, ctrl_t* ctrl);
+
+// Returns a hash seed.
+//
+// The seed consists of the ctrl_ pointer, which adds enough entropy to ensure
+// non-determinism of iteration order in most cases.
+inline size_t HashSeed(const ctrl_t* ctrl) {
+  // The low bits of the pointer have little or no entropy because of
+  // alignment. We shift the pointer to try to use higher entropy bits. A
+  // good number seems to be 12 bits, because that aligns with page size.
+  return reinterpret_cast<uintptr_t>(ctrl) >> 12;
+}
+
+inline size_t H1(size_t hash, const ctrl_t* ctrl) {
+  return (hash >> 7) ^ HashSeed(ctrl);
+}
+inline ctrl_t H2(size_t hash) { return hash & 0x7F; }
+
+inline bool IsEmpty(ctrl_t c) { return c == kEmpty; }
+inline bool IsFull(ctrl_t c) { return c >= 0; }
+inline bool IsDeleted(ctrl_t c) { return c == kDeleted; }
+inline bool IsEmptyOrDeleted(ctrl_t c) { return c < kSentinel; }
+
+#if SWISSTABLE_HAVE_SSE2
+struct Group {
+  static constexpr size_t kWidth = 16;  // the number of slots per group
+
+  explicit Group(const ctrl_t* pos) {
+    ctrl = _mm_loadu_si128(reinterpret_cast<const __m128i*>(pos));
+  }
+
+  // Returns a bitmask representing the positions of slots that match hash.
+  BitMask<uint32_t, kWidth> Match(h2_t hash) const {
+    auto match = _mm_set1_epi8(hash);
+    return BitMask<uint32_t, kWidth>(
+        _mm_movemask_epi8(_mm_cmpeq_epi8(match, ctrl)));
+  }
+
+  // Returns a bitmask representing the positions of empty slots.
+  BitMask<uint32_t, kWidth> MatchEmpty() const {
+#if SWISSTABLE_HAVE_SSSE3
+    // This only works because kEmpty is -128.
+    return BitMask<uint32_t, kWidth>(
+        _mm_movemask_epi8(_mm_sign_epi8(ctrl, ctrl)));
+#else
+    return Match(kEmpty);
+#endif
+  }
+
+  // Returns a bitmask representing the positions of empty or deleted slots.
+  BitMask<uint32_t, kWidth> MatchEmptyOrDeleted() const {
+    auto special = _mm_set1_epi8(kSentinel);
+    return BitMask<uint32_t, kWidth>(
+        _mm_movemask_epi8(_mm_cmpgt_epi8(special, ctrl)));
+  }
+
+  // Returns the number of trailing empty or deleted elements in the group.
+  uint32_t CountLeadingEmptyOrDeleted() const {
+    auto special = _mm_set1_epi8(kSentinel);
+    return TrailingZeros(_mm_movemask_epi8(_mm_cmpgt_epi8(special, ctrl)) + 1);
+  }
+
+  void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
+    auto msbs = _mm_set1_epi8(0x80);
+    auto x126 = _mm_set1_epi8(126);
+#if SWISSTABLE_HAVE_SSSE3
+    auto res = _mm_or_si128(_mm_shuffle_epi8(x126, ctrl), msbs);
+#else
+    auto zero = _mm_setzero_si128();
+    auto special_mask = _mm_cmpgt_epi8(zero, ctrl);
+    auto res = _mm_or_si128(msbs, _mm_andnot_si128(special_mask, x126));
+#endif
+    _mm_storeu_si128(reinterpret_cast<__m128i*>(dst), res);
+  }
+
+  __m128i ctrl;
+};
+#else
+struct Group {
+  static constexpr size_t kWidth = 8;
+
+  explicit Group(const ctrl_t* pos) : ctrl(little_endian::Load64(pos)) {}
+
+  BitMask<uint64_t, kWidth, 3> Match(h2_t hash) const {
+    // For the technique, see:
+    // http://graphics.stanford.edu/~seander/bithacks.html##ValueInWord
+    // (Determine if a word has a byte equal to n).
+    //
+    // Caveat: there are false positives but:
+    // - they only occur if there is a real match
+    // - they never occur on kEmpty, kDeleted, kSentinel
+    // - they will be handled gracefully by subsequent checks in code
+    //
+    // Example:
+    //   v = 0x1716151413121110
+    //   hash = 0x12
+    //   retval = (v - lsbs) & ~v & msbs = 0x0000000080800000
+    constexpr uint64_t msbs = 0x8080808080808080ULL;
+    constexpr uint64_t lsbs = 0x0101010101010101ULL;
+    auto x = ctrl ^ (lsbs * hash);
+    return BitMask<uint64_t, kWidth, 3>((x - lsbs) & ~x & msbs);
+  }
+
+  BitMask<uint64_t, kWidth, 3> MatchEmpty() const {
+    constexpr uint64_t msbs = 0x8080808080808080ULL;
+    return BitMask<uint64_t, kWidth, 3>((ctrl & (~ctrl << 6)) & msbs);
+  }
+
+  BitMask<uint64_t, kWidth, 3> MatchEmptyOrDeleted() const {
+    constexpr uint64_t msbs = 0x8080808080808080ULL;
+    return BitMask<uint64_t, kWidth, 3>((ctrl & (~ctrl << 7)) & msbs);
+  }
+
+  uint32_t CountLeadingEmptyOrDeleted() const {
+    constexpr uint64_t gaps = 0x00FEFEFEFEFEFEFEULL;
+    return (TrailingZeros(((~ctrl & (ctrl >> 7)) | gaps) + 1) + 7) >> 3;
+  }
+
+  void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
+    constexpr uint64_t msbs = 0x8080808080808080ULL;
+    constexpr uint64_t lsbs = 0x0101010101010101ULL;
+    auto x = ctrl & msbs;
+    auto res = (~x + (x >> 7)) & ~lsbs;
+    little_endian::Store64(dst, res);
+  }
+
+  uint64_t ctrl;
+};
+#endif  // SWISSTABLE_HAVE_SSE2
+
+template <class Policy, class Hash, class Eq, class Alloc>
+class raw_hash_set;
+
+
+inline bool IsValidCapacity(size_t n) {
+  return ((n + 1) & n) == 0 && n >= Group::kWidth - 1;
+}
+
+// PRECONDITION:
+//   IsValidCapacity(capacity)
+//   ctrl[capacity] == kSentinel
+//   ctrl[i] != kSentinel for all i < capacity
+// Applies mapping for every byte in ctrl:
+//   DELETED -> EMPTY
+//   EMPTY -> EMPTY
+//   FULL -> DELETED
+inline void ConvertDeletedToEmptyAndFullToDeleted(
+    ctrl_t* ctrl, size_t capacity) {
+  assert(ctrl[capacity] == kSentinel);
+  assert(IsValidCapacity(capacity));
+  for (ctrl_t* pos = ctrl; pos != ctrl + capacity + 1; pos += Group::kWidth) {
+    Group{pos}.ConvertSpecialToEmptyAndFullToDeleted(pos);
+  }
+  // Copy the cloned ctrl bytes.
+  std::memcpy(ctrl + capacity + 1, ctrl, Group::kWidth);
+  ctrl[capacity] = kSentinel;
+}
+
+// Rounds up the capacity to the next power of 2 minus 1 and ensures it is
+// greater or equal to Group::kWidth - 1.
+inline size_t NormalizeCapacity(size_t n) {
+  constexpr size_t kMinCapacity = Group::kWidth - 1;
+  return n <= kMinCapacity
+             ? kMinCapacity
+             : std::numeric_limits<size_t>::max() >> LeadingZeros(n);
+}
+
+// The node_handle concept from C++17.
+// We specialize node_handle for sets and maps. node_handle_base holds the
+// common API of both.
+template <typename Policy, typename Alloc>
+class node_handle_base {
+ protected:
+  using PolicyTraits = hash_policy_traits<Policy>;
+  using slot_type = typename PolicyTraits::slot_type;
+
+ public:
+  using allocator_type = Alloc;
+
+  constexpr node_handle_base() {}
+  node_handle_base(node_handle_base&& other) noexcept {
+    *this = std::move(other);
+  }
+  ~node_handle_base() { destroy(); }
+  node_handle_base& operator=(node_handle_base&& other) {
+    destroy();
+    if (!other.empty()) {
+      alloc_ = other.alloc_;
+      PolicyTraits::transfer(alloc(), slot(), other.slot());
+      other.reset();
+    }
+    return *this;
+  }
+
+  bool empty() const noexcept { return !alloc_; }
+  explicit operator bool() const noexcept { return !empty(); }
+  allocator_type get_allocator() const { return *alloc_; }
+
+ protected:
+  template <typename, typename, typename, typename>
+  friend class raw_hash_set;
+
+  node_handle_base(const allocator_type& a, slot_type* s) : alloc_(a) {
+    PolicyTraits::transfer(alloc(), slot(), s);
+  }
+
+  void destroy() {
+    if (!empty()) {
+      PolicyTraits::destroy(alloc(), slot());
+      reset();
+    }
+  }
+
+  void reset() {
+    assert(alloc_.has_value());
+    alloc_ = absl::nullopt;
+  }
+
+  slot_type* slot() const {
+    assert(!empty());
+    return reinterpret_cast<slot_type*>(std::addressof(slot_space_));
+  }
+  allocator_type* alloc() { return std::addressof(*alloc_); }
+
+ private:
+  absl::optional<allocator_type> alloc_;
+  mutable absl::aligned_storage_t<sizeof(slot_type), alignof(slot_type)>
+      slot_space_;
+};
+
+// For sets.
+template <typename Policy, typename Alloc, typename = void>
+class node_handle : public node_handle_base<Policy, Alloc> {
+  using Base = typename node_handle::node_handle_base;
+
+ public:
+  using value_type = typename Base::PolicyTraits::value_type;
+
+  constexpr node_handle() {}
+
+  value_type& value() const {
+    return Base::PolicyTraits::element(this->slot());
+  }
+
+ private:
+  template <typename, typename, typename, typename>
+  friend class raw_hash_set;
+
+  node_handle(const Alloc& a, typename Base::slot_type* s) : Base(a, s) {}
+};
+
+// For maps.
+template <typename Policy, typename Alloc>
+class node_handle<Policy, Alloc, absl::void_t<typename Policy::mapped_type>>
+    : public node_handle_base<Policy, Alloc> {
+  using Base = typename node_handle::node_handle_base;
+
+ public:
+  using key_type = typename Policy::key_type;
+  using mapped_type = typename Policy::mapped_type;
+
+  constexpr node_handle() {}
+
+  auto key() const -> decltype(Base::PolicyTraits::key(this->slot())) {
+    return Base::PolicyTraits::key(this->slot());
+  }
+
+  mapped_type& mapped() const {
+    return Base::PolicyTraits::value(
+        &Base::PolicyTraits::element(this->slot()));
+  }
+
+ private:
+  template <typename, typename, typename, typename>
+  friend class raw_hash_set;
+
+  node_handle(const Alloc& a, typename Base::slot_type* s) : Base(a, s) {}
+};
+
+// Implement the insert_return_type<> concept of C++17.
+template <class Iterator, class NodeType>
+struct insert_return_type {
+  Iterator position;
+  bool inserted;
+  NodeType node;
+};
+
+// Helper trait to allow or disallow arbitrary keys when the hash and
+// eq functions are transparent.
+// It is very important that the inner template is an alias and that the type it
+// produces is not a dependent type. Otherwise, type deduction would fail.
+template <bool is_transparent>
+struct KeyArg {
+  // Transparent. Forward `K`.
+  template <typename K, typename key_type>
+  using type = K;
+};
+
+template <>
+struct KeyArg<false> {
+  // Not transparent. Always use `key_type`.
+  template <typename K, typename key_type>
+  using type = key_type;
+};
+
+// Policy: a policy defines how to perform different operations on
+// the slots of the hashtable (see hash_policy_traits.h for the full interface
+// of policy).
+//
+// Hash: a (possibly polymorphic) functor that hashes keys of the hashtable. The
+// functor should accept a key and return size_t as hash. For best performance
+// it is important that the hash function provides high entropy across all bits
+// of the hash.
+//
+// Eq: a (possibly polymorphic) functor that compares two keys for equality. It
+// should accept two (of possibly different type) keys and return a bool: true
+// if they are equal, false if they are not. If two keys compare equal, then
+// their hash values as defined by Hash MUST be equal.
+//
+// Allocator: an Allocator [http://devdocs.io/cpp/concept/allocator] with which
+// the storage of the hashtable will be allocated and the elements will be
+// constructed and destroyed.
+template <class Policy, class Hash, class Eq, class Alloc>
+class raw_hash_set {
+  using PolicyTraits = hash_policy_traits<Policy>;
+  using KeyArgImpl = container_internal::KeyArg<IsTransparent<Eq>::value &&
+                                                IsTransparent<Hash>::value>;
+
+ public:
+  using init_type = typename PolicyTraits::init_type;
+  using key_type = typename PolicyTraits::key_type;
+  // TODO(sbenza): Hide slot_type as it is an implementation detail. Needs user
+  // code fixes!
+  using slot_type = typename PolicyTraits::slot_type;
+  using allocator_type = Alloc;
+  using size_type = size_t;
+  using difference_type = ptrdiff_t;
+  using hasher = Hash;
+  using key_equal = Eq;
+  using policy_type = Policy;
+  using value_type = typename PolicyTraits::value_type;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using pointer = typename absl::allocator_traits<
+      allocator_type>::template rebind_traits<value_type>::pointer;
+  using const_pointer = typename absl::allocator_traits<
+      allocator_type>::template rebind_traits<value_type>::const_pointer;
+
+  // Alias used for heterogeneous lookup functions.
+  // `key_arg<K>` evaluates to `K` when the functors are tranparent and to
+  // `key_type` otherwise. It permits template argument deduction on `K` for the
+  // transparent case.
+  template <class K>
+  using key_arg = typename KeyArgImpl::template type<K, key_type>;
+
+ private:
+  // Give an early error when key_type is not hashable/eq.
+  auto KeyTypeCanBeHashed(const Hash& h, const key_type& k) -> decltype(h(k));
+  auto KeyTypeCanBeEq(const Eq& eq, const key_type& k) -> decltype(eq(k, k));
+
+  using Layout = absl::container_internal::Layout<ctrl_t, slot_type>;
+
+  static Layout MakeLayout(size_t capacity) {
+    assert(IsValidCapacity(capacity));
+    return Layout(capacity + Group::kWidth + 1, capacity);
+  }
+
+  using AllocTraits = absl::allocator_traits<allocator_type>;
+  using SlotAlloc = typename absl::allocator_traits<
+      allocator_type>::template rebind_alloc<slot_type>;
+  using SlotAllocTraits = typename absl::allocator_traits<
+      allocator_type>::template rebind_traits<slot_type>;
+
+  static_assert(std::is_lvalue_reference<reference>::value,
+                "Policy::element() must return a reference");
+
+  template <typename T>
+  struct SameAsElementReference
+      : std::is_same<typename std::remove_cv<
+                         typename std::remove_reference<reference>::type>::type,
+                     typename std::remove_cv<
+                         typename std::remove_reference<T>::type>::type> {};
+
+  // An enabler for insert(T&&): T must be convertible to init_type or be the
+  // same as [cv] value_type [ref].
+  // Note: we separate SameAsElementReference into its own type to avoid using
+  // reference unless we need to. MSVC doesn't seem to like it in some
+  // cases.
+  template <class T>
+  using RequiresInsertable = typename std::enable_if<
+      absl::disjunction<std::is_convertible<T, init_type>,
+                        SameAsElementReference<T>>::value,
+      int>::type;
+
+  // RequiresNotInit is a workaround for gcc prior to 7.1.
+  // See https://godbolt.org/g/Y4xsUh.
+  template <class T>
+  using RequiresNotInit =
+      typename std::enable_if<!std::is_same<T, init_type>::value, int>::type;
+
+  template <class... Ts>
+  using IsDecomposable = IsDecomposable<void, PolicyTraits, Hash, Eq, Ts...>;
+
+ public:
+  static_assert(std::is_same<pointer, value_type*>::value,
+                "Allocators with custom pointer types are not supported");
+  static_assert(std::is_same<const_pointer, const value_type*>::value,
+                "Allocators with custom pointer types are not supported");
+
+  class iterator {
+    friend class raw_hash_set;
+
+   public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = typename raw_hash_set::value_type;
+    using reference =
+        absl::conditional_t<PolicyTraits::constant_iterators::value,
+                            const value_type&, value_type&>;
+    using pointer = absl::remove_reference_t<reference>*;
+    using difference_type = typename raw_hash_set::difference_type;
+
+    iterator() {}
+
+    // PRECONDITION: not an end() iterator.
+    reference operator*() const { return PolicyTraits::element(slot_); }
+
+    // PRECONDITION: not an end() iterator.
+    pointer operator->() const { return &operator*(); }
+
+    // PRECONDITION: not an end() iterator.
+    iterator& operator++() {
+      ++ctrl_;
+      ++slot_;
+      skip_empty_or_deleted();
+      return *this;
+    }
+    // PRECONDITION: not an end() iterator.
+    iterator operator++(int) {
+      auto tmp = *this;
+      ++*this;
+      return tmp;
+    }
+
+    friend bool operator==(const iterator& a, const iterator& b) {
+      return a.ctrl_ == b.ctrl_;
+    }
+    friend bool operator!=(const iterator& a, const iterator& b) {
+      return !(a == b);
+    }
+
+   private:
+    iterator(ctrl_t* ctrl) : ctrl_(ctrl) {}  // for end()
+    iterator(ctrl_t* ctrl, slot_type* slot) : ctrl_(ctrl), slot_(slot) {}
+
+    void skip_empty_or_deleted() {
+      while (IsEmptyOrDeleted(*ctrl_)) {
+        // ctrl is not necessarily aligned to Group::kWidth. It is also likely
+        // to read past the space for ctrl bytes and into slots. This is ok
+        // because ctrl has sizeof() == 1 and slot has sizeof() >= 1 so there
+        // is no way to read outside the combined slot array.
+        uint32_t shift = Group{ctrl_}.CountLeadingEmptyOrDeleted();
+        ctrl_ += shift;
+        slot_ += shift;
+      }
+    }
+
+    ctrl_t* ctrl_ = nullptr;
+    slot_type* slot_;
+  };
+
+  class const_iterator {
+    friend class raw_hash_set;
+
+   public:
+    using iterator_category = typename iterator::iterator_category;
+    using value_type = typename raw_hash_set::value_type;
+    using reference = typename raw_hash_set::const_reference;
+    using pointer = typename raw_hash_set::const_pointer;
+    using difference_type = typename raw_hash_set::difference_type;
+
+    const_iterator() {}
+    // Implicit construction from iterator.
+    const_iterator(iterator i) : inner_(std::move(i)) {}
+
+    reference operator*() const { return *inner_; }
+    pointer operator->() const { return inner_.operator->(); }
+
+    const_iterator& operator++() {
+      ++inner_;
+      return *this;
+    }
+    const_iterator operator++(int) { return inner_++; }
+
+    friend bool operator==(const const_iterator& a, const const_iterator& b) {
+      return a.inner_ == b.inner_;
+    }
+    friend bool operator!=(const const_iterator& a, const const_iterator& b) {
+      return !(a == b);
+    }
+
+   private:
+    const_iterator(const ctrl_t* ctrl, const slot_type* slot)
+        : inner_(const_cast<ctrl_t*>(ctrl), const_cast<slot_type*>(slot)) {}
+
+    iterator inner_;
+  };
+
+  using node_type = container_internal::node_handle<Policy, Alloc>;
+
+  raw_hash_set() noexcept(
+      std::is_nothrow_default_constructible<hasher>::value&&
+          std::is_nothrow_default_constructible<key_equal>::value&&
+              std::is_nothrow_default_constructible<allocator_type>::value) {}
+
+  explicit raw_hash_set(size_t bucket_count, const hasher& hash = hasher(),
+                        const key_equal& eq = key_equal(),
+                        const allocator_type& alloc = allocator_type())
+      : ctrl_(EmptyGroup()), settings_(0, hash, eq, alloc) {
+    if (bucket_count) {
+      capacity_ = NormalizeCapacity(bucket_count);
+      growth_left() = static_cast<size_t>(capacity_ * kMaxLoadFactor);
+      initialize_slots();
+    }
+  }
+
+  raw_hash_set(size_t bucket_count, const hasher& hash,
+               const allocator_type& alloc)
+      : raw_hash_set(bucket_count, hash, key_equal(), alloc) {}
+
+  raw_hash_set(size_t bucket_count, const allocator_type& alloc)
+      : raw_hash_set(bucket_count, hasher(), key_equal(), alloc) {}
+
+  explicit raw_hash_set(const allocator_type& alloc)
+      : raw_hash_set(0, hasher(), key_equal(), alloc) {}
+
+  template <class InputIter>
+  raw_hash_set(InputIter first, InputIter last, size_t bucket_count = 0,
+               const hasher& hash = hasher(), const key_equal& eq = key_equal(),
+               const allocator_type& alloc = allocator_type())
+      : raw_hash_set(bucket_count, hash, eq, alloc) {
+    insert(first, last);
+  }
+
+  template <class InputIter>
+  raw_hash_set(InputIter first, InputIter last, size_t bucket_count,
+               const hasher& hash, const allocator_type& alloc)
+      : raw_hash_set(first, last, bucket_count, hash, key_equal(), alloc) {}
+
+  template <class InputIter>
+  raw_hash_set(InputIter first, InputIter last, size_t bucket_count,
+               const allocator_type& alloc)
+      : raw_hash_set(first, last, bucket_count, hasher(), key_equal(), alloc) {}
+
+  template <class InputIter>
+  raw_hash_set(InputIter first, InputIter last, const allocator_type& alloc)
+      : raw_hash_set(first, last, 0, hasher(), key_equal(), alloc) {}
+
+  // Instead of accepting std::initializer_list<value_type> as the first
+  // argument like std::unordered_set<value_type> does, we have two overloads
+  // that accept std::initializer_list<T> and std::initializer_list<init_type>.
+  // This is advantageous for performance.
+  //
+  //   // Turns {"abc", "def"} into std::initializer_list<std::string>, then copies
+  //   // the strings into the set.
+  //   std::unordered_set<std::string> s = {"abc", "def"};
+  //
+  //   // Turns {"abc", "def"} into std::initializer_list<const char*>, then
+  //   // copies the strings into the set.
+  //   absl::flat_hash_set<std::string> s = {"abc", "def"};
+  //
+  // The same trick is used in insert().
+  //
+  // The enabler is necessary to prevent this constructor from triggering where
+  // the copy constructor is meant to be called.
+  //
+  //   absl::flat_hash_set<int> a, b{a};
+  //
+  // RequiresNotInit<T> is a workaround for gcc prior to 7.1.
+  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
+  raw_hash_set(std::initializer_list<T> init, size_t bucket_count = 0,
+               const hasher& hash = hasher(), const key_equal& eq = key_equal(),
+               const allocator_type& alloc = allocator_type())
+      : raw_hash_set(init.begin(), init.end(), bucket_count, hash, eq, alloc) {}
+
+  raw_hash_set(std::initializer_list<init_type> init, size_t bucket_count = 0,
+               const hasher& hash = hasher(), const key_equal& eq = key_equal(),
+               const allocator_type& alloc = allocator_type())
+      : raw_hash_set(init.begin(), init.end(), bucket_count, hash, eq, alloc) {}
+
+  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
+  raw_hash_set(std::initializer_list<T> init, size_t bucket_count,
+               const hasher& hash, const allocator_type& alloc)
+      : raw_hash_set(init, bucket_count, hash, key_equal(), alloc) {}
+
+  raw_hash_set(std::initializer_list<init_type> init, size_t bucket_count,
+               const hasher& hash, const allocator_type& alloc)
+      : raw_hash_set(init, bucket_count, hash, key_equal(), alloc) {}
+
+  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
+  raw_hash_set(std::initializer_list<T> init, size_t bucket_count,
+               const allocator_type& alloc)
+      : raw_hash_set(init, bucket_count, hasher(), key_equal(), alloc) {}
+
+  raw_hash_set(std::initializer_list<init_type> init, size_t bucket_count,
+               const allocator_type& alloc)
+      : raw_hash_set(init, bucket_count, hasher(), key_equal(), alloc) {}
+
+  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
+  raw_hash_set(std::initializer_list<T> init, const allocator_type& alloc)
+      : raw_hash_set(init, 0, hasher(), key_equal(), alloc) {}
+
+  raw_hash_set(std::initializer_list<init_type> init,
+               const allocator_type& alloc)
+      : raw_hash_set(init, 0, hasher(), key_equal(), alloc) {}
+
+  raw_hash_set(const raw_hash_set& that)
+      : raw_hash_set(that, AllocTraits::select_on_container_copy_construction(
+                               that.alloc_ref())) {}
+
+  raw_hash_set(const raw_hash_set& that, const allocator_type& a)
+      : raw_hash_set(0, that.hash_ref(), that.eq_ref(), a) {
+    reserve(that.size());
+    // Because the table is guaranteed to be empty, we can do something faster
+    // than a full `insert`.
+    for (const auto& v : that) {
+      const size_t hash = PolicyTraits::apply(HashElement{hash_ref()}, v);
+      const size_t i = find_first_non_full(hash);
+      set_ctrl(i, H2(hash));
+      emplace_at(i, v);
+    }
+    size_ = that.size();
+    growth_left() -= that.size();
+  }
+
+  raw_hash_set(raw_hash_set&& that) noexcept(
+      std::is_nothrow_copy_constructible<hasher>::value&&
+          std::is_nothrow_copy_constructible<key_equal>::value&&
+              std::is_nothrow_copy_constructible<allocator_type>::value)
+      : ctrl_(absl::exchange(that.ctrl_, EmptyGroup())),
+        slots_(absl::exchange(that.slots_, nullptr)),
+        size_(absl::exchange(that.size_, 0)),
+        capacity_(absl::exchange(that.capacity_, 0)),
+        // Hash, equality and allocator are copied instead of moved because
+        // `that` must be left valid. If Hash is std::function<Key>, moving it
+        // would create a nullptr functor that cannot be called.
+        settings_(that.settings_) {
+    // growth_left was copied above, reset the one from `that`.
+    that.growth_left() = 0;
+  }
+
+  raw_hash_set(raw_hash_set&& that, const allocator_type& a)
+      : ctrl_(EmptyGroup()),
+        slots_(nullptr),
+        size_(0),
+        capacity_(0),
+        settings_(0, that.hash_ref(), that.eq_ref(), a) {
+    if (a == that.alloc_ref()) {
+      std::swap(ctrl_, that.ctrl_);
+      std::swap(slots_, that.slots_);
+      std::swap(size_, that.size_);
+      std::swap(capacity_, that.capacity_);
+      std::swap(growth_left(), that.growth_left());
+    } else {
+      reserve(that.size());
+      // Note: this will copy elements of dense_set and unordered_set instead of
+      // moving them. This can be fixed if it ever becomes an issue.
+      for (auto& elem : that) insert(std::move(elem));
+    }
+  }
+
+  raw_hash_set& operator=(const raw_hash_set& that) {
+    raw_hash_set tmp(that,
+                     AllocTraits::propagate_on_container_copy_assignment::value
+                         ? that.alloc_ref()
+                         : alloc_ref());
+    swap(tmp);
+    return *this;
+  }
+
+  raw_hash_set& operator=(raw_hash_set&& that) noexcept(
+      absl::allocator_traits<allocator_type>::is_always_equal::value&&
+          std::is_nothrow_move_assignable<hasher>::value&&
+              std::is_nothrow_move_assignable<key_equal>::value) {
+    // TODO(sbenza): We should only use the operations from the noexcept clause
+    // to make sure we actually adhere to that contract.
+    return move_assign(
+        std::move(that),
+        typename AllocTraits::propagate_on_container_move_assignment());
+  }
+
+  ~raw_hash_set() { destroy_slots(); }
+
+  iterator begin() {
+    auto it = iterator_at(0);
+    it.skip_empty_or_deleted();
+    return it;
+  }
+  iterator end() { return {ctrl_ + capacity_}; }
+
+  const_iterator begin() const {
+    return const_cast<raw_hash_set*>(this)->begin();
+  }
+  const_iterator end() const { return const_cast<raw_hash_set*>(this)->end(); }
+  const_iterator cbegin() const { return begin(); }
+  const_iterator cend() const { return end(); }
+
+  bool empty() const { return !size(); }
+  size_t size() const { return size_; }
+  size_t capacity() const { return capacity_; }
+  size_t max_size() const { return std::numeric_limits<size_t>::max(); }
+
+  void clear() {
+    // Iterating over this container is O(bucket_count()). When bucket_count()
+    // is much greater than size(), iteration becomes prohibitively expensive.
+    // For clear() it is more important to reuse the allocated array when the
+    // container is small because allocation takes comparatively long time
+    // compared to destruction of the elements of the container. So we pick the
+    // largest bucket_count() threshold for which iteration is still fast and
+    // past that we simply deallocate the array.
+    if (capacity_ > 127) {
+      destroy_slots();
+    } else if (capacity_) {
+      for (size_t i = 0; i != capacity_; ++i) {
+        if (IsFull(ctrl_[i])) {
+          PolicyTraits::destroy(&alloc_ref(), slots_ + i);
+        }
+      }
+      size_ = 0;
+      reset_ctrl();
+      growth_left() = static_cast<size_t>(capacity_ * kMaxLoadFactor);
+    }
+    assert(empty());
+  }
+
+  // This overload kicks in when the argument is an rvalue of insertable and
+  // decomposable type other than init_type.
+  //
+  //   flat_hash_map<std::string, int> m;
+  //   m.insert(std::make_pair("abc", 42));
+  template <class T, RequiresInsertable<T> = 0,
+            typename std::enable_if<IsDecomposable<T>::value, int>::type = 0,
+            T* = nullptr>
+  std::pair<iterator, bool> insert(T&& value) {
+    return emplace(std::forward<T>(value));
+  }
+
+  // This overload kicks in when the argument is a bitfield or an lvalue of
+  // insertable and decomposable type.
+  //
+  //   union { int n : 1; };
+  //   flat_hash_set<int> s;
+  //   s.insert(n);
+  //
+  //   flat_hash_set<std::string> s;
+  //   const char* p = "hello";
+  //   s.insert(p);
+  //
+  // TODO(romanp): Once we stop supporting gcc 5.1 and below, replace
+  // RequiresInsertable<T> with RequiresInsertable<const T&>.
+  // We are hitting this bug: https://godbolt.org/g/1Vht4f.
+  template <
+      class T, RequiresInsertable<T> = 0,
+      typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
+  std::pair<iterator, bool> insert(const T& value) {
+    return emplace(value);
+  }
+
+  // This overload kicks in when the argument is an rvalue of init_type. Its
+  // purpose is to handle brace-init-list arguments.
+  //
+  //   flat_hash_set<std::string, int> s;
+  //   s.insert({"abc", 42});
+  std::pair<iterator, bool> insert(init_type&& value) {
+    return emplace(std::move(value));
+  }
+
+  template <class T, RequiresInsertable<T> = 0,
+            typename std::enable_if<IsDecomposable<T>::value, int>::type = 0,
+            T* = nullptr>
+  iterator insert(const_iterator, T&& value) {
+    return insert(std::forward<T>(value)).first;
+  }
+
+  // TODO(romanp): Once we stop supporting gcc 5.1 and below, replace
+  // RequiresInsertable<T> with RequiresInsertable<const T&>.
+  // We are hitting this bug: https://godbolt.org/g/1Vht4f.
+  template <
+      class T, RequiresInsertable<T> = 0,
+      typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
+  iterator insert(const_iterator, const T& value) {
+    return insert(value).first;
+  }
+
+  iterator insert(const_iterator, init_type&& value) {
+    return insert(std::move(value)).first;
+  }
+
+  template <class InputIt>
+  void insert(InputIt first, InputIt last) {
+    for (; first != last; ++first) insert(*first);
+  }
+
+  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<const T&> = 0>
+  void insert(std::initializer_list<T> ilist) {
+    insert(ilist.begin(), ilist.end());
+  }
+
+  void insert(std::initializer_list<init_type> ilist) {
+    insert(ilist.begin(), ilist.end());
+  }
+
+  insert_return_type<iterator, node_type> insert(node_type&& node) {
+    if (!node) return {end(), false, node_type()};
+    const auto& elem = PolicyTraits::element(node.slot());
+    auto res = PolicyTraits::apply(
+        InsertSlot<false>{*this, std::move(*node.slot())}, elem);
+    if (res.second) {
+      node.reset();
+      return {res.first, true, node_type()};
+    } else {
+      return {res.first, false, std::move(node)};
+    }
+  }
+
+  iterator insert(const_iterator, node_type&& node) {
+    return insert(std::move(node)).first;
+  }
+
+  // This overload kicks in if we can deduce the key from args. This enables us
+  // to avoid constructing value_type if an entry with the same key already
+  // exists.
+  //
+  // For example:
+  //
+  //   flat_hash_map<std::string, std::string> m = {{"abc", "def"}};
+  //   // Creates no std::string copies and makes no heap allocations.
+  //   m.emplace("abc", "xyz");
+  template <class... Args, typename std::enable_if<
+                               IsDecomposable<Args...>::value, int>::type = 0>
+  std::pair<iterator, bool> emplace(Args&&... args) {
+    return PolicyTraits::apply(EmplaceDecomposable{*this},
+                               std::forward<Args>(args)...);
+  }
+
+  // This overload kicks in if we cannot deduce the key from args. It constructs
+  // value_type unconditionally and then either moves it into the table or
+  // destroys.
+  template <class... Args, typename std::enable_if<
+                               !IsDecomposable<Args...>::value, int>::type = 0>
+  std::pair<iterator, bool> emplace(Args&&... args) {
+    typename std::aligned_storage<sizeof(slot_type), alignof(slot_type)>::type
+        raw;
+    slot_type* slot = reinterpret_cast<slot_type*>(&raw);
+
+    PolicyTraits::construct(&alloc_ref(), slot, std::forward<Args>(args)...);
+    const auto& elem = PolicyTraits::element(slot);
+    return PolicyTraits::apply(InsertSlot<true>{*this, std::move(*slot)}, elem);
+  }
+
+  template <class... Args>
+  iterator emplace_hint(const_iterator, Args&&... args) {
+    return emplace(std::forward<Args>(args)...).first;
+  }
+
+  // Extension API: support for lazy emplace.
+  //
+  // Looks up key in the table. If found, returns the iterator to the element.
+  // Otherwise calls f with one argument of type raw_hash_set::constructor. f
+  // MUST call raw_hash_set::constructor with arguments as if a
+  // raw_hash_set::value_type is constructed, otherwise the behavior is
+  // undefined.
+  //
+  // For example:
+  //
+  //   std::unordered_set<ArenaString> s;
+  //   // Makes ArenaStr even if "abc" is in the map.
+  //   s.insert(ArenaString(&arena, "abc"));
+  //
+  //   flat_hash_set<ArenaStr> s;
+  //   // Makes ArenaStr only if "abc" is not in the map.
+  //   s.lazy_emplace("abc", [&](const constructor& ctor) {
+  //     ctor(&arena, "abc");
+  //   });
+  //
+  // WARNING: This API is currently experimental. If there is a way to implement
+  // the same thing with the rest of the API, prefer that.
+  class constructor {
+    friend class raw_hash_set;
+
+   public:
+    template <class... Args>
+    void operator()(Args&&... args) const {
+      assert(*slot_);
+      PolicyTraits::construct(alloc_, *slot_, std::forward<Args>(args)...);
+      *slot_ = nullptr;
+    }
+
+   private:
+    constructor(allocator_type* a, slot_type** slot) : alloc_(a), slot_(slot) {}
+
+    allocator_type* alloc_;
+    slot_type** slot_;
+  };
+
+  template <class K = key_type, class F>
+  iterator lazy_emplace(const key_arg<K>& key, F&& f) {
+    auto res = find_or_prepare_insert(key);
+    if (res.second) {
+      slot_type* slot = slots_ + res.first;
+      std::forward<F>(f)(constructor(&alloc_ref(), &slot));
+      assert(!slot);
+    }
+    return iterator_at(res.first);
+  }
+
+  // Extension API: support for heterogeneous keys.
+  //
+  //   std::unordered_set<std::string> s;
+  //   // Turns "abc" into std::string.
+  //   s.erase("abc");
+  //
+  //   flat_hash_set<std::string> s;
+  //   // Uses "abc" directly without copying it into std::string.
+  //   s.erase("abc");
+  template <class K = key_type>
+  size_type erase(const key_arg<K>& key) {
+    auto it = find(key);
+    if (it == end()) return 0;
+    erase(it);
+    return 1;
+  }
+
+  // Erases the element pointed to by `it`.  Unlike `std::unordered_set::erase`,
+  // this method returns void to reduce algorithmic complexity to O(1).  In
+  // order to erase while iterating across a map, use the following idiom (which
+  // also works for standard containers):
+  //
+  // for (auto it = m.begin(), end = m.end(); it != end;) {
+  //   if (<pred>) {
+  //     m.erase(it++);
+  //   } else {
+  //     ++it;
+  //   }
+  // }
+  void erase(const_iterator cit) { erase(cit.inner_); }
+
+  // This overload is necessary because otherwise erase<K>(const K&) would be
+  // a better match if non-const iterator is passed as an argument.
+  void erase(iterator it) {
+    assert(it != end());
+    PolicyTraits::destroy(&alloc_ref(), it.slot_);
+    erase_meta_only(it);
+  }
+
+  iterator erase(const_iterator first, const_iterator last) {
+    while (first != last) {
+      erase(first++);
+    }
+    return last.inner_;
+  }
+
+  // Moves elements from `src` into `this`.
+  // If the element already exists in `this`, it is left unmodified in `src`.
+  template <typename H, typename E>
+  void merge(raw_hash_set<Policy, H, E, Alloc>& src) {  // NOLINT
+    assert(this != &src);
+    for (auto it = src.begin(), e = src.end(); it != e; ++it) {
+      if (PolicyTraits::apply(InsertSlot<false>{*this, std::move(*it.slot_)},
+                              PolicyTraits::element(it.slot_))
+              .second) {
+        src.erase_meta_only(it);
+      }
+    }
+  }
+
+  template <typename H, typename E>
+  void merge(raw_hash_set<Policy, H, E, Alloc>&& src) {
+    merge(src);
+  }
+
+  node_type extract(const_iterator position) {
+    node_type node(alloc_ref(), position.inner_.slot_);
+    erase_meta_only(position);
+    return node;
+  }
+
+  template <
+      class K = key_type,
+      typename std::enable_if<!std::is_same<K, iterator>::value, int>::type = 0>
+  node_type extract(const key_arg<K>& key) {
+    auto it = find(key);
+    return it == end() ? node_type() : extract(const_iterator{it});
+  }
+
+  void swap(raw_hash_set& that) noexcept(
+      IsNoThrowSwappable<hasher>() && IsNoThrowSwappable<key_equal>() &&
+      (!AllocTraits::propagate_on_container_swap::value ||
+       IsNoThrowSwappable<allocator_type>())) {
+    using std::swap;
+    swap(ctrl_, that.ctrl_);
+    swap(slots_, that.slots_);
+    swap(size_, that.size_);
+    swap(capacity_, that.capacity_);
+    swap(growth_left(), that.growth_left());
+    swap(hash_ref(), that.hash_ref());
+    swap(eq_ref(), that.eq_ref());
+    if (AllocTraits::propagate_on_container_swap::value) {
+      swap(alloc_ref(), that.alloc_ref());
+    } else {
+      // If the allocators do not compare equal it is officially undefined
+      // behavior. We choose to do nothing.
+    }
+  }
+
+  void rehash(size_t n) {
+    if (n == 0 && capacity_ == 0) return;
+    if (n == 0 && size_ == 0) return destroy_slots();
+    auto m = NormalizeCapacity(std::max(
+        n, static_cast<size_t>(std::ceil(size() / kMaxLoadFactor))));
+    // n == 0 unconditionally rehashes as per the standard.
+    if (n == 0 || m > capacity_) {
+      resize(m);
+    }
+  }
+
+  void reserve(size_t n) {
+    rehash(static_cast<size_t>(std::ceil(n / kMaxLoadFactor)));
+  }
+
+  // Extension API: support for heterogeneous keys.
+  //
+  //   std::unordered_set<std::string> s;
+  //   // Turns "abc" into std::string.
+  //   s.count("abc");
+  //
+  //   ch_set<std::string> s;
+  //   // Uses "abc" directly without copying it into std::string.
+  //   s.count("abc");
+  template <class K = key_type>
+  size_t count(const key_arg<K>& key) const {
+    return find(key) == end() ? 0 : 1;
+  }
+
+  // Issues CPU prefetch instructions for the memory needed to find or insert
+  // a key.  Like all lookup functions, this support heterogeneous keys.
+  //
+  // NOTE: This is a very low level operation and should not be used without
+  // specific benchmarks indicating its importance.
+  template <class K = key_type>
+  void prefetch(const key_arg<K>& key) const {
+    (void)key;
+#if defined(__GNUC__)
+    auto seq = probe(hash_ref()(key));
+    __builtin_prefetch(static_cast<const void*>(ctrl_ + seq.offset()));
+    __builtin_prefetch(static_cast<const void*>(slots_ + seq.offset()));
+#endif  // __GNUC__
+  }
+
+  // The API of find() has two extensions.
+  //
+  // 1. The hash can be passed by the user. It must be equal to the hash of the
+  // key.
+  //
+  // 2. The type of the key argument doesn't have to be key_type. This is so
+  // called heterogeneous key support.
+  template <class K = key_type>
+  iterator find(const key_arg<K>& key, size_t hash) {
+    auto seq = probe(hash);
+    while (true) {
+      Group g{ctrl_ + seq.offset()};
+      for (int i : g.Match(H2(hash))) {
+        if (ABSL_PREDICT_TRUE(PolicyTraits::apply(
+                EqualElement<K>{key, eq_ref()},
+                PolicyTraits::element(slots_ + seq.offset(i)))))
+          return iterator_at(seq.offset(i));
+      }
+      if (ABSL_PREDICT_TRUE(g.MatchEmpty())) return end();
+      seq.next();
+    }
+  }
+  template <class K = key_type>
+  iterator find(const key_arg<K>& key) {
+    return find(key, hash_ref()(key));
+  }
+
+  template <class K = key_type>
+  const_iterator find(const key_arg<K>& key, size_t hash) const {
+    return const_cast<raw_hash_set*>(this)->find(key, hash);
+  }
+  template <class K = key_type>
+  const_iterator find(const key_arg<K>& key) const {
+    return find(key, hash_ref()(key));
+  }
+
+  template <class K = key_type>
+  bool contains(const key_arg<K>& key) const {
+    return find(key) != end();
+  }
+
+  template <class K = key_type>
+  std::pair<iterator, iterator> equal_range(const key_arg<K>& key) {
+    auto it = find(key);
+    if (it != end()) return {it, std::next(it)};
+    return {it, it};
+  }
+  template <class K = key_type>
+  std::pair<const_iterator, const_iterator> equal_range(
+      const key_arg<K>& key) const {
+    auto it = find(key);
+    if (it != end()) return {it, std::next(it)};
+    return {it, it};
+  }
+
+  size_t bucket_count() const { return capacity_; }
+  float load_factor() const {
+    return capacity_ ? static_cast<double>(size()) / capacity_ : 0.0;
+  }
+  float max_load_factor() const { return 1.0f; }
+  void max_load_factor(float) {
+    // Does nothing.
+  }
+
+  hasher hash_function() const { return hash_ref(); }
+  key_equal key_eq() const { return eq_ref(); }
+  allocator_type get_allocator() const { return alloc_ref(); }
+
+  friend bool operator==(const raw_hash_set& a, const raw_hash_set& b) {
+    if (a.size() != b.size()) return false;
+    const raw_hash_set* outer = &a;
+    const raw_hash_set* inner = &b;
+    if (outer->capacity() > inner->capacity()) std::swap(outer, inner);
+    for (const value_type& elem : *outer)
+      if (!inner->has_element(elem)) return false;
+    return true;
+  }
+
+  friend bool operator!=(const raw_hash_set& a, const raw_hash_set& b) {
+    return !(a == b);
+  }
+
+  friend void swap(raw_hash_set& a,
+                   raw_hash_set& b) noexcept(noexcept(a.swap(b))) {
+    a.swap(b);
+  }
+
+ private:
+  template <class Container, typename Enabler>
+  friend struct absl::container_internal::hashtable_debug_internal::
+      HashtableDebugAccess;
+
+  struct FindElement {
+    template <class K, class... Args>
+    const_iterator operator()(const K& key, Args&&...) const {
+      return s.find(key);
+    }
+    const raw_hash_set& s;
+  };
+
+  struct HashElement {
+    template <class K, class... Args>
+    size_t operator()(const K& key, Args&&...) const {
+      return h(key);
+    }
+    const hasher& h;
+  };
+
+  template <class K1>
+  struct EqualElement {
+    template <class K2, class... Args>
+    bool operator()(const K2& lhs, Args&&...) const {
+      return eq(lhs, rhs);
+    }
+    const K1& rhs;
+    const key_equal& eq;
+  };
+
+  struct EmplaceDecomposable {
+    template <class K, class... Args>
+    std::pair<iterator, bool> operator()(const K& key, Args&&... args) const {
+      auto res = s.find_or_prepare_insert(key);
+      if (res.second) {
+        s.emplace_at(res.first, std::forward<Args>(args)...);
+      }
+      return {s.iterator_at(res.first), res.second};
+    }
+    raw_hash_set& s;
+  };
+
+  template <bool do_destroy>
+  struct InsertSlot {
+    template <class K, class... Args>
+    std::pair<iterator, bool> operator()(const K& key, Args&&...) && {
+      auto res = s.find_or_prepare_insert(key);
+      if (res.second) {
+        PolicyTraits::transfer(&s.alloc_ref(), s.slots_ + res.first, &slot);
+      } else if (do_destroy) {
+        PolicyTraits::destroy(&s.alloc_ref(), &slot);
+      }
+      return {s.iterator_at(res.first), res.second};
+    }
+    raw_hash_set& s;
+    // Constructed slot. Either moved into place or destroyed.
+    slot_type&& slot;
+  };
+
+  // "erases" the object from the container, except that it doesn't actually
+  // destroy the object. It only updates all the metadata of the class.
+  // This can be used in conjunction with Policy::transfer to move the object to
+  // another place.
+  void erase_meta_only(const_iterator it) {
+    assert(IsFull(*it.inner_.ctrl_) && "erasing a dangling iterator");
+    --size_;
+    const size_t index = it.inner_.ctrl_ - ctrl_;
+    const size_t index_before = (index - Group::kWidth) & capacity_;
+    const auto empty_after = Group(it.inner_.ctrl_).MatchEmpty();
+    const auto empty_before = Group(ctrl_ + index_before).MatchEmpty();
+
+    // We count how many consecutive non empties we have to the right and to the
+    // left of `it`. If the sum is >= kWidth then there is at least one probe
+    // window that might have seen a full group.
+    bool was_never_full =
+        empty_before && empty_after &&
+        static_cast<size_t>(empty_after.TrailingZeros() +
+                            empty_before.LeadingZeros()) < Group::kWidth;
+
+    set_ctrl(index, was_never_full ? kEmpty : kDeleted);
+    growth_left() += was_never_full;
+  }
+
+  void initialize_slots() {
+    assert(capacity_);
+    auto layout = MakeLayout(capacity_);
+    char* mem = static_cast<char*>(
+        Allocate<Layout::Alignment()>(&alloc_ref(), layout.AllocSize()));
+    ctrl_ = reinterpret_cast<ctrl_t*>(layout.template Pointer<0>(mem));
+    slots_ = layout.template Pointer<1>(mem);
+    reset_ctrl();
+    growth_left() = static_cast<size_t>(capacity_ * kMaxLoadFactor) - size_;
+  }
+
+  void destroy_slots() {
+    if (!capacity_) return;
+    for (size_t i = 0; i != capacity_; ++i) {
+      if (IsFull(ctrl_[i])) {
+        PolicyTraits::destroy(&alloc_ref(), slots_ + i);
+      }
+    }
+    auto layout = MakeLayout(capacity_);
+    // Unpoison before returning the memory to the allocator.
+    SanitizerUnpoisonMemoryRegion(slots_, sizeof(slot_type) * capacity_);
+    Deallocate<Layout::Alignment()>(&alloc_ref(), ctrl_, layout.AllocSize());
+    ctrl_ = EmptyGroup();
+    slots_ = nullptr;
+    size_ = 0;
+    capacity_ = 0;
+    growth_left() = 0;
+  }
+
+  void resize(size_t new_capacity) {
+    assert(IsValidCapacity(new_capacity));
+    auto* old_ctrl = ctrl_;
+    auto* old_slots = slots_;
+    const size_t old_capacity = capacity_;
+    capacity_ = new_capacity;
+    initialize_slots();
+
+    for (size_t i = 0; i != old_capacity; ++i) {
+      if (IsFull(old_ctrl[i])) {
+        size_t hash = PolicyTraits::apply(HashElement{hash_ref()},
+                                          PolicyTraits::element(old_slots + i));
+        size_t new_i = find_first_non_full(hash);
+        set_ctrl(new_i, H2(hash));
+        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, old_slots + i);
+      }
+    }
+    if (old_capacity) {
+      SanitizerUnpoisonMemoryRegion(old_slots,
+                                    sizeof(slot_type) * old_capacity);
+      auto layout = MakeLayout(old_capacity);
+      Deallocate<Layout::Alignment()>(&alloc_ref(), old_ctrl,
+                                      layout.AllocSize());
+    }
+  }
+
+  void drop_deletes_without_resize() ABSL_ATTRIBUTE_NOINLINE {
+    assert(IsValidCapacity(capacity_));
+    // Algorithm:
+    // - mark all DELETED slots as EMPTY
+    // - mark all FULL slots as DELETED
+    // - for each slot marked as DELETED
+    //     hash = Hash(element)
+    //     target = find_first_non_full(hash)
+    //     if target is in the same group
+    //       mark slot as FULL
+    //     else if target is EMPTY
+    //       transfer element to target
+    //       mark slot as EMPTY
+    //       mark target as FULL
+    //     else if target is DELETED
+    //       swap current element with target element
+    //       mark target as FULL
+    //       repeat procedure for current slot with moved from element (target)
+    ConvertDeletedToEmptyAndFullToDeleted(ctrl_, capacity_);
+    typename std::aligned_storage<sizeof(slot_type), alignof(slot_type)>::type
+        raw;
+    slot_type* slot = reinterpret_cast<slot_type*>(&raw);
+    for (size_t i = 0; i != capacity_; ++i) {
+      if (!IsDeleted(ctrl_[i])) continue;
+      size_t hash = PolicyTraits::apply(HashElement{hash_ref()},
+                                        PolicyTraits::element(slots_ + i));
+      size_t new_i = find_first_non_full(hash);
+
+      // Verify if the old and new i fall within the same group wrt the hash.
+      // If they do, we don't need to move the object as it falls already in the
+      // best probe we can.
+      const auto probe_index = [&](size_t pos) {
+        return ((pos - probe(hash).offset()) & capacity_) / Group::kWidth;
+      };
+
+      // Element doesn't move.
+      if (ABSL_PREDICT_TRUE(probe_index(new_i) == probe_index(i))) {
+        set_ctrl(i, H2(hash));
+        continue;
+      }
+      if (IsEmpty(ctrl_[new_i])) {
+        // Transfer element to the empty spot.
+        // set_ctrl poisons/unpoisons the slots so we have to call it at the
+        // right time.
+        set_ctrl(new_i, H2(hash));
+        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, slots_ + i);
+        set_ctrl(i, kEmpty);
+      } else {
+        assert(IsDeleted(ctrl_[new_i]));
+        set_ctrl(new_i, H2(hash));
+        // Until we are done rehashing, DELETED marks previously FULL slots.
+        // Swap i and new_i elements.
+        PolicyTraits::transfer(&alloc_ref(), slot, slots_ + i);
+        PolicyTraits::transfer(&alloc_ref(), slots_ + i, slots_ + new_i);
+        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, slot);
+        --i;  // repeat
+      }
+    }
+    growth_left() = static_cast<size_t>(capacity_ * kMaxLoadFactor) - size_;
+  }
+
+  void rehash_and_grow_if_necessary() {
+    if (capacity_ == 0) {
+      resize(Group::kWidth - 1);
+    } else if (size() <= kMaxLoadFactor / 2 * capacity_) {
+      // Squash DELETED without growing if there is enough capacity.
+      drop_deletes_without_resize();
+    } else {
+      // Otherwise grow the container.
+      resize(capacity_ * 2 + 1);
+    }
+  }
+
+  bool has_element(const value_type& elem) const {
+    size_t hash = PolicyTraits::apply(HashElement{hash_ref()}, elem);
+    auto seq = probe(hash);
+    while (true) {
+      Group g{ctrl_ + seq.offset()};
+      for (int i : g.Match(H2(hash))) {
+        if (ABSL_PREDICT_TRUE(PolicyTraits::element(slots_ + seq.offset(i)) ==
+                              elem))
+          return true;
+      }
+      if (ABSL_PREDICT_TRUE(g.MatchEmpty())) return false;
+      seq.next();
+      assert(seq.index() < capacity_ && "full table!");
+    }
+    return false;
+  }
+
+  // Probes the raw_hash_set with the probe sequence for hash and returns the
+  // pointer to the first empty or deleted slot.
+  // NOTE: this function must work with tables having both kEmpty and kDelete
+  // in one group. Such tables appears during drop_deletes_without_resize.
+  //
+  // This function is very useful when insertions happen and:
+  // - the input is already a set
+  // - there are enough slots
+  // - the element with the hash is not in the table
+  size_t find_first_non_full(size_t hash) {
+    auto seq = probe(hash);
+    while (true) {
+      Group g{ctrl_ + seq.offset()};
+      auto mask = g.MatchEmptyOrDeleted();
+      if (mask) {
+#if !defined(NDEBUG)
+        // We want to force small tables to have random entries too, so
+        // in debug build we will randomly insert in either the front or back of
+        // the group.
+        // TODO(kfm,sbenza): revisit after we do unconditional mixing
+        if (ShouldInsertBackwards(hash, ctrl_))
+          return seq.offset(mask.HighestBitSet());
+        else
+          return seq.offset(mask.LowestBitSet());
+#else
+        return seq.offset(mask.LowestBitSet());
+#endif
+      }
+      assert(seq.index() < capacity_ && "full table!");
+      seq.next();
+    }
+  }
+
+  // TODO(alkis): Optimize this assuming *this and that don't overlap.
+  raw_hash_set& move_assign(raw_hash_set&& that, std::true_type) {
+    raw_hash_set tmp(std::move(that));
+    swap(tmp);
+    return *this;
+  }
+  raw_hash_set& move_assign(raw_hash_set&& that, std::false_type) {
+    raw_hash_set tmp(std::move(that), alloc_ref());
+    swap(tmp);
+    return *this;
+  }
+
+ protected:
+  template <class K>
+  std::pair<size_t, bool> find_or_prepare_insert(const K& key) {
+    auto hash = hash_ref()(key);
+    auto seq = probe(hash);
+    while (true) {
+      Group g{ctrl_ + seq.offset()};
+      for (int i : g.Match(H2(hash))) {
+        if (ABSL_PREDICT_TRUE(PolicyTraits::apply(
+                EqualElement<K>{key, eq_ref()},
+                PolicyTraits::element(slots_ + seq.offset(i)))))
+          return {seq.offset(i), false};
+      }
+      if (ABSL_PREDICT_TRUE(g.MatchEmpty())) break;
+      seq.next();
+    }
+    return {prepare_insert(hash), true};
+  }
+
+  size_t prepare_insert(size_t hash) ABSL_ATTRIBUTE_NOINLINE {
+    size_t target = find_first_non_full(hash);
+    if (ABSL_PREDICT_FALSE(growth_left() == 0 && !IsDeleted(ctrl_[target]))) {
+      rehash_and_grow_if_necessary();
+      target = find_first_non_full(hash);
+    }
+    ++size_;
+    growth_left() -= IsEmpty(ctrl_[target]);
+    set_ctrl(target, H2(hash));
+    return target;
+  }
+
+  // Constructs the value in the space pointed by the iterator. This only works
+  // after an unsuccessful find_or_prepare_insert() and before any other
+  // modifications happen in the raw_hash_set.
+  //
+  // PRECONDITION: i is an index returned from find_or_prepare_insert(k), where
+  // k is the key decomposed from `forward<Args>(args)...`, and the bool
+  // returned by find_or_prepare_insert(k) was true.
+  // POSTCONDITION: *m.iterator_at(i) == value_type(forward<Args>(args)...).
+  template <class... Args>
+  void emplace_at(size_t i, Args&&... args) {
+    PolicyTraits::construct(&alloc_ref(), slots_ + i,
+                            std::forward<Args>(args)...);
+
+    assert(PolicyTraits::apply(FindElement{*this}, *iterator_at(i)) ==
+               iterator_at(i) &&
+           "constructed value does not match the lookup key");
+  }
+
+  iterator iterator_at(size_t i) { return {ctrl_ + i, slots_ + i}; }
+  const_iterator iterator_at(size_t i) const { return {ctrl_ + i, slots_ + i}; }
+
+ private:
+  friend struct RawHashSetTestOnlyAccess;
+
+  probe_seq<Group::kWidth> probe(size_t hash) const {
+    return probe_seq<Group::kWidth>(H1(hash, ctrl_), capacity_);
+  }
+
+  // Reset all ctrl bytes back to kEmpty, except the sentinel.
+  void reset_ctrl() {
+    std::memset(ctrl_, kEmpty, capacity_ + Group::kWidth);
+    ctrl_[capacity_] = kSentinel;
+    SanitizerPoisonMemoryRegion(slots_, sizeof(slot_type) * capacity_);
+  }
+
+  // Sets the control byte, and if `i < Group::kWidth`, set the cloned byte at
+  // the end too.
+  void set_ctrl(size_t i, ctrl_t h) {
+    assert(i < capacity_);
+
+    if (IsFull(h)) {
+      SanitizerUnpoisonObject(slots_ + i);
+    } else {
+      SanitizerPoisonObject(slots_ + i);
+    }
+
+    ctrl_[i] = h;
+    ctrl_[((i - Group::kWidth) & capacity_) + Group::kWidth] = h;
+  }
+
+  size_t& growth_left() { return settings_.template get<0>(); }
+
+  hasher& hash_ref() { return settings_.template get<1>(); }
+  const hasher& hash_ref() const { return settings_.template get<1>(); }
+  key_equal& eq_ref() { return settings_.template get<2>(); }
+  const key_equal& eq_ref() const { return settings_.template get<2>(); }
+  allocator_type& alloc_ref() { return settings_.template get<3>(); }
+  const allocator_type& alloc_ref() const {
+    return settings_.template get<3>();
+  }
+
+  // On average each group has 2 empty slot (for the vectorized case).
+  static constexpr float kMaxLoadFactor = 14.0 / 16.0;
+
+  // TODO(alkis): Investigate removing some of these fields:
+  // - ctrl/slots can be derived from each other
+  // - size can be moved into the slot array
+  ctrl_t* ctrl_ = EmptyGroup();    // [(capacity + 1) * ctrl_t]
+  slot_type* slots_ = nullptr;     // [capacity * slot_type]
+  size_t size_ = 0;                // number of full slots
+  size_t capacity_ = 0;            // total number of slots
+  absl::container_internal::CompressedTuple<size_t /* growth_left */, hasher,
+                                            key_equal, allocator_type>
+      settings_{0, hasher{}, key_equal{}, allocator_type{}};
+};
+
+namespace hashtable_debug_internal {
+template <typename Set>
+struct HashtableDebugAccess<Set, absl::void_t<typename Set::raw_hash_set>> {
+  using Traits = typename Set::PolicyTraits;
+  using Slot = typename Traits::slot_type;
+
+  static size_t GetNumProbes(const Set& set,
+                             const typename Set::key_type& key) {
+    size_t num_probes = 0;
+    size_t hash = set.hash_ref()(key);
+    auto seq = set.probe(hash);
+    while (true) {
+      container_internal::Group g{set.ctrl_ + seq.offset()};
+      for (int i : g.Match(container_internal::H2(hash))) {
+        if (Traits::apply(
+                typename Set::template EqualElement<typename Set::key_type>{
+                    key, set.eq_ref()},
+                Traits::element(set.slots_ + seq.offset(i))))
+          return num_probes;
+        ++num_probes;
+      }
+      if (g.MatchEmpty()) return num_probes;
+      seq.next();
+      ++num_probes;
+    }
+  }
+
+  static size_t AllocatedByteSize(const Set& c) {
+    size_t capacity = c.capacity_;
+    if (capacity == 0) return 0;
+    auto layout = Set::MakeLayout(capacity);
+    size_t m = layout.AllocSize();
+
+    size_t per_slot = Traits::space_used(static_cast<const Slot*>(nullptr));
+    if (per_slot != ~size_t{}) {
+      m += per_slot * c.size();
+    } else {
+      for (size_t i = 0; i != capacity; ++i) {
+        if (container_internal::IsFull(c.ctrl_[i])) {
+          m += Traits::space_used(c.slots_ + i);
+        }
+      }
+    }
+    return m;
+  }
+
+  static size_t LowerBoundAllocatedByteSize(size_t size) {
+    size_t capacity = container_internal::NormalizeCapacity(
+        std::ceil(size / Set::kMaxLoadFactor));
+    if (capacity == 0) return 0;
+    auto layout = Set::MakeLayout(capacity);
+    size_t m = layout.AllocSize();
+    size_t per_slot = Traits::space_used(static_cast<const Slot*>(nullptr));
+    if (per_slot != ~size_t{}) {
+      m += per_slot * size;
+    }
+    return m;
+  }
+};
+
+}  // namespace hashtable_debug_internal
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_H_
diff --git a/absl/container/internal/raw_hash_set_allocator_test.cc b/absl/container/internal/raw_hash_set_allocator_test.cc
new file mode 100644
index 000000000000..891fa450fe08
--- /dev/null
+++ b/absl/container/internal/raw_hash_set_allocator_test.cc
@@ -0,0 +1,428 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <limits>
+#include <scoped_allocator>
+
+#include "gtest/gtest.h"
+#include "absl/container/internal/raw_hash_set.h"
+#include "absl/container/internal/tracked.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+enum AllocSpec {
+  kPropagateOnCopy = 1,
+  kPropagateOnMove = 2,
+  kPropagateOnSwap = 4,
+};
+
+struct AllocState {
+  size_t num_allocs = 0;
+  std::set<void*> owned;
+};
+
+template <class T,
+          int Spec = kPropagateOnCopy | kPropagateOnMove | kPropagateOnSwap>
+class CheckedAlloc {
+ public:
+  template <class, int>
+  friend class CheckedAlloc;
+
+  using value_type = T;
+
+  CheckedAlloc() {}
+  explicit CheckedAlloc(size_t id) : id_(id) {}
+  CheckedAlloc(const CheckedAlloc&) = default;
+  CheckedAlloc& operator=(const CheckedAlloc&) = default;
+
+  template <class U>
+  CheckedAlloc(const CheckedAlloc<U, Spec>& that)
+      : id_(that.id_), state_(that.state_) {}
+
+  template <class U>
+  struct rebind {
+    using other = CheckedAlloc<U, Spec>;
+  };
+
+  using propagate_on_container_copy_assignment =
+      std::integral_constant<bool, (Spec & kPropagateOnCopy) != 0>;
+
+  using propagate_on_container_move_assignment =
+      std::integral_constant<bool, (Spec & kPropagateOnMove) != 0>;
+
+  using propagate_on_container_swap =
+      std::integral_constant<bool, (Spec & kPropagateOnSwap) != 0>;
+
+  CheckedAlloc select_on_container_copy_construction() const {
+    if (Spec & kPropagateOnCopy) return *this;
+    return {};
+  }
+
+  T* allocate(size_t n) {
+    T* ptr = std::allocator<T>().allocate(n);
+    track_alloc(ptr);
+    return ptr;
+  }
+  void deallocate(T* ptr, size_t n) {
+    memset(ptr, 0, n * sizeof(T));  // The freed memory must be unpoisoned.
+    track_dealloc(ptr);
+    return std::allocator<T>().deallocate(ptr, n);
+  }
+
+  friend bool operator==(const CheckedAlloc& a, const CheckedAlloc& b) {
+    return a.id_ == b.id_;
+  }
+  friend bool operator!=(const CheckedAlloc& a, const CheckedAlloc& b) {
+    return !(a == b);
+  }
+
+  size_t num_allocs() const { return state_->num_allocs; }
+
+  void swap(CheckedAlloc& that) {
+    using std::swap;
+    swap(id_, that.id_);
+    swap(state_, that.state_);
+  }
+
+  friend void swap(CheckedAlloc& a, CheckedAlloc& b) { a.swap(b); }
+
+  friend std::ostream& operator<<(std::ostream& o, const CheckedAlloc& a) {
+    return o << "alloc(" << a.id_ << ")";
+  }
+
+ private:
+  void track_alloc(void* ptr) {
+    AllocState* state = state_.get();
+    ++state->num_allocs;
+    if (!state->owned.insert(ptr).second)
+      ADD_FAILURE() << *this << " got previously allocated memory: " << ptr;
+  }
+  void track_dealloc(void* ptr) {
+    if (state_->owned.erase(ptr) != 1)
+      ADD_FAILURE() << *this
+                    << " deleting memory owned by another allocator: " << ptr;
+  }
+
+  size_t id_ = std::numeric_limits<size_t>::max();
+
+  std::shared_ptr<AllocState> state_ = std::make_shared<AllocState>();
+};
+
+struct Identity {
+  int32_t operator()(int32_t v) const { return v; }
+};
+
+struct Policy {
+  using slot_type = Tracked<int32_t>;
+  using init_type = Tracked<int32_t>;
+  using key_type = int32_t;
+
+  template <class allocator_type, class... Args>
+  static void construct(allocator_type* alloc, slot_type* slot,
+                        Args&&... args) {
+    std::allocator_traits<allocator_type>::construct(
+        *alloc, slot, std::forward<Args>(args)...);
+  }
+
+  template <class allocator_type>
+  static void destroy(allocator_type* alloc, slot_type* slot) {
+    std::allocator_traits<allocator_type>::destroy(*alloc, slot);
+  }
+
+  template <class allocator_type>
+  static void transfer(allocator_type* alloc, slot_type* new_slot,
+                       slot_type* old_slot) {
+    construct(alloc, new_slot, std::move(*old_slot));
+    destroy(alloc, old_slot);
+  }
+
+  template <class F>
+  static auto apply(F&& f, int32_t v) -> decltype(std::forward<F>(f)(v, v)) {
+    return std::forward<F>(f)(v, v);
+  }
+
+  template <class F>
+  static auto apply(F&& f, const slot_type& v)
+      -> decltype(std::forward<F>(f)(v.val(), v)) {
+    return std::forward<F>(f)(v.val(), v);
+  }
+
+  template <class F>
+  static auto apply(F&& f, slot_type&& v)
+      -> decltype(std::forward<F>(f)(v.val(), std::move(v))) {
+    return std::forward<F>(f)(v.val(), std::move(v));
+  }
+
+  static slot_type& element(slot_type* slot) { return *slot; }
+};
+
+template <int Spec>
+struct PropagateTest : public ::testing::Test {
+  using Alloc = CheckedAlloc<Tracked<int32_t>, Spec>;
+
+  using Table = raw_hash_set<Policy, Identity, std::equal_to<int32_t>, Alloc>;
+
+  PropagateTest() {
+    EXPECT_EQ(a1, t1.get_allocator());
+    EXPECT_NE(a2, t1.get_allocator());
+  }
+
+  Alloc a1 = Alloc(1);
+  Table t1 = Table(0, a1);
+  Alloc a2 = Alloc(2);
+};
+
+using PropagateOnAll =
+    PropagateTest<kPropagateOnCopy | kPropagateOnMove | kPropagateOnSwap>;
+using NoPropagateOnCopy = PropagateTest<kPropagateOnMove | kPropagateOnSwap>;
+using NoPropagateOnMove = PropagateTest<kPropagateOnCopy | kPropagateOnSwap>;
+
+TEST_F(PropagateOnAll, Empty) { EXPECT_EQ(0, a1.num_allocs()); }
+
+TEST_F(PropagateOnAll, InsertAllocates) {
+  auto it = t1.insert(0).first;
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, InsertDecomposes) {
+  auto it = t1.insert(0).first;
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+
+  EXPECT_FALSE(t1.insert(0).second);
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, RehashMoves) {
+  auto it = t1.insert(0).first;
+  EXPECT_EQ(0, it->num_moves());
+  t1.rehash(2 * t1.capacity());
+  EXPECT_EQ(2, a1.num_allocs());
+  it = t1.find(0);
+  EXPECT_EQ(1, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, CopyConstructor) {
+  auto it = t1.insert(0).first;
+  Table u(t1);
+  EXPECT_EQ(2, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(NoPropagateOnCopy, CopyConstructor) {
+  auto it = t1.insert(0).first;
+  Table u(t1);
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(1, u.get_allocator().num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, CopyConstructorWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(t1, a1);
+  EXPECT_EQ(2, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(NoPropagateOnCopy, CopyConstructorWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(t1, a1);
+  EXPECT_EQ(2, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, CopyConstructorWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(t1, a2);
+  EXPECT_EQ(a2, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(1, a2.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(NoPropagateOnCopy, CopyConstructorWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(t1, a2);
+  EXPECT_EQ(a2, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(1, a2.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, MoveConstructor) {
+  auto it = t1.insert(0).first;
+  Table u(std::move(t1));
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(NoPropagateOnMove, MoveConstructor) {
+  auto it = t1.insert(0).first;
+  Table u(std::move(t1));
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, MoveConstructorWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(std::move(t1), a1);
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(NoPropagateOnMove, MoveConstructorWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(std::move(t1), a1);
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, MoveConstructorWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(std::move(t1), a2);
+  it = u.find(0);
+  EXPECT_EQ(a2, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(1, a2.num_allocs());
+  EXPECT_EQ(1, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(NoPropagateOnMove, MoveConstructorWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(std::move(t1), a2);
+  it = u.find(0);
+  EXPECT_EQ(a2, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(1, a2.num_allocs());
+  EXPECT_EQ(1, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, CopyAssignmentWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a1);
+  u = t1;
+  EXPECT_EQ(2, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(NoPropagateOnCopy, CopyAssignmentWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a1);
+  u = t1;
+  EXPECT_EQ(2, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, CopyAssignmentWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a2);
+  u = t1;
+  EXPECT_EQ(a1, u.get_allocator());
+  EXPECT_EQ(2, a1.num_allocs());
+  EXPECT_EQ(0, a2.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(NoPropagateOnCopy, CopyAssignmentWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a2);
+  u = t1;
+  EXPECT_EQ(a2, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(1, a2.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(1, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, MoveAssignmentWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a1);
+  u = std::move(t1);
+  EXPECT_EQ(a1, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(NoPropagateOnMove, MoveAssignmentWithSameAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a1);
+  u = std::move(t1);
+  EXPECT_EQ(a1, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, MoveAssignmentWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a2);
+  u = std::move(t1);
+  EXPECT_EQ(a1, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, a2.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(NoPropagateOnMove, MoveAssignmentWithDifferentAlloc) {
+  auto it = t1.insert(0).first;
+  Table u(0, a2);
+  u = std::move(t1);
+  it = u.find(0);
+  EXPECT_EQ(a2, u.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(1, a2.num_allocs());
+  EXPECT_EQ(1, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+TEST_F(PropagateOnAll, Swap) {
+  auto it = t1.insert(0).first;
+  Table u(0, a2);
+  u.swap(t1);
+  EXPECT_EQ(a1, u.get_allocator());
+  EXPECT_EQ(a2, t1.get_allocator());
+  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_EQ(0, a2.num_allocs());
+  EXPECT_EQ(0, it->num_moves());
+  EXPECT_EQ(0, it->num_copies());
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/raw_hash_set_test.cc b/absl/container/internal/raw_hash_set_test.cc
new file mode 100644
index 000000000000..f59a19b4a62d
--- /dev/null
+++ b/absl/container/internal/raw_hash_set_test.cc
@@ -0,0 +1,1961 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/raw_hash_set.h"
+
+#include <array>
+#include <cmath>
+#include <cstdint>
+#include <deque>
+#include <functional>
+#include <memory>
+#include <numeric>
+#include <random>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/internal/cycleclock.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hash_function_defaults.h"
+#include "absl/container/internal/hash_policy_testing.h"
+#include "absl/container/internal/hashtable_debug.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+
+struct RawHashSetTestOnlyAccess {
+  template <typename C>
+  static auto GetSlots(const C& c) -> decltype(c.slots_) {
+    return c.slots_;
+  }
+};
+
+namespace {
+
+using ::testing::DoubleNear;
+using ::testing::ElementsAre;
+using ::testing::Optional;
+using ::testing::Pair;
+using ::testing::UnorderedElementsAre;
+
+TEST(Util, NormalizeCapacity) {
+  constexpr size_t kMinCapacity = Group::kWidth - 1;
+  EXPECT_EQ(kMinCapacity, NormalizeCapacity(0));
+  EXPECT_EQ(kMinCapacity, NormalizeCapacity(1));
+  EXPECT_EQ(kMinCapacity, NormalizeCapacity(2));
+  EXPECT_EQ(kMinCapacity, NormalizeCapacity(kMinCapacity));
+  EXPECT_EQ(kMinCapacity * 2 + 1, NormalizeCapacity(kMinCapacity + 1));
+  EXPECT_EQ(kMinCapacity * 2 + 1, NormalizeCapacity(kMinCapacity + 2));
+}
+
+TEST(Util, probe_seq) {
+  probe_seq<16> seq(0, 127);
+  auto gen = [&]() {
+    size_t res = seq.offset();
+    seq.next();
+    return res;
+  };
+  std::vector<size_t> offsets(8);
+  std::generate_n(offsets.begin(), 8, gen);
+  EXPECT_THAT(offsets, ElementsAre(0, 16, 48, 96, 32, 112, 80, 64));
+  seq = probe_seq<16>(128, 127);
+  std::generate_n(offsets.begin(), 8, gen);
+  EXPECT_THAT(offsets, ElementsAre(0, 16, 48, 96, 32, 112, 80, 64));
+}
+
+TEST(BitMask, Smoke) {
+  EXPECT_FALSE((BitMask<uint8_t, 8>(0)));
+  EXPECT_TRUE((BitMask<uint8_t, 8>(5)));
+
+  EXPECT_THAT((BitMask<uint8_t, 8>(0)), ElementsAre());
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x1)), ElementsAre(0));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x2)), ElementsAre(1));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x3)), ElementsAre(0, 1));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x4)), ElementsAre(2));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x5)), ElementsAre(0, 2));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x55)), ElementsAre(0, 2, 4, 6));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0xAA)), ElementsAre(1, 3, 5, 7));
+}
+
+TEST(BitMask, WithShift) {
+  // See the non-SSE version of Group for details on what this math is for.
+  uint64_t ctrl = 0x1716151413121110;
+  uint64_t hash = 0x12;
+  constexpr uint64_t msbs = 0x8080808080808080ULL;
+  constexpr uint64_t lsbs = 0x0101010101010101ULL;
+  auto x = ctrl ^ (lsbs * hash);
+  uint64_t mask = (x - lsbs) & ~x & msbs;
+  EXPECT_EQ(0x0000000080800000, mask);
+
+  BitMask<uint64_t, 8, 3> b(mask);
+  EXPECT_EQ(*b, 2);
+}
+
+TEST(BitMask, LeadingTrailing) {
+  EXPECT_EQ((BitMask<uint32_t, 16>(0b0001101001000000).LeadingZeros()), 3);
+  EXPECT_EQ((BitMask<uint32_t, 16>(0b0001101001000000).TrailingZeros()), 6);
+
+  EXPECT_EQ((BitMask<uint32_t, 16>(0b0000000000000001).LeadingZeros()), 15);
+  EXPECT_EQ((BitMask<uint32_t, 16>(0b0000000000000001).TrailingZeros()), 0);
+
+  EXPECT_EQ((BitMask<uint32_t, 16>(0b1000000000000000).LeadingZeros()), 0);
+  EXPECT_EQ((BitMask<uint32_t, 16>(0b1000000000000000).TrailingZeros()), 15);
+
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000008080808000).LeadingZeros()), 3);
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000008080808000).TrailingZeros()), 1);
+
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000000000000080).LeadingZeros()), 7);
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000000000000080).TrailingZeros()), 0);
+
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x8000000000000000).LeadingZeros()), 0);
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x8000000000000000).TrailingZeros()), 7);
+}
+
+TEST(Group, EmptyGroup) {
+  for (h2_t h = 0; h != 128; ++h) EXPECT_FALSE(Group{EmptyGroup()}.Match(h));
+}
+
+#if SWISSTABLE_HAVE_SSE2
+TEST(Group, Match) {
+  ctrl_t group[] = {kEmpty, 1, kDeleted, 3, kEmpty, 5, kSentinel, 7,
+                    7,      5, 3,        1, 1,      1, 1,         1};
+  EXPECT_THAT(Group{group}.Match(0), ElementsAre());
+  EXPECT_THAT(Group{group}.Match(1), ElementsAre(1, 11, 12, 13, 14, 15));
+  EXPECT_THAT(Group{group}.Match(3), ElementsAre(3, 10));
+  EXPECT_THAT(Group{group}.Match(5), ElementsAre(5, 9));
+  EXPECT_THAT(Group{group}.Match(7), ElementsAre(7, 8));
+}
+
+TEST(Group, MatchEmpty) {
+  ctrl_t group[] = {kEmpty, 1, kDeleted, 3, kEmpty, 5, kSentinel, 7,
+                    7,      5, 3,        1, 1,      1, 1,         1};
+  EXPECT_THAT(Group{group}.MatchEmpty(), ElementsAre(0, 4));
+}
+
+TEST(Group, MatchEmptyOrDeleted) {
+  ctrl_t group[] = {kEmpty, 1, kDeleted, 3, kEmpty, 5, kSentinel, 7,
+                    7,      5, 3,        1, 1,      1, 1,         1};
+  EXPECT_THAT(Group{group}.MatchEmptyOrDeleted(), ElementsAre(0, 2, 4));
+}
+#else
+TEST(Group, Match) {
+  ctrl_t group[] = {kEmpty, 1, 2, kDeleted, 2, 1, kSentinel, 1};
+  EXPECT_THAT(Group{group}.Match(0), ElementsAre());
+  EXPECT_THAT(Group{group}.Match(1), ElementsAre(1, 5, 7));
+  EXPECT_THAT(Group{group}.Match(2), ElementsAre(2, 4));
+}
+TEST(Group, MatchEmpty) {
+  ctrl_t group[] = {kEmpty, 1, 2, kDeleted, 2, 1, kSentinel, 1};
+  EXPECT_THAT(Group{group}.MatchEmpty(), ElementsAre(0));
+}
+
+TEST(Group, MatchEmptyOrDeleted) {
+  ctrl_t group[] = {kEmpty, 1, 2, kDeleted, 2, 1, kSentinel, 1};
+  EXPECT_THAT(Group{group}.MatchEmptyOrDeleted(), ElementsAre(0, 3));
+}
+#endif
+
+TEST(Batch, DropDeletes) {
+  constexpr size_t kCapacity = 63;
+  constexpr size_t kGroupWidth = container_internal::Group::kWidth;
+  std::vector<ctrl_t> ctrl(kCapacity + 1 + kGroupWidth);
+  ctrl[kCapacity] = kSentinel;
+  std::vector<ctrl_t> pattern = {kEmpty, 2, kDeleted, 2, kEmpty, 1, kDeleted};
+  for (size_t i = 0; i != kCapacity; ++i) {
+    ctrl[i] = pattern[i % pattern.size()];
+    if (i < kGroupWidth - 1)
+      ctrl[i + kCapacity + 1] = pattern[i % pattern.size()];
+  }
+  ConvertDeletedToEmptyAndFullToDeleted(ctrl.data(), kCapacity);
+  ASSERT_EQ(ctrl[kCapacity], kSentinel);
+  for (size_t i = 0; i < kCapacity + 1 + kGroupWidth; ++i) {
+    ctrl_t expected = pattern[i % (kCapacity + 1) % pattern.size()];
+    if (i == kCapacity) expected = kSentinel;
+    if (expected == kDeleted) expected = kEmpty;
+    if (IsFull(expected)) expected = kDeleted;
+    EXPECT_EQ(ctrl[i], expected)
+        << i << " " << int{pattern[i % pattern.size()]};
+  }
+}
+
+TEST(Group, CountLeadingEmptyOrDeleted) {
+  const std::vector<ctrl_t> empty_examples = {kEmpty, kDeleted};
+  const std::vector<ctrl_t> full_examples = {0, 1, 2, 3, 5, 9, 127, kSentinel};
+
+  for (ctrl_t empty : empty_examples) {
+    std::vector<ctrl_t> e(Group::kWidth, empty);
+    EXPECT_EQ(Group::kWidth, Group{e.data()}.CountLeadingEmptyOrDeleted());
+    for (ctrl_t full : full_examples) {
+      for (size_t i = 0; i != Group::kWidth; ++i) {
+        std::vector<ctrl_t> f(Group::kWidth, empty);
+        f[i] = full;
+        EXPECT_EQ(i, Group{f.data()}.CountLeadingEmptyOrDeleted());
+      }
+      std::vector<ctrl_t> f(Group::kWidth, empty);
+      f[Group::kWidth * 2 / 3] = full;
+      f[Group::kWidth / 2] = full;
+      EXPECT_EQ(
+          Group::kWidth / 2, Group{f.data()}.CountLeadingEmptyOrDeleted());
+    }
+  }
+}
+
+struct IntPolicy {
+  using slot_type = int64_t;
+  using key_type = int64_t;
+  using init_type = int64_t;
+
+  static void construct(void*, int64_t* slot, int64_t v) { *slot = v; }
+  static void destroy(void*, int64_t*) {}
+  static void transfer(void*, int64_t* new_slot, int64_t* old_slot) {
+    *new_slot = *old_slot;
+  }
+
+  static int64_t& element(slot_type* slot) { return *slot; }
+
+  template <class F>
+  static auto apply(F&& f, int64_t x) -> decltype(std::forward<F>(f)(x, x)) {
+    return std::forward<F>(f)(x, x);
+  }
+};
+
+class StringPolicy {
+  template <class F, class K, class V,
+            class = typename std::enable_if<
+                std::is_convertible<const K&, absl::string_view>::value>::type>
+  decltype(std::declval<F>()(
+      std::declval<const absl::string_view&>(), std::piecewise_construct,
+      std::declval<std::tuple<K>>(),
+      std::declval<V>())) static apply_impl(F&& f,
+                                            std::pair<std::tuple<K>, V> p) {
+    const absl::string_view& key = std::get<0>(p.first);
+    return std::forward<F>(f)(key, std::piecewise_construct, std::move(p.first),
+                              std::move(p.second));
+  }
+
+ public:
+  struct slot_type {
+    struct ctor {};
+
+    template <class... Ts>
+    slot_type(ctor, Ts&&... ts) : pair(std::forward<Ts>(ts)...) {}
+
+    std::pair<std::string, std::string> pair;
+  };
+
+  using key_type = std::string;
+  using init_type = std::pair<std::string, std::string>;
+
+  template <class allocator_type, class... Args>
+  static void construct(allocator_type* alloc, slot_type* slot, Args... args) {
+    std::allocator_traits<allocator_type>::construct(
+        *alloc, slot, typename slot_type::ctor(), std::forward<Args>(args)...);
+  }
+
+  template <class allocator_type>
+  static void destroy(allocator_type* alloc, slot_type* slot) {
+    std::allocator_traits<allocator_type>::destroy(*alloc, slot);
+  }
+
+  template <class allocator_type>
+  static void transfer(allocator_type* alloc, slot_type* new_slot,
+                       slot_type* old_slot) {
+    construct(alloc, new_slot, std::move(old_slot->pair));
+    destroy(alloc, old_slot);
+  }
+
+  static std::pair<std::string, std::string>& element(slot_type* slot) {
+    return slot->pair;
+  }
+
+  template <class F, class... Args>
+  static auto apply(F&& f, Args&&... args)
+      -> decltype(apply_impl(std::forward<F>(f),
+                             PairArgs(std::forward<Args>(args)...))) {
+    return apply_impl(std::forward<F>(f),
+                      PairArgs(std::forward<Args>(args)...));
+  }
+};
+
+struct StringHash : absl::Hash<absl::string_view> {
+  using is_transparent = void;
+};
+struct StringEq : std::equal_to<absl::string_view> {
+  using is_transparent = void;
+};
+
+struct StringTable
+    : raw_hash_set<StringPolicy, StringHash, StringEq, std::allocator<int>> {
+  using Base = typename StringTable::raw_hash_set;
+  StringTable() {}
+  using Base::Base;
+};
+
+struct IntTable
+    : raw_hash_set<IntPolicy, container_internal::hash_default_hash<int64_t>,
+                   std::equal_to<int64_t>, std::allocator<int64_t>> {
+  using Base = typename IntTable::raw_hash_set;
+  IntTable() {}
+  using Base::Base;
+};
+
+struct BadFastHash {
+  template <class T>
+  size_t operator()(const T&) const {
+    return 0;
+  }
+};
+
+struct BadTable : raw_hash_set<IntPolicy, BadFastHash, std::equal_to<int>,
+                               std::allocator<int>> {
+  using Base = typename BadTable::raw_hash_set;
+  BadTable() {}
+  using Base::Base;
+};
+
+TEST(Table, EmptyFunctorOptimization) {
+  static_assert(std::is_empty<std::equal_to<absl::string_view>>::value, "");
+  static_assert(std::is_empty<std::allocator<int>>::value, "");
+
+  struct MockTable {
+    void* ctrl;
+    void* slots;
+    size_t size;
+    size_t capacity;
+    size_t growth_left;
+  };
+  struct StatelessHash {
+    size_t operator()(absl::string_view) const { return 0; }
+  };
+  struct StatefulHash : StatelessHash {
+    size_t dummy;
+  };
+
+  EXPECT_EQ(
+      sizeof(MockTable),
+      sizeof(
+          raw_hash_set<StringPolicy, StatelessHash,
+                       std::equal_to<absl::string_view>, std::allocator<int>>));
+
+  EXPECT_EQ(
+      sizeof(MockTable) + sizeof(StatefulHash),
+      sizeof(
+          raw_hash_set<StringPolicy, StatefulHash,
+                       std::equal_to<absl::string_view>, std::allocator<int>>));
+}
+
+TEST(Table, Empty) {
+  IntTable t;
+  EXPECT_EQ(0, t.size());
+  EXPECT_TRUE(t.empty());
+}
+
+#ifdef __GNUC__
+template <class T>
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void DoNotOptimize(const T& v) {
+  asm volatile("" : : "r,m"(v) : "memory");
+}
+#endif
+
+TEST(Table, Prefetch) {
+  IntTable t;
+  t.emplace(1);
+  // Works for both present and absent keys.
+  t.prefetch(1);
+  t.prefetch(2);
+
+  // Do not run in debug mode, when prefetch is not implemented, or when
+  // sanitizers are enabled.
+#if defined(NDEBUG) && defined(__GNUC__) && !defined(ADDRESS_SANITIZER) && \
+    !defined(MEMORY_SANITIZER) && !defined(THREAD_SANITIZER) &&            \
+    !defined(UNDEFINED_BEHAVIOR_SANITIZER)
+  const auto now = [] { return absl::base_internal::CycleClock::Now(); };
+
+  static constexpr int size = 1000000;
+  for (int i = 0; i < size; ++i) t.insert(i);
+
+  int64_t no_prefetch = 0, prefetch = 0;
+  for (int iter = 0; iter < 10; ++iter) {
+    int64_t time = now();
+    for (int i = 0; i < size; ++i) {
+      DoNotOptimize(t.find(i));
+    }
+    no_prefetch += now() - time;
+
+    time = now();
+    for (int i = 0; i < size; ++i) {
+      t.prefetch(i + 20);
+      DoNotOptimize(t.find(i));
+    }
+    prefetch += now() - time;
+  }
+
+  // no_prefetch is at least 30% slower.
+  EXPECT_GE(1.0 * no_prefetch / prefetch, 1.3);
+#endif
+}
+
+TEST(Table, LookupEmpty) {
+  IntTable t;
+  auto it = t.find(0);
+  EXPECT_TRUE(it == t.end());
+}
+
+TEST(Table, Insert1) {
+  IntTable t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_THAT(*res.first, 0);
+  EXPECT_EQ(1, t.size());
+  EXPECT_THAT(*t.find(0), 0);
+}
+
+TEST(Table, Insert2) {
+  IntTable t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_THAT(*res.first, 0);
+  EXPECT_EQ(1, t.size());
+  EXPECT_TRUE(t.find(1) == t.end());
+  res = t.emplace(1);
+  EXPECT_TRUE(res.second);
+  EXPECT_THAT(*res.first, 1);
+  EXPECT_EQ(2, t.size());
+  EXPECT_THAT(*t.find(0), 0);
+  EXPECT_THAT(*t.find(1), 1);
+}
+
+TEST(Table, InsertCollision) {
+  BadTable t;
+  EXPECT_TRUE(t.find(1) == t.end());
+  auto res = t.emplace(1);
+  EXPECT_TRUE(res.second);
+  EXPECT_THAT(*res.first, 1);
+  EXPECT_EQ(1, t.size());
+
+  EXPECT_TRUE(t.find(2) == t.end());
+  res = t.emplace(2);
+  EXPECT_THAT(*res.first, 2);
+  EXPECT_TRUE(res.second);
+  EXPECT_EQ(2, t.size());
+
+  EXPECT_THAT(*t.find(1), 1);
+  EXPECT_THAT(*t.find(2), 2);
+}
+
+// Test that we do not add existent element in case we need to search through
+// many groups with deleted elements
+TEST(Table, InsertCollisionAndFindAfterDelete) {
+  BadTable t;  // all elements go to the same group.
+  // Have at least 2 groups with Group::kWidth collisions
+  // plus some extra collisions in the last group.
+  constexpr size_t kNumInserts = Group::kWidth * 2 + 5;
+  for (size_t i = 0; i < kNumInserts; ++i) {
+    auto res = t.emplace(i);
+    EXPECT_TRUE(res.second);
+    EXPECT_THAT(*res.first, i);
+    EXPECT_EQ(i + 1, t.size());
+  }
+
+  // Remove elements one by one and check
+  // that we still can find all other elements.
+  for (size_t i = 0; i < kNumInserts; ++i) {
+    EXPECT_EQ(1, t.erase(i)) << i;
+    for (size_t j = i + 1; j < kNumInserts; ++j) {
+      EXPECT_THAT(*t.find(j), j);
+      auto res = t.emplace(j);
+      EXPECT_FALSE(res.second) << i << " " << j;
+      EXPECT_THAT(*res.first, j);
+      EXPECT_EQ(kNumInserts - i - 1, t.size());
+    }
+  }
+  EXPECT_TRUE(t.empty());
+}
+
+TEST(Table, LazyEmplace) {
+  StringTable t;
+  bool called = false;
+  auto it = t.lazy_emplace("abc", [&](const StringTable::constructor& f) {
+    called = true;
+    f("abc", "ABC");
+  });
+  EXPECT_TRUE(called);
+  EXPECT_THAT(*it, Pair("abc", "ABC"));
+  called = false;
+  it = t.lazy_emplace("abc", [&](const StringTable::constructor& f) {
+    called = true;
+    f("abc", "DEF");
+  });
+  EXPECT_FALSE(called);
+  EXPECT_THAT(*it, Pair("abc", "ABC"));
+}
+
+TEST(Table, ContainsEmpty) {
+  IntTable t;
+
+  EXPECT_FALSE(t.contains(0));
+}
+
+TEST(Table, Contains1) {
+  IntTable t;
+
+  EXPECT_TRUE(t.insert(0).second);
+  EXPECT_TRUE(t.contains(0));
+  EXPECT_FALSE(t.contains(1));
+
+  EXPECT_EQ(1, t.erase(0));
+  EXPECT_FALSE(t.contains(0));
+}
+
+TEST(Table, Contains2) {
+  IntTable t;
+
+  EXPECT_TRUE(t.insert(0).second);
+  EXPECT_TRUE(t.contains(0));
+  EXPECT_FALSE(t.contains(1));
+
+  t.clear();
+  EXPECT_FALSE(t.contains(0));
+}
+
+int decompose_constructed;
+struct DecomposeType {
+  DecomposeType(int i) : i(i) {  // NOLINT
+    ++decompose_constructed;
+  }
+
+  explicit DecomposeType(const char* d) : DecomposeType(*d) {}
+
+  int i;
+};
+
+struct DecomposeHash {
+  using is_transparent = void;
+  size_t operator()(DecomposeType a) const { return a.i; }
+  size_t operator()(int a) const { return a; }
+  size_t operator()(const char* a) const { return *a; }
+};
+
+struct DecomposeEq {
+  using is_transparent = void;
+  bool operator()(DecomposeType a, DecomposeType b) const { return a.i == b.i; }
+  bool operator()(DecomposeType a, int b) const { return a.i == b; }
+  bool operator()(DecomposeType a, const char* b) const { return a.i == *b; }
+};
+
+struct DecomposePolicy {
+  using slot_type = DecomposeType;
+  using key_type = DecomposeType;
+  using init_type = DecomposeType;
+
+  template <typename T>
+  static void construct(void*, DecomposeType* slot, T&& v) {
+    *slot = DecomposeType(std::forward<T>(v));
+  }
+  static void destroy(void*, DecomposeType*) {}
+  static DecomposeType& element(slot_type* slot) { return *slot; }
+
+  template <class F, class T>
+  static auto apply(F&& f, const T& x) -> decltype(std::forward<F>(f)(x, x)) {
+    return std::forward<F>(f)(x, x);
+  }
+};
+
+template <typename Hash, typename Eq>
+void TestDecompose(bool construct_three) {
+  DecomposeType elem{0};
+  const int one = 1;
+  const char* three_p = "3";
+  const auto& three = three_p;
+
+  raw_hash_set<DecomposePolicy, Hash, Eq, std::allocator<int>> set1;
+
+  decompose_constructed = 0;
+  int expected_constructed = 0;
+  EXPECT_EQ(expected_constructed, decompose_constructed);
+  set1.insert(elem);
+  EXPECT_EQ(expected_constructed, decompose_constructed);
+  set1.insert(1);
+  EXPECT_EQ(++expected_constructed, decompose_constructed);
+  set1.emplace("3");
+  EXPECT_EQ(++expected_constructed, decompose_constructed);
+  EXPECT_EQ(expected_constructed, decompose_constructed);
+
+  {  // insert(T&&)
+    set1.insert(1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // insert(const T&)
+    set1.insert(one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // insert(hint, T&&)
+    set1.insert(set1.begin(), 1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // insert(hint, const T&)
+    set1.insert(set1.begin(), one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // emplace(...)
+    set1.emplace(1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace("3");
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace(one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace(three);
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // emplace_hint(...)
+    set1.emplace_hint(set1.begin(), 1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace_hint(set1.begin(), "3");
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace_hint(set1.begin(), one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace_hint(set1.begin(), three);
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+}
+
+TEST(Table, Decompose) {
+  TestDecompose<DecomposeHash, DecomposeEq>(false);
+
+  struct TransparentHashIntOverload {
+    size_t operator()(DecomposeType a) const { return a.i; }
+    size_t operator()(int a) const { return a; }
+  };
+  struct TransparentEqIntOverload {
+    bool operator()(DecomposeType a, DecomposeType b) const {
+      return a.i == b.i;
+    }
+    bool operator()(DecomposeType a, int b) const { return a.i == b; }
+  };
+  TestDecompose<TransparentHashIntOverload, DecomposeEq>(true);
+  TestDecompose<TransparentHashIntOverload, TransparentEqIntOverload>(true);
+  TestDecompose<DecomposeHash, TransparentEqIntOverload>(true);
+}
+
+// Returns the largest m such that a table with m elements has the same number
+// of buckets as a table with n elements.
+size_t MaxDensitySize(size_t n) {
+  IntTable t;
+  t.reserve(n);
+  for (size_t i = 0; i != n; ++i) t.emplace(i);
+  const size_t c = t.bucket_count();
+  while (c == t.bucket_count()) t.emplace(n++);
+  return t.size() - 1;
+}
+
+struct Modulo1000Hash {
+  size_t operator()(int x) const { return x % 1000; }
+};
+
+struct Modulo1000HashTable
+    : public raw_hash_set<IntPolicy, Modulo1000Hash, std::equal_to<int>,
+                          std::allocator<int>> {};
+
+// Test that rehash with no resize happen in case of many deleted slots.
+TEST(Table, RehashWithNoResize) {
+  Modulo1000HashTable t;
+  // Adding the same length (and the same hash) strings
+  // to have at least kMinFullGroups groups
+  // with Group::kWidth collisions. Then feel upto MaxDensitySize;
+  const size_t kMinFullGroups = 7;
+  std::vector<int> keys;
+  for (size_t i = 0; i < MaxDensitySize(Group::kWidth * kMinFullGroups); ++i) {
+    int k = i * 1000;
+    t.emplace(k);
+    keys.push_back(k);
+  }
+  const size_t capacity = t.capacity();
+
+  // Remove elements from all groups except the first and the last one.
+  // All elements removed from full groups will be marked as kDeleted.
+  const size_t erase_begin = Group::kWidth / 2;
+  const size_t erase_end = (t.size() / Group::kWidth - 1) * Group::kWidth;
+  for (size_t i = erase_begin; i < erase_end; ++i) {
+    EXPECT_EQ(1, t.erase(keys[i])) << i;
+  }
+  keys.erase(keys.begin() + erase_begin, keys.begin() + erase_end);
+
+  auto last_key = keys.back();
+  size_t last_key_num_probes = GetHashtableDebugNumProbes(t, last_key);
+
+  // Make sure that we have to make a lot of probes for last key.
+  ASSERT_GT(last_key_num_probes, kMinFullGroups);
+
+  int x = 1;
+  // Insert and erase one element, before inplace rehash happen.
+  while (last_key_num_probes == GetHashtableDebugNumProbes(t, last_key)) {
+    t.emplace(x);
+    ASSERT_EQ(capacity, t.capacity());
+    // All elements should be there.
+    ASSERT_TRUE(t.find(x) != t.end()) << x;
+    for (const auto& k : keys) {
+      ASSERT_TRUE(t.find(k) != t.end()) << k;
+    }
+    t.erase(x);
+    ++x;
+  }
+}
+
+TEST(Table, InsertEraseStressTest) {
+  IntTable t;
+  const size_t kMinElementCount = 250;
+  std::deque<int> keys;
+  size_t i = 0;
+  for (; i < MaxDensitySize(kMinElementCount); ++i) {
+    t.emplace(i);
+    keys.push_back(i);
+  }
+  const size_t kNumIterations = 1000000;
+  for (; i < kNumIterations; ++i) {
+    ASSERT_EQ(1, t.erase(keys.front()));
+    keys.pop_front();
+    t.emplace(i);
+    keys.push_back(i);
+  }
+}
+
+TEST(Table, InsertOverloads) {
+  StringTable t;
+  // These should all trigger the insert(init_type) overload.
+  t.insert({{}, {}});
+  t.insert({"ABC", {}});
+  t.insert({"DEF", "!!!"});
+
+  EXPECT_THAT(t, UnorderedElementsAre(Pair("", ""), Pair("ABC", ""),
+                                      Pair("DEF", "!!!")));
+}
+
+TEST(Table, LargeTable) {
+  IntTable t;
+  for (int64_t i = 0; i != 100000; ++i) t.emplace(i << 40);
+  for (int64_t i = 0; i != 100000; ++i) ASSERT_EQ(i << 40, *t.find(i << 40));
+}
+
+// Timeout if copy is quadratic as it was in Rust.
+TEST(Table, EnsureNonQuadraticAsInRust) {
+  static const size_t kLargeSize = 1 << 15;
+
+  IntTable t;
+  for (size_t i = 0; i != kLargeSize; ++i) {
+    t.insert(i);
+  }
+
+  // If this is quadratic, the test will timeout.
+  IntTable t2;
+  for (const auto& entry : t) t2.insert(entry);
+}
+
+TEST(Table, ClearBug) {
+  IntTable t;
+  constexpr size_t capacity = container_internal::Group::kWidth - 1;
+  constexpr size_t max_size = capacity / 2;
+  for (size_t i = 0; i < max_size; ++i) {
+    t.insert(i);
+  }
+  ASSERT_EQ(capacity, t.capacity());
+  intptr_t original = reinterpret_cast<intptr_t>(&*t.find(2));
+  t.clear();
+  ASSERT_EQ(capacity, t.capacity());
+  for (size_t i = 0; i < max_size; ++i) {
+    t.insert(i);
+  }
+  ASSERT_EQ(capacity, t.capacity());
+  intptr_t second = reinterpret_cast<intptr_t>(&*t.find(2));
+  // We are checking that original and second are close enough to each other
+  // that they are probably still in the same group.  This is not strictly
+  // guaranteed.
+  EXPECT_LT(std::abs(original - second),
+            capacity * sizeof(IntTable::value_type));
+}
+
+TEST(Table, Erase) {
+  IntTable t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_EQ(1, t.size());
+  t.erase(res.first);
+  EXPECT_EQ(0, t.size());
+  EXPECT_TRUE(t.find(0) == t.end());
+}
+
+// Collect N bad keys by following algorithm:
+// 1. Create an empty table and reserve it to 2 * N.
+// 2. Insert N random elements.
+// 3. Take first Group::kWidth - 1 to bad_keys array.
+// 4. Clear the table without resize.
+// 5. Go to point 2 while N keys not collected
+std::vector<int64_t> CollectBadMergeKeys(size_t N) {
+  static constexpr int kGroupSize = Group::kWidth - 1;
+
+  auto topk_range = [](size_t b, size_t e, IntTable* t) -> std::vector<int64_t> {
+    for (size_t i = b; i != e; ++i) {
+      t->emplace(i);
+    }
+    std::vector<int64_t> res;
+    res.reserve(kGroupSize);
+    auto it = t->begin();
+    for (size_t i = b; i != e && i != b + kGroupSize; ++i, ++it) {
+      res.push_back(*it);
+    }
+    return res;
+  };
+
+  std::vector<int64_t> bad_keys;
+  bad_keys.reserve(N);
+  IntTable t;
+  t.reserve(N * 2);
+
+  for (size_t b = 0; bad_keys.size() < N; b += N) {
+    auto keys = topk_range(b, b + N, &t);
+    bad_keys.insert(bad_keys.end(), keys.begin(), keys.end());
+    t.erase(t.begin(), t.end());
+    EXPECT_TRUE(t.empty());
+  }
+  return bad_keys;
+}
+
+struct ProbeStats {
+  // Number of elements with specific probe length over all tested tables.
+  std::vector<size_t> all_probes_histogram;
+  // Ratios total_probe_length/size for every tested table.
+  std::vector<double> single_table_ratios;
+
+  friend ProbeStats operator+(const ProbeStats& a, const ProbeStats& b) {
+    ProbeStats res = a;
+    res.all_probes_histogram.resize(std::max(res.all_probes_histogram.size(),
+                                             b.all_probes_histogram.size()));
+    std::transform(b.all_probes_histogram.begin(), b.all_probes_histogram.end(),
+                   res.all_probes_histogram.begin(),
+                   res.all_probes_histogram.begin(), std::plus<size_t>());
+    res.single_table_ratios.insert(res.single_table_ratios.end(),
+                                   b.single_table_ratios.begin(),
+                                   b.single_table_ratios.end());
+    return res;
+  }
+
+  // Average ratio total_probe_length/size over tables.
+  double AvgRatio() const {
+    return std::accumulate(single_table_ratios.begin(),
+                           single_table_ratios.end(), 0.0) /
+           single_table_ratios.size();
+  }
+
+  // Maximum ratio total_probe_length/size over tables.
+  double MaxRatio() const {
+    return *std::max_element(single_table_ratios.begin(),
+                             single_table_ratios.end());
+  }
+
+  // Percentile ratio total_probe_length/size over tables.
+  double PercentileRatio(double Percentile = 0.95) const {
+    auto r = single_table_ratios;
+    auto mid = r.begin() + static_cast<size_t>(r.size() * Percentile);
+    if (mid != r.end()) {
+      std::nth_element(r.begin(), mid, r.end());
+      return *mid;
+    } else {
+      return MaxRatio();
+    }
+  }
+
+  // Maximum probe length over all elements and all tables.
+  size_t MaxProbe() const { return all_probes_histogram.size(); }
+
+  // Fraction of elements with specified probe length.
+  std::vector<double> ProbeNormalizedHistogram() const {
+    double total_elements = std::accumulate(all_probes_histogram.begin(),
+                                            all_probes_histogram.end(), 0ull);
+    std::vector<double> res;
+    for (size_t p : all_probes_histogram) {
+      res.push_back(p / total_elements);
+    }
+    return res;
+  }
+
+  size_t PercentileProbe(double Percentile = 0.99) const {
+    size_t idx = 0;
+    for (double p : ProbeNormalizedHistogram()) {
+      if (Percentile > p) {
+        Percentile -= p;
+        ++idx;
+      } else {
+        return idx;
+      }
+    }
+    return idx;
+  }
+
+  friend std::ostream& operator<<(std::ostream& out, const ProbeStats& s) {
+    out << "{AvgRatio:" << s.AvgRatio() << ", MaxRatio:" << s.MaxRatio()
+        << ", PercentileRatio:" << s.PercentileRatio()
+        << ", MaxProbe:" << s.MaxProbe() << ", Probes=[";
+    for (double p : s.ProbeNormalizedHistogram()) {
+      out << p << ",";
+    }
+    out << "]}";
+
+    return out;
+  }
+};
+
+struct ExpectedStats {
+  double avg_ratio;
+  double max_ratio;
+  std::vector<std::pair<double, double>> pecentile_ratios;
+  std::vector<std::pair<double, double>> pecentile_probes;
+
+  friend std::ostream& operator<<(std::ostream& out, const ExpectedStats& s) {
+    out << "{AvgRatio:" << s.avg_ratio << ", MaxRatio:" << s.max_ratio
+        << ", PercentileRatios: [";
+    for (auto el : s.pecentile_ratios) {
+      out << el.first << ":" << el.second << ", ";
+    }
+    out << "], PercentileProbes: [";
+    for (auto el : s.pecentile_probes) {
+      out << el.first << ":" << el.second << ", ";
+    }
+    out << "]}";
+
+    return out;
+  }
+};
+
+void VerifyStats(size_t size, const ExpectedStats& exp,
+                 const ProbeStats& stats) {
+  EXPECT_LT(stats.AvgRatio(), exp.avg_ratio) << size << " " << stats;
+  EXPECT_LT(stats.MaxRatio(), exp.max_ratio) << size << " " << stats;
+  for (auto pr : exp.pecentile_ratios) {
+    EXPECT_LE(stats.PercentileRatio(pr.first), pr.second)
+        << size << " " << pr.first << " " << stats;
+  }
+
+  for (auto pr : exp.pecentile_probes) {
+    EXPECT_LE(stats.PercentileProbe(pr.first), pr.second)
+        << size << " " << pr.first << " " << stats;
+  }
+}
+
+using ProbeStatsPerSize = std::map<size_t, ProbeStats>;
+
+// Collect total ProbeStats on num_iters iterations of the following algorithm:
+// 1. Create new table and reserve it to keys.size() * 2
+// 2. Insert all keys xored with seed
+// 3. Collect ProbeStats from final table.
+ProbeStats CollectProbeStatsOnKeysXoredWithSeed(const std::vector<int64_t>& keys,
+                                                size_t num_iters) {
+  const size_t reserve_size = keys.size() * 2;
+
+  ProbeStats stats;
+
+  int64_t seed = 0x71b1a19b907d6e33;
+  while (num_iters--) {
+    seed = static_cast<int64_t>(static_cast<uint64_t>(seed) * 17 + 13);
+    IntTable t1;
+    t1.reserve(reserve_size);
+    for (const auto& key : keys) {
+      t1.emplace(key ^ seed);
+    }
+
+    auto probe_histogram = GetHashtableDebugNumProbesHistogram(t1);
+    stats.all_probes_histogram.resize(
+        std::max(stats.all_probes_histogram.size(), probe_histogram.size()));
+    std::transform(probe_histogram.begin(), probe_histogram.end(),
+                   stats.all_probes_histogram.begin(),
+                   stats.all_probes_histogram.begin(), std::plus<size_t>());
+
+    size_t total_probe_seq_length = 0;
+    for (size_t i = 0; i < probe_histogram.size(); ++i) {
+      total_probe_seq_length += i * probe_histogram[i];
+    }
+    stats.single_table_ratios.push_back(total_probe_seq_length * 1.0 /
+                                        keys.size());
+    t1.erase(t1.begin(), t1.end());
+  }
+  return stats;
+}
+
+ExpectedStats XorSeedExpectedStats() {
+  constexpr bool kRandomizesInserts =
+#if NDEBUG
+      false;
+#else   // NDEBUG
+      true;
+#endif  // NDEBUG
+
+  // The effective load factor is larger in non-opt mode because we insert
+  // elements out of order.
+  switch (container_internal::Group::kWidth) {
+    case 8:
+      if (kRandomizesInserts) {
+  return {0.05,
+          1.0,
+          {{0.95, 0.5}},
+          {{0.95, 0}, {0.99, 2}, {0.999, 4}, {0.9999, 10}}};
+      } else {
+  return {0.05,
+          2.0,
+          {{0.95, 0.1}},
+          {{0.95, 0}, {0.99, 2}, {0.999, 4}, {0.9999, 10}}};
+      }
+      break;
+    case 16:
+      if (kRandomizesInserts) {
+        return {0.1,
+                1.0,
+                {{0.95, 0.1}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
+      } else {
+        return {0.05,
+                1.0,
+                {{0.95, 0.05}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 4}, {0.9999, 10}}};
+      }
+      break;
+    default:
+      ABSL_RAW_LOG(FATAL, "%s", "Unknown Group width");
+  }
+  return {};
+}
+TEST(Table, DISABLED_EnsureNonQuadraticTopNXorSeedByProbeSeqLength) {
+  ProbeStatsPerSize stats;
+  std::vector<size_t> sizes = {Group::kWidth << 5, Group::kWidth << 10};
+  for (size_t size : sizes) {
+    stats[size] =
+        CollectProbeStatsOnKeysXoredWithSeed(CollectBadMergeKeys(size), 200);
+  }
+  auto expected = XorSeedExpectedStats();
+  for (size_t size : sizes) {
+    auto& stat = stats[size];
+    VerifyStats(size, expected, stat);
+  }
+}
+
+// Collect total ProbeStats on num_iters iterations of the following algorithm:
+// 1. Create new table
+// 2. Select 10% of keys and insert 10 elements key * 17 + j * 13
+// 3. Collect ProbeStats from final table
+ProbeStats CollectProbeStatsOnLinearlyTransformedKeys(
+    const std::vector<int64_t>& keys, size_t num_iters) {
+  ProbeStats stats;
+
+  std::random_device rd;
+  std::mt19937 rng(rd());
+  auto linear_transform = [](size_t x, size_t y) { return x * 17 + y * 13; };
+  std::uniform_int_distribution<size_t> dist(0, keys.size()-1);
+  while (num_iters--) {
+    IntTable t1;
+    size_t num_keys = keys.size() / 10;
+    size_t start = dist(rng);
+    for (size_t i = 0; i != num_keys; ++i) {
+      for (size_t j = 0; j != 10; ++j) {
+        t1.emplace(linear_transform(keys[(i + start) % keys.size()], j));
+      }
+    }
+
+    auto probe_histogram = GetHashtableDebugNumProbesHistogram(t1);
+    stats.all_probes_histogram.resize(
+        std::max(stats.all_probes_histogram.size(), probe_histogram.size()));
+    std::transform(probe_histogram.begin(), probe_histogram.end(),
+                   stats.all_probes_histogram.begin(),
+                   stats.all_probes_histogram.begin(), std::plus<size_t>());
+
+    size_t total_probe_seq_length = 0;
+    for (size_t i = 0; i < probe_histogram.size(); ++i) {
+      total_probe_seq_length += i * probe_histogram[i];
+    }
+    stats.single_table_ratios.push_back(total_probe_seq_length * 1.0 /
+                                        t1.size());
+    t1.erase(t1.begin(), t1.end());
+  }
+  return stats;
+}
+
+ExpectedStats LinearTransformExpectedStats() {
+  constexpr bool kRandomizesInserts =
+#if NDEBUG
+      false;
+#else   // NDEBUG
+      true;
+#endif  // NDEBUG
+
+  // The effective load factor is larger in non-opt mode because we insert
+  // elements out of order.
+  switch (container_internal::Group::kWidth) {
+    case 8:
+      if (kRandomizesInserts) {
+        return {0.1,
+                0.5,
+                {{0.95, 0.3}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
+      } else {
+        return {0.15,
+                0.5,
+                {{0.95, 0.3}},
+                {{0.95, 0}, {0.99, 3}, {0.999, 15}, {0.9999, 25}}};
+      }
+      break;
+    case 16:
+      if (kRandomizesInserts) {
+        return {0.1,
+                0.4,
+                {{0.95, 0.3}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
+      } else {
+        return {0.05,
+                0.2,
+                {{0.95, 0.1}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 6}, {0.9999, 10}}};
+      }
+      break;
+    default:
+      ABSL_RAW_LOG(FATAL, "%s", "Unknown Group width");
+  }
+  return {};
+}
+TEST(Table, DISABLED_EnsureNonQuadraticTopNLinearTransformByProbeSeqLength) {
+  ProbeStatsPerSize stats;
+  std::vector<size_t> sizes = {Group::kWidth << 5, Group::kWidth << 10};
+  for (size_t size : sizes) {
+    stats[size] = CollectProbeStatsOnLinearlyTransformedKeys(
+        CollectBadMergeKeys(size), 300);
+  }
+  auto expected = LinearTransformExpectedStats();
+  for (size_t size : sizes) {
+    auto& stat = stats[size];
+    VerifyStats(size, expected, stat);
+  }
+}
+
+TEST(Table, EraseCollision) {
+  BadTable t;
+
+  // 1 2 3
+  t.emplace(1);
+  t.emplace(2);
+  t.emplace(3);
+  EXPECT_THAT(*t.find(1), 1);
+  EXPECT_THAT(*t.find(2), 2);
+  EXPECT_THAT(*t.find(3), 3);
+  EXPECT_EQ(3, t.size());
+
+  // 1 DELETED 3
+  t.erase(t.find(2));
+  EXPECT_THAT(*t.find(1), 1);
+  EXPECT_TRUE(t.find(2) == t.end());
+  EXPECT_THAT(*t.find(3), 3);
+  EXPECT_EQ(2, t.size());
+
+  // DELETED DELETED 3
+  t.erase(t.find(1));
+  EXPECT_TRUE(t.find(1) == t.end());
+  EXPECT_TRUE(t.find(2) == t.end());
+  EXPECT_THAT(*t.find(3), 3);
+  EXPECT_EQ(1, t.size());
+
+  // DELETED DELETED DELETED
+  t.erase(t.find(3));
+  EXPECT_TRUE(t.find(1) == t.end());
+  EXPECT_TRUE(t.find(2) == t.end());
+  EXPECT_TRUE(t.find(3) == t.end());
+  EXPECT_EQ(0, t.size());
+}
+
+TEST(Table, EraseInsertProbing) {
+  BadTable t(100);
+
+  // 1 2 3 4
+  t.emplace(1);
+  t.emplace(2);
+  t.emplace(3);
+  t.emplace(4);
+
+  // 1 DELETED 3 DELETED
+  t.erase(t.find(2));
+  t.erase(t.find(4));
+
+  // 1 10 3 11 12
+  t.emplace(10);
+  t.emplace(11);
+  t.emplace(12);
+
+  EXPECT_EQ(5, t.size());
+  EXPECT_THAT(t, UnorderedElementsAre(1, 10, 3, 11, 12));
+}
+
+TEST(Table, Clear) {
+  IntTable t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  t.clear();
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_EQ(1, t.size());
+  t.clear();
+  EXPECT_EQ(0, t.size());
+  EXPECT_TRUE(t.find(0) == t.end());
+}
+
+TEST(Table, Swap) {
+  IntTable t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_EQ(1, t.size());
+  IntTable u;
+  t.swap(u);
+  EXPECT_EQ(0, t.size());
+  EXPECT_EQ(1, u.size());
+  EXPECT_TRUE(t.find(0) == t.end());
+  EXPECT_THAT(*u.find(0), 0);
+}
+
+TEST(Table, Rehash) {
+  IntTable t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  t.emplace(0);
+  t.emplace(1);
+  EXPECT_EQ(2, t.size());
+  t.rehash(128);
+  EXPECT_EQ(2, t.size());
+  EXPECT_THAT(*t.find(0), 0);
+  EXPECT_THAT(*t.find(1), 1);
+}
+
+TEST(Table, RehashDoesNotRehashWhenNotNecessary) {
+  IntTable t;
+  t.emplace(0);
+  t.emplace(1);
+  auto* p = &*t.find(0);
+  t.rehash(1);
+  EXPECT_EQ(p, &*t.find(0));
+}
+
+TEST(Table, RehashZeroDoesNotAllocateOnEmptyTable) {
+  IntTable t;
+  t.rehash(0);
+  EXPECT_EQ(0, t.bucket_count());
+}
+
+TEST(Table, RehashZeroDeallocatesEmptyTable) {
+  IntTable t;
+  t.emplace(0);
+  t.clear();
+  EXPECT_NE(0, t.bucket_count());
+  t.rehash(0);
+  EXPECT_EQ(0, t.bucket_count());
+}
+
+TEST(Table, RehashZeroForcesRehash) {
+  IntTable t;
+  t.emplace(0);
+  t.emplace(1);
+  auto* p = &*t.find(0);
+  t.rehash(0);
+  EXPECT_NE(p, &*t.find(0));
+}
+
+TEST(Table, ConstructFromInitList) {
+  using P = std::pair<std::string, std::string>;
+  struct Q {
+    operator P() const { return {}; }
+  };
+  StringTable t = {P(), Q(), {}, {{}, {}}};
+}
+
+TEST(Table, CopyConstruct) {
+  IntTable t;
+  t.max_load_factor(.321f);
+  t.emplace(0);
+  EXPECT_EQ(1, t.size());
+  {
+    IntTable u(t);
+    EXPECT_EQ(1, u.size());
+    EXPECT_EQ(t.max_load_factor(), u.max_load_factor());
+    EXPECT_THAT(*u.find(0), 0);
+  }
+  {
+    IntTable u{t};
+    EXPECT_EQ(1, u.size());
+    EXPECT_EQ(t.max_load_factor(), u.max_load_factor());
+    EXPECT_THAT(*u.find(0), 0);
+  }
+  {
+    IntTable u = t;
+    EXPECT_EQ(1, u.size());
+    EXPECT_EQ(t.max_load_factor(), u.max_load_factor());
+    EXPECT_THAT(*u.find(0), 0);
+  }
+}
+
+TEST(Table, CopyConstructWithAlloc) {
+  StringTable t;
+  t.max_load_factor(.321f);
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u(t, Alloc<std::pair<std::string, std::string>>());
+  EXPECT_EQ(1, u.size());
+  EXPECT_EQ(t.max_load_factor(), u.max_load_factor());
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+struct ExplicitAllocIntTable
+    : raw_hash_set<IntPolicy, container_internal::hash_default_hash<int64_t>,
+                   std::equal_to<int64_t>, Alloc<int64_t>> {
+  ExplicitAllocIntTable() {}
+};
+
+TEST(Table, AllocWithExplicitCtor) {
+  ExplicitAllocIntTable t;
+  EXPECT_EQ(0, t.size());
+}
+
+TEST(Table, MoveConstruct) {
+  {
+    StringTable t;
+    t.max_load_factor(.321f);
+    const float lf = t.max_load_factor();
+    t.emplace("a", "b");
+    EXPECT_EQ(1, t.size());
+
+    StringTable u(std::move(t));
+    EXPECT_EQ(1, u.size());
+    EXPECT_EQ(lf, u.max_load_factor());
+    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  }
+  {
+    StringTable t;
+    t.max_load_factor(.321f);
+    const float lf = t.max_load_factor();
+    t.emplace("a", "b");
+    EXPECT_EQ(1, t.size());
+
+    StringTable u{std::move(t)};
+    EXPECT_EQ(1, u.size());
+    EXPECT_EQ(lf, u.max_load_factor());
+    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  }
+  {
+    StringTable t;
+    t.max_load_factor(.321f);
+    const float lf = t.max_load_factor();
+    t.emplace("a", "b");
+    EXPECT_EQ(1, t.size());
+
+    StringTable u = std::move(t);
+    EXPECT_EQ(1, u.size());
+    EXPECT_EQ(lf, u.max_load_factor());
+    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  }
+}
+
+TEST(Table, MoveConstructWithAlloc) {
+  StringTable t;
+  t.max_load_factor(.321f);
+  const float lf = t.max_load_factor();
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u(std::move(t), Alloc<std::pair<std::string, std::string>>());
+  EXPECT_EQ(1, u.size());
+  EXPECT_EQ(lf, u.max_load_factor());
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, CopyAssign) {
+  StringTable t;
+  t.max_load_factor(.321f);
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u;
+  u = t;
+  EXPECT_EQ(1, u.size());
+  EXPECT_EQ(t.max_load_factor(), u.max_load_factor());
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, CopySelfAssign) {
+  StringTable t;
+  t.max_load_factor(.321f);
+  const float lf = t.max_load_factor();
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  t = *&t;
+  EXPECT_EQ(1, t.size());
+  EXPECT_EQ(lf, t.max_load_factor());
+  EXPECT_THAT(*t.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, MoveAssign) {
+  StringTable t;
+  t.max_load_factor(.321f);
+  const float lf = t.max_load_factor();
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u;
+  u = std::move(t);
+  EXPECT_EQ(1, u.size());
+  EXPECT_EQ(lf, u.max_load_factor());
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, Equality) {
+  StringTable t;
+  std::vector<std::pair<std::string, std::string>> v = {{"a", "b"}, {"aa", "bb"}};
+  t.insert(std::begin(v), std::end(v));
+  StringTable u = t;
+  EXPECT_EQ(u, t);
+}
+
+TEST(Table, Equality2) {
+  StringTable t;
+  std::vector<std::pair<std::string, std::string>> v1 = {{"a", "b"}, {"aa", "bb"}};
+  t.insert(std::begin(v1), std::end(v1));
+  StringTable u;
+  std::vector<std::pair<std::string, std::string>> v2 = {{"a", "a"}, {"aa", "aa"}};
+  u.insert(std::begin(v2), std::end(v2));
+  EXPECT_NE(u, t);
+}
+
+TEST(Table, Equality3) {
+  StringTable t;
+  std::vector<std::pair<std::string, std::string>> v1 = {{"b", "b"}, {"bb", "bb"}};
+  t.insert(std::begin(v1), std::end(v1));
+  StringTable u;
+  std::vector<std::pair<std::string, std::string>> v2 = {{"a", "a"}, {"aa", "aa"}};
+  u.insert(std::begin(v2), std::end(v2));
+  EXPECT_NE(u, t);
+}
+
+TEST(Table, NumDeletedRegression) {
+  IntTable t;
+  t.emplace(0);
+  t.erase(t.find(0));
+  // construct over a deleted slot.
+  t.emplace(0);
+  t.clear();
+}
+
+TEST(Table, FindFullDeletedRegression) {
+  IntTable t;
+  for (int i = 0; i < 1000; ++i) {
+    t.emplace(i);
+    t.erase(t.find(i));
+  }
+  EXPECT_EQ(0, t.size());
+}
+
+TEST(Table, ReplacingDeletedSlotDoesNotRehash) {
+  size_t n;
+  {
+    // Compute n such that n is the maximum number of elements before rehash.
+    IntTable t;
+    t.emplace(0);
+    size_t c = t.bucket_count();
+    for (n = 1; c == t.bucket_count(); ++n) t.emplace(n);
+    --n;
+  }
+  IntTable t;
+  t.rehash(n);
+  const size_t c = t.bucket_count();
+  for (size_t i = 0; i != n; ++i) t.emplace(i);
+  EXPECT_EQ(c, t.bucket_count()) << "rehashing threshold = " << n;
+  t.erase(0);
+  t.emplace(0);
+  EXPECT_EQ(c, t.bucket_count()) << "rehashing threshold = " << n;
+}
+
+TEST(Table, NoThrowMoveConstruct) {
+  ASSERT_TRUE(
+      std::is_nothrow_copy_constructible<absl::Hash<absl::string_view>>::value);
+  ASSERT_TRUE(std::is_nothrow_copy_constructible<
+              std::equal_to<absl::string_view>>::value);
+  ASSERT_TRUE(std::is_nothrow_copy_constructible<std::allocator<int>>::value);
+  EXPECT_TRUE(std::is_nothrow_move_constructible<StringTable>::value);
+}
+
+TEST(Table, NoThrowMoveAssign) {
+  ASSERT_TRUE(
+      std::is_nothrow_move_assignable<absl::Hash<absl::string_view>>::value);
+  ASSERT_TRUE(
+      std::is_nothrow_move_assignable<std::equal_to<absl::string_view>>::value);
+  ASSERT_TRUE(std::is_nothrow_move_assignable<std::allocator<int>>::value);
+  ASSERT_TRUE(
+      absl::allocator_traits<std::allocator<int>>::is_always_equal::value);
+  EXPECT_TRUE(std::is_nothrow_move_assignable<StringTable>::value);
+}
+
+TEST(Table, NoThrowSwappable) {
+  ASSERT_TRUE(
+      container_internal::IsNoThrowSwappable<absl::Hash<absl::string_view>>());
+  ASSERT_TRUE(container_internal::IsNoThrowSwappable<
+              std::equal_to<absl::string_view>>());
+  ASSERT_TRUE(container_internal::IsNoThrowSwappable<std::allocator<int>>());
+  EXPECT_TRUE(container_internal::IsNoThrowSwappable<StringTable>());
+}
+
+TEST(Table, HeterogeneousLookup) {
+  struct Hash {
+    size_t operator()(int64_t i) const { return i; }
+    size_t operator()(double i) const {
+      ADD_FAILURE();
+      return i;
+    }
+  };
+  struct Eq {
+    bool operator()(int64_t a, int64_t b) const { return a == b; }
+    bool operator()(double a, int64_t b) const {
+      ADD_FAILURE();
+      return a == b;
+    }
+    bool operator()(int64_t a, double b) const {
+      ADD_FAILURE();
+      return a == b;
+    }
+    bool operator()(double a, double b) const {
+      ADD_FAILURE();
+      return a == b;
+    }
+  };
+
+  struct THash {
+    using is_transparent = void;
+    size_t operator()(int64_t i) const { return i; }
+    size_t operator()(double i) const { return i; }
+  };
+  struct TEq {
+    using is_transparent = void;
+    bool operator()(int64_t a, int64_t b) const { return a == b; }
+    bool operator()(double a, int64_t b) const { return a == b; }
+    bool operator()(int64_t a, double b) const { return a == b; }
+    bool operator()(double a, double b) const { return a == b; }
+  };
+
+  raw_hash_set<IntPolicy, Hash, Eq, Alloc<int64_t>> s{0, 1, 2};
+  // It will convert to int64_t before the query.
+  EXPECT_EQ(1, *s.find(double{1.1}));
+
+  raw_hash_set<IntPolicy, THash, TEq, Alloc<int64_t>> ts{0, 1, 2};
+  // It will try to use the double, and fail to find the object.
+  EXPECT_TRUE(ts.find(1.1) == ts.end());
+}
+
+template <class Table>
+using CallFind = decltype(std::declval<Table&>().find(17));
+
+template <class Table>
+using CallErase = decltype(std::declval<Table&>().erase(17));
+
+template <class Table>
+using CallExtract = decltype(std::declval<Table&>().extract(17));
+
+template <class Table>
+using CallPrefetch = decltype(std::declval<Table&>().prefetch(17));
+
+template <class Table>
+using CallCount = decltype(std::declval<Table&>().count(17));
+
+template <template <typename> class C, class Table, class = void>
+struct VerifyResultOf : std::false_type {};
+
+template <template <typename> class C, class Table>
+struct VerifyResultOf<C, Table, absl::void_t<C<Table>>> : std::true_type {};
+
+TEST(Table, HeterogeneousLookupOverloads) {
+  using NonTransparentTable =
+      raw_hash_set<StringPolicy, absl::Hash<absl::string_view>,
+                   std::equal_to<absl::string_view>, std::allocator<int>>;
+
+  EXPECT_FALSE((VerifyResultOf<CallFind, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallErase, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallExtract, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallPrefetch, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallCount, NonTransparentTable>()));
+
+  using TransparentTable = raw_hash_set<
+      StringPolicy,
+      absl::container_internal::hash_default_hash<absl::string_view>,
+      absl::container_internal::hash_default_eq<absl::string_view>,
+      std::allocator<int>>;
+
+  EXPECT_TRUE((VerifyResultOf<CallFind, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallErase, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallExtract, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallPrefetch, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallCount, TransparentTable>()));
+}
+
+// TODO(alkis): Expand iterator tests.
+TEST(Iterator, IsDefaultConstructible) {
+  StringTable::iterator i;
+  EXPECT_TRUE(i == StringTable::iterator());
+}
+
+TEST(ConstIterator, IsDefaultConstructible) {
+  StringTable::const_iterator i;
+  EXPECT_TRUE(i == StringTable::const_iterator());
+}
+
+TEST(Iterator, ConvertsToConstIterator) {
+  StringTable::iterator i;
+  EXPECT_TRUE(i == StringTable::const_iterator());
+}
+
+TEST(Iterator, Iterates) {
+  IntTable t;
+  for (size_t i = 3; i != 6; ++i) EXPECT_TRUE(t.emplace(i).second);
+  EXPECT_THAT(t, UnorderedElementsAre(3, 4, 5));
+}
+
+TEST(Table, Merge) {
+  StringTable t1, t2;
+  t1.emplace("0", "-0");
+  t1.emplace("1", "-1");
+  t2.emplace("0", "~0");
+  t2.emplace("2", "~2");
+
+  EXPECT_THAT(t1, UnorderedElementsAre(Pair("0", "-0"), Pair("1", "-1")));
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair("0", "~0"), Pair("2", "~2")));
+
+  t1.merge(t2);
+  EXPECT_THAT(t1, UnorderedElementsAre(Pair("0", "-0"), Pair("1", "-1"),
+                                       Pair("2", "~2")));
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair("0", "~0")));
+}
+
+TEST(Nodes, EmptyNodeType) {
+  using node_type = StringTable::node_type;
+  node_type n;
+  EXPECT_FALSE(n);
+  EXPECT_TRUE(n.empty());
+
+  EXPECT_TRUE((std::is_same<node_type::allocator_type,
+                            StringTable::allocator_type>::value));
+}
+
+TEST(Nodes, ExtractInsert) {
+  constexpr char k0[] = "Very long std::string zero.";
+  constexpr char k1[] = "Very long std::string one.";
+  constexpr char k2[] = "Very long std::string two.";
+  StringTable t = {{k0, ""}, {k1, ""}, {k2, ""}};
+  EXPECT_THAT(t,
+              UnorderedElementsAre(Pair(k0, ""), Pair(k1, ""), Pair(k2, "")));
+
+  auto node = t.extract(k0);
+  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
+  EXPECT_TRUE(node);
+  EXPECT_FALSE(node.empty());
+
+  StringTable t2;
+  auto res = t2.insert(std::move(node));
+  EXPECT_TRUE(res.inserted);
+  EXPECT_THAT(*res.position, Pair(k0, ""));
+  EXPECT_FALSE(res.node);
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair(k0, "")));
+
+  // Not there.
+  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
+  node = t.extract("Not there!");
+  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
+  EXPECT_FALSE(node);
+
+  // Inserting nothing.
+  res = t2.insert(std::move(node));
+  EXPECT_FALSE(res.inserted);
+  EXPECT_EQ(res.position, t2.end());
+  EXPECT_FALSE(res.node);
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair(k0, "")));
+
+  t.emplace(k0, "1");
+  node = t.extract(k0);
+
+  // Insert duplicate.
+  res = t2.insert(std::move(node));
+  EXPECT_FALSE(res.inserted);
+  EXPECT_THAT(*res.position, Pair(k0, ""));
+  EXPECT_TRUE(res.node);
+  EXPECT_FALSE(node);
+}
+
+StringTable MakeSimpleTable(size_t size) {
+  StringTable t;
+  for (size_t i = 0; i < size; ++i) t.emplace(std::string(1, 'A' + i), "");
+  return t;
+}
+
+std::string OrderOfIteration(const StringTable& t) {
+  std::string order;
+  for (auto& p : t) order += p.first;
+  return order;
+}
+
+TEST(Table, IterationOrderChangesByInstance) {
+  // Needs to be more than kWidth elements to be able to affect order.
+  const StringTable reference = MakeSimpleTable(20);
+
+  // Since order is non-deterministic we can't just try once and verify.
+  // We'll try until we find that order changed. It should not take many tries
+  // for that.
+  // Important: we have to keep the old tables around. Otherwise tcmalloc will
+  // just give us the same blocks and we would be doing the same order again.
+  std::vector<StringTable> garbage;
+  for (int i = 0; i < 10; ++i) {
+    auto trial = MakeSimpleTable(20);
+    if (OrderOfIteration(trial) != OrderOfIteration(reference)) {
+      // We are done.
+      return;
+    }
+    garbage.push_back(std::move(trial));
+  }
+  FAIL();
+}
+
+TEST(Table, IterationOrderChangesOnRehash) {
+  // Since order is non-deterministic we can't just try once and verify.
+  // We'll try until we find that order changed. It should not take many tries
+  // for that.
+  // Important: we have to keep the old tables around. Otherwise tcmalloc will
+  // just give us the same blocks and we would be doing the same order again.
+  std::vector<StringTable> garbage;
+  for (int i = 0; i < 10; ++i) {
+    // Needs to be more than kWidth elements to be able to affect order.
+    StringTable t = MakeSimpleTable(20);
+    const std::string reference = OrderOfIteration(t);
+    // Force rehash to the same size.
+    t.rehash(0);
+    std::string trial = OrderOfIteration(t);
+    if (trial != reference) {
+      // We are done.
+      return;
+    }
+    garbage.push_back(std::move(t));
+  }
+  FAIL();
+}
+
+TEST(Table, IterationOrderChangesForSmallTables) {
+  // Since order is non-deterministic we can't just try once and verify.
+  // We'll try until we find that order changed.
+  // Important: we have to keep the old tables around. Otherwise tcmalloc will
+  // just give us the same blocks and we would be doing the same order again.
+  StringTable reference_table = MakeSimpleTable(5);
+  const std::string reference = OrderOfIteration(reference_table);
+  std::vector<StringTable> garbage;
+  for (int i = 0; i < 50; ++i) {
+    StringTable t = MakeSimpleTable(5);
+    std::string trial = OrderOfIteration(t);
+    if (trial != reference) {
+      // We are done.
+      return;
+    }
+    garbage.push_back(std::move(t));
+  }
+  FAIL() << "Iteration order remained the same across many attempts.";
+}
+
+// Fill the table to 3 different load factors (min, median, max) and evaluate
+// the percentage of perfect hits using the debug API.
+template <class Table, class AddFn>
+std::vector<double> CollectPerfectRatios(Table t, AddFn add) {
+  using Key = typename Table::key_type;
+
+  // First, fill enough to have a good distribution.
+  constexpr size_t kMinSize = 10000;
+  std::vector<Key> keys;
+  while (t.size() < kMinSize) keys.push_back(add(t));
+  // Then, insert until we reach min load factor.
+  double lf = t.load_factor();
+  while (lf <= t.load_factor()) keys.push_back(add(t));
+
+  // We are now at min load factor. Take a snapshot.
+  size_t perfect = 0;
+  auto update_perfect = [&](Key k) {
+    perfect += GetHashtableDebugNumProbes(t, k) == 0;
+  };
+  for (const auto& k : keys) update_perfect(k);
+
+  std::vector<double> perfect_ratios;
+  // Keep going until we hit max load factor.
+  while (t.load_factor() < .6) {
+    perfect_ratios.push_back(1.0 * perfect / t.size());
+    update_perfect(add(t));
+  }
+  while (t.load_factor() > .5) {
+    perfect_ratios.push_back(1.0 * perfect / t.size());
+    update_perfect(add(t));
+  }
+  return perfect_ratios;
+}
+
+std::vector<std::pair<double, double>> StringTablePefectRatios() {
+  constexpr bool kRandomizesInserts =
+#if NDEBUG
+      false;
+#else   // NDEBUG
+      true;
+#endif  // NDEBUG
+
+  // The effective load factor is larger in non-opt mode because we insert
+  // elements out of order.
+  switch (container_internal::Group::kWidth) {
+    case 8:
+      if (kRandomizesInserts) {
+        return {{0.986, 0.02}, {0.95, 0.02}, {0.89, 0.02}};
+      } else {
+        return {{0.995, 0.01}, {0.97, 0.01}, {0.89, 0.02}};
+      }
+      break;
+    case 16:
+      if (kRandomizesInserts) {
+        return {{0.973, 0.01}, {0.965, 0.01}, {0.92, 0.02}};
+      } else {
+        return {{0.995, 0.005}, {0.99, 0.005}, {0.94, 0.01}};
+      }
+      break;
+    default:
+      // Ignore anything else.
+      return {};
+  }
+}
+
+// This is almost a change detector, but it allows us to know how we are
+// affecting the probe distribution.
+TEST(Table, EffectiveLoadFactorStrings) {
+  std::vector<double> perfect_ratios =
+      CollectPerfectRatios(StringTable(), [](StringTable& t) {
+        return t.emplace(std::to_string(t.size()), "").first->first;
+      });
+
+  auto ratios = StringTablePefectRatios();
+  if (ratios.empty()) return;
+
+  EXPECT_THAT(perfect_ratios.front(),
+              DoubleNear(ratios[0].first, ratios[0].second));
+  EXPECT_THAT(perfect_ratios[perfect_ratios.size() / 2],
+              DoubleNear(ratios[1].first, ratios[1].second));
+  EXPECT_THAT(perfect_ratios.back(),
+              DoubleNear(ratios[2].first, ratios[2].second));
+}
+
+std::vector<std::pair<double, double>> IntTablePefectRatios() {
+  constexpr bool kRandomizesInserts =
+#ifdef NDEBUG
+      false;
+#else   // NDEBUG
+      true;
+#endif  // NDEBUG
+
+  // The effective load factor is larger in non-opt mode because we insert
+  // elements out of order.
+  switch (container_internal::Group::kWidth) {
+    case 8:
+      if (kRandomizesInserts) {
+        return {{0.99, 0.02}, {0.985, 0.02}, {0.95, 0.05}};
+      } else {
+        return {{0.99, 0.01}, {0.99, 0.01}, {0.95, 0.02}};
+      }
+      break;
+    case 16:
+      if (kRandomizesInserts) {
+        return {{0.98, 0.02}, {0.978, 0.02}, {0.96, 0.02}};
+      } else {
+        return {{0.998, 0.003}, {0.995, 0.01}, {0.975, 0.02}};
+      }
+      break;
+    default:
+      // Ignore anything else.
+      return {};
+  }
+}
+
+// This is almost a change detector, but it allows us to know how we are
+// affecting the probe distribution.
+TEST(Table, EffectiveLoadFactorInts) {
+  std::vector<double> perfect_ratios = CollectPerfectRatios(
+      IntTable(), [](IntTable& t) { return *t.emplace(t.size()).first; });
+
+  auto ratios = IntTablePefectRatios();
+  if (ratios.empty()) return;
+
+  EXPECT_THAT(perfect_ratios.front(),
+              DoubleNear(ratios[0].first, ratios[0].second));
+  EXPECT_THAT(perfect_ratios[perfect_ratios.size() / 2],
+              DoubleNear(ratios[1].first, ratios[1].second));
+  EXPECT_THAT(perfect_ratios.back(),
+              DoubleNear(ratios[2].first, ratios[2].second));
+}
+
+// Confirm that we assert if we try to erase() end().
+TEST(Table, EraseOfEndAsserts) {
+  // Use an assert with side-effects to figure out if they are actually enabled.
+  bool assert_enabled = false;
+  assert([&]() {
+    assert_enabled = true;
+    return true;
+  }());
+  if (!assert_enabled) return;
+
+  IntTable t;
+  // Extra simple "regexp" as regexp support is highly varied across platforms.
+  constexpr char kDeathMsg[] = "it != end";
+  EXPECT_DEATH(t.erase(t.end()), kDeathMsg);
+}
+
+#ifdef ADDRESS_SANITIZER
+TEST(Sanitizer, PoisoningUnused) {
+  IntTable t;
+  // Insert something to force an allocation.
+  int64_t& v1 = *t.insert(0).first;
+
+  // Make sure there is something to test.
+  ASSERT_GT(t.capacity(), 1);
+
+  int64_t* slots = RawHashSetTestOnlyAccess::GetSlots(t);
+  for (size_t i = 0; i < t.capacity(); ++i) {
+    EXPECT_EQ(slots + i != &v1, __asan_address_is_poisoned(slots + i));
+  }
+}
+
+TEST(Sanitizer, PoisoningOnErase) {
+  IntTable t;
+  int64_t& v = *t.insert(0).first;
+
+  EXPECT_FALSE(__asan_address_is_poisoned(&v));
+  t.erase(0);
+  EXPECT_TRUE(__asan_address_is_poisoned(&v));
+}
+#endif  // ADDRESS_SANITIZER
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/tracked.h b/absl/container/internal/tracked.h
new file mode 100644
index 000000000000..7d14af03138d
--- /dev/null
+++ b/absl/container/internal/tracked.h
@@ -0,0 +1,78 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_TRACKED_H_
+#define ABSL_CONTAINER_INTERNAL_TRACKED_H_
+
+#include <stddef.h>
+#include <memory>
+#include <utility>
+
+namespace absl {
+namespace container_internal {
+
+// A class that tracks its copies and moves so that it can be queried in tests.
+template <class T>
+class Tracked {
+ public:
+  Tracked() {}
+  // NOLINTNEXTLINE(runtime/explicit)
+  Tracked(const T& val) : val_(val) {}
+  Tracked(const Tracked& that)
+      : val_(that.val_),
+        num_moves_(that.num_moves_),
+        num_copies_(that.num_copies_) {
+    ++(*num_copies_);
+  }
+  Tracked(Tracked&& that)
+      : val_(std::move(that.val_)),
+        num_moves_(std::move(that.num_moves_)),
+        num_copies_(std::move(that.num_copies_)) {
+    ++(*num_moves_);
+  }
+  Tracked& operator=(const Tracked& that) {
+    val_ = that.val_;
+    num_moves_ = that.num_moves_;
+    num_copies_ = that.num_copies_;
+    ++(*num_copies_);
+  }
+  Tracked& operator=(Tracked&& that) {
+    val_ = std::move(that.val_);
+    num_moves_ = std::move(that.num_moves_);
+    num_copies_ = std::move(that.num_copies_);
+    ++(*num_moves_);
+  }
+
+  const T& val() const { return val_; }
+
+  friend bool operator==(const Tracked& a, const Tracked& b) {
+    return a.val_ == b.val_;
+  }
+  friend bool operator!=(const Tracked& a, const Tracked& b) {
+    return !(a == b);
+  }
+
+  size_t num_copies() { return *num_copies_; }
+  size_t num_moves() { return *num_moves_; }
+
+ private:
+  T val_;
+  std::shared_ptr<size_t> num_moves_ = std::make_shared<size_t>(0);
+  std::shared_ptr<size_t> num_copies_ = std::make_shared<size_t>(0);
+};
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_TRACKED_H_
diff --git a/absl/container/internal/unordered_map_constructor_test.h b/absl/container/internal/unordered_map_constructor_test.h
new file mode 100644
index 000000000000..2ffb646cb264
--- /dev/null
+++ b/absl/container/internal/unordered_map_constructor_test.h
@@ -0,0 +1,404 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_CONSTRUCTOR_TEST_H_
+#define ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_CONSTRUCTOR_TEST_H_
+
+#include <algorithm>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+
+namespace absl {
+namespace container_internal {
+
+template <class UnordMap>
+class ConstructorTest : public ::testing::Test {};
+
+TYPED_TEST_CASE_P(ConstructorTest);
+
+TYPED_TEST_P(ConstructorTest, NoArgs) {
+  TypeParam m;
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCount) {
+  TypeParam m(123);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHash) {
+  using H = typename TypeParam::hasher;
+  H hasher;
+  TypeParam m(123, hasher);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHashEqual) {
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  H hasher;
+  E equal;
+  TypeParam m(123, hasher, equal);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHashEqualAlloc) {
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountAlloc) {
+#if defined(UNORDERED_MAP_CXX14) || defined(UNORDERED_MAP_CXX17)
+  using A = typename TypeParam::allocator_type;
+  A alloc(0);
+  TypeParam m(123, alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHashAlloc) {
+#if defined(UNORDERED_MAP_CXX14) || defined(UNORDERED_MAP_CXX17)
+  using H = typename TypeParam::hasher;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  A alloc(0);
+  TypeParam m(123, hasher, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, BucketAlloc) {
+#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
+  using A = typename TypeParam::allocator_type;
+  A alloc(0);
+  TypeParam m(alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(m, ::testing::UnorderedElementsAre());
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end(), 123, hasher, equal, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, InputIteratorBucketAlloc) {
+#if defined(UNORDERED_MAP_CXX14) || defined(UNORDERED_MAP_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using A = typename TypeParam::allocator_type;
+  A alloc(0);
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end(), 123, alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashAlloc) {
+#if defined(UNORDERED_MAP_CXX14) || defined(UNORDERED_MAP_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  A alloc(0);
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end(), 123, hasher, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, CopyConstructor) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam n(m);
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, CopyConstructorAlloc) {
+#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam n(m, A(11));
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_NE(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+#endif
+}
+
+// TODO(alkis): Test non-propagating allocators on copy constructors.
+
+TYPED_TEST_P(ConstructorTest, MoveConstructor) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam t(m);
+  TypeParam n(std::move(t));
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, MoveConstructorAlloc) {
+#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam t(m);
+  TypeParam n(std::move(t), A(1));
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_NE(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+#endif
+}
+
+// TODO(alkis): Test non-propagating allocators on move constructors.
+
+TYPED_TEST_P(ConstructorTest, InitializerListBucketHashEqualAlloc) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(values, 123, hasher, equal, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, InitializerListBucketAlloc) {
+#if defined(UNORDERED_MAP_CXX14) || defined(UNORDERED_MAP_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using A = typename TypeParam::allocator_type;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  A alloc(0);
+  TypeParam m(values, 123, alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, InitializerListBucketHashAlloc) {
+#if defined(UNORDERED_MAP_CXX14) || defined(UNORDERED_MAP_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  A alloc(0);
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m(values, 123, hasher, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, Assignment) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
+  TypeParam n;
+  n = m;
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m, n);
+}
+
+// TODO(alkis): Test [non-]propagating allocators on move/copy assignments
+// (it depends on traits).
+
+TYPED_TEST_P(ConstructorTest, MoveAssignment) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
+  TypeParam t(m);
+  TypeParam n;
+  n = std::move(t);
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerList) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m;
+  m = values;
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentOverwritesExisting) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()});
+  TypeParam n({gen()});
+  n = m;
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, MoveAssignmentOverwritesExisting) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()});
+  TypeParam t(m);
+  TypeParam n({gen()});
+  n = std::move(t);
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerListOverwritesExisting) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m;
+  m = values;
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentOnSelf) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m(values);
+  m = *&m;  // Avoid -Wself-assign
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+// We cannot test self move as standard states that it leaves standard
+// containers in unspecified state (and in practice in causes memory-leak
+// according to heap-checker!).
+
+REGISTER_TYPED_TEST_CASE_P(
+    ConstructorTest, NoArgs, BucketCount, BucketCountHash, BucketCountHashEqual,
+    BucketCountHashEqualAlloc, BucketCountAlloc, BucketCountHashAlloc,
+    BucketAlloc, InputIteratorBucketHashEqualAlloc, InputIteratorBucketAlloc,
+    InputIteratorBucketHashAlloc, CopyConstructor, CopyConstructorAlloc,
+    MoveConstructor, MoveConstructorAlloc, InitializerListBucketHashEqualAlloc,
+    InitializerListBucketAlloc, InitializerListBucketHashAlloc, Assignment,
+    MoveAssignment, AssignmentFromInitializerList,
+    AssignmentOverwritesExisting, MoveAssignmentOverwritesExisting,
+    AssignmentFromInitializerListOverwritesExisting, AssignmentOnSelf);
+
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_CONSTRUCTOR_TEST_H_
diff --git a/absl/container/internal/unordered_map_lookup_test.h b/absl/container/internal/unordered_map_lookup_test.h
new file mode 100644
index 000000000000..1f1b6b489b30
--- /dev/null
+++ b/absl/container/internal/unordered_map_lookup_test.h
@@ -0,0 +1,114 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_LOOKUP_TEST_H_
+#define ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_LOOKUP_TEST_H_
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+
+namespace absl {
+namespace container_internal {
+
+template <class UnordMap>
+class LookupTest : public ::testing::Test {};
+
+TYPED_TEST_CASE_P(LookupTest);
+
+TYPED_TEST_P(LookupTest, At) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  for (const auto& p : values) {
+    const auto& val = m.at(p.first);
+    EXPECT_EQ(p.second, val) << ::testing::PrintToString(p.first);
+  }
+}
+
+TYPED_TEST_P(LookupTest, OperatorBracket) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  for (const auto& p : values) {
+    auto& val = m[p.first];
+    EXPECT_EQ(V(), val) << ::testing::PrintToString(p.first);
+    val = p.second;
+  }
+  for (const auto& p : values)
+    EXPECT_EQ(p.second, m[p.first]) << ::testing::PrintToString(p.first);
+}
+
+TYPED_TEST_P(LookupTest, Count) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  for (const auto& p : values)
+    EXPECT_EQ(0, m.count(p.first)) << ::testing::PrintToString(p.first);
+  m.insert(values.begin(), values.end());
+  for (const auto& p : values)
+    EXPECT_EQ(1, m.count(p.first)) << ::testing::PrintToString(p.first);
+}
+
+TYPED_TEST_P(LookupTest, Find) {
+  using std::get;
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  for (const auto& p : values)
+    EXPECT_TRUE(m.end() == m.find(p.first))
+        << ::testing::PrintToString(p.first);
+  m.insert(values.begin(), values.end());
+  for (const auto& p : values) {
+    auto it = m.find(p.first);
+    EXPECT_TRUE(m.end() != it) << ::testing::PrintToString(p.first);
+    EXPECT_EQ(p.second, get<1>(*it)) << ::testing::PrintToString(p.first);
+  }
+}
+
+TYPED_TEST_P(LookupTest, EqualRange) {
+  using std::get;
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  for (const auto& p : values) {
+    auto r = m.equal_range(p.first);
+    ASSERT_EQ(0, std::distance(r.first, r.second));
+  }
+  m.insert(values.begin(), values.end());
+  for (const auto& p : values) {
+    auto r = m.equal_range(p.first);
+    ASSERT_EQ(1, std::distance(r.first, r.second));
+    EXPECT_EQ(p.second, get<1>(*r.first)) << ::testing::PrintToString(p.first);
+  }
+}
+
+REGISTER_TYPED_TEST_CASE_P(LookupTest, At, OperatorBracket, Count, Find,
+                           EqualRange);
+
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_LOOKUP_TEST_H_
diff --git a/absl/container/internal/unordered_map_modifiers_test.h b/absl/container/internal/unordered_map_modifiers_test.h
new file mode 100644
index 000000000000..b6c633ae2735
--- /dev/null
+++ b/absl/container/internal/unordered_map_modifiers_test.h
@@ -0,0 +1,272 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
+#define ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+
+namespace absl {
+namespace container_internal {
+
+template <class UnordMap>
+class ModifiersTest : public ::testing::Test {};
+
+TYPED_TEST_CASE_P(ModifiersTest);
+
+TYPED_TEST_P(ModifiersTest, Clear) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  m.clear();
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAre());
+  EXPECT_TRUE(m.empty());
+}
+
+TYPED_TEST_P(ModifiersTest, Insert) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  auto p = m.insert(val);
+  EXPECT_TRUE(p.second);
+  EXPECT_EQ(val, *p.first);
+  T val2 = {val.first, hash_internal::Generator<V>()()};
+  p = m.insert(val2);
+  EXPECT_FALSE(p.second);
+  EXPECT_EQ(val, *p.first);
+}
+
+TYPED_TEST_P(ModifiersTest, InsertHint) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  auto it = m.insert(m.end(), val);
+  EXPECT_TRUE(it != m.end());
+  EXPECT_EQ(val, *it);
+  T val2 = {val.first, hash_internal::Generator<V>()()};
+  it = m.insert(it, val2);
+  EXPECT_TRUE(it != m.end());
+  EXPECT_EQ(val, *it);
+}
+
+TYPED_TEST_P(ModifiersTest, InsertRange) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  m.insert(values.begin(), values.end());
+  ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+TYPED_TEST_P(ModifiersTest, InsertOrAssign) {
+#ifdef UNORDERED_MAP_CXX17
+  using std::get;
+  using K = typename TypeParam::key_type;
+  using V = typename TypeParam::mapped_type;
+  K k = hash_internal::Generator<K>()();
+  V val = hash_internal::Generator<V>()();
+  TypeParam m;
+  auto p = m.insert_or_assign(k, val);
+  EXPECT_TRUE(p.second);
+  EXPECT_EQ(k, get<0>(*p.first));
+  EXPECT_EQ(val, get<1>(*p.first));
+  V val2 = hash_internal::Generator<V>()();
+  p = m.insert_or_assign(k, val2);
+  EXPECT_FALSE(p.second);
+  EXPECT_EQ(k, get<0>(*p.first));
+  EXPECT_EQ(val2, get<1>(*p.first));
+#endif
+}
+
+TYPED_TEST_P(ModifiersTest, InsertOrAssignHint) {
+#ifdef UNORDERED_MAP_CXX17
+  using std::get;
+  using K = typename TypeParam::key_type;
+  using V = typename TypeParam::mapped_type;
+  K k = hash_internal::Generator<K>()();
+  V val = hash_internal::Generator<V>()();
+  TypeParam m;
+  auto it = m.insert_or_assign(m.end(), k, val);
+  EXPECT_TRUE(it != m.end());
+  EXPECT_EQ(k, get<0>(*it));
+  EXPECT_EQ(val, get<1>(*it));
+  V val2 = hash_internal::Generator<V>()();
+  it = m.insert_or_assign(it, k, val2);
+  EXPECT_EQ(k, get<0>(*it));
+  EXPECT_EQ(val2, get<1>(*it));
+#endif
+}
+
+TYPED_TEST_P(ModifiersTest, Emplace) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
+  // with test traits/policy.
+  auto p = m.emplace(val);
+  EXPECT_TRUE(p.second);
+  EXPECT_EQ(val, *p.first);
+  T val2 = {val.first, hash_internal::Generator<V>()()};
+  p = m.emplace(val2);
+  EXPECT_FALSE(p.second);
+  EXPECT_EQ(val, *p.first);
+}
+
+TYPED_TEST_P(ModifiersTest, EmplaceHint) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
+  // with test traits/policy.
+  auto it = m.emplace_hint(m.end(), val);
+  EXPECT_EQ(val, *it);
+  T val2 = {val.first, hash_internal::Generator<V>()()};
+  it = m.emplace_hint(it, val2);
+  EXPECT_EQ(val, *it);
+}
+
+TYPED_TEST_P(ModifiersTest, TryEmplace) {
+#ifdef UNORDERED_MAP_CXX17
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
+  // with test traits/policy.
+  auto p = m.try_emplace(val.first, val.second);
+  EXPECT_TRUE(p.second);
+  EXPECT_EQ(val, *p.first);
+  T val2 = {val.first, hash_internal::Generator<V>()()};
+  p = m.try_emplace(val2.first, val2.second);
+  EXPECT_FALSE(p.second);
+  EXPECT_EQ(val, *p.first);
+#endif
+}
+
+TYPED_TEST_P(ModifiersTest, TryEmplaceHint) {
+#ifdef UNORDERED_MAP_CXX17
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
+  // with test traits/policy.
+  auto it = m.try_emplace(m.end(), val.first, val.second);
+  EXPECT_EQ(val, *it);
+  T val2 = {val.first, hash_internal::Generator<V>()()};
+  it = m.try_emplace(it, val2.first, val2.second);
+  EXPECT_EQ(val, *it);
+#endif
+}
+
+template <class V>
+using IfNotVoid = typename std::enable_if<!std::is_void<V>::value, V>::type;
+
+// In openmap we chose not to return the iterator from erase because that's
+// more expensive. As such we adapt erase to return an iterator here.
+struct EraseFirst {
+  template <class Map>
+  auto operator()(Map* m, int) const
+      -> IfNotVoid<decltype(m->erase(m->begin()))> {
+    return m->erase(m->begin());
+  }
+  template <class Map>
+  typename Map::iterator operator()(Map* m, ...) const {
+    auto it = m->begin();
+    m->erase(it++);
+    return it;
+  }
+};
+
+TYPED_TEST_P(ModifiersTest, Erase) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using std::get;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  auto& first = *m.begin();
+  std::vector<T> values2;
+  for (const auto& val : values)
+    if (get<0>(val) != get<0>(first)) values2.push_back(val);
+  auto it = EraseFirst()(&m, 0);
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(1, std::count(values2.begin(), values2.end(), *it));
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values2.begin(),
+                                                             values2.end()));
+}
+
+TYPED_TEST_P(ModifiersTest, EraseRange) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  auto it = m.erase(m.begin(), m.end());
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAre());
+  EXPECT_TRUE(it == m.end());
+}
+
+TYPED_TEST_P(ModifiersTest, EraseKey) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_EQ(1, m.erase(values[0].first));
+  EXPECT_EQ(0, std::count(m.begin(), m.end(), values[0]));
+  EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values.begin() + 1,
+                                                             values.end()));
+}
+
+TYPED_TEST_P(ModifiersTest, Swap) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> v1;
+  std::vector<T> v2;
+  std::generate_n(std::back_inserter(v1), 5, hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(v2), 5, hash_internal::Generator<T>());
+  TypeParam m1(v1.begin(), v1.end());
+  TypeParam m2(v2.begin(), v2.end());
+  EXPECT_THAT(items(m1), ::testing::UnorderedElementsAreArray(v1));
+  EXPECT_THAT(items(m2), ::testing::UnorderedElementsAreArray(v2));
+  m1.swap(m2);
+  EXPECT_THAT(items(m1), ::testing::UnorderedElementsAreArray(v2));
+  EXPECT_THAT(items(m2), ::testing::UnorderedElementsAreArray(v1));
+}
+
+// TODO(alkis): Write tests for extract.
+// TODO(alkis): Write tests for merge.
+
+REGISTER_TYPED_TEST_CASE_P(ModifiersTest, Clear, Insert, InsertHint,
+                           InsertRange, InsertOrAssign, InsertOrAssignHint,
+                           Emplace, EmplaceHint, TryEmplace, TryEmplaceHint,
+                           Erase, EraseRange, EraseKey, Swap);
+
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
diff --git a/absl/container/internal/unordered_map_test.cc b/absl/container/internal/unordered_map_test.cc
new file mode 100644
index 000000000000..40e799cda89c
--- /dev/null
+++ b/absl/container/internal/unordered_map_test.cc
@@ -0,0 +1,38 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <unordered_map>
+
+#include "absl/container/internal/unordered_map_constructor_test.h"
+#include "absl/container/internal/unordered_map_lookup_test.h"
+#include "absl/container/internal/unordered_map_modifiers_test.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using MapTypes = ::testing::Types<
+    std::unordered_map<int, int, StatefulTestingHash, StatefulTestingEqual,
+                       Alloc<std::pair<const int, int>>>,
+    std::unordered_map<std::string, std::string, StatefulTestingHash,
+                       StatefulTestingEqual,
+                       Alloc<std::pair<const std::string, std::string>>>>;
+
+INSTANTIATE_TYPED_TEST_CASE_P(UnorderedMap, ConstructorTest, MapTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(UnorderedMap, LookupTest, MapTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(UnorderedMap, ModifiersTest, MapTypes);
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/internal/unordered_set_constructor_test.h b/absl/container/internal/unordered_set_constructor_test.h
new file mode 100644
index 000000000000..cb593704685c
--- /dev/null
+++ b/absl/container/internal/unordered_set_constructor_test.h
@@ -0,0 +1,408 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_SET_CONSTRUCTOR_TEST_H_
+#define ABSL_CONTAINER_INTERNAL_UNORDERED_SET_CONSTRUCTOR_TEST_H_
+
+#include <algorithm>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+
+namespace absl {
+namespace container_internal {
+
+template <class UnordMap>
+class ConstructorTest : public ::testing::Test {};
+
+TYPED_TEST_CASE_P(ConstructorTest);
+
+TYPED_TEST_P(ConstructorTest, NoArgs) {
+  TypeParam m;
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCount) {
+  TypeParam m(123);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHash) {
+  using H = typename TypeParam::hasher;
+  H hasher;
+  TypeParam m(123, hasher);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHashEqual) {
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  H hasher;
+  E equal;
+  TypeParam m(123, hasher, equal);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHashEqualAlloc) {
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+
+  const auto& cm = m;
+  EXPECT_EQ(cm.hash_function(), hasher);
+  EXPECT_EQ(cm.key_eq(), equal);
+  EXPECT_EQ(cm.get_allocator(), alloc);
+  EXPECT_TRUE(cm.empty());
+  EXPECT_THAT(keys(cm), ::testing::UnorderedElementsAre());
+  EXPECT_GE(cm.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountAlloc) {
+#if defined(UNORDERED_SET_CXX14) || defined(UNORDERED_SET_CXX17)
+  using A = typename TypeParam::allocator_type;
+  A alloc(0);
+  TypeParam m(123, alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, BucketCountHashAlloc) {
+#if defined(UNORDERED_SET_CXX14) || defined(UNORDERED_SET_CXX17)
+  using H = typename TypeParam::hasher;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  A alloc(0);
+  TypeParam m(123, hasher, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, BucketAlloc) {
+#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
+  using A = typename TypeParam::allocator_type;
+  A alloc(0);
+  TypeParam m(alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_TRUE(m.empty());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  std::vector<T> values;
+  for (size_t i = 0; i != 10; ++i)
+    values.push_back(hash_internal::Generator<T>()());
+  TypeParam m(values.begin(), values.end(), 123, hasher, equal, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, InputIteratorBucketAlloc) {
+#if defined(UNORDERED_SET_CXX14) || defined(UNORDERED_SET_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using A = typename TypeParam::allocator_type;
+  A alloc(0);
+  std::vector<T> values;
+  for (size_t i = 0; i != 10; ++i)
+    values.push_back(hash_internal::Generator<T>()());
+  TypeParam m(values.begin(), values.end(), 123, alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashAlloc) {
+#if defined(UNORDERED_SET_CXX14) || defined(UNORDERED_SET_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  A alloc(0);
+  std::vector<T> values;
+  for (size_t i = 0; i != 10; ++i)
+    values.push_back(hash_internal::Generator<T>()());
+  TypeParam m(values.begin(), values.end(), 123, hasher, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, CopyConstructor) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam n(m);
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, CopyConstructorAlloc) {
+#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam n(m, A(11));
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_NE(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+#endif
+}
+
+// TODO(alkis): Test non-propagating allocators on copy constructors.
+
+TYPED_TEST_P(ConstructorTest, MoveConstructor) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam t(m);
+  TypeParam n(std::move(t));
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, MoveConstructorAlloc) {
+#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(123, hasher, equal, alloc);
+  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  TypeParam t(m);
+  TypeParam n(std::move(t), A(1));
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_NE(m.get_allocator(), n.get_allocator());
+  EXPECT_EQ(m, n);
+#endif
+}
+
+// TODO(alkis): Test non-propagating allocators on move constructors.
+
+TYPED_TEST_P(ConstructorTest, InitializerListBucketHashEqualAlloc) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  TypeParam m(values, 123, hasher, equal, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.key_eq(), equal);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+}
+
+TYPED_TEST_P(ConstructorTest, InitializerListBucketAlloc) {
+#if defined(UNORDERED_SET_CXX14) || defined(UNORDERED_SET_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using A = typename TypeParam::allocator_type;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  A alloc(0);
+  TypeParam m(values, 123, alloc);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, InitializerListBucketHashAlloc) {
+#if defined(UNORDERED_SET_CXX14) || defined(UNORDERED_SET_CXX17)
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  A alloc(0);
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m(values, 123, hasher, alloc);
+  EXPECT_EQ(m.hash_function(), hasher);
+  EXPECT_EQ(m.get_allocator(), alloc);
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_GE(m.bucket_count(), 123);
+#endif
+}
+
+TYPED_TEST_P(ConstructorTest, Assignment) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
+  TypeParam n;
+  n = m;
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m, n);
+}
+
+// TODO(alkis): Test [non-]propagating allocators on move/copy assignments
+// (it depends on traits).
+
+TYPED_TEST_P(ConstructorTest, MoveAssignment) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using H = typename TypeParam::hasher;
+  using E = typename TypeParam::key_equal;
+  using A = typename TypeParam::allocator_type;
+  H hasher;
+  E equal;
+  A alloc(0);
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
+  TypeParam t(m);
+  TypeParam n;
+  n = std::move(t);
+  EXPECT_EQ(m.hash_function(), n.hash_function());
+  EXPECT_EQ(m.key_eq(), n.key_eq());
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerList) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m;
+  m = values;
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentOverwritesExisting) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()});
+  TypeParam n({gen()});
+  n = m;
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, MoveAssignmentOverwritesExisting) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  TypeParam m({gen(), gen(), gen()});
+  TypeParam t(m);
+  TypeParam n({gen()});
+  n = std::move(t);
+  EXPECT_EQ(m, n);
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerListOverwritesExisting) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m;
+  m = values;
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+TYPED_TEST_P(ConstructorTest, AssignmentOnSelf) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  hash_internal::Generator<T> gen;
+  std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
+  TypeParam m(values);
+  m = *&m;  // Avoid -Wself-assign.
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+REGISTER_TYPED_TEST_CASE_P(
+    ConstructorTest, NoArgs, BucketCount, BucketCountHash, BucketCountHashEqual,
+    BucketCountHashEqualAlloc, BucketCountAlloc, BucketCountHashAlloc,
+    BucketAlloc, InputIteratorBucketHashEqualAlloc, InputIteratorBucketAlloc,
+    InputIteratorBucketHashAlloc, CopyConstructor, CopyConstructorAlloc,
+    MoveConstructor, MoveConstructorAlloc, InitializerListBucketHashEqualAlloc,
+    InitializerListBucketAlloc, InitializerListBucketHashAlloc, Assignment,
+    MoveAssignment, AssignmentFromInitializerList,
+    AssignmentOverwritesExisting, MoveAssignmentOverwritesExisting,
+    AssignmentFromInitializerListOverwritesExisting, AssignmentOnSelf);
+
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_CONSTRUCTOR_TEST_H_
diff --git a/absl/container/internal/unordered_set_lookup_test.h b/absl/container/internal/unordered_set_lookup_test.h
new file mode 100644
index 000000000000..aca9c6a5df7b
--- /dev/null
+++ b/absl/container/internal/unordered_set_lookup_test.h
@@ -0,0 +1,88 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_SET_LOOKUP_TEST_H_
+#define ABSL_CONTAINER_INTERNAL_UNORDERED_SET_LOOKUP_TEST_H_
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+
+namespace absl {
+namespace container_internal {
+
+template <class UnordSet>
+class LookupTest : public ::testing::Test {};
+
+TYPED_TEST_CASE_P(LookupTest);
+
+TYPED_TEST_P(LookupTest, Count) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  for (const auto& v : values)
+    EXPECT_EQ(0, m.count(v)) << ::testing::PrintToString(v);
+  m.insert(values.begin(), values.end());
+  for (const auto& v : values)
+    EXPECT_EQ(1, m.count(v)) << ::testing::PrintToString(v);
+}
+
+TYPED_TEST_P(LookupTest, Find) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  for (const auto& v : values)
+    EXPECT_TRUE(m.end() == m.find(v)) << ::testing::PrintToString(v);
+  m.insert(values.begin(), values.end());
+  for (const auto& v : values) {
+    typename TypeParam::iterator it = m.find(v);
+    static_assert(std::is_same<const typename TypeParam::value_type&,
+                               decltype(*it)>::value,
+                  "");
+    static_assert(std::is_same<const typename TypeParam::value_type*,
+                               decltype(it.operator->())>::value,
+                  "");
+    EXPECT_TRUE(m.end() != it) << ::testing::PrintToString(v);
+    EXPECT_EQ(v, *it) << ::testing::PrintToString(v);
+  }
+}
+
+TYPED_TEST_P(LookupTest, EqualRange) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  for (const auto& v : values) {
+    auto r = m.equal_range(v);
+    ASSERT_EQ(0, std::distance(r.first, r.second));
+  }
+  m.insert(values.begin(), values.end());
+  for (const auto& v : values) {
+    auto r = m.equal_range(v);
+    ASSERT_EQ(1, std::distance(r.first, r.second));
+    EXPECT_EQ(v, *r.first);
+  }
+}
+
+REGISTER_TYPED_TEST_CASE_P(LookupTest, Count, Find, EqualRange);
+
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_LOOKUP_TEST_H_
diff --git a/absl/container/internal/unordered_set_modifiers_test.h b/absl/container/internal/unordered_set_modifiers_test.h
new file mode 100644
index 000000000000..9beacf331697
--- /dev/null
+++ b/absl/container/internal/unordered_set_modifiers_test.h
@@ -0,0 +1,187 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MODIFIERS_TEST_H_
+#define ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MODIFIERS_TEST_H_
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+
+namespace absl {
+namespace container_internal {
+
+template <class UnordSet>
+class ModifiersTest : public ::testing::Test {};
+
+TYPED_TEST_CASE_P(ModifiersTest);
+
+TYPED_TEST_P(ModifiersTest, Clear) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  m.clear();
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_TRUE(m.empty());
+}
+
+TYPED_TEST_P(ModifiersTest, Insert) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  auto p = m.insert(val);
+  EXPECT_TRUE(p.second);
+  EXPECT_EQ(val, *p.first);
+  p = m.insert(val);
+  EXPECT_FALSE(p.second);
+}
+
+TYPED_TEST_P(ModifiersTest, InsertHint) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  auto it = m.insert(m.end(), val);
+  EXPECT_TRUE(it != m.end());
+  EXPECT_EQ(val, *it);
+  it = m.insert(it, val);
+  EXPECT_TRUE(it != m.end());
+  EXPECT_EQ(val, *it);
+}
+
+TYPED_TEST_P(ModifiersTest, InsertRange) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m;
+  m.insert(values.begin(), values.end());
+  ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+}
+
+TYPED_TEST_P(ModifiersTest, Emplace) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
+  // with test traits/policy.
+  auto p = m.emplace(val);
+  EXPECT_TRUE(p.second);
+  EXPECT_EQ(val, *p.first);
+  p = m.emplace(val);
+  EXPECT_FALSE(p.second);
+  EXPECT_EQ(val, *p.first);
+}
+
+TYPED_TEST_P(ModifiersTest, EmplaceHint) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
+  // with test traits/policy.
+  auto it = m.emplace_hint(m.end(), val);
+  EXPECT_EQ(val, *it);
+  it = m.emplace_hint(it, val);
+  EXPECT_EQ(val, *it);
+}
+
+template <class V>
+using IfNotVoid = typename std::enable_if<!std::is_void<V>::value, V>::type;
+
+// In openmap we chose not to return the iterator from erase because that's
+// more expensive. As such we adapt erase to return an iterator here.
+struct EraseFirst {
+  template <class Map>
+  auto operator()(Map* m, int) const
+      -> IfNotVoid<decltype(m->erase(m->begin()))> {
+    return m->erase(m->begin());
+  }
+  template <class Map>
+  typename Map::iterator operator()(Map* m, ...) const {
+    auto it = m->begin();
+    m->erase(it++);
+    return it;
+  }
+};
+
+TYPED_TEST_P(ModifiersTest, Erase) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  std::vector<T> values2;
+  for (const auto& val : values)
+    if (val != *m.begin()) values2.push_back(val);
+  auto it = EraseFirst()(&m, 0);
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(1, std::count(values2.begin(), values2.end(), *it));
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values2.begin(),
+                                                            values2.end()));
+}
+
+TYPED_TEST_P(ModifiersTest, EraseRange) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  auto it = m.erase(m.begin(), m.end());
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
+  EXPECT_TRUE(it == m.end());
+}
+
+TYPED_TEST_P(ModifiersTest, EraseKey) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> values;
+  std::generate_n(std::back_inserter(values), 10,
+                  hash_internal::Generator<T>());
+  TypeParam m(values.begin(), values.end());
+  ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
+  EXPECT_EQ(1, m.erase(values[0]));
+  EXPECT_EQ(0, std::count(m.begin(), m.end(), values[0]));
+  EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values.begin() + 1,
+                                                            values.end()));
+}
+
+TYPED_TEST_P(ModifiersTest, Swap) {
+  using T = hash_internal::GeneratedType<TypeParam>;
+  std::vector<T> v1;
+  std::vector<T> v2;
+  std::generate_n(std::back_inserter(v1), 5, hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(v2), 5, hash_internal::Generator<T>());
+  TypeParam m1(v1.begin(), v1.end());
+  TypeParam m2(v2.begin(), v2.end());
+  EXPECT_THAT(keys(m1), ::testing::UnorderedElementsAreArray(v1));
+  EXPECT_THAT(keys(m2), ::testing::UnorderedElementsAreArray(v2));
+  m1.swap(m2);
+  EXPECT_THAT(keys(m1), ::testing::UnorderedElementsAreArray(v2));
+  EXPECT_THAT(keys(m2), ::testing::UnorderedElementsAreArray(v1));
+}
+
+// TODO(alkis): Write tests for extract.
+// TODO(alkis): Write tests for merge.
+
+REGISTER_TYPED_TEST_CASE_P(ModifiersTest, Clear, Insert, InsertHint,
+                           InsertRange, Emplace, EmplaceHint, Erase, EraseRange,
+                           EraseKey, Swap);
+
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MODIFIERS_TEST_H_
diff --git a/absl/container/internal/unordered_set_test.cc b/absl/container/internal/unordered_set_test.cc
new file mode 100644
index 000000000000..1281ce53d5e2
--- /dev/null
+++ b/absl/container/internal/unordered_set_test.cc
@@ -0,0 +1,37 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <unordered_set>
+
+#include "absl/container/internal/unordered_set_constructor_test.h"
+#include "absl/container/internal/unordered_set_lookup_test.h"
+#include "absl/container/internal/unordered_set_modifiers_test.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using SetTypes =
+    ::testing::Types<std::unordered_set<int, StatefulTestingHash,
+                                        StatefulTestingEqual, Alloc<int>>,
+                     std::unordered_set<std::string, StatefulTestingHash,
+                                        StatefulTestingEqual, Alloc<std::string>>>;
+
+INSTANTIATE_TYPED_TEST_CASE_P(UnorderedSet, ConstructorTest, SetTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(UnorderedSet, LookupTest, SetTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(UnorderedSet, ModifiersTest, SetTypes);
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/node_hash_map.h b/absl/container/node_hash_map.h
new file mode 100644
index 000000000000..3c7de1e8885f
--- /dev/null
+++ b/absl/container/node_hash_map.h
@@ -0,0 +1,530 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: node_hash_map.h
+// -----------------------------------------------------------------------------
+//
+// An `absl::node_hash_map<K, V>` is an unordered associative container of
+// unique keys and associated values designed to be a more efficient replacement
+// for `std::unordered_map`. Like `unordered_map`, search, insertion, and
+// deletion of map elements can be done as an `O(1)` operation. However,
+// `node_hash_map` (and other unordered associative containers known as the
+// collection of Abseil "Swiss tables") contain other optimizations that result
+// in both memory and computation advantages.
+//
+// In most cases, your default choice for a hash map should be a map of type
+// `flat_hash_map`. However, if you need pointer stability and cannot store
+// a `flat_hash_map` with `unique_ptr` elements, a `node_hash_map` may be a
+// valid alternative. As well, if you are migrating your code from using
+// `std::unordered_map`, a `node_hash_map` provides a more straightforward
+// migration, because it guarantees pointer stability. Consider migrating to
+// `node_hash_map` and perhaps converting to a more efficient `flat_hash_map`
+// upon further review.
+
+#ifndef ABSL_CONTAINER_NODE_HASH_MAP_H_
+#define ABSL_CONTAINER_NODE_HASH_MAP_H_
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
+#include "absl/container/internal/node_hash_policy.h"
+#include "absl/container/internal/raw_hash_map.h"  // IWYU pragma: export
+#include "absl/memory/memory.h"
+
+namespace absl {
+namespace container_internal {
+template <class Key, class Value>
+class NodeHashMapPolicy;
+}  // namespace container_internal
+
+// -----------------------------------------------------------------------------
+// absl::node_hash_map
+// -----------------------------------------------------------------------------
+//
+// An `absl::node_hash_map<K, V>` is an unordered associative container which
+// has been optimized for both speed and memory footprint in most common use
+// cases. Its interface is similar to that of `std::unordered_map<K, V>` with
+// the following notable differences:
+//
+// * Supports heterogeneous lookup, through `find()`, `operator[]()` and
+//   `insert()`, provided that the map is provided a compatible heterogeneous
+//   hashing function and equality operator.
+// * Contains a `capacity()` member function indicating the number of element
+//   slots (open, deleted, and empty) within the hash map.
+// * Returns `void` from the `erase(iterator)` overload.
+//
+// By default, `node_hash_map` uses the `absl::Hash` hashing framework.
+// All fundamental and Abseil types that support the `absl::Hash` framework have
+// a compatible equality operator for comparing insertions into `node_hash_map`.
+// If your type is not yet supported by the `asbl::Hash` framework, see
+// absl/hash/hash.h for information on extending Abseil hashing to user-defined
+// types.
+//
+// Example:
+//
+//   // Create a node hash map of three strings (that map to strings)
+//   absl::node_hash_map<std::string, std::string> ducks =
+//     {{"a", "huey"}, {"b", "dewey"}, {"c", "louie"}};
+//
+//  // Insert a new element into the node hash map
+//  ducks.insert({"d", "donald"}};
+//
+//  // Force a rehash of the node hash map
+//  ducks.rehash(0);
+//
+//  // Find the element with the key "b"
+//  std::string search_key = "b";
+//  auto result = ducks.find(search_key);
+//  if (result != ducks.end()) {
+//    std::cout << "Result: " << search_key->second << std::endl;
+//  }
+template <class Key, class Value,
+          class Hash = absl::container_internal::hash_default_hash<Key>,
+          class Eq = absl::container_internal::hash_default_eq<Key>,
+          class Alloc = std::allocator<std::pair<const Key, Value>>>
+class node_hash_map
+    : public absl::container_internal::raw_hash_map<
+          absl::container_internal::NodeHashMapPolicy<Key, Value>, Hash, Eq,
+          Alloc> {
+  using Base = typename node_hash_map::raw_hash_map;
+
+ public:
+  node_hash_map() {}
+  using Base::Base;
+
+  // node_hash_map::begin()
+  //
+  // Returns an iterator to the beginning of the `node_hash_map`.
+  using Base::begin;
+
+  // node_hash_map::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `node_hash_map`.
+  using Base::cbegin;
+
+  // node_hash_map::cend()
+  //
+  // Returns a const iterator to the end of the `node_hash_map`.
+  using Base::cend;
+
+  // node_hash_map::end()
+  //
+  // Returns an iterator to the end of the `node_hash_map`.
+  using Base::end;
+
+  // node_hash_map::capacity()
+  //
+  // Returns the number of element slots (assigned, deleted, and empty)
+  // available within the `node_hash_map`.
+  //
+  // NOTE: this member function is particular to `absl::node_hash_map` and is
+  // not provided in the `std::unordered_map` API.
+  using Base::capacity;
+
+  // node_hash_map::empty()
+  //
+  // Returns whether or not the `node_hash_map` is empty.
+  using Base::empty;
+
+  // node_hash_map::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `node_hash_map` under current memory constraints. This value can be thought
+  // of as the largest value of `std::distance(begin(), end())` for a
+  // `node_hash_map<K, V>`.
+  using Base::max_size;
+
+  // node_hash_map::size()
+  //
+  // Returns the number of elements currently within the `node_hash_map`.
+  using Base::size;
+
+  // node_hash_map::clear()
+  //
+  // Removes all elements from the `node_hash_map`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  //
+  // NOTE: this operation may shrink the underlying buffer. To avoid shrinking
+  // the underlying buffer call `erase(begin(), end())`.
+  using Base::clear;
+
+  // node_hash_map::erase()
+  //
+  // Erases elements within the `node_hash_map`. Erasing does not trigger a
+  // rehash. Overloads are listed below.
+  //
+  // void erase(const_iterator pos):
+  //
+  //   Erases the element at `position` of the `node_hash_map`, returning
+  //   `void`.
+  //
+  //   NOTE: this return behavior is different than that of STL containers in
+  //   general and `std::unordered_map` in particular.
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning an
+  //   iterator pointing to `last`.
+  //
+  // size_type erase(const key_type& key):
+  //
+  //   Erases the element with the matching key, if it exists.
+  using Base::erase;
+
+  // node_hash_map::insert()
+  //
+  // Inserts an element of the specified value into the `node_hash_map`,
+  // returning an iterator pointing to the newly inserted element, provided that
+  // an element with the given key does not already exist. If rehashing occurs
+  // due to the insertion, all iterators are invalidated. Overloads are listed
+  // below.
+  //
+  // std::pair<iterator,bool> insert(const init_type& value):
+  //
+  //   Inserts a value into the `node_hash_map`. Returns a pair consisting of an
+  //   iterator to the inserted element (or to the element that prevented the
+  //   insertion) and a `bool` denoting whether the insertion took place.
+  //
+  // std::pair<iterator,bool> insert(T&& value):
+  // std::pair<iterator,bool> insert(init_type&& value ):
+  //
+  //   Inserts a moveable value into the `node_hash_map`. Returns a `std::pair`
+  //   consisting of an iterator to the inserted element (or to the element that
+  //   prevented the insertion) and a `bool` denoting whether the insertion took
+  //   place.
+  //
+  // iterator insert(const_iterator hint, const init_type& value):
+  // iterator insert(const_iterator hint, T&& value):
+  // iterator insert(const_iterator hint, init_type&& value );
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element, or to the existing element that prevented the
+  //   insertion.
+  //
+  // void insert(InputIterator first, InputIterator last ):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently, for `node_hash_map` we guarantee the
+  //   first match is inserted.
+  //
+  // void insert(std::initializer_list<init_type> ilist ):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently within the initializer list, for
+  //   `node_hash_map` we guarantee the first match is inserted.
+  using Base::insert;
+
+  // node_hash_map::insert_or_assign()
+  //
+  // Inserts an element of the specified value into the `node_hash_map` provided
+  // that a value with the given key does not already exist, or replaces it with
+  // the element value if a key for that value already exists, returning an
+  // iterator pointing to the newly inserted element. If rehashing occurs due to
+  // the insertion, all iterators are invalidated. Overloads are listed
+  // below.
+  //
+  // std::pair<iterator, bool> insert_or_assign(const init_type& k, T&& obj):
+  // std::pair<iterator, bool> insert_or_assign(init_type&& k, T&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `node_hash_map`.
+  //
+  // iterator insert_or_assign(const_iterator hint,
+  //                           const init_type& k, T&& obj):
+  // iterator insert_or_assign(const_iterator hint, init_type&& k, T&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `node_hash_map` using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search.
+  using Base::insert_or_assign;
+
+  // node_hash_map::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `node_hash_map`, provided that no element with the given key
+  // already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace;
+
+  // node_hash_map::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `node_hash_map`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search, and only inserts
+  // provided that no element with the given key already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace_hint;
+
+  // node_hash_map::try_emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `node_hash_map`, provided that no element with the given key
+  // already exists. Unlike `emplace()`, if an element with the given key
+  // already exists, we guarantee that no element is constructed.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  // Overloads are listed below.
+  //
+  //   std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args):
+  //   std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args):
+  //
+  // Inserts (via copy or move) the element of the specified key into the
+  // `node_hash_map`.
+  //
+  //   iterator try_emplace(const_iterator hint,
+  //                        const init_type& k, Args&&... args):
+  //   iterator try_emplace(const_iterator hint, init_type&& k, Args&&... args):
+  //
+  // Inserts (via copy or move) the element of the specified key into the
+  // `node_hash_map` using the position of `hint` as a non-binding suggestion
+  // for where to begin the insertion search.
+  using Base::try_emplace;
+
+  // node_hash_map::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the key,value pair of the element at the indicated position and
+  //   returns a node handle owning that extracted data.
+  //
+  // node_type extract(const key_type& x):
+  //
+  //   Extracts the key,value pair of the element with a key matching the passed
+  //   key value and returns a node handle owning that extracted data. If the
+  //   `node_hash_map` does not contain an element with a matching key, this
+  //   function returns an empty node handle.
+  using Base::extract;
+
+  // node_hash_map::merge()
+  //
+  // Extracts elements from a given `source` node hash map into this
+  // `node_hash_map`. If the destination `node_hash_map` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // node_hash_map::swap(node_hash_map& other)
+  //
+  // Exchanges the contents of this `node_hash_map` with those of the `other`
+  // node hash map, avoiding invocation of any move, copy, or swap operations on
+  // individual elements.
+  //
+  // All iterators and references on the `node_hash_map` remain valid, excepting
+  // for the past-the-end iterator, which is invalidated.
+  //
+  // `swap()` requires that the node hash map's hashing and key equivalence
+  // functions be Swappable, and are exchaged using unqualified calls to
+  // non-member `swap()`. If the map's allocator has
+  // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
+  // set to `true`, the allocators are also exchanged using an unqualified call
+  // to non-member `swap()`; otherwise, the allocators are not swapped.
+  using Base::swap;
+
+  // node_hash_map::rehash(count)
+  //
+  // Rehashes the `node_hash_map`, setting the number of slots to be at least
+  // the passed value. If the new number of slots increases the load factor more
+  // than the current maximum load factor
+  // (`count` < `size()` / `max_load_factor()`), then the new number of slots
+  // will be at least `size()` / `max_load_factor()`.
+  //
+  // To force a rehash, pass rehash(0).
+  using Base::rehash;
+
+  // node_hash_map::reserve(count)
+  //
+  // Sets the number of slots in the `node_hash_map` to the number needed to
+  // accommodate at least `count` total elements without exceeding the current
+  // maximum load factor, and may rehash the container if needed.
+  using Base::reserve;
+
+  // node_hash_map::at()
+  //
+  // Returns a reference to the mapped value of the element with key equivalent
+  // to the passed key.
+  using Base::at;
+
+  // node_hash_map::contains()
+  //
+  // Determines whether an element with a key comparing equal to the given `key`
+  // exists within the `node_hash_map`, returning `true` if so or `false`
+  // otherwise.
+  using Base::contains;
+
+  // node_hash_map::count(const Key& key) const
+  //
+  // Returns the number of elements with a key comparing equal to the given
+  // `key` within the `node_hash_map`. note that this function will return
+  // either `1` or `0` since duplicate keys are not allowed within a
+  // `node_hash_map`.
+  using Base::count;
+
+  // node_hash_map::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `node_hash_map`.
+  using Base::equal_range;
+
+  // node_hash_map::find()
+  //
+  // Finds an element with the passed `key` within the `node_hash_map`.
+  using Base::find;
+
+  // node_hash_map::operator[]()
+  //
+  // Returns a reference to the value mapped to the passed key within the
+  // `node_hash_map`, performing an `insert()` if the key does not already
+  // exist. If an insertion occurs and results in a rehashing of the container,
+  // all iterators are invalidated. Otherwise iterators are not affected and
+  // references are not invalidated. Overloads are listed below.
+  //
+  // T& operator[](const Key& key ):
+  //
+  //   Inserts an init_type object constructed in-place if the element with the
+  //   given key does not exist.
+  //
+  // T& operator[]( Key&& key ):
+  //
+  //   Inserts an init_type object constructed in-place provided that an element
+  //   with the given key does not exist.
+  using Base::operator[];
+
+  // node_hash_map::bucket_count()
+  //
+  // Returns the number of "buckets" within the `node_hash_map`.
+  using Base::bucket_count;
+
+  // node_hash_map::load_factor()
+  //
+  // Returns the current load factor of the `node_hash_map` (the average number
+  // of slots occupied with a value within the hash map).
+  using Base::load_factor;
+
+  // node_hash_map::max_load_factor()
+  //
+  // Manages the maximum load factor of the `node_hash_map`. Overloads are
+  // listed below.
+  //
+  // float node_hash_map::max_load_factor()
+  //
+  //   Returns the current maximum load factor of the `node_hash_map`.
+  //
+  // void node_hash_map::max_load_factor(float ml)
+  //
+  //   Sets the maximum load factor of the `node_hash_map` to the passed value.
+  //
+  //   NOTE: This overload is provided only for API compatibility with the STL;
+  //   `node_hash_map` will ignore any set load factor and manage its rehashing
+  //   internally as an implementation detail.
+  using Base::max_load_factor;
+
+  // node_hash_map::get_allocator()
+  //
+  // Returns the allocator function associated with this `node_hash_map`.
+  using Base::get_allocator;
+
+  // node_hash_map::hash_function()
+  //
+  // Returns the hashing function used to hash the keys within this
+  // `node_hash_map`.
+  using Base::hash_function;
+
+  // node_hash_map::key_eq()
+  //
+  // Returns the function used for comparing keys equality.
+  using Base::key_eq;
+
+  ABSL_DEPRECATED("Call `hash_function()` instead.")
+  typename Base::hasher hash_funct() { return this->hash_function(); }
+
+  ABSL_DEPRECATED("Call `rehash()` instead.")
+  void resize(typename Base::size_type hint) { this->rehash(hint); }
+};
+
+namespace container_internal {
+
+template <class Key, class Value>
+class NodeHashMapPolicy
+    : public absl::container_internal::node_hash_policy<
+          std::pair<const Key, Value>&, NodeHashMapPolicy<Key, Value>> {
+  using value_type = std::pair<const Key, Value>;
+
+ public:
+  using key_type = Key;
+  using mapped_type = Value;
+  using init_type = std::pair</*non const*/ key_type, mapped_type>;
+
+  template <class Allocator, class... Args>
+  static value_type* new_element(Allocator* alloc, Args&&... args) {
+    using PairAlloc = typename absl::allocator_traits<
+        Allocator>::template rebind_alloc<value_type>;
+    PairAlloc pair_alloc(*alloc);
+    value_type* res =
+        absl::allocator_traits<PairAlloc>::allocate(pair_alloc, 1);
+    absl::allocator_traits<PairAlloc>::construct(pair_alloc, res,
+                                                 std::forward<Args>(args)...);
+    return res;
+  }
+
+  template <class Allocator>
+  static void delete_element(Allocator* alloc, value_type* pair) {
+    using PairAlloc = typename absl::allocator_traits<
+        Allocator>::template rebind_alloc<value_type>;
+    PairAlloc pair_alloc(*alloc);
+    absl::allocator_traits<PairAlloc>::destroy(pair_alloc, pair);
+    absl::allocator_traits<PairAlloc>::deallocate(pair_alloc, pair, 1);
+  }
+
+  template <class F, class... Args>
+  static decltype(absl::container_internal::DecomposePair(
+      std::declval<F>(), std::declval<Args>()...))
+  apply(F&& f, Args&&... args) {
+    return absl::container_internal::DecomposePair(std::forward<F>(f),
+                                                   std::forward<Args>(args)...);
+  }
+
+  static size_t element_space_used(const value_type*) {
+    return sizeof(value_type);
+  }
+
+  static Value& value(value_type* elem) { return elem->second; }
+  static const Value& value(const value_type* elem) { return elem->second; }
+};
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_NODE_HASH_MAP_H_
diff --git a/absl/container/node_hash_map_test.cc b/absl/container/node_hash_map_test.cc
new file mode 100644
index 000000000000..bd789644c24e
--- /dev/null
+++ b/absl/container/node_hash_map_test.cc
@@ -0,0 +1,218 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/node_hash_map.h"
+
+#include "absl/container/internal/tracked.h"
+#include "absl/container/internal/unordered_map_constructor_test.h"
+#include "absl/container/internal/unordered_map_lookup_test.h"
+#include "absl/container/internal/unordered_map_modifiers_test.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+
+using ::testing::Field;
+using ::testing::Pair;
+using ::testing::UnorderedElementsAre;
+
+using MapTypes = ::testing::Types<
+    absl::node_hash_map<int, int, StatefulTestingHash, StatefulTestingEqual,
+                        Alloc<std::pair<const int, int>>>,
+    absl::node_hash_map<std::string, std::string, StatefulTestingHash,
+                        StatefulTestingEqual,
+                        Alloc<std::pair<const std::string, std::string>>>>;
+
+INSTANTIATE_TYPED_TEST_CASE_P(NodeHashMap, ConstructorTest, MapTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(NodeHashMap, LookupTest, MapTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(NodeHashMap, ModifiersTest, MapTypes);
+
+using M = absl::node_hash_map<std::string, Tracked<int>>;
+
+TEST(NodeHashMap, Emplace) {
+  M m;
+  Tracked<int> t(53);
+  m.emplace("a", t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(1, t.num_copies());
+
+  m.emplace(std::string("a"), t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(1, t.num_copies());
+
+  std::string a("a");
+  m.emplace(a, t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(1, t.num_copies());
+
+  const std::string ca("a");
+  m.emplace(a, t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(1, t.num_copies());
+
+  m.emplace(std::make_pair("a", t));
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(2, t.num_copies());
+
+  m.emplace(std::make_pair(std::string("a"), t));
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(3, t.num_copies());
+
+  std::pair<std::string, Tracked<int>> p("a", t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(4, t.num_copies());
+  m.emplace(p);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(4, t.num_copies());
+
+  const std::pair<std::string, Tracked<int>> cp("a", t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(5, t.num_copies());
+  m.emplace(cp);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(5, t.num_copies());
+
+  std::pair<const std::string, Tracked<int>> pc("a", t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(6, t.num_copies());
+  m.emplace(pc);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(6, t.num_copies());
+
+  const std::pair<const std::string, Tracked<int>> cpc("a", t);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(7, t.num_copies());
+  m.emplace(cpc);
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(7, t.num_copies());
+
+  m.emplace(std::piecewise_construct, std::forward_as_tuple("a"),
+            std::forward_as_tuple(t));
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(7, t.num_copies());
+
+  m.emplace(std::piecewise_construct, std::forward_as_tuple(std::string("a")),
+            std::forward_as_tuple(t));
+  ASSERT_EQ(0, t.num_moves());
+  ASSERT_EQ(7, t.num_copies());
+}
+
+TEST(NodeHashMap, AssignRecursive) {
+  struct Tree {
+    // Verify that unordered_map<K, IncompleteType> can be instantiated.
+    absl::node_hash_map<int, Tree> children;
+  };
+  Tree root;
+  const Tree& child = root.children.emplace().first->second;
+  // Verify that `lhs = rhs` doesn't read rhs after clearing lhs.
+  root = child;
+}
+
+TEST(FlatHashMap, MoveOnlyKey) {
+  struct Key {
+    Key() = default;
+    Key(Key&&) = default;
+    Key& operator=(Key&&) = default;
+  };
+  struct Eq {
+    bool operator()(const Key&, const Key&) const { return true; }
+  };
+  struct Hash {
+    size_t operator()(const Key&) const { return 0; }
+  };
+  absl::node_hash_map<Key, int, Hash, Eq> m;
+  m[Key()];
+}
+
+struct NonMovableKey {
+  explicit NonMovableKey(int i) : i(i) {}
+  NonMovableKey(NonMovableKey&&) = delete;
+  int i;
+};
+struct NonMovableKeyHash {
+  using is_transparent = void;
+  size_t operator()(const NonMovableKey& k) const { return k.i; }
+  size_t operator()(int k) const { return k; }
+};
+struct NonMovableKeyEq {
+  using is_transparent = void;
+  bool operator()(const NonMovableKey& a, const NonMovableKey& b) const {
+    return a.i == b.i;
+  }
+  bool operator()(const NonMovableKey& a, int b) const { return a.i == b; }
+};
+
+TEST(NodeHashMap, MergeExtractInsert) {
+  absl::node_hash_map<NonMovableKey, int, NonMovableKeyHash, NonMovableKeyEq>
+      set1, set2;
+  set1.emplace(std::piecewise_construct, std::make_tuple(7),
+               std::make_tuple(-7));
+  set1.emplace(std::piecewise_construct, std::make_tuple(17),
+               std::make_tuple(-17));
+
+  set2.emplace(std::piecewise_construct, std::make_tuple(7),
+               std::make_tuple(-70));
+  set2.emplace(std::piecewise_construct, std::make_tuple(19),
+               std::make_tuple(-190));
+
+  auto Elem = [](int key, int value) {
+    return Pair(Field(&NonMovableKey::i, key), value);
+  };
+
+  EXPECT_THAT(set1, UnorderedElementsAre(Elem(7, -7), Elem(17, -17)));
+  EXPECT_THAT(set2, UnorderedElementsAre(Elem(7, -70), Elem(19, -190)));
+
+  // NonMovableKey is neither copyable nor movable. We should still be able to
+  // move nodes around.
+  static_assert(!std::is_move_constructible<NonMovableKey>::value, "");
+  set1.merge(set2);
+
+  EXPECT_THAT(set1,
+              UnorderedElementsAre(Elem(7, -7), Elem(17, -17), Elem(19, -190)));
+  EXPECT_THAT(set2, UnorderedElementsAre(Elem(7, -70)));
+
+  auto node = set1.extract(7);
+  EXPECT_TRUE(node);
+  EXPECT_EQ(node.key().i, 7);
+  EXPECT_EQ(node.mapped(), -7);
+  EXPECT_THAT(set1, UnorderedElementsAre(Elem(17, -17), Elem(19, -190)));
+
+  auto insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_FALSE(insert_result.inserted);
+  EXPECT_TRUE(insert_result.node);
+  EXPECT_EQ(insert_result.node.key().i, 7);
+  EXPECT_EQ(insert_result.node.mapped(), -7);
+  EXPECT_THAT(*insert_result.position, Elem(7, -70));
+  EXPECT_THAT(set2, UnorderedElementsAre(Elem(7, -70)));
+
+  node = set1.extract(17);
+  EXPECT_TRUE(node);
+  EXPECT_EQ(node.key().i, 17);
+  EXPECT_EQ(node.mapped(), -17);
+  EXPECT_THAT(set1, UnorderedElementsAre(Elem(19, -190)));
+
+  node.mapped() = 23;
+
+  insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_TRUE(insert_result.inserted);
+  EXPECT_FALSE(insert_result.node);
+  EXPECT_THAT(*insert_result.position, Elem(17, 23));
+  EXPECT_THAT(set2, UnorderedElementsAre(Elem(7, -70), Elem(17, 23)));
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/container/node_hash_set.h b/absl/container/node_hash_set.h
new file mode 100644
index 000000000000..927a454cffbe
--- /dev/null
+++ b/absl/container/node_hash_set.h
@@ -0,0 +1,439 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: node_hash_set.h
+// -----------------------------------------------------------------------------
+//
+// An `absl::node_hash_set<T>` is an unordered associative container designed to
+// be a more efficient replacement for `std::unordered_set`. Like
+// `unordered_set`, search, insertion, and deletion of map elements can be done
+// as an `O(1)` operation. However, `node_hash_set` (and other unordered
+// associative containers known as the collection of Abseil "Swiss tables")
+// contain other optimizations that result in both memory and computation
+// advantages.
+//
+// In most cases, your default choice for a hash table should be a map of type
+// `flat_hash_map` or a set of type `flat_hash_set`. However, if you need
+// pointer stability, a `node_hash_set` should be your preferred choice. As
+// well, if you are migrating your code from using `std::unordered_set`, a
+// `node_hash_set` should be an easy migration. Consider migrating to
+// `node_hash_set` and perhaps converting to a more efficient `flat_hash_set`
+// upon further review.
+
+#ifndef ABSL_CONTAINER_NODE_HASH_SET_H_
+#define ABSL_CONTAINER_NODE_HASH_SET_H_
+
+#include <type_traits>
+
+#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
+#include "absl/container/internal/node_hash_policy.h"
+#include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
+#include "absl/memory/memory.h"
+
+namespace absl {
+namespace container_internal {
+template <typename T>
+struct NodeHashSetPolicy;
+}  // namespace container_internal
+
+// -----------------------------------------------------------------------------
+// absl::node_hash_set
+// -----------------------------------------------------------------------------
+//
+// An `absl::node_hash_set<T>` is an unordered associative container which
+// has been optimized for both speed and memory footprint in most common use
+// cases. Its interface is similar to that of `std::unordered_set<T>` with the
+// following notable differences:
+//
+// * Supports heterogeneous lookup, through `find()`, `operator[]()` and
+//   `insert()`, provided that the map is provided a compatible heterogeneous
+//   hashing function and equality operator.
+// * Contains a `capacity()` member function indicating the number of element
+//   slots (open, deleted, and empty) within the hash set.
+// * Returns `void` from the `erase(iterator)` overload.
+//
+// By default, `node_hash_set` uses the `absl::Hash` hashing framework.
+// All fundamental and Abseil types that support the `absl::Hash` framework have
+// a compatible equality operator for comparing insertions into `node_hash_set`.
+// If your type is not yet supported by the `asbl::Hash` framework, see
+// absl/hash/hash.h for information on extending Abseil hashing to user-defined
+// types.
+//
+// Example:
+//
+//   // Create a node hash set of three strings
+//   absl::node_hash_map<std::string, std::string> ducks =
+//     {"huey", "dewey"}, "louie"};
+//
+//  // Insert a new element into the node hash map
+//  ducks.insert("donald"};
+//
+//  // Force a rehash of the node hash map
+//  ducks.rehash(0);
+//
+//  // See if "dewey" is present
+//  if (ducks.contains("dewey")) {
+//    std::cout << "We found dewey!" << std::endl;
+//  }
+template <class T, class Hash = absl::container_internal::hash_default_hash<T>,
+          class Eq = absl::container_internal::hash_default_eq<T>,
+          class Alloc = std::allocator<T>>
+class node_hash_set
+    : public absl::container_internal::raw_hash_set<
+          absl::container_internal::NodeHashSetPolicy<T>, Hash, Eq, Alloc> {
+  using Base = typename node_hash_set::raw_hash_set;
+
+ public:
+  node_hash_set() {}
+  using Base::Base;
+
+  // node_hash_set::begin()
+  //
+  // Returns an iterator to the beginning of the `node_hash_set`.
+  using Base::begin;
+
+  // node_hash_set::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `node_hash_set`.
+  using Base::cbegin;
+
+  // node_hash_set::cend()
+  //
+  // Returns a const iterator to the end of the `node_hash_set`.
+  using Base::cend;
+
+  // node_hash_set::end()
+  //
+  // Returns an iterator to the end of the `node_hash_set`.
+  using Base::end;
+
+  // node_hash_set::capacity()
+  //
+  // Returns the number of element slots (assigned, deleted, and empty)
+  // available within the `node_hash_set`.
+  //
+  // NOTE: this member function is particular to `absl::node_hash_set` and is
+  // not provided in the `std::unordered_map` API.
+  using Base::capacity;
+
+  // node_hash_set::empty()
+  //
+  // Returns whether or not the `node_hash_set` is empty.
+  using Base::empty;
+
+  // node_hash_set::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `node_hash_set` under current memory constraints. This value can be thought
+  // of the largest value of `std::distance(begin(), end())` for a
+  // `node_hash_set<T>`.
+  using Base::max_size;
+
+  // node_hash_set::size()
+  //
+  // Returns the number of elements currently within the `node_hash_set`.
+  using Base::size;
+
+  // node_hash_set::clear()
+  //
+  // Removes all elements from the `node_hash_set`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  //
+  // NOTE: this operation may shrink the underlying buffer. To avoid shrinking
+  // the underlying buffer call `erase(begin(), end())`.
+  using Base::clear;
+
+  // node_hash_set::erase()
+  //
+  // Erases elements within the `node_hash_set`. Erasing does not trigger a
+  // rehash. Overloads are listed below.
+  //
+  // void erase(const_iterator pos):
+  //
+  //   Erases the element at `position` of the `node_hash_set`, returning
+  //   `void`.
+  //
+  //   NOTE: this return behavior is different than that of STL containers in
+  //   general and `std::unordered_map` in particular.
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning an
+  //   iterator pointing to `last`.
+  //
+  // size_type erase(const key_type& key):
+  //
+  //   Erases the element with the matching key, if it exists.
+  using Base::erase;
+
+  // node_hash_set::insert()
+  //
+  // Inserts an element of the specified value into the `node_hash_set`,
+  // returning an iterator pointing to the newly inserted element, provided that
+  // an element with the given key does not already exist. If rehashing occurs
+  // due to the insertion, all iterators are invalidated. Overloads are listed
+  // below.
+  //
+  // std::pair<iterator,bool> insert(const T& value):
+  //
+  //   Inserts a value into the `node_hash_set`. Returns a pair consisting of an
+  //   iterator to the inserted element (or to the element that prevented the
+  //   insertion) and a bool denoting whether the insertion took place.
+  //
+  // std::pair<iterator,bool> insert(T&& value):
+  //
+  //   Inserts a moveable value into the `node_hash_set`. Returns a pair
+  //   consisting of an iterator to the inserted element (or to the element that
+  //   prevented the insertion) and a bool denoting whether the insertion took
+  //   place.
+  //
+  // iterator insert(const_iterator hint, const T& value):
+  // iterator insert(const_iterator hint, T&& value):
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element, or to the existing element that prevented the
+  //   insertion.
+  //
+  // void insert(InputIterator first, InputIterator last ):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently, for `node_hash_set` we guarantee the
+  //   first match is inserted.
+  //
+  // void insert(std::initializer_list<T> ilist ):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  //
+  //   NOTE: Although the STL does not specify which element may be inserted if
+  //   multiple keys compare equivalently within the initializer list, for
+  //   `node_hash_set` we guarantee the first match is inserted.
+  using Base::insert;
+
+  // node_hash_set::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `node_hash_set`, provided that no element with the given key
+  // already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace;
+
+  // node_hash_set::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `node_hash_set`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search, and only inserts
+  // provided that no element with the given key already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If rehashing occurs due to the insertion, all iterators are invalidated.
+  using Base::emplace_hint;
+
+  // node_hash_set::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the element at the indicated position and returns a node handle
+  //   owning that extracted data.
+  //
+  // node_type extract(const key_type& x):
+  //
+  //   Extracts the element with the key matching the passed key value and
+  //   returns a node handle owning that extracted data. If the `node_hash_set`
+  //   does not contain an element with a matching key, this function returns an
+  // empty node handle.
+  using Base::extract;
+
+  // node_hash_set::merge()
+  //
+  // Extracts elements from a given `source` flat hash map into this
+  // `node_hash_set`. If the destination `node_hash_set` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // node_hash_set::swap(node_hash_set& other)
+  //
+  // Exchanges the contents of this `node_hash_set` with those of the `other`
+  // flat hash map, avoiding invocation of any move, copy, or swap operations on
+  // individual elements.
+  //
+  // All iterators and references on the `node_hash_set` remain valid, excepting
+  // for the past-the-end iterator, which is invalidated.
+  //
+  // `swap()` requires that the flat hash set's hashing and key equivalence
+  // functions be Swappable, and are exchaged using unqualified calls to
+  // non-member `swap()`. If the map's allocator has
+  // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
+  // set to `true`, the allocators are also exchanged using an unqualified call
+  // to non-member `swap()`; otherwise, the allocators are not swapped.
+  using Base::swap;
+
+  // node_hash_set::rehash(count)
+  //
+  // Rehashes the `node_hash_set`, setting the number of slots to be at least
+  // the passed value. If the new number of slots increases the load factor more
+  // than the current maximum load factor
+  // (`count` < `size()` / `max_load_factor()`), then the new number of slots
+  // will be at least `size()` / `max_load_factor()`.
+  //
+  // To force a rehash, pass rehash(0).
+  //
+  // NOTE: unlike behavior in `std::unordered_set`, references are also
+  // invalidated upon a `rehash()`.
+  using Base::rehash;
+
+  // node_hash_set::reserve(count)
+  //
+  // Sets the number of slots in the `node_hash_set` to the number needed to
+  // accommodate at least `count` total elements without exceeding the current
+  // maximum load factor, and may rehash the container if needed.
+  using Base::reserve;
+
+  // node_hash_set::contains()
+  //
+  // Determines whether an element comparing equal to the given `key` exists
+  // within the `node_hash_set`, returning `true` if so or `false` otherwise.
+  using Base::contains;
+
+  // node_hash_set::count(const Key& key) const
+  //
+  // Returns the number of elements comparing equal to the given `key` within
+  // the `node_hash_set`. note that this function will return either `1` or `0`
+  // since duplicate elements are not allowed within a `node_hash_set`.
+  using Base::count;
+
+  // node_hash_set::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `node_hash_set`.
+  using Base::equal_range;
+
+  // node_hash_set::find()
+  //
+  // Finds an element with the passed `key` within the `node_hash_set`.
+  using Base::find;
+
+  // node_hash_set::bucket_count()
+  //
+  // Returns the number of "buckets" within the `node_hash_set`. Note that
+  // because a flat hash map contains all elements within its internal storage,
+  // this value simply equals the current capacity of the `node_hash_set`.
+  using Base::bucket_count;
+
+  // node_hash_set::load_factor()
+  //
+  // Returns the current load factor of the `node_hash_set` (the average number
+  // of slots occupied with a value within the hash map).
+  using Base::load_factor;
+
+  // node_hash_set::max_load_factor()
+  //
+  // Manages the maximum load factor of the `node_hash_set`. Overloads are
+  // listed below.
+  //
+  // float node_hash_set::max_load_factor()
+  //
+  //   Returns the current maximum load factor of the `node_hash_set`.
+  //
+  // void node_hash_set::max_load_factor(float ml)
+  //
+  //   Sets the maximum load factor of the `node_hash_set` to the passed value.
+  //
+  //   NOTE: This overload is provided only for API compatibility with the STL;
+  //   `node_hash_set` will ignore any set load factor and manage its rehashing
+  //   internally as an implementation detail.
+  using Base::max_load_factor;
+
+  // node_hash_set::get_allocator()
+  //
+  // Returns the allocator function associated with this `node_hash_set`.
+  using Base::get_allocator;
+
+  // node_hash_set::hash_function()
+  //
+  // Returns the hashing function used to hash the keys within this
+  // `node_hash_set`.
+  using Base::hash_function;
+
+  // node_hash_set::key_eq()
+  //
+  // Returns the function used for comparing keys equality.
+  using Base::key_eq;
+
+  ABSL_DEPRECATED("Call `hash_function()` instead.")
+  typename Base::hasher hash_funct() { return this->hash_function(); }
+
+  ABSL_DEPRECATED("Call `rehash()` instead.")
+  void resize(typename Base::size_type hint) { this->rehash(hint); }
+};
+
+namespace container_internal {
+
+template <class T>
+struct NodeHashSetPolicy
+    : absl::container_internal::node_hash_policy<T&, NodeHashSetPolicy<T>> {
+  using key_type = T;
+  using init_type = T;
+  using constant_iterators = std::true_type;
+
+  template <class Allocator, class... Args>
+  static T* new_element(Allocator* alloc, Args&&... args) {
+    using ValueAlloc =
+        typename absl::allocator_traits<Allocator>::template rebind_alloc<T>;
+    ValueAlloc value_alloc(*alloc);
+    T* res = absl::allocator_traits<ValueAlloc>::allocate(value_alloc, 1);
+    absl::allocator_traits<ValueAlloc>::construct(value_alloc, res,
+                                                  std::forward<Args>(args)...);
+    return res;
+  }
+
+  template <class Allocator>
+  static void delete_element(Allocator* alloc, T* elem) {
+    using ValueAlloc =
+        typename absl::allocator_traits<Allocator>::template rebind_alloc<T>;
+    ValueAlloc value_alloc(*alloc);
+    absl::allocator_traits<ValueAlloc>::destroy(value_alloc, elem);
+    absl::allocator_traits<ValueAlloc>::deallocate(value_alloc, elem, 1);
+  }
+
+  template <class F, class... Args>
+  static decltype(absl::container_internal::DecomposeValue(
+      std::declval<F>(), std::declval<Args>()...))
+  apply(F&& f, Args&&... args) {
+    return absl::container_internal::DecomposeValue(
+        std::forward<F>(f), std::forward<Args>(args)...);
+  }
+
+  static size_t element_space_used(const T*) { return sizeof(T); }
+};
+}  // namespace container_internal
+}  // namespace absl
+#endif  // ABSL_CONTAINER_NODE_HASH_SET_H_
diff --git a/absl/container/node_hash_set_test.cc b/absl/container/node_hash_set_test.cc
new file mode 100644
index 000000000000..7e498f0f779b
--- /dev/null
+++ b/absl/container/node_hash_set_test.cc
@@ -0,0 +1,103 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/node_hash_set.h"
+
+#include "absl/container/internal/unordered_set_constructor_test.h"
+#include "absl/container/internal/unordered_set_lookup_test.h"
+#include "absl/container/internal/unordered_set_modifiers_test.h"
+
+namespace absl {
+namespace container_internal {
+namespace {
+using ::absl::container_internal::hash_internal::Enum;
+using ::absl::container_internal::hash_internal::EnumClass;
+using ::testing::Pointee;
+using ::testing::UnorderedElementsAre;
+
+using SetTypes = ::testing::Types<
+    node_hash_set<int, StatefulTestingHash, StatefulTestingEqual, Alloc<int>>,
+    node_hash_set<std::string, StatefulTestingHash, StatefulTestingEqual,
+                  Alloc<int>>,
+    node_hash_set<Enum, StatefulTestingHash, StatefulTestingEqual, Alloc<Enum>>,
+    node_hash_set<EnumClass, StatefulTestingHash, StatefulTestingEqual,
+                  Alloc<EnumClass>>>;
+
+INSTANTIATE_TYPED_TEST_CASE_P(NodeHashSet, ConstructorTest, SetTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(NodeHashSet, LookupTest, SetTypes);
+INSTANTIATE_TYPED_TEST_CASE_P(NodeHashSet, ModifiersTest, SetTypes);
+
+TEST(NodeHashSet, MoveableNotCopyableCompiles) {
+  node_hash_set<std::unique_ptr<void*>> t;
+  node_hash_set<std::unique_ptr<void*>> u;
+  u = std::move(t);
+}
+
+TEST(NodeHashSet, MergeExtractInsert) {
+  struct Hash {
+    size_t operator()(const std::unique_ptr<int>& p) const { return *p; }
+  };
+  struct Eq {
+    bool operator()(const std::unique_ptr<int>& a,
+                    const std::unique_ptr<int>& b) const {
+      return *a == *b;
+    }
+  };
+  absl::node_hash_set<std::unique_ptr<int>, Hash, Eq> set1, set2;
+  set1.insert(absl::make_unique<int>(7));
+  set1.insert(absl::make_unique<int>(17));
+
+  set2.insert(absl::make_unique<int>(7));
+  set2.insert(absl::make_unique<int>(19));
+
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17)));
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(19)));
+
+  set1.merge(set2);
+
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17), Pointee(19)));
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
+
+  auto node = set1.extract(absl::make_unique<int>(7));
+  EXPECT_TRUE(node);
+  EXPECT_THAT(node.value(), Pointee(7));
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(17), Pointee(19)));
+
+  auto insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_FALSE(insert_result.inserted);
+  EXPECT_TRUE(insert_result.node);
+  EXPECT_THAT(insert_result.node.value(), Pointee(7));
+  EXPECT_EQ(**insert_result.position, 7);
+  EXPECT_NE(insert_result.position->get(), insert_result.node.value().get());
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
+
+  node = set1.extract(absl::make_unique<int>(17));
+  EXPECT_TRUE(node);
+  EXPECT_THAT(node.value(), Pointee(17));
+  EXPECT_THAT(set1, UnorderedElementsAre(Pointee(19)));
+
+  node.value() = absl::make_unique<int>(23);
+
+  insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_TRUE(insert_result.inserted);
+  EXPECT_FALSE(insert_result.node);
+  EXPECT_EQ(**insert_result.position, 23);
+  EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(23)));
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace absl
diff --git a/absl/copts.bzl b/absl/copts.bzl
index e4b425b08e71..717f5a457b4d 100644
--- a/absl/copts.bzl
+++ b/absl/copts.bzl
@@ -117,6 +117,7 @@ MSVC_FLAGS = [
     "/W3",
     "/wd4005",  # macro-redefinition
     "/wd4068",  # unknown pragma
+    "/wd4180",  # qualifier applied to function type has no meaning; ignored
     "/wd4244",  # conversion from 'type1' to 'type2', possible loss of data
     "/wd4267",  # conversion from 'size_t' to 'type', possible loss of data
     "/wd4800",  # forcing value to bool 'true' or 'false' (performance warning)
diff --git a/absl/hash/BUILD.bazel b/absl/hash/BUILD.bazel
new file mode 100644
index 000000000000..50aa5506c11c
--- /dev/null
+++ b/absl/hash/BUILD.bazel
@@ -0,0 +1,114 @@
+#
+# Copyright 2018 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load(
+    "//absl:copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+cc_library(
+    name = "hash",
+    srcs = [
+        "internal/hash.cc",
+        "internal/hash.h",
+    ],
+    hdrs = ["hash.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":city",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/container:fixed_array",
+        "//absl/meta:type_traits",
+        "//absl/numeric:int128",
+        "//absl/strings",
+        "//absl/types:optional",
+        "//absl/types:variant",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "hash_testing",
+    testonly = 1,
+    hdrs = ["hash_testing.h"],
+    deps = [
+        ":spy_hash_state",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/types:variant",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "hash_test",
+    srcs = ["hash_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":hash",
+        ":hash_testing",
+        "//absl/base:core_headers",
+        "//absl/container:flat_hash_set",
+        "//absl/hash:spy_hash_state",
+        "//absl/meta:type_traits",
+        "//absl/numeric:int128",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "spy_hash_state",
+    testonly = 1,
+    hdrs = ["internal/spy_hash_state.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":hash",
+        "//absl/strings",
+        "//absl/strings:str_format",
+    ],
+)
+
+cc_library(
+    name = "city",
+    srcs = ["internal/city.cc"],
+    hdrs = [
+        "internal/city.h",
+        "internal/city_crc.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+    ],
+)
+
+cc_test(
+    name = "city_test",
+    srcs = ["internal/city_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":city",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
diff --git a/absl/hash/CMakeLists.txt b/absl/hash/CMakeLists.txt
new file mode 100644
index 000000000000..35081e3774f8
--- /dev/null
+++ b/absl/hash/CMakeLists.txt
@@ -0,0 +1,80 @@
+#
+# Copyright 2018 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+list(APPEND HASH_PUBLIC_HEADERS
+  "hash.h"
+)
+
+list(APPEND HASH_INTERNAL_HEADERS
+  "internal/city.h"
+  "internal/city_crc.h"
+  "internal/hash.h"
+)
+
+# absl_hash library
+list(APPEND HASH_SRC
+  "internal/city.cc"
+  "internal/hash.cc"
+  ${HASH_PUBLIC_HEADERS}
+  ${HASH_INTERNAL_HEADERS}
+)
+
+set(HASH_PUBLIC_LIBRARIES absl::hash absl::container absl::strings absl::str_format absl::utility)
+
+absl_library(
+  TARGET
+    absl_hash
+  SOURCES
+    ${HASH_SRC}
+  PUBLIC_LIBRARIES
+    ${HASH_PUBLIC_LIBRARIES}
+  EXPORT_NAME
+    hash
+)
+
+#
+## TESTS
+#
+
+# testing support
+set(HASH_TEST_HEADERS hash_testing.h internal/spy_hash_state.h)
+set(HASH_TEST_PUBLIC_LIBRARIES absl::hash absl::container absl::numeric absl::strings absl::str_format)
+
+# hash_test
+set(HASH_TEST_SRC "hash_test.cc" ${HASH_TEST_HEADERS})
+
+absl_test(
+  TARGET
+    hash_test
+  SOURCES
+    ${HASH_TEST_SRC}
+  PUBLIC_LIBRARIES
+    ${HASH_TEST_PUBLIC_LIBRARIES}
+)
+
+# hash_test
+set(CITY_TEST_SRC "internal/city_test.cc")
+
+absl_test(
+  TARGET
+    city_test
+  SOURCES
+    ${CITY_TEST_SRC}
+  PUBLIC_LIBRARIES
+    ${HASH_TEST_PUBLIC_LIBRARIES}
+)
+
+
diff --git a/absl/hash/hash.h b/absl/hash/hash.h
new file mode 100644
index 000000000000..c7ba4c2b7a5c
--- /dev/null
+++ b/absl/hash/hash.h
@@ -0,0 +1,312 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: hash.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the Abseil `hash` library and the Abseil hashing
+// framework. This framework consists of the following:
+//
+//   * The `absl::Hash` functor, which is used to invoke the hasher within the
+//     Abseil hashing framework. `absl::Hash<T>` supports most basic types and
+//     a number of Abseil types out of the box.
+//   * `AbslHashValue`, an extension point that allows you to extend types to
+//     support Abseil hashing without requiring you to define a hashing
+//     algorithm.
+//   * `HashState`, a type-erased class which implement the manipulation of the
+//     hash state (H) itself. containing member functions `combine()` and
+//     `combine_contiguous()`, which you can use to contribute to an existing
+//     hash state when hashing your types.
+//
+// Unlike `std::hash` or other hashing frameworks, the Abseil hashing framework
+// provides most of its utility by abstracting away the hash algorithm (and its
+// implementation) entirely. Instead, a type invokes the Abseil hashing
+// framework by simply combining its state with the state of known, hashable
+// types. Hashing of that combined state is separately done by `absl::Hash`.
+//
+// Example:
+//
+//   // Suppose we have a class `Circle` for which we want to add hashing
+//   class Circle {
+//     public:
+//       ...
+//     private:
+//       std::pair<int, int> center_;
+//       int radius_;
+//     };
+//
+//   // To add hashing support to `Circle`, we simply need to add an ordinary
+//   // function `AbslHashValue()`, and return the combined hash state of the
+//   // existing hash state and the class state:
+//
+//     template <typename H>
+//     friend H AbslHashValue(H h, const Circle& c) {
+//       return H::combine(std::move(h), c.center_, c.radius_);
+//     }
+//
+// For more information, see Adding Type Support to `absl::Hash` below.
+//
+#ifndef ABSL_HASH_HASH_H_
+#define ABSL_HASH_HASH_H_
+
+#include "absl/hash/internal/hash.h"
+
+namespace absl {
+
+// -----------------------------------------------------------------------------
+// `absl::Hash`
+// -----------------------------------------------------------------------------
+//
+// `absl::Hash<T>` is a convenient general-purpose hash functor for a type `T`
+// satisfying any of the following conditions (in order):
+//
+//  * T is an arithmetic or pointer type
+//  * T defines an overload for `AbslHashValue(H, const T&)` for an arbitrary
+//    hash state `H`.
+//  - T defines a specialization of `HASH_NAMESPACE::hash<T>`
+//  - T defines a specialization of `std::hash<T>`
+//
+// `absl::Hash` intrinsically supports the following types:
+//
+//   * All integral types (including bool)
+//   * All enum types
+//   * All floating-point types (although hashing them is discouraged)
+//   * All pointer types, including nullptr_t
+//   * std::pair<T1, T2>, if T1 and T2 are hashable
+//   * std::tuple<Ts...>, if all the Ts... are hashable
+//   * std::unique_ptr and std::shared_ptr
+//   * All string-like types including:
+//     * std::string
+//     * std::string_view (as well as any instance of std::basic_string that
+//       uses char and std::char_traits)
+//  * All the standard sequence containers (provided the elements are hashable)
+//  * All the standard ordered associative containers (provided the elements are
+//    hashable)
+//  * absl types such as the following:
+//    * absl::string_view
+//    * absl::InlinedVector
+//    * absl::FixedArray
+//    * absl::unit128
+//    * absl::Time, absl::Duration, and absl::TimeZone
+//
+// Note: the list above is not meant to be exhaustive. Additional type support
+// may be added, in which case the above list will be updated.
+//
+// -----------------------------------------------------------------------------
+// absl::Hash Invocation Evaluation
+// -----------------------------------------------------------------------------
+//
+// When invoked, `absl::Hash<T>` searches for supplied hash functions in the
+// following order:
+//
+//   * Natively supported types out of the box (see above)
+//   * Types for which an `AbslHashValue()` overload is provided (such as
+//     user-defined types). See "Adding Type Support to `absl::Hash`" below.
+//   * Types which define a `HASH_NAMESPACE::hash<T>` specialization (aka
+//     `__gnu_cxx::hash<T>` for gcc/Clang or `stdext::hash<T>` for MSVC)
+//   * Types which define a `std::hash<T>` specialization
+//
+// The fallback to legacy hash functions exists mainly for backwards
+// compatibility. If you have a choice, prefer defining an `AbslHashValue`
+// overload instead of specializing any legacy hash functors.
+//
+// -----------------------------------------------------------------------------
+// The Hash State Concept, and using `HashState` for Type Erasure
+// -----------------------------------------------------------------------------
+//
+// The `absl::Hash` framework relies on the Concept of a "hash state." Such a
+// hash state is used in several places:
+//
+// * Within existing implementations of `absl::Hash<T>` to store the hashed
+//   state of an object. Note that it is up to the implementation how it stores
+//   such state. A hash table, for example, may mix the state to produce an
+//   integer value; a testing framework may simply hold a vector of that state.
+// * Within implementations of `AbslHashValue()` used to extend user-defined
+//   types. (See "Adding Type Support to absl::Hash" below.)
+// * Inside a `HashState`, providing type erasure for the concept of a hash
+//   state, which you can use to extend the `absl::Hash` framework for types
+//   that are otherwise difficult to extend using `AbslHashValue()`. (See the
+//   `HashState` class below.)
+//
+// The "hash state" concept contains two member functions for mixing hash state:
+//
+// * `H::combine()`
+//
+//   Combines an arbitrary number of values into a hash state, returning the
+//   updated state. Note that the existing hash state is move-only and must be
+//   passed by value.
+//
+//   Each of the value types T must be hashable by H.
+//
+//   NOTE:
+//
+//     state = H::combine(std::move(state), value1, value2, value3);
+//
+//   must be guaranteed to produce the same hash expansion as
+//
+//     state = H::combine(std::move(state), value1);
+//     state = H::combine(std::move(state), value2);
+//     state = H::combine(std::move(state), value3);
+//
+// * `H::combine_contiguous()`
+//
+//    Combines a contiguous array of `size` elements into a hash state,
+//    returning the updated state. Note that the existing hash state is
+//    move-only and must be passed by value.
+//
+//    NOTE:
+//
+//      state = H::combine_contiguous(std::move(state), data, size);
+//
+//    need NOT be guaranteed to produce the same hash expansion as a loop
+//    (it may perform internal optimizations). If you need this guarantee, use a
+//    loop instead.
+//
+// -----------------------------------------------------------------------------
+// Adding Type Support to `absl::Hash`
+// -----------------------------------------------------------------------------
+//
+// To add support for your user-defined type, add a proper `AbslHashValue()`
+// overload as a free (non-member) function. The overload will take an
+// existing hash state and should combine that state with state from the type.
+//
+// Example:
+//
+//   template <typename H>
+//   H AbslHashValue(H state, const MyType& v) {
+//     return H::combine(std::move(state), v.field1, ..., v.fieldN);
+//   }
+//
+// where `(field1, ..., fieldN)` are the members you would use on your
+// `operator==` to define equality.
+//
+// Notice that `AbslHashValue` is not a class member, but an ordinary function.
+// An `AbslHashValue` overload for a type should only be declared in the same
+// file and namespace as said type. The proper `AbslHashValue` implementation
+// for a given type will be discovered via ADL.
+//
+// Note: unlike `std::hash', `absl::Hash` should never be specialized. It must
+// only be extended by adding `AbslHashValue()` overloads.
+//
+template <typename T>
+using Hash = absl::hash_internal::Hash<T>;
+
+// HashState
+//
+// A type erased version of the hash state concept, for use in user-defined
+// `AbslHashValue` implementations that can't use templates (such as PImpl
+// classes, virtual functions, etc.). The type erasure adds overhead so it
+// should be avoided unless necessary.
+//
+// Note: This wrapper will only erase calls to:
+//     combine_contiguous(H, const unsigned char*, size_t)
+//
+// All other calls will be handled internally and will not invoke overloads
+// provided by the wrapped class.
+//
+// Users of this class should still define a template `AbslHashValue` function,
+// but can use `absl::HashState::Create(&state)` to erase the type of the hash
+// state and dispatch to their private hashing logic.
+//
+// This state can be used like any other hash state. In particular, you can call
+// `HashState::combine()` and `HashState::combine_contiguous()` on it.
+//
+// Example:
+//
+//   class Interface {
+//    public:
+//     template <typename H>
+//     friend H AbslHashValue(H state, const Interface& value) {
+//       state = H::combine(std::move(state), std::type_index(typeid(*this)));
+//       value.HashValue(absl::HashState::Create(&state));
+//       return state;
+//     }
+//    private:
+//     virtual void HashValue(absl::HashState state) const = 0;
+//  };
+//
+//   class Impl : Interface {
+//    private:
+//     void HashValue(absl::HashState state) const override {
+//       absl::HashState::combine(std::move(state), v1_, v2_);
+//     }
+//     int v1_;
+//     string v2_;
+//   };
+class HashState : public hash_internal::HashStateBase<HashState> {
+ public:
+  // HashState::Create()
+  //
+  // Create a new `HashState` instance that wraps `state`. All calls to
+  // `combine()` and `combine_contiguous()` on the new instance will be
+  // redirected to the original `state` object. The `state` object must outlive
+  // the `HashState` instance.
+  template <typename T>
+  static HashState Create(T* state) {
+    HashState s;
+    s.Init(state);
+    return s;
+  }
+
+  HashState(const HashState&) = delete;
+  HashState& operator=(const HashState&) = delete;
+  HashState(HashState&&) = default;
+  HashState& operator=(HashState&&) = default;
+
+  // HashState::combine()
+  //
+  // Combines an arbitrary number of values into a hash state, returning the
+  // updated state.
+  using HashState::HashStateBase::combine;
+
+  // HashState::combine_contiguous()
+  //
+  // Combines a contiguous array of `size` elements into a hash state, returning
+  // the updated state.
+  static HashState combine_contiguous(HashState hash_state,
+                                      const unsigned char* first, size_t size) {
+    hash_state.combine_contiguous_(hash_state.state_, first, size);
+    return hash_state;
+  }
+  using HashState::HashStateBase::combine_contiguous;
+
+ private:
+  HashState() = default;
+
+  template <typename T>
+  static void CombineContiguousImpl(void* p, const unsigned char* first,
+                                    size_t size) {
+    T& state = *static_cast<T*>(p);
+    state = T::combine_contiguous(std::move(state), first, size);
+  }
+
+  template <typename T>
+  void Init(T* state) {
+    state_ = state;
+    combine_contiguous_ = &CombineContiguousImpl<T>;
+  }
+
+  // Do not erase an already erased state.
+  void Init(HashState* state) {
+    state_ = state->state_;
+    combine_contiguous_ = state->combine_contiguous_;
+  }
+
+  void* state_;
+  void (*combine_contiguous_)(void*, const unsigned char*, size_t);
+};
+
+}  // namespace absl
+#endif  // ABSL_HASH_HASH_H_
diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
new file mode 100644
index 000000000000..7b6fb2e7ac1c
--- /dev/null
+++ b/absl/hash/hash_test.cc
@@ -0,0 +1,425 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/hash/hash.h"
+
+#include <array>
+#include <cstring>
+#include <deque>
+#include <forward_list>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <list>
+#include <map>
+#include <memory>
+#include <numeric>
+#include <random>
+#include <set>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/hash/internal/spy_hash_state.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/int128.h"
+
+namespace {
+
+using absl::Hash;
+using absl::hash_internal::SpyHashState;
+
+template <typename T>
+class HashValueIntTest : public testing::Test {
+};
+TYPED_TEST_CASE_P(HashValueIntTest);
+
+template <typename T>
+SpyHashState SpyHash(const T& value) {
+  return SpyHashState::combine(SpyHashState(), value);
+}
+
+// Helper trait to verify if T is hashable. We use absl::Hash's poison status to
+// detect it.
+template <typename T>
+using is_hashable = std::is_default_constructible<absl::Hash<T>>;
+
+TYPED_TEST_P(HashValueIntTest, BasicUsage) {
+  EXPECT_TRUE((is_hashable<TypeParam>::value));
+
+  TypeParam n = 42;
+  EXPECT_EQ(SpyHash(n), SpyHash(TypeParam{42}));
+  EXPECT_NE(SpyHash(n), SpyHash(TypeParam{0}));
+  EXPECT_NE(SpyHash(std::numeric_limits<TypeParam>::max()),
+            SpyHash(std::numeric_limits<TypeParam>::min()));
+}
+
+TYPED_TEST_P(HashValueIntTest, FastPath) {
+  // Test the fast-path to make sure the values are the same.
+  TypeParam n = 42;
+  EXPECT_EQ(absl::Hash<TypeParam>{}(n),
+            absl::Hash<std::tuple<TypeParam>>{}(std::tuple<TypeParam>(n)));
+}
+
+REGISTER_TYPED_TEST_CASE_P(HashValueIntTest, BasicUsage, FastPath);
+using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t, uint32_t,
+                                uint64_t, size_t>;
+INSTANTIATE_TYPED_TEST_CASE_P(My, HashValueIntTest, IntTypes);
+
+template <typename T, typename = void>
+struct IsHashCallble : std::false_type {};
+
+template <typename T>
+struct IsHashCallble<T, absl::void_t<decltype(std::declval<absl::Hash<T>>()(
+                            std::declval<const T&>()))>> : std::true_type {};
+
+template <typename T, typename = void>
+struct IsAggregateInitializable : std::false_type {};
+
+template <typename T>
+struct IsAggregateInitializable<T, absl::void_t<decltype(T{})>>
+    : std::true_type {};
+
+TEST(IsHashableTest, ValidHash) {
+  EXPECT_TRUE((is_hashable<int>::value));
+  EXPECT_TRUE(std::is_default_constructible<absl::Hash<int>>::value);
+  EXPECT_TRUE(std::is_copy_constructible<absl::Hash<int>>::value);
+  EXPECT_TRUE(std::is_move_constructible<absl::Hash<int>>::value);
+  EXPECT_TRUE(absl::is_copy_assignable<absl::Hash<int>>::value);
+  EXPECT_TRUE(absl::is_move_assignable<absl::Hash<int>>::value);
+  EXPECT_TRUE(IsHashCallble<int>::value);
+  EXPECT_TRUE(IsAggregateInitializable<absl::Hash<int>>::value);
+}
+#if ABSL_HASH_INTERNAL_CAN_POISON_ && !defined(__APPLE__)
+TEST(IsHashableTest, PoisonHash) {
+  struct X {};
+  EXPECT_FALSE((is_hashable<X>::value));
+  EXPECT_FALSE(std::is_default_constructible<absl::Hash<X>>::value);
+  EXPECT_FALSE(std::is_copy_constructible<absl::Hash<X>>::value);
+  EXPECT_FALSE(std::is_move_constructible<absl::Hash<X>>::value);
+  EXPECT_FALSE(absl::is_copy_assignable<absl::Hash<X>>::value);
+  EXPECT_FALSE(absl::is_move_assignable<absl::Hash<X>>::value);
+  EXPECT_FALSE(IsHashCallble<X>::value);
+  EXPECT_FALSE(IsAggregateInitializable<absl::Hash<X>>::value);
+}
+#endif  // ABSL_HASH_INTERNAL_CAN_POISON_
+
+// Hashable types
+//
+// These types exist simply to exercise various AbslHashValue behaviors, so
+// they are named by what their AbslHashValue overload does.
+struct NoOp {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, NoOp n) {
+    return std::move(h);
+  }
+};
+
+struct EmptyCombine {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, EmptyCombine e) {
+    return HashCode::combine(std::move(h));
+  }
+};
+
+template <typename Int>
+struct CombineIterative {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, CombineIterative c) {
+    for (int i = 0; i < 5; ++i) {
+      h = HashCode::combine(std::move(h), Int(i));
+    }
+    return h;
+  }
+};
+
+template <typename Int>
+struct CombineVariadic {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, CombineVariadic c) {
+    return HashCode::combine(std::move(h), Int(0), Int(1), Int(2), Int(3),
+                             Int(4));
+  }
+};
+
+using InvokeTag = absl::hash_internal::InvokeHashTag;
+template <InvokeTag T>
+using InvokeTagConstant = std::integral_constant<InvokeTag, T>;
+
+template <InvokeTag... Tags>
+struct MinTag;
+
+template <InvokeTag a, InvokeTag b, InvokeTag... Tags>
+struct MinTag<a, b, Tags...> : MinTag<(a < b ? a : b), Tags...> {};
+
+template <InvokeTag a>
+struct MinTag<a> : InvokeTagConstant<a> {};
+
+template <InvokeTag... Tags>
+struct CustomHashType {
+  size_t value;
+};
+
+template <InvokeTag allowed, InvokeTag... tags>
+struct EnableIfContained
+    : std::enable_if<absl::disjunction<
+          std::integral_constant<bool, allowed == tags>...>::value> {};
+
+template <
+    typename H, InvokeTag... Tags,
+    typename = typename EnableIfContained<InvokeTag::kHashValue, Tags...>::type>
+H AbslHashValue(H state, CustomHashType<Tags...> t) {
+  static_assert(MinTag<Tags...>::value == InvokeTag::kHashValue, "");
+  return H::combine(std::move(state),
+                    t.value + static_cast<int>(InvokeTag::kHashValue));
+}
+
+}  // namespace
+
+namespace absl {
+namespace hash_internal {
+template <InvokeTag... Tags>
+struct is_uniquely_represented<
+    CustomHashType<Tags...>,
+    typename EnableIfContained<InvokeTag::kUniquelyRepresented, Tags...>::type>
+    : std::true_type {};
+}  // namespace hash_internal
+}  // namespace absl
+
+#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+namespace ABSL_INTERNAL_LEGACY_HASH_NAMESPACE {
+template <InvokeTag... Tags>
+struct hash<CustomHashType<Tags...>> {
+  template <InvokeTag... TagsIn, typename = typename EnableIfContained<
+                                     InvokeTag::kLegacyHash, TagsIn...>::type>
+  size_t operator()(CustomHashType<TagsIn...> t) const {
+    static_assert(MinTag<Tags...>::value == InvokeTag::kLegacyHash, "");
+    return t.value + static_cast<int>(InvokeTag::kLegacyHash);
+  }
+};
+}  // namespace ABSL_INTERNAL_LEGACY_HASH_NAMESPACE
+#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+
+namespace std {
+template <InvokeTag... Tags>  // NOLINT
+struct hash<CustomHashType<Tags...>> {
+  template <InvokeTag... TagsIn, typename = typename EnableIfContained<
+                                     InvokeTag::kStdHash, TagsIn...>::type>
+  size_t operator()(CustomHashType<TagsIn...> t) const {
+    static_assert(MinTag<Tags...>::value == InvokeTag::kStdHash, "");
+    return t.value + static_cast<int>(InvokeTag::kStdHash);
+  }
+};
+}  // namespace std
+
+namespace {
+
+template <typename... T>
+void TestCustomHashType(InvokeTagConstant<InvokeTag::kNone>, T...) {
+  using type = CustomHashType<T::value...>;
+  SCOPED_TRACE(testing::PrintToString(std::vector<InvokeTag>{T::value...}));
+  EXPECT_TRUE(is_hashable<type>());
+  EXPECT_TRUE(is_hashable<const type>());
+  EXPECT_TRUE(is_hashable<const type&>());
+
+  const size_t offset = static_cast<int>(std::min({T::value...}));
+  EXPECT_EQ(SpyHash(type{7}), SpyHash(size_t{7 + offset}));
+}
+
+void TestCustomHashType(InvokeTagConstant<InvokeTag::kNone>) {
+#if ABSL_HASH_INTERNAL_CAN_POISON_
+  // is_hashable is false if we don't support any of the hooks.
+  using type = CustomHashType<>;
+  EXPECT_FALSE(is_hashable<type>());
+  EXPECT_FALSE(is_hashable<const type>());
+  EXPECT_FALSE(is_hashable<const type&>());
+#endif  // ABSL_HASH_INTERNAL_CAN_POISON_
+}
+
+template <InvokeTag Tag, typename... T>
+void TestCustomHashType(InvokeTagConstant<Tag> tag, T... t) {
+  constexpr auto next = static_cast<InvokeTag>(static_cast<int>(Tag) + 1);
+  TestCustomHashType(InvokeTagConstant<next>(), tag, t...);
+  TestCustomHashType(InvokeTagConstant<next>(), t...);
+}
+
+TEST(HashTest, CustomHashType) {
+  TestCustomHashType(InvokeTagConstant<InvokeTag{}>());
+}
+
+TEST(HashTest, NoOpsAreEquivalent) {
+  EXPECT_EQ(Hash<NoOp>()({}), Hash<NoOp>()({}));
+  EXPECT_EQ(Hash<NoOp>()({}), Hash<EmptyCombine>()({}));
+}
+
+template <typename T>
+class HashIntTest : public testing::Test {
+};
+TYPED_TEST_CASE_P(HashIntTest);
+
+TYPED_TEST_P(HashIntTest, BasicUsage) {
+  EXPECT_NE(Hash<NoOp>()({}), Hash<TypeParam>()(0));
+  EXPECT_NE(Hash<NoOp>()({}),
+            Hash<TypeParam>()(std::numeric_limits<TypeParam>::max()));
+  if (std::numeric_limits<TypeParam>::min() != 0) {
+    EXPECT_NE(Hash<NoOp>()({}),
+              Hash<TypeParam>()(std::numeric_limits<TypeParam>::min()));
+  }
+
+  EXPECT_EQ(Hash<CombineIterative<TypeParam>>()({}),
+            Hash<CombineVariadic<TypeParam>>()({}));
+}
+
+REGISTER_TYPED_TEST_CASE_P(HashIntTest, BasicUsage);
+using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t, uint32_t,
+                                uint64_t, size_t>;
+INSTANTIATE_TYPED_TEST_CASE_P(My, HashIntTest, IntTypes);
+
+struct StructWithPadding {
+  char c;
+  int i;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const StructWithPadding& s) {
+    return H::combine(std::move(hash_state), s.c, s.i);
+  }
+};
+
+static_assert(sizeof(StructWithPadding) > sizeof(char) + sizeof(int),
+              "StructWithPadding doesn't have padding");
+static_assert(std::is_standard_layout<StructWithPadding>::value, "");
+
+// This check has to be disabled because libstdc++ doesn't support it.
+// static_assert(std::is_trivially_constructible<StructWithPadding>::value, "");
+
+template <typename T>
+struct ArraySlice {
+  T* begin;
+  T* end;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const ArraySlice& slice) {
+    for (auto t = slice.begin; t != slice.end; ++t) {
+      hash_state = H::combine(std::move(hash_state), *t);
+    }
+    return hash_state;
+  }
+};
+
+TEST(HashTest, HashNonUniquelyRepresentedType) {
+  // Create equal StructWithPadding objects that are known to have non-equal
+  // padding bytes.
+  static const size_t kNumStructs = 10;
+  unsigned char buffer1[kNumStructs * sizeof(StructWithPadding)];
+  std::memset(buffer1, 0, sizeof(buffer1));
+  auto* s1 = reinterpret_cast<StructWithPadding*>(buffer1);
+
+  unsigned char buffer2[kNumStructs * sizeof(StructWithPadding)];
+  std::memset(buffer2, 255, sizeof(buffer2));
+  auto* s2 = reinterpret_cast<StructWithPadding*>(buffer2);
+  for (int i = 0; i < kNumStructs; ++i) {
+    SCOPED_TRACE(i);
+    s1[i].c = s2[i].c = '0' + i;
+    s1[i].i = s2[i].i = i;
+    ASSERT_FALSE(memcmp(buffer1 + i * sizeof(StructWithPadding),
+                        buffer2 + i * sizeof(StructWithPadding),
+                        sizeof(StructWithPadding)) == 0)
+        << "Bug in test code: objects do not have unequal"
+        << " object representations";
+  }
+
+  EXPECT_EQ(Hash<StructWithPadding>()(s1[0]), Hash<StructWithPadding>()(s2[0]));
+  EXPECT_EQ(Hash<ArraySlice<StructWithPadding>>()({s1, s1 + kNumStructs}),
+            Hash<ArraySlice<StructWithPadding>>()({s2, s2 + kNumStructs}));
+}
+
+TEST(HashTest, StandardHashContainerUsage) {
+  std::unordered_map<int, std::string, Hash<int>> map = {{0, "foo"}, { 42, "bar" }};
+
+  EXPECT_NE(map.find(0), map.end());
+  EXPECT_EQ(map.find(1), map.end());
+  EXPECT_NE(map.find(0u), map.end());
+}
+
+struct ConvertibleFromNoOp {
+  ConvertibleFromNoOp(NoOp) {}  // NOLINT(runtime/explicit)
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, ConvertibleFromNoOp) {
+    return H::combine(std::move(hash_state), 1);
+  }
+};
+
+TEST(HashTest, HeterogeneousCall) {
+  EXPECT_NE(Hash<ConvertibleFromNoOp>()(NoOp()),
+            Hash<NoOp>()(NoOp()));
+}
+
+TEST(IsUniquelyRepresentedTest, SanityTest) {
+  using absl::hash_internal::is_uniquely_represented;
+
+  EXPECT_TRUE(is_uniquely_represented<unsigned char>::value);
+  EXPECT_TRUE(is_uniquely_represented<int>::value);
+  EXPECT_FALSE(is_uniquely_represented<bool>::value);
+  EXPECT_FALSE(is_uniquely_represented<int*>::value);
+}
+
+struct IntAndString {
+  int i;
+  std::string s;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, IntAndString int_and_string) {
+    return H::combine(std::move(hash_state), int_and_string.s,
+                      int_and_string.i);
+  }
+};
+
+TEST(HashTest, SmallValueOn64ByteBoundary) {
+  Hash<IntAndString>()(IntAndString{0, std::string(63, '0')});
+}
+
+struct TypeErased {
+  size_t n;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const TypeErased& v) {
+    v.HashValue(absl::HashState::Create(&hash_state));
+    return hash_state;
+  }
+
+  void HashValue(absl::HashState state) const {
+    absl::HashState::combine(std::move(state), n);
+  }
+};
+
+TEST(HashTest, TypeErased) {
+  EXPECT_TRUE((is_hashable<TypeErased>::value));
+  EXPECT_TRUE((is_hashable<std::pair<TypeErased, int>>::value));
+
+  EXPECT_EQ(SpyHash(TypeErased{7}), SpyHash(size_t{7}));
+  EXPECT_NE(SpyHash(TypeErased{7}), SpyHash(size_t{13}));
+
+  EXPECT_EQ(SpyHash(std::make_pair(TypeErased{7}, 17)),
+            SpyHash(std::make_pair(size_t{7}, 17)));
+}
+
+}  // namespace
diff --git a/absl/hash/hash_testing.h b/absl/hash/hash_testing.h
new file mode 100644
index 000000000000..1e3cda64467d
--- /dev/null
+++ b/absl/hash/hash_testing.h
@@ -0,0 +1,372 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_HASH_HASH_TESTING_H_
+#define ABSL_HASH_HASH_TESTING_H_
+
+#include <initializer_list>
+#include <tuple>
+#include <type_traits>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/hash/internal/spy_hash_state.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/str_cat.h"
+#include "absl/types/variant.h"
+
+namespace absl {
+
+// Run the absl::Hash algorithm over all the elements passed in and verify that
+// their hash expansion is congruent with their `==` operator.
+//
+// It is used in conjunction with EXPECT_TRUE. Failures will output information
+// on what requirement failed and on which objects.
+//
+// Users should pass a collection of types as either an initializer list or a
+// container of cases.
+//
+//   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+//       {v1, v2, ..., vN}));
+//
+//   std::vector<MyType> cases;
+//   // Fill cases...
+//   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(cases));
+//
+// Users can pass a variety of types for testing heterogeneous lookup with
+// `std::make_tuple`:
+//
+//   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+//       std::make_tuple(v1, v2, ..., vN)));
+//
+//
+// Ideally, the values passed should provide enough coverage of the `==`
+// operator and the AbslHashValue implementations.
+// For dynamically sized types, the empty state should usually be included in
+// the values.
+//
+// The function accepts an optional comparator function, in case that `==` is
+// not enough for the values provided.
+//
+// Usage:
+//
+//   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+//       std::make_tuple(v1, v2, ..., vN), MyCustomEq{}));
+//
+// It checks the following requirements:
+//   1. The expansion for a value is deterministic.
+//   2. For any two objects `a` and `b` in the sequence, if `a == b` evaluates
+//      to true, then their hash expansion must be equal.
+//   3. If `a == b` evaluates to false their hash expansion must be unequal.
+//   4. If `a == b` evaluates to false neither hash expansion can be a
+//      suffix of the other.
+//   5. AbslHashValue overloads should not be called by the user. They are only
+//      meant to be called by the framework. Users should call H::combine() and
+//      H::combine_contiguous().
+//   6. No moved-from instance of the hash state is used in the implementation
+//      of AbslHashValue.
+//
+// The values do not have to have the same type. This can be useful for
+// equivalent types that support heterogeneous lookup.
+//
+// A possible reason for breaking (2) is combining state in the hash expansion
+// that was not used in `==`.
+// For example:
+//
+// struct Bad2 {
+//   int a, b;
+//   template <typename H>
+//   friend H AbslHashValue(H state, Bad2 x) {
+//     // Uses a and b.
+//     return H::combine(x.a, x.b);
+//   }
+//   friend bool operator==(Bad2 x, Bad2 y) {
+//     // Only uses a.
+//     return x.a == y.a;
+//   }
+// };
+//
+// As for (3), breaking this usually means that there is state being passed to
+// the `==` operator that is not used in the hash expansion.
+// For example:
+//
+// struct Bad3 {
+//   int a, b;
+//   template <typename H>
+//   friend H AbslHashValue(H state, Bad3 x) {
+//     // Only uses a.
+//     return H::combine(x.a);
+//   }
+//   friend bool operator==(Bad3 x, Bad3 y) {
+//     // Uses a and b.
+//     return x.a == y.a && x.b == y.b;
+//   }
+// };
+//
+// Finally, a common way to break 4 is by combining dynamic ranges without
+// combining the size of the range.
+// For example:
+//
+// struct Bad4 {
+//   int *p, size;
+//   template <typename H>
+//   friend H AbslHashValue(H state, Bad4 x) {
+//     return H::combine_range(x.p, x.p + x.size);
+//   }
+//   friend bool operator==(Bad4 x, Bad4 y) {
+//     return std::equal(x.p, x.p + x.size, y.p, y.p + y.size);
+//   }
+// };
+//
+// An easy solution to this is to combine the size after combining the range,
+// like so:
+//   template <typename H>
+//   friend H AbslHashValue(H state, Bad4 x) {
+//     return H::combine(H::combine_range(x.p, x.p + x.size), x.size);
+//   }
+//
+template <int&... ExplicitBarrier, typename Container>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(const Container& values);
+
+template <int&... ExplicitBarrier, typename Container, typename Eq>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals);
+
+template <int&..., typename T>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values);
+
+template <int&..., typename T, typename Eq>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values,
+                                      Eq equals);
+
+namespace hash_internal {
+
+struct PrintVisitor {
+  size_t index;
+  template <typename T>
+  std::string operator()(const T* value) const {
+    return absl::StrCat("#", index, "(", testing::PrintToString(*value), ")");
+  }
+};
+
+template <typename Eq>
+struct EqVisitor {
+  Eq eq;
+  template <typename T, typename U>
+  bool operator()(const T* t, const U* u) const {
+    return eq(*t, *u);
+  }
+};
+
+struct ExpandVisitor {
+  template <typename T>
+  SpyHashState operator()(const T* value) const {
+    return SpyHashState::combine(SpyHashState(), *value);
+  }
+};
+
+template <typename Container, typename Eq>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
+  using V = typename Container::value_type;
+
+  struct Info {
+    const V& value;
+    size_t index;
+    std::string ToString() const { return absl::visit(PrintVisitor{index}, value); }
+    SpyHashState expand() const { return absl::visit(ExpandVisitor{}, value); }
+  };
+
+  using EqClass = std::vector<Info>;
+  std::vector<EqClass> classes;
+
+  // Gather the values in equivalence classes.
+  size_t i = 0;
+  for (const auto& value : values) {
+    EqClass* c = nullptr;
+    for (auto& eqclass : classes) {
+      if (absl::visit(EqVisitor<Eq>{equals}, value, eqclass[0].value)) {
+        c = &eqclass;
+        break;
+      }
+    }
+    if (c == nullptr) {
+      classes.emplace_back();
+      c = &classes.back();
+    }
+    c->push_back({value, i});
+    ++i;
+
+    // Verify potential errors captured by SpyHashState.
+    if (auto error = c->back().expand().error()) {
+      return testing::AssertionFailure() << *error;
+    }
+  }
+
+  if (classes.size() < 2) {
+    return testing::AssertionFailure()
+           << "At least two equivalence classes are expected.";
+  }
+
+  // We assume that equality is correctly implemented.
+  // Now we verify that AbslHashValue is also correctly implemented.
+
+  for (const auto& c : classes) {
+    // All elements of the equivalence class must have the same hash expansion.
+    const SpyHashState expected = c[0].expand();
+    for (const Info& v : c) {
+      if (v.expand() != v.expand()) {
+        return testing::AssertionFailure()
+               << "Hash expansion for " << v.ToString()
+               << " is non-deterministic.";
+      }
+      if (v.expand() != expected) {
+        return testing::AssertionFailure()
+               << "Values " << c[0].ToString() << " and " << v.ToString()
+               << " evaluate as equal but have an unequal hash expansion.";
+      }
+    }
+
+    // Elements from other classes must have different hash expansion.
+    for (const auto& c2 : classes) {
+      if (&c == &c2) continue;
+      const SpyHashState c2_hash = c2[0].expand();
+      switch (SpyHashState::Compare(expected, c2_hash)) {
+        case SpyHashState::CompareResult::kEqual:
+          return testing::AssertionFailure()
+                 << "Values " << c[0].ToString() << " and " << c2[0].ToString()
+                 << " evaluate as unequal but have an equal hash expansion.";
+        case SpyHashState::CompareResult::kBSuffixA:
+          return testing::AssertionFailure()
+                 << "Hash expansion of " << c2[0].ToString()
+                 << " is a suffix of the hash expansion of " << c[0].ToString()
+                 << ".";
+        case SpyHashState::CompareResult::kASuffixB:
+          return testing::AssertionFailure()
+                 << "Hash expansion of " << c[0].ToString()
+                 << " is a suffix of the hash expansion of " << c2[0].ToString()
+                 << ".";
+        case SpyHashState::CompareResult::kUnequal:
+          break;
+      }
+    }
+  }
+  return testing::AssertionSuccess();
+}
+
+template <typename... T>
+struct TypeSet {
+  template <typename U, bool = disjunction<std::is_same<T, U>...>::value>
+  struct Insert {
+    using type = TypeSet<U, T...>;
+  };
+  template <typename U>
+  struct Insert<U, true> {
+    using type = TypeSet;
+  };
+
+  template <template <typename...> class C>
+  using apply = C<T...>;
+};
+
+template <typename... T>
+struct MakeTypeSet : TypeSet<>{};
+template <typename T, typename... Ts>
+struct MakeTypeSet<T, Ts...> : MakeTypeSet<Ts...>::template Insert<T>::type {};
+
+template <typename... T>
+using VariantForTypes = typename MakeTypeSet<
+    const typename std::decay<T>::type*...>::template apply<absl::variant>;
+
+template <typename Container>
+struct ContainerAsVector {
+  using V = absl::variant<const typename Container::value_type*>;
+  using Out = std::vector<V>;
+
+  static Out Do(const Container& values) {
+    Out out;
+    for (const auto& v : values) out.push_back(&v);
+    return out;
+  }
+};
+
+template <typename... T>
+struct ContainerAsVector<std::tuple<T...>> {
+  using V = VariantForTypes<T...>;
+  using Out = std::vector<V>;
+
+  template <size_t... I>
+  static Out DoImpl(const std::tuple<T...>& tuple, absl::index_sequence<I...>) {
+    return Out{&std::get<I>(tuple)...};
+  }
+
+  static Out Do(const std::tuple<T...>& values) {
+    return DoImpl(values, absl::index_sequence_for<T...>());
+  }
+};
+
+template <>
+struct ContainerAsVector<std::tuple<>> {
+  static std::vector<VariantForTypes<int>> Do(std::tuple<>) { return {}; }
+};
+
+struct DefaultEquals {
+  template <typename T, typename U>
+  bool operator()(const T& t, const U& u) const {
+    return t == u;
+  }
+};
+
+}  // namespace hash_internal
+
+template <int&..., typename Container>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(const Container& values) {
+  return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
+      hash_internal::ContainerAsVector<Container>::Do(values),
+      hash_internal::DefaultEquals{});
+}
+
+template <int&..., typename Container, typename Eq>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
+  return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
+      hash_internal::ContainerAsVector<Container>::Do(values),
+      equals);
+}
+
+template <int&..., typename T>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values) {
+  return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
+      hash_internal::ContainerAsVector<std::initializer_list<T>>::Do(values),
+      hash_internal::DefaultEquals{});
+}
+
+template <int&..., typename T, typename Eq>
+ABSL_MUST_USE_RESULT testing::AssertionResult
+VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values,
+                                      Eq equals) {
+  return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
+      hash_internal::ContainerAsVector<std::initializer_list<T>>::Do(values),
+      equals);
+}
+
+}  // namespace absl
+
+#endif  // ABSL_HASH_HASH_TESTING_H_
diff --git a/absl/hash/internal/city.cc b/absl/hash/internal/city.cc
new file mode 100644
index 000000000000..591017abbea4
--- /dev/null
+++ b/absl/hash/internal/city.cc
@@ -0,0 +1,589 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This file provides CityHash64() and related functions.
+//
+// It's probably possible to create even faster hash functions by
+// writing a program that systematically explores some of the space of
+// possible hash functions, by using SIMD instructions, or by
+// compromising on hash quality.
+
+#include "absl/hash/internal/city.h"
+
+#include <string.h>  // for memcpy and memset
+#include <algorithm>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/unaligned_access.h"
+#include "absl/base/optimization.h"
+
+namespace absl {
+namespace hash_internal {
+
+#ifdef ABSL_IS_BIG_ENDIAN
+#define uint32_in_expected_order(x) (absl::gbswap_32(x))
+#define uint64_in_expected_order(x) (absl::gbswap_64(x))
+#else
+#define uint32_in_expected_order(x) (x)
+#define uint64_in_expected_order(x) (x)
+#endif
+
+static uint64_t Fetch64(const char *p) {
+  return uint64_in_expected_order(ABSL_INTERNAL_UNALIGNED_LOAD64(p));
+}
+
+static uint32_t Fetch32(const char *p) {
+  return uint32_in_expected_order(ABSL_INTERNAL_UNALIGNED_LOAD32(p));
+}
+
+// Some primes between 2^63 and 2^64 for various uses.
+static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
+static const uint64_t k1 = 0xb492b66fbe98f273ULL;
+static const uint64_t k2 = 0x9ae16a3b2f90404fULL;
+
+// Magic numbers for 32-bit hashing.  Copied from Murmur3.
+static const uint32_t c1 = 0xcc9e2d51;
+static const uint32_t c2 = 0x1b873593;
+
+// A 32-bit to 32-bit integer hash copied from Murmur3.
+static uint32_t fmix(uint32_t h) {
+  h ^= h >> 16;
+  h *= 0x85ebca6b;
+  h ^= h >> 13;
+  h *= 0xc2b2ae35;
+  h ^= h >> 16;
+  return h;
+}
+
+static uint32_t Rotate32(uint32_t val, int shift) {
+  // Avoid shifting by 32: doing so yields an undefined result.
+  return shift == 0 ? val : ((val >> shift) | (val << (32 - shift)));
+}
+
+#undef PERMUTE3
+#define PERMUTE3(a, b, c) \
+  do {                    \
+    std::swap(a, b);      \
+    std::swap(a, c);      \
+  } while (0)
+
+static uint32_t Mur(uint32_t a, uint32_t h) {
+  // Helper from Murmur3 for combining two 32-bit values.
+  a *= c1;
+  a = Rotate32(a, 17);
+  a *= c2;
+  h ^= a;
+  h = Rotate32(h, 19);
+  return h * 5 + 0xe6546b64;
+}
+
+static uint32_t Hash32Len13to24(const char *s, size_t len) {
+  uint32_t a = Fetch32(s - 4 + (len >> 1));
+  uint32_t b = Fetch32(s + 4);
+  uint32_t c = Fetch32(s + len - 8);
+  uint32_t d = Fetch32(s + (len >> 1));
+  uint32_t e = Fetch32(s);
+  uint32_t f = Fetch32(s + len - 4);
+  uint32_t h = len;
+
+  return fmix(Mur(f, Mur(e, Mur(d, Mur(c, Mur(b, Mur(a, h)))))));
+}
+
+static uint32_t Hash32Len0to4(const char *s, size_t len) {
+  uint32_t b = 0;
+  uint32_t c = 9;
+  for (size_t i = 0; i < len; i++) {
+    signed char v = s[i];
+    b = b * c1 + v;
+    c ^= b;
+  }
+  return fmix(Mur(b, Mur(len, c)));
+}
+
+static uint32_t Hash32Len5to12(const char *s, size_t len) {
+  uint32_t a = len, b = len * 5, c = 9, d = b;
+  a += Fetch32(s);
+  b += Fetch32(s + len - 4);
+  c += Fetch32(s + ((len >> 1) & 4));
+  return fmix(Mur(c, Mur(b, Mur(a, d))));
+}
+
+uint32_t CityHash32(const char *s, size_t len) {
+  if (len <= 24) {
+    return len <= 12
+               ? (len <= 4 ? Hash32Len0to4(s, len) : Hash32Len5to12(s, len))
+               : Hash32Len13to24(s, len);
+  }
+
+  // len > 24
+  uint32_t h = len, g = c1 * len, f = g;
+  uint32_t a0 = Rotate32(Fetch32(s + len - 4) * c1, 17) * c2;
+  uint32_t a1 = Rotate32(Fetch32(s + len - 8) * c1, 17) * c2;
+  uint32_t a2 = Rotate32(Fetch32(s + len - 16) * c1, 17) * c2;
+  uint32_t a3 = Rotate32(Fetch32(s + len - 12) * c1, 17) * c2;
+  uint32_t a4 = Rotate32(Fetch32(s + len - 20) * c1, 17) * c2;
+  h ^= a0;
+  h = Rotate32(h, 19);
+  h = h * 5 + 0xe6546b64;
+  h ^= a2;
+  h = Rotate32(h, 19);
+  h = h * 5 + 0xe6546b64;
+  g ^= a1;
+  g = Rotate32(g, 19);
+  g = g * 5 + 0xe6546b64;
+  g ^= a3;
+  g = Rotate32(g, 19);
+  g = g * 5 + 0xe6546b64;
+  f += a4;
+  f = Rotate32(f, 19);
+  f = f * 5 + 0xe6546b64;
+  size_t iters = (len - 1) / 20;
+  do {
+    uint32_t a0 = Rotate32(Fetch32(s) * c1, 17) * c2;
+    uint32_t a1 = Fetch32(s + 4);
+    uint32_t a2 = Rotate32(Fetch32(s + 8) * c1, 17) * c2;
+    uint32_t a3 = Rotate32(Fetch32(s + 12) * c1, 17) * c2;
+    uint32_t a4 = Fetch32(s + 16);
+    h ^= a0;
+    h = Rotate32(h, 18);
+    h = h * 5 + 0xe6546b64;
+    f += a1;
+    f = Rotate32(f, 19);
+    f = f * c1;
+    g += a2;
+    g = Rotate32(g, 18);
+    g = g * 5 + 0xe6546b64;
+    h ^= a3 + a1;
+    h = Rotate32(h, 19);
+    h = h * 5 + 0xe6546b64;
+    g ^= a4;
+    g = absl::gbswap_32(g) * 5;
+    h += a4 * 5;
+    h = absl::gbswap_32(h);
+    f += a0;
+    PERMUTE3(f, h, g);
+    s += 20;
+  } while (--iters != 0);
+  g = Rotate32(g, 11) * c1;
+  g = Rotate32(g, 17) * c1;
+  f = Rotate32(f, 11) * c1;
+  f = Rotate32(f, 17) * c1;
+  h = Rotate32(h + g, 19);
+  h = h * 5 + 0xe6546b64;
+  h = Rotate32(h, 17) * c1;
+  h = Rotate32(h + f, 19);
+  h = h * 5 + 0xe6546b64;
+  h = Rotate32(h, 17) * c1;
+  return h;
+}
+
+// Bitwise right rotate.  Normally this will compile to a single
+// instruction, especially if the shift is a manifest constant.
+static uint64_t Rotate(uint64_t val, int shift) {
+  // Avoid shifting by 64: doing so yields an undefined result.
+  return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
+}
+
+static uint64_t ShiftMix(uint64_t val) { return val ^ (val >> 47); }
+
+static uint64_t HashLen16(uint64_t u, uint64_t v) {
+  return Hash128to64(uint128(u, v));
+}
+
+static uint64_t HashLen16(uint64_t u, uint64_t v, uint64_t mul) {
+  // Murmur-inspired hashing.
+  uint64_t a = (u ^ v) * mul;
+  a ^= (a >> 47);
+  uint64_t b = (v ^ a) * mul;
+  b ^= (b >> 47);
+  b *= mul;
+  return b;
+}
+
+static uint64_t HashLen0to16(const char *s, size_t len) {
+  if (len >= 8) {
+    uint64_t mul = k2 + len * 2;
+    uint64_t a = Fetch64(s) + k2;
+    uint64_t b = Fetch64(s + len - 8);
+    uint64_t c = Rotate(b, 37) * mul + a;
+    uint64_t d = (Rotate(a, 25) + b) * mul;
+    return HashLen16(c, d, mul);
+  }
+  if (len >= 4) {
+    uint64_t mul = k2 + len * 2;
+    uint64_t a = Fetch32(s);
+    return HashLen16(len + (a << 3), Fetch32(s + len - 4), mul);
+  }
+  if (len > 0) {
+    uint8_t a = s[0];
+    uint8_t b = s[len >> 1];
+    uint8_t c = s[len - 1];
+    uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
+    uint32_t z = len + (static_cast<uint32_t>(c) << 2);
+    return ShiftMix(y * k2 ^ z * k0) * k2;
+  }
+  return k2;
+}
+
+// This probably works well for 16-byte strings as well, but it may be overkill
+// in that case.
+static uint64_t HashLen17to32(const char *s, size_t len) {
+  uint64_t mul = k2 + len * 2;
+  uint64_t a = Fetch64(s) * k1;
+  uint64_t b = Fetch64(s + 8);
+  uint64_t c = Fetch64(s + len - 8) * mul;
+  uint64_t d = Fetch64(s + len - 16) * k2;
+  return HashLen16(Rotate(a + b, 43) + Rotate(c, 30) + d,
+                   a + Rotate(b + k2, 18) + c, mul);
+}
+
+// Return a 16-byte hash for 48 bytes.  Quick and dirty.
+// Callers do best to use "random-looking" values for a and b.
+static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(uint64_t w, uint64_t x,
+                                                        uint64_t y, uint64_t z,
+                                                        uint64_t a, uint64_t b) {
+  a += w;
+  b = Rotate(b + a + z, 21);
+  uint64_t c = a;
+  a += x;
+  a += y;
+  b += Rotate(a, 44);
+  return std::make_pair(a + z, b + c);
+}
+
+// Return a 16-byte hash for s[0] ... s[31], a, and b.  Quick and dirty.
+static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(const char *s, uint64_t a,
+                                                        uint64_t b) {
+  return WeakHashLen32WithSeeds(Fetch64(s), Fetch64(s + 8), Fetch64(s + 16),
+                                Fetch64(s + 24), a, b);
+}
+
+// Return an 8-byte hash for 33 to 64 bytes.
+static uint64_t HashLen33to64(const char *s, size_t len) {
+  uint64_t mul = k2 + len * 2;
+  uint64_t a = Fetch64(s) * k2;
+  uint64_t b = Fetch64(s + 8);
+  uint64_t c = Fetch64(s + len - 24);
+  uint64_t d = Fetch64(s + len - 32);
+  uint64_t e = Fetch64(s + 16) * k2;
+  uint64_t f = Fetch64(s + 24) * 9;
+  uint64_t g = Fetch64(s + len - 8);
+  uint64_t h = Fetch64(s + len - 16) * mul;
+  uint64_t u = Rotate(a + g, 43) + (Rotate(b, 30) + c) * 9;
+  uint64_t v = ((a + g) ^ d) + f + 1;
+  uint64_t w = absl::gbswap_64((u + v) * mul) + h;
+  uint64_t x = Rotate(e + f, 42) + c;
+  uint64_t y = (absl::gbswap_64((v + w) * mul) + g) * mul;
+  uint64_t z = e + f + c;
+  a = absl::gbswap_64((x + z) * mul + y) + b;
+  b = ShiftMix((z + a) * mul + d + h) * mul;
+  return b + x;
+}
+
+uint64_t CityHash64(const char *s, size_t len) {
+  if (len <= 32) {
+    if (len <= 16) {
+      return HashLen0to16(s, len);
+    } else {
+      return HashLen17to32(s, len);
+    }
+  } else if (len <= 64) {
+    return HashLen33to64(s, len);
+  }
+
+  // For strings over 64 bytes we hash the end first, and then as we
+  // loop we keep 56 bytes of state: v, w, x, y, and z.
+  uint64_t x = Fetch64(s + len - 40);
+  uint64_t y = Fetch64(s + len - 16) + Fetch64(s + len - 56);
+  uint64_t z = HashLen16(Fetch64(s + len - 48) + len, Fetch64(s + len - 24));
+  std::pair<uint64_t, uint64_t> v = WeakHashLen32WithSeeds(s + len - 64, len, z);
+  std::pair<uint64_t, uint64_t> w = WeakHashLen32WithSeeds(s + len - 32, y + k1, x);
+  x = x * k1 + Fetch64(s);
+
+  // Decrease len to the nearest multiple of 64, and operate on 64-byte chunks.
+  len = (len - 1) & ~static_cast<size_t>(63);
+  do {
+    x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
+    y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
+    x ^= w.second;
+    y += v.first + Fetch64(s + 40);
+    z = Rotate(z + w.first, 33) * k1;
+    v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
+    w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
+    std::swap(z, x);
+    s += 64;
+    len -= 64;
+  } while (len != 0);
+  return HashLen16(HashLen16(v.first, w.first) + ShiftMix(y) * k1 + z,
+                   HashLen16(v.second, w.second) + x);
+}
+
+uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed) {
+  return CityHash64WithSeeds(s, len, k2, seed);
+}
+
+uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
+                           uint64_t seed1) {
+  return HashLen16(CityHash64(s, len) - seed0, seed1);
+}
+
+// A subroutine for CityHash128().  Returns a decent 128-bit hash for strings
+// of any length representable in signed long.  Based on City and Murmur.
+static uint128 CityMurmur(const char *s, size_t len, uint128 seed) {
+  uint64_t a = Uint128Low64(seed);
+  uint64_t b = Uint128High64(seed);
+  uint64_t c = 0;
+  uint64_t d = 0;
+  int64_t l = len - 16;
+  if (l <= 0) {  // len <= 16
+    a = ShiftMix(a * k1) * k1;
+    c = b * k1 + HashLen0to16(s, len);
+    d = ShiftMix(a + (len >= 8 ? Fetch64(s) : c));
+  } else {  // len > 16
+    c = HashLen16(Fetch64(s + len - 8) + k1, a);
+    d = HashLen16(b + len, c + Fetch64(s + len - 16));
+    a += d;
+    do {
+      a ^= ShiftMix(Fetch64(s) * k1) * k1;
+      a *= k1;
+      b ^= a;
+      c ^= ShiftMix(Fetch64(s + 8) * k1) * k1;
+      c *= k1;
+      d ^= c;
+      s += 16;
+      l -= 16;
+    } while (l > 0);
+  }
+  a = HashLen16(a, c);
+  b = HashLen16(d, b);
+  return uint128(a ^ b, HashLen16(b, a));
+}
+
+uint128 CityHash128WithSeed(const char *s, size_t len, uint128 seed) {
+  if (len < 128) {
+    return CityMurmur(s, len, seed);
+  }
+
+  // We expect len >= 128 to be the common case.  Keep 56 bytes of state:
+  // v, w, x, y, and z.
+  std::pair<uint64_t, uint64_t> v, w;
+  uint64_t x = Uint128Low64(seed);
+  uint64_t y = Uint128High64(seed);
+  uint64_t z = len * k1;
+  v.first = Rotate(y ^ k1, 49) * k1 + Fetch64(s);
+  v.second = Rotate(v.first, 42) * k1 + Fetch64(s + 8);
+  w.first = Rotate(y + z, 35) * k1 + x;
+  w.second = Rotate(x + Fetch64(s + 88), 53) * k1;
+
+  // This is the same inner loop as CityHash64(), manually unrolled.
+  do {
+    x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
+    y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
+    x ^= w.second;
+    y += v.first + Fetch64(s + 40);
+    z = Rotate(z + w.first, 33) * k1;
+    v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
+    w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
+    std::swap(z, x);
+    s += 64;
+    x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
+    y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
+    x ^= w.second;
+    y += v.first + Fetch64(s + 40);
+    z = Rotate(z + w.first, 33) * k1;
+    v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
+    w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
+    std::swap(z, x);
+    s += 64;
+    len -= 128;
+  } while (ABSL_PREDICT_TRUE(len >= 128));
+  x += Rotate(v.first + z, 49) * k0;
+  y = y * k0 + Rotate(w.second, 37);
+  z = z * k0 + Rotate(w.first, 27);
+  w.first *= 9;
+  v.first *= k0;
+  // If 0 < len < 128, hash up to 4 chunks of 32 bytes each from the end of s.
+  for (size_t tail_done = 0; tail_done < len;) {
+    tail_done += 32;
+    y = Rotate(x + y, 42) * k0 + v.second;
+    w.first += Fetch64(s + len - tail_done + 16);
+    x = x * k0 + w.first;
+    z += w.second + Fetch64(s + len - tail_done);
+    w.second += v.first;
+    v = WeakHashLen32WithSeeds(s + len - tail_done, v.first + z, v.second);
+    v.first *= k0;
+  }
+  // At this point our 56 bytes of state should contain more than
+  // enough information for a strong 128-bit hash.  We use two
+  // different 56-byte-to-8-byte hashes to get a 16-byte final result.
+  x = HashLen16(x, v.first);
+  y = HashLen16(y + z, w.first);
+  return uint128(HashLen16(x + v.second, w.second) + y,
+                 HashLen16(x + w.second, y + v.second));
+}
+
+uint128 CityHash128(const char *s, size_t len) {
+  return len >= 16
+             ? CityHash128WithSeed(s + 16, len - 16,
+                                   uint128(Fetch64(s), Fetch64(s + 8) + k0))
+             : CityHash128WithSeed(s, len, uint128(k0, k1));
+}
+}  // namespace hash_internal
+}  // namespace absl
+
+#ifdef __SSE4_2__
+#include <nmmintrin.h>
+#include "absl/hash/internal/city_crc.h"
+
+namespace absl {
+namespace hash_internal {
+
+// Requires len >= 240.
+static void CityHashCrc256Long(const char *s, size_t len, uint32_t seed,
+                               uint64_t *result) {
+  uint64_t a = Fetch64(s + 56) + k0;
+  uint64_t b = Fetch64(s + 96) + k0;
+  uint64_t c = result[0] = HashLen16(b, len);
+  uint64_t d = result[1] = Fetch64(s + 120) * k0 + len;
+  uint64_t e = Fetch64(s + 184) + seed;
+  uint64_t f = 0;
+  uint64_t g = 0;
+  uint64_t h = c + d;
+  uint64_t x = seed;
+  uint64_t y = 0;
+  uint64_t z = 0;
+
+  // 240 bytes of input per iter.
+  size_t iters = len / 240;
+  len -= iters * 240;
+  do {
+#undef CHUNK
+#define CHUNK(r)               \
+  PERMUTE3(x, z, y);           \
+  b += Fetch64(s);             \
+  c += Fetch64(s + 8);         \
+  d += Fetch64(s + 16);        \
+  e += Fetch64(s + 24);        \
+  f += Fetch64(s + 32);        \
+  a += b;                      \
+  h += f;                      \
+  b += c;                      \
+  f += d;                      \
+  g += e;                      \
+  e += z;                      \
+  g += x;                      \
+  z = _mm_crc32_u64(z, b + g); \
+  y = _mm_crc32_u64(y, e + h); \
+  x = _mm_crc32_u64(x, f + a); \
+  e = Rotate(e, r);            \
+  c += e;                      \
+  s += 40
+
+    CHUNK(0);
+    PERMUTE3(a, h, c);
+    CHUNK(33);
+    PERMUTE3(a, h, f);
+    CHUNK(0);
+    PERMUTE3(b, h, f);
+    CHUNK(42);
+    PERMUTE3(b, h, d);
+    CHUNK(0);
+    PERMUTE3(b, h, e);
+    CHUNK(33);
+    PERMUTE3(a, h, e);
+  } while (--iters > 0);
+
+  while (len >= 40) {
+    CHUNK(29);
+    e ^= Rotate(a, 20);
+    h += Rotate(b, 30);
+    g ^= Rotate(c, 40);
+    f += Rotate(d, 34);
+    PERMUTE3(c, h, g);
+    len -= 40;
+  }
+  if (len > 0) {
+    s = s + len - 40;
+    CHUNK(33);
+    e ^= Rotate(a, 43);
+    h += Rotate(b, 42);
+    g ^= Rotate(c, 41);
+    f += Rotate(d, 40);
+  }
+  result[0] ^= h;
+  result[1] ^= g;
+  g += h;
+  a = HashLen16(a, g + z);
+  x += y << 32;
+  b += x;
+  c = HashLen16(c, z) + h;
+  d = HashLen16(d, e + result[0]);
+  g += e;
+  h += HashLen16(x, f);
+  e = HashLen16(a, d) + g;
+  z = HashLen16(b, c) + a;
+  y = HashLen16(g, h) + c;
+  result[0] = e + z + y + x;
+  a = ShiftMix((a + y) * k0) * k0 + b;
+  result[1] += a + result[0];
+  a = ShiftMix(a * k0) * k0 + c;
+  result[2] = a + result[1];
+  a = ShiftMix((a + e) * k0) * k0;
+  result[3] = a + result[2];
+}
+
+// Requires len < 240.
+static void CityHashCrc256Short(const char *s, size_t len, uint64_t *result) {
+  char buf[240];
+  memcpy(buf, s, len);
+  memset(buf + len, 0, 240 - len);
+  CityHashCrc256Long(buf, 240, ~static_cast<uint32_t>(len), result);
+}
+
+void CityHashCrc256(const char *s, size_t len, uint64_t *result) {
+  if (ABSL_PREDICT_TRUE(len >= 240)) {
+    CityHashCrc256Long(s, len, 0, result);
+  } else {
+    CityHashCrc256Short(s, len, result);
+  }
+}
+
+uint128 CityHashCrc128WithSeed(const char *s, size_t len, uint128 seed) {
+  if (len <= 900) {
+    return CityHash128WithSeed(s, len, seed);
+  } else {
+    uint64_t result[4];
+    CityHashCrc256(s, len, result);
+    uint64_t u = Uint128High64(seed) + result[0];
+    uint64_t v = Uint128Low64(seed) + result[1];
+    return uint128(HashLen16(u, v + result[2]),
+                   HashLen16(Rotate(v, 32), u * k0 + result[3]));
+  }
+}
+
+uint128 CityHashCrc128(const char *s, size_t len) {
+  if (len <= 900) {
+    return CityHash128(s, len);
+  } else {
+    uint64_t result[4];
+    CityHashCrc256(s, len, result);
+    return uint128(result[2], result[3]);
+  }
+}
+
+}  // namespace hash_internal
+}  // namespace absl
+
+#endif
diff --git a/absl/hash/internal/city.h b/absl/hash/internal/city.h
new file mode 100644
index 000000000000..55b37b875f0b
--- /dev/null
+++ b/absl/hash/internal/city.h
@@ -0,0 +1,108 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// http://code.google.com/p/cityhash/
+//
+// This file provides a few functions for hashing strings.  All of them are
+// high-quality functions in the sense that they pass standard tests such
+// as Austin Appleby's SMHasher.  They are also fast.
+//
+// For 64-bit x86 code, on short strings, we don't know of anything faster than
+// CityHash64 that is of comparable quality.  We believe our nearest competitor
+// is Murmur3.  For 64-bit x86 code, CityHash64 is an excellent choice for hash
+// tables and most other hashing (excluding cryptography).
+//
+// For 64-bit x86 code, on long strings, the picture is more complicated.
+// On many recent Intel CPUs, such as Nehalem, Westmere, Sandy Bridge, etc.,
+// CityHashCrc128 appears to be faster than all competitors of comparable
+// quality.  CityHash128 is also good but not quite as fast.  We believe our
+// nearest competitor is Bob Jenkins' Spooky.  We don't have great data for
+// other 64-bit CPUs, but for long strings we know that Spooky is slightly
+// faster than CityHash on some relatively recent AMD x86-64 CPUs, for example.
+// Note that CityHashCrc128 is declared in citycrc.h.
+//
+// For 32-bit x86 code, we don't know of anything faster than CityHash32 that
+// is of comparable quality.  We believe our nearest competitor is Murmur3A.
+// (On 64-bit CPUs, it is typically faster to use the other CityHash variants.)
+//
+// Functions in the CityHash family are not suitable for cryptography.
+//
+// Please see CityHash's README file for more details on our performance
+// measurements and so on.
+//
+// WARNING: This code has been only lightly tested on big-endian platforms!
+// It is known to work well on little-endian platforms that have a small penalty
+// for unaligned reads, such as current Intel and AMD moderate-to-high-end CPUs.
+// It should work on all 32-bit and 64-bit platforms that allow unaligned reads;
+// bug reports are welcome.
+//
+// By the way, for some hash functions, given strings a and b, the hash
+// of a+b is easily derived from the hashes of a and b.  This property
+// doesn't hold for any hash functions in this file.
+
+#ifndef ABSL_HASH_INTERNAL_CITY_H_
+#define ABSL_HASH_INTERNAL_CITY_H_
+
+#include <stdint.h>
+#include <stdlib.h>  // for size_t.
+#include <utility>
+
+
+namespace absl {
+namespace hash_internal {
+
+typedef std::pair<uint64_t, uint64_t> uint128;
+
+inline uint64_t Uint128Low64(const uint128 &x) { return x.first; }
+inline uint64_t Uint128High64(const uint128 &x) { return x.second; }
+
+// Hash function for a byte array.
+uint64_t CityHash64(const char *s, size_t len);
+
+// Hash function for a byte array.  For convenience, a 64-bit seed is also
+// hashed into the result.
+uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed);
+
+// Hash function for a byte array.  For convenience, two seeds are also
+// hashed into the result.
+uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
+                           uint64_t seed1);
+
+// Hash function for a byte array.
+uint128 CityHash128(const char *s, size_t len);
+
+// Hash function for a byte array.  For convenience, a 128-bit seed is also
+// hashed into the result.
+uint128 CityHash128WithSeed(const char *s, size_t len, uint128 seed);
+
+// Hash function for a byte array.  Most useful in 32-bit binaries.
+uint32_t CityHash32(const char *s, size_t len);
+
+// Hash 128 input bits down to 64 bits of output.
+// This is intended to be a reasonably good hash function.
+inline uint64_t Hash128to64(const uint128 &x) {
+  // Murmur-inspired hashing.
+  const uint64_t kMul = 0x9ddfea08eb382d69ULL;
+  uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
+  a ^= (a >> 47);
+  uint64_t b = (Uint128High64(x) ^ a) * kMul;
+  b ^= (b >> 47);
+  b *= kMul;
+  return b;
+}
+
+}  // namespace hash_internal
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_CITY_H_
diff --git a/absl/hash/internal/city_crc.h b/absl/hash/internal/city_crc.h
new file mode 100644
index 000000000000..6be6557d213f
--- /dev/null
+++ b/absl/hash/internal/city_crc.h
@@ -0,0 +1,41 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This file declares the subset of the CityHash functions that require
+// _mm_crc32_u64().  See the CityHash README for details.
+//
+// Functions in the CityHash family are not suitable for cryptography.
+
+#ifndef ABSL_HASH_INTERNAL_CITY_CRC_H_
+#define ABSL_HASH_INTERNAL_CITY_CRC_H_
+
+#include "absl/hash/internal/city.h"
+
+namespace absl {
+namespace hash_internal {
+
+// Hash function for a byte array.
+uint128 CityHashCrc128(const char *s, size_t len);
+
+// Hash function for a byte array.  For convenience, a 128-bit seed is also
+// hashed into the result.
+uint128 CityHashCrc128WithSeed(const char *s, size_t len, uint128 seed);
+
+// Hash function for a byte array.  Sets result[0] ... result[3].
+void CityHashCrc256(const char *s, size_t len, uint64_t *result);
+
+}  // namespace hash_internal
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_CITY_CRC_H_
diff --git a/absl/hash/internal/city_test.cc b/absl/hash/internal/city_test.cc
new file mode 100644
index 000000000000..678da53d34d0
--- /dev/null
+++ b/absl/hash/internal/city_test.cc
@@ -0,0 +1,1812 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/hash/internal/city.h"
+
+#include <string.h>
+#include <cstdio>
+#include <iostream>
+#include "gtest/gtest.h"
+#ifdef __SSE4_2__
+#include "absl/hash/internal/city_crc.h"
+#endif
+
+namespace absl {
+namespace hash_internal {
+
+static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
+static const uint64_t kSeed0 = 1234567;
+static const uint64_t kSeed1 = k0;
+static const uint128 kSeed128(kSeed0, kSeed1);
+static const int kDataSize = 1 << 20;
+static const int kTestSize = 300;
+
+static char data[kDataSize];
+
+// Initialize data to pseudorandom values.
+void setup() {
+  uint64_t a = 9;
+  uint64_t b = 777;
+  for (int i = 0; i < kDataSize; i++) {
+    a += b;
+    b += a;
+    a = (a ^ (a >> 41)) * k0;
+    b = (b ^ (b >> 41)) * k0 + i;
+    uint8_t u = b >> 37;
+    memcpy(data + i, &u, 1);  // uint8_t -> char
+  }
+}
+
+#define C(x) 0x##x##ULL
+static const uint64_t testdata[kTestSize][16] = {
+    {C(9ae16a3b2f90404f), C(75106db890237a4a), C(3feac5f636039766),
+     C(3df09dfc64c09a2b), C(3cb540c392e51e29), C(6b56343feac0663),
+     C(5b7bc50fd8e8ad92), C(3df09dfc64c09a2b), C(3cb540c392e51e29),
+     C(6b56343feac0663), C(5b7bc50fd8e8ad92), C(95162f24e6a5f930),
+     C(6808bdf4f1eb06e0), C(b3b1f3a67b624d82), C(c9a62f12bd4cd80b),
+     C(dc56d17a)},
+    {C(541150e87f415e96), C(1aef0d24b3148a1a), C(bacc300e1e82345a),
+     C(c3cdc41e1df33513), C(2c138ff2596d42f6), C(f58e9082aed3055f),
+     C(162e192b2957163d), C(c3cdc41e1df33513), C(2c138ff2596d42f6),
+     C(f58e9082aed3055f), C(162e192b2957163d), C(fb99e85e0d16f90c),
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+     C(3cc79a9e334e1f84), C(b1b7ec44f9302176), C(5cb476450dc0c297),
+     C(dc5ef652521ef6a2), C(3cc79a9e334e1f84), C(769e2a283dbcc651),
+     C(9f24b105c8511d3f), C(c31c15575de2f27e), C(ecfecf32c3ae2d66),
+     C(dea4e3dd)},
+    {C(392058251cf22acc), C(944ec4475ead4620), C(b330a10b5cb94166),
+     C(54bc9bee7cbe1767), C(485820bdbe442431), C(54d6120ea2972e90),
+     C(f437a0341f29b72a), C(54bc9bee7cbe1767), C(485820bdbe442431),
+     C(54d6120ea2972e90), C(f437a0341f29b72a), C(8f30885c784d5704),
+     C(aa95376b16c7906a), C(e826928cfaf93dc3), C(20e8f54d1c16d7d8),
+     C(c6064f22)},
+    {C(adf5c1e5d6419947), C(2a9747bc659d28aa), C(95c5b8cb1f5d62c),
+     C(80973ea532b0f310), C(a471829aa9c17dd9), C(c2ff3479394804ab),
+     C(6bf44f8606753636), C(80973ea532b0f310), C(a471829aa9c17dd9),
+     C(c2ff3479394804ab), C(6bf44f8606753636), C(5184d2973e6dd827),
+     C(121b96369a332d9a), C(5c25d3475ab69e50), C(26d2961d62884168),
+     C(743bed9c)},
+    {C(6bc1db2c2bee5aba), C(e63b0ed635307398), C(7b2eca111f30dbbc),
+     C(230d2b3e47f09830), C(ec8624a821c1caf4), C(ea6ec411cdbf1cb1),
+     C(5f38ae82af364e27), C(230d2b3e47f09830), C(ec8624a821c1caf4),
+     C(ea6ec411cdbf1cb1), C(5f38ae82af364e27), C(a519ef515ea7187c),
+     C(6bad5efa7ebae05f), C(748abacb11a74a63), C(a28eef963d1396eb),
+     C(fce254d5)},
+    {C(b00f898229efa508), C(83b7590ad7f6985c), C(2780e70a0592e41d),
+     C(7122413bdbc94035), C(e7f90fae33bf7763), C(4b6bd0fb30b12387),
+     C(557359c0c44f48ca), C(7122413bdbc94035), C(e7f90fae33bf7763),
+     C(4b6bd0fb30b12387), C(557359c0c44f48ca), C(d5656c3d6bc5f0d),
+     C(983ff8e5e784da99), C(628479671b445bf), C(e179a1e27ce68f5d), C(e47ec9d1)},
+    {C(b56eb769ce0d9a8c), C(ce196117bfbcaf04), C(b26c3c3797d66165),
+     C(5ed12338f630ab76), C(fab19fcb319116d), C(167f5f42b521724b),
+     C(c4aa56c409568d74), C(5ed12338f630ab76), C(fab19fcb319116d),
+     C(167f5f42b521724b), C(c4aa56c409568d74), C(75fff4b42f8e9778),
+     C(94218f94710c1ea3), C(b7b05efb738b06a6), C(83fff2deabf9cd3), C(334a145c)},
+    {C(70c0637675b94150), C(259e1669305b0a15), C(46e1dd9fd387a58d),
+     C(fca4e5bc9292788e), C(cd509dc1facce41c), C(bbba575a59d82fe),
+     C(4e2e71c15b45d4d3), C(fca4e5bc9292788e), C(cd509dc1facce41c),
+     C(bbba575a59d82fe), C(4e2e71c15b45d4d3), C(5dc54582ead999c),
+     C(72612d1571963c6f), C(30318a9d2d3d1829), C(785dd00f4cc9c9a0),
+     C(adec1e3c)},
+    {C(74c0b8a6821faafe), C(abac39d7491370e7), C(faf0b2a48a4e6aed),
+     C(967e970df9673d2a), C(d465247cffa415c0), C(33a1df0ca1107722),
+     C(49fc2a10adce4a32), C(967e970df9673d2a), C(d465247cffa415c0),
+     C(33a1df0ca1107722), C(49fc2a10adce4a32), C(c5707e079a284308),
+     C(573028266635dda6), C(f786f5eee6127fa0), C(b30d79cebfb51266),
+     C(f6a9fbf8)},
+    {C(5fb5e48ac7b7fa4f), C(a96170f08f5acbc7), C(bbf5c63d4f52a1e5),
+     C(6cc09e60700563e9), C(d18f23221e964791), C(ffc23eeef7af26eb),
+     C(693a954a3622a315), C(815308a32a9b0daf), C(efb2ab27bf6fd0bd),
+     C(9f1ffc0986111118), C(f9a3aa1778ea3985), C(698fe54b2b93933b),
+     C(dacc2b28404d0f10), C(815308a32a9b0daf), C(efb2ab27bf6fd0bd),
+     C(5398210c)},
+};
+
+void TestUnchanging(const uint64_t* expected, int offset, int len) {
+  const uint128 u = CityHash128(data + offset, len);
+  const uint128 v = CityHash128WithSeed(data + offset, len, kSeed128);
+  EXPECT_EQ(expected[0], CityHash64(data + offset, len));
+  EXPECT_EQ(expected[15], CityHash32(data + offset, len));
+  EXPECT_EQ(expected[1], CityHash64WithSeed(data + offset, len, kSeed0));
+  EXPECT_EQ(expected[2],
+            CityHash64WithSeeds(data + offset, len, kSeed0, kSeed1));
+  EXPECT_EQ(expected[3], Uint128Low64(u));
+  EXPECT_EQ(expected[4], Uint128High64(u));
+  EXPECT_EQ(expected[5], Uint128Low64(v));
+  EXPECT_EQ(expected[6], Uint128High64(v));
+#ifdef __SSE4_2__
+  const uint128 y = CityHashCrc128(data + offset, len);
+  const uint128 z = CityHashCrc128WithSeed(data + offset, len, kSeed128);
+  uint64_t crc256_results[4];
+  CityHashCrc256(data + offset, len, crc256_results);
+  EXPECT_EQ(expected[7], Uint128Low64(y));
+  EXPECT_EQ(expected[8], Uint128High64(y));
+  EXPECT_EQ(expected[9], Uint128Low64(z));
+  EXPECT_EQ(expected[10], Uint128High64(z));
+  for (int i = 0; i < 4; i++) {
+    EXPECT_EQ(expected[11 + i], crc256_results[i]);
+  }
+#endif
+}
+
+TEST(CityHashTest, Unchanging) {
+  setup();
+  int i = 0;
+  for (; i < kTestSize - 1; i++) {
+    TestUnchanging(testdata[i], i * i, i);
+  }
+  TestUnchanging(testdata[i], 0, kDataSize);
+}
+
+}  // namespace hash_internal
+}  // namespace absl
diff --git a/absl/hash/internal/hash.cc b/absl/hash/internal/hash.cc
new file mode 100644
index 000000000000..4bf64096e7ea
--- /dev/null
+++ b/absl/hash/internal/hash.cc
@@ -0,0 +1,23 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/hash/internal/hash.h"
+
+namespace absl {
+namespace hash_internal {
+
+ABSL_CONST_INIT const void* const CityHashState::kSeed = &kSeed;
+
+}  // namespace hash_internal
+}  // namespace absl
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
new file mode 100644
index 000000000000..4543d679a837
--- /dev/null
+++ b/absl/hash/internal/hash.h
@@ -0,0 +1,885 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: hash.h
+// -----------------------------------------------------------------------------
+//
+#ifndef ABSL_HASH_INTERNAL_HASH_H_
+#define ABSL_HASH_INTERNAL_HASH_H_
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <cstring>
+#include <deque>
+#include <forward_list>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <list>
+#include <map>
+#include <memory>
+#include <set>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "absl/base/internal/endian.h"
+#include "absl/base/port.h"
+#include "absl/container/fixed_array.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/int128.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+#include "absl/types/variant.h"
+#include "absl/utility/utility.h"
+#include "absl/hash/internal/city.h"
+
+namespace absl {
+namespace hash_internal {
+
+// HashStateBase
+//
+// A hash state object represents an intermediate state in the computation
+// of an unspecified hash algorithm. `HashStateBase` provides a CRTP style
+// base class for hash state implementations. Developers adding type support
+// for `absl::Hash` should not rely on any parts of the state object other than
+// the following member functions:
+//
+//   * HashStateBase::combine()
+//   * HashStateBase::combine_contiguous()
+//
+// A derived hash state class of type `H` must provide a static member function
+// with a signature similar to the following:
+//
+//    `static H combine_contiguous(H state, const unsigned char*, size_t)`.
+//
+// `HashStateBase` will provide a complete implementations for a hash state
+// object in terms of this method.
+//
+// Example:
+//
+//   // Use CRTP to define your derived class.
+//   struct MyHashState : HashStateBase<MyHashState> {
+//       static H combine_contiguous(H state, const unsigned char*, size_t);
+//       using MyHashState::HashStateBase::combine;
+//       using MyHashState::HashStateBase::combine_contiguous;
+//   };
+template <typename H>
+class HashStateBase {
+ public:
+  // HashStateBase::combine()
+  //
+  // Combines an arbitrary number of values into a hash state, returning the
+  // updated state.
+  //
+  // Each of the value types `T` must be separately hashable by the Abseil
+  // hashing framework.
+  //
+  // NOTE:
+  //
+  //   state = H::combine(std::move(state), value1, value2, value3);
+  //
+  // is guaranteed to produce the same hash expansion as:
+  //
+  //   state = H::combine(std::move(state), value1);
+  //   state = H::combine(std::move(state), value2);
+  //   state = H::combine(std::move(state), value3);
+  template <typename T, typename... Ts>
+  static H combine(H state, const T& value, const Ts&... values);
+  static H combine(H state) { return state; }
+
+  // HashStateBase::combine_contiguous()
+  //
+  // Combines a contiguous array of `size` elements into a hash state, returning
+  // the updated state.
+  //
+  // NOTE:
+  //
+  //   state = H::combine_contiguous(std::move(state), data, size);
+  //
+  // is NOT guaranteed to produce the same hash expansion as a for-loop (it may
+  // perform internal optimizations).  If you need this guarantee, use the
+  // for-loop instead.
+  template <typename T>
+  static H combine_contiguous(H state, const T* data, size_t size);
+};
+
+// is_uniquely_represented
+//
+// `is_uniquely_represented<T>` is a trait class that indicates whether `T`
+// is uniquely represented.
+//
+// A type is "uniquely represented" if two equal values of that type are
+// guaranteed to have the same bytes in their underlying storage. In other
+// words, if `a == b`, then `memcmp(&a, &b, sizeof(T))` is guaranteed to be
+// zero. This property cannot be detected automatically, so this trait is false
+// by default, but can be specialized by types that wish to assert that they are
+// uniquely represented. This makes them eligible for certain optimizations.
+//
+// If you have any doubt whatsoever, do not specialize this template.
+// The default is completely safe, and merely disables some optimizations
+// that will not matter for most types. Specializing this template,
+// on the other hand, can be very hazardous.
+//
+// To be uniquely represented, a type must not have multiple ways of
+// representing the same value; for example, float and double are not
+// uniquely represented, because they have distinct representations for
+// +0 and -0. Furthermore, the type's byte representation must consist
+// solely of user-controlled data, with no padding bits and no compiler-
+// controlled data such as vptrs or sanitizer metadata. This is usually
+// very difficult to guarantee, because in most cases the compiler can
+// insert data and padding bits at its own discretion.
+//
+// If you specialize this template for a type `T`, you must do so in the file
+// that defines that type (or in this file). If you define that specialization
+// anywhere else, `is_uniquely_represented<T>` could have different meanings
+// in different places.
+//
+// The Enable parameter is meaningless; it is provided as a convenience,
+// to support certain SFINAE techniques when defining specializations.
+template <typename T, typename Enable = void>
+struct is_uniquely_represented : std::false_type {};
+
+// is_uniquely_represented<unsigned char>
+//
+// unsigned char is a synonym for "byte", so it is guaranteed to be
+// uniquely represented.
+template <>
+struct is_uniquely_represented<unsigned char> : std::true_type {};
+
+// is_uniquely_represented for non-standard integral types
+//
+// Integral types other than bool should be uniquely represented on any
+// platform that this will plausibly be ported to.
+template <typename Integral>
+struct is_uniquely_represented<
+    Integral, typename std::enable_if<std::is_integral<Integral>::value>::type>
+    : std::true_type {};
+
+// is_uniquely_represented<bool>
+//
+//
+template <>
+struct is_uniquely_represented<bool> : std::false_type {};
+
+// hash_bytes()
+//
+// Convenience function that combines `hash_state` with the byte representation
+// of `value`.
+template <typename H, typename T>
+H hash_bytes(H hash_state, const T& value) {
+  const unsigned char* start = reinterpret_cast<const unsigned char*>(&value);
+  return H::combine_contiguous(std::move(hash_state), start, sizeof(value));
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for Basic Types
+// -----------------------------------------------------------------------------
+
+// Note: Default `AbslHashValue` implementations live in `hash_internal`. This
+// allows us to block lexical scope lookup when doing an unqualified call to
+// `AbslHashValue` below. User-defined implementations of `AbslHashValue` can
+// only be found via ADL.
+
+// AbslHashValue() for hashing bool values
+//
+// We use SFINAE to ensure that this overload only accepts bool, not types that
+// are convertible to bool.
+template <typename H, typename B>
+typename std::enable_if<std::is_same<B, bool>::value, H>::type AbslHashValue(
+    H hash_state, B value) {
+  return H::combine(std::move(hash_state),
+                    static_cast<unsigned char>(value ? 1 : 0));
+}
+
+// AbslHashValue() for hashing enum values
+template <typename H, typename Enum>
+typename std::enable_if<std::is_enum<Enum>::value, H>::type AbslHashValue(
+    H hash_state, Enum e) {
+  // In practice, we could almost certainly just invoke hash_bytes directly,
+  // but it's possible that a sanitizer might one day want to
+  // store data in the unused bits of an enum. To avoid that risk, we
+  // convert to the underlying type before hashing. Hopefully this will get
+  // optimized away; if not, we can reopen discussion with c-toolchain-team.
+  return H::combine(std::move(hash_state),
+                    static_cast<typename std::underlying_type<Enum>::type>(e));
+}
+// AbslHashValue() for hashing floating-point values
+template <typename H, typename Float>
+typename std::enable_if<std::is_floating_point<Float>::value, H>::type
+AbslHashValue(H hash_state, Float value) {
+  return hash_internal::hash_bytes(std::move(hash_state),
+                                   value == 0 ? 0 : value);
+}
+
+// Long double has the property that it might have extra unused bytes in it.
+// For example, in x86 sizeof(long double)==16 but it only really uses 80-bits
+// of it. This means we can't use hash_bytes on a long double and have to
+// convert it to something else first.
+template <typename H>
+H AbslHashValue(H hash_state, long double value) {
+  const int category = std::fpclassify(value);
+  switch (category) {
+    case FP_INFINITE:
+      // Add the sign bit to differentiate between +Inf and -Inf
+      hash_state = H::combine(std::move(hash_state), std::signbit(value));
+      break;
+
+    case FP_NAN:
+    case FP_ZERO:
+    default:
+      // Category is enough for these.
+      break;
+
+    case FP_NORMAL:
+    case FP_SUBNORMAL:
+      // We can't convert `value` directly to double because this would have
+      // undefined behavior if the value is out of range.
+      // std::frexp gives us a value in the range (-1, -.5] or [.5, 1) that is
+      // guaranteed to be in range for `double`. The truncation is
+      // implementation defined, but that works as long as it is deterministic.
+      int exp;
+      auto mantissa = static_cast<double>(std::frexp(value, &exp));
+      hash_state = H::combine(std::move(hash_state), mantissa, exp);
+  }
+
+  return H::combine(std::move(hash_state), category);
+}
+
+// AbslHashValue() for hashing pointers
+template <typename H, typename T>
+H AbslHashValue(H hash_state, T* ptr) {
+  return hash_internal::hash_bytes(std::move(hash_state), ptr);
+}
+
+// AbslHashValue() for hashing nullptr_t
+template <typename H>
+H AbslHashValue(H hash_state, std::nullptr_t) {
+  return H::combine(std::move(hash_state), static_cast<void*>(nullptr));
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for Composite Types
+// -----------------------------------------------------------------------------
+
+// is_hashable()
+//
+// Trait class which returns true if T is hashable by the absl::Hash framework.
+// Used for the AbslHashValue implementations for composite types below.
+template <typename T>
+struct is_hashable;
+
+// AbslHashValue() for hashing pairs
+template <typename H, typename T1, typename T2>
+typename std::enable_if<is_hashable<T1>::value && is_hashable<T2>::value,
+                        H>::type
+AbslHashValue(H hash_state, const std::pair<T1, T2>& p) {
+  return H::combine(std::move(hash_state), p.first, p.second);
+}
+
+// hash_tuple()
+//
+// Helper function for hashing a tuple. The third argument should
+// be an index_sequence running from 0 to tuple_size<Tuple> - 1.
+template <typename H, typename Tuple, size_t... Is>
+H hash_tuple(H hash_state, const Tuple& t, absl::index_sequence<Is...>) {
+  return H::combine(std::move(hash_state), std::get<Is>(t)...);
+}
+
+// AbslHashValue for hashing tuples
+template <typename H, typename... Ts>
+#if _MSC_VER
+// This SFINAE gets MSVC confused under some conditions. Let's just disable it
+// for now.
+H
+#else
+typename std::enable_if<absl::conjunction<is_hashable<Ts>...>::value, H>::type
+#endif
+AbslHashValue(H hash_state, const std::tuple<Ts...>& t) {
+  return hash_internal::hash_tuple(std::move(hash_state), t,
+                                   absl::make_index_sequence<sizeof...(Ts)>());
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for Pointers
+// -----------------------------------------------------------------------------
+
+// AbslHashValue for hashing unique_ptr
+template <typename H, typename T, typename D>
+H AbslHashValue(H hash_state, const std::unique_ptr<T, D>& ptr) {
+  return H::combine(std::move(hash_state), ptr.get());
+}
+
+// AbslHashValue for hashing shared_ptr
+template <typename H, typename T>
+H AbslHashValue(H hash_state, const std::shared_ptr<T>& ptr) {
+  return H::combine(std::move(hash_state), ptr.get());
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for String-Like Types
+// -----------------------------------------------------------------------------
+
+// AbslHashValue for hashing strings
+//
+// All the string-like types supported here provide the same hash expansion for
+// the same character sequence. These types are:
+//
+//  - `std::string` (and std::basic_string<char, std::char_traits<char>, A> for
+//      any allocator A)
+//  - `absl::string_view` and `std::string_view`
+//
+// For simplicity, we currently support only `char` strings. This support may
+// be broadened, if necessary, but with some caution - this overload would
+// misbehave in cases where the traits' `eq()` member isn't equivalent to `==`
+// on the underlying character type.
+template <typename H>
+H AbslHashValue(H hash_state, absl::string_view str) {
+  return H::combine(
+      H::combine_contiguous(std::move(hash_state), str.data(), str.size()),
+      str.size());
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for Sequence Containers
+// -----------------------------------------------------------------------------
+
+// AbslHashValue for hashing std::array
+template <typename H, typename T, size_t N>
+typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+    H hash_state, const std::array<T, N>& array) {
+  return H::combine_contiguous(std::move(hash_state), array.data(),
+                               array.size());
+}
+
+// AbslHashValue for hashing std::deque
+template <typename H, typename T, typename Allocator>
+typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+    H hash_state, const std::deque<T, Allocator>& deque) {
+  // TODO(gromer): investigate a more efficient implementation taking
+  // advantage of the chunk structure.
+  for (const auto& t : deque) {
+    hash_state = H::combine(std::move(hash_state), t);
+  }
+  return H::combine(std::move(hash_state), deque.size());
+}
+
+// AbslHashValue for hashing std::forward_list
+template <typename H, typename T, typename Allocator>
+typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+    H hash_state, const std::forward_list<T, Allocator>& list) {
+  size_t size = 0;
+  for (const T& t : list) {
+    hash_state = H::combine(std::move(hash_state), t);
+    ++size;
+  }
+  return H::combine(std::move(hash_state), size);
+}
+
+// AbslHashValue for hashing std::list
+template <typename H, typename T, typename Allocator>
+typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+    H hash_state, const std::list<T, Allocator>& list) {
+  for (const auto& t : list) {
+    hash_state = H::combine(std::move(hash_state), t);
+  }
+  return H::combine(std::move(hash_state), list.size());
+}
+
+// AbslHashValue for hashing std::vector
+//
+// Do not use this for vector<bool>. It does not have a .data(), and a fallback
+// for std::hash<> is most likely faster.
+template <typename H, typename T, typename Allocator>
+typename std::enable_if<is_hashable<T>::value && !std::is_same<T, bool>::value,
+                        H>::type
+AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
+  return H::combine(H::combine_contiguous(std::move(hash_state), vector.data(),
+                                          vector.size()),
+                    vector.size());
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for Ordered Associative Containers
+// -----------------------------------------------------------------------------
+
+// AbslHashValue for hashing std::map
+template <typename H, typename Key, typename T, typename Compare,
+          typename Allocator>
+typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
+                        H>::type
+AbslHashValue(H hash_state, const std::map<Key, T, Compare, Allocator>& map) {
+  for (const auto& t : map) {
+    hash_state = H::combine(std::move(hash_state), t);
+  }
+  return H::combine(std::move(hash_state), map.size());
+}
+
+// AbslHashValue for hashing std::multimap
+template <typename H, typename Key, typename T, typename Compare,
+          typename Allocator>
+typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
+                        H>::type
+AbslHashValue(H hash_state,
+              const std::multimap<Key, T, Compare, Allocator>& map) {
+  for (const auto& t : map) {
+    hash_state = H::combine(std::move(hash_state), t);
+  }
+  return H::combine(std::move(hash_state), map.size());
+}
+
+// AbslHashValue for hashing std::set
+template <typename H, typename Key, typename Compare, typename Allocator>
+typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
+    H hash_state, const std::set<Key, Compare, Allocator>& set) {
+  for (const auto& t : set) {
+    hash_state = H::combine(std::move(hash_state), t);
+  }
+  return H::combine(std::move(hash_state), set.size());
+}
+
+// AbslHashValue for hashing std::multiset
+template <typename H, typename Key, typename Compare, typename Allocator>
+typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
+    H hash_state, const std::multiset<Key, Compare, Allocator>& set) {
+  for (const auto& t : set) {
+    hash_state = H::combine(std::move(hash_state), t);
+  }
+  return H::combine(std::move(hash_state), set.size());
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for Wrapper Types
+// -----------------------------------------------------------------------------
+
+// AbslHashValue for hashing absl::optional
+template <typename H, typename T>
+typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+    H hash_state, const absl::optional<T>& opt) {
+  if (opt) hash_state = H::combine(std::move(hash_state), *opt);
+  return H::combine(std::move(hash_state), opt.has_value());
+}
+
+// VariantVisitor
+template <typename H>
+struct VariantVisitor {
+  H&& hash_state;
+  template <typename T>
+  H operator()(const T& t) const {
+    return H::combine(std::move(hash_state), t);
+  }
+};
+
+// AbslHashValue for hashing absl::variant
+template <typename H, typename... T>
+typename std::enable_if<conjunction<is_hashable<T>...>::value, H>::type
+AbslHashValue(H hash_state, const absl::variant<T...>& v) {
+  if (!v.valueless_by_exception()) {
+    hash_state = absl::visit(VariantVisitor<H>{std::move(hash_state)}, v);
+  }
+  return H::combine(std::move(hash_state), v.index());
+}
+// -----------------------------------------------------------------------------
+
+// hash_range_or_bytes()
+//
+// Mixes all values in the range [data, data+size) into the hash state.
+// This overload accepts only uniquely-represented types, and hashes them by
+// hashing the entire range of bytes.
+template <typename H, typename T>
+typename std::enable_if<is_uniquely_represented<T>::value, H>::type
+hash_range_or_bytes(H hash_state, const T* data, size_t size) {
+  const auto* bytes = reinterpret_cast<const unsigned char*>(data);
+  return H::combine_contiguous(std::move(hash_state), bytes, sizeof(T) * size);
+}
+
+// hash_range_or_bytes()
+template <typename H, typename T>
+typename std::enable_if<!is_uniquely_represented<T>::value, H>::type
+hash_range_or_bytes(H hash_state, const T* data, size_t size) {
+  for (const auto end = data + size; data < end; ++data) {
+    hash_state = H::combine(std::move(hash_state), *data);
+  }
+  return hash_state;
+}
+
+// InvokeHashTag
+//
+// InvokeHash(H, const T&) invokes the appropriate hash implementation for a
+// hasher of type `H` and a value of type `T`. If `T` is not hashable, there
+// will be no matching overload of InvokeHash().
+// Note: Some platforms (eg MSVC) do not support the detect idiom on
+// std::hash. In those platforms the last fallback will be std::hash and
+// InvokeHash() will always have a valid overload even if std::hash<T> is not
+// valid.
+//
+// We try the following options in order:
+//   * If is_uniquely_represented, hash bytes directly.
+//   * ADL AbslHashValue(H, const T&) call.
+//   * std::hash<T>
+
+// In MSVC we can't probe std::hash or stdext::hash because it triggers a
+// static_assert instead of failing substitution.
+#if defined(_MSC_VER)
+#undef ABSL_HASH_INTERNAL_CAN_POISON_
+#else   // _MSC_VER
+#define ABSL_HASH_INTERNAL_CAN_POISON_ 1
+#endif  // _MSC_VER
+
+#if defined(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE) && \
+    ABSL_HASH_INTERNAL_CAN_POISON_
+#define ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_ 1
+#endif
+
+enum class InvokeHashTag {
+  kUniquelyRepresented,
+  kHashValue,
+#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+  kLegacyHash,
+#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+  kStdHash,
+  kNone
+};
+
+// HashSelect
+//
+// Type trait to select the appropriate hash implementation to use.
+// HashSelect<T>::value is an instance of InvokeHashTag that indicates the best
+// available hashing mechanism.
+// See `Note` above about MSVC.
+template <typename T>
+struct HashSelect {
+ private:
+  struct State : HashStateBase<State> {
+    static State combine_contiguous(State hash_state, const unsigned char*,
+                                    size_t);
+    using State::HashStateBase::combine_contiguous;
+  };
+
+  // `Probe<V, Tag>::value` evaluates to `V<T>::value` if it is a valid
+  // expression, and `false` otherwise.
+  // `Probe<V, Tag>::tag` always evaluates to `Tag`.
+  template <template <typename> class V, InvokeHashTag Tag>
+  struct Probe {
+   private:
+    template <typename U, typename std::enable_if<V<U>::value, int>::type = 0>
+    static std::true_type Test(int);
+    template <typename U>
+    static std::false_type Test(char);
+
+   public:
+    static constexpr InvokeHashTag kTag = Tag;
+    static constexpr bool value = decltype(
+        Test<absl::remove_const_t<absl::remove_reference_t<T>>>(0))::value;
+  };
+
+  template <typename U>
+  using ProbeUniquelyRepresented = is_uniquely_represented<U>;
+
+  template <typename U>
+  using ProbeHashValue =
+      std::is_same<State, decltype(AbslHashValue(std::declval<State>(),
+                                                 std::declval<const U&>()))>;
+
+#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+  template <typename U>
+  using ProbeLegacyHash =
+      std::is_convertible<decltype(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE::hash<
+                                   U>()(std::declval<const U&>())),
+                          size_t>;
+#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+
+  template <typename U>
+  using ProbeStdHash =
+#if ABSL_HASH_INTERNAL_CAN_POISON_
+      std::is_convertible<decltype(std::hash<U>()(std::declval<const U&>())),
+                          size_t>;
+#else   // ABSL_HASH_INTERNAL_CAN_POISON_
+      std::true_type;
+#endif  // ABSL_HASH_INTERNAL_CAN_POISON_
+
+  template <typename U>
+  using ProbeNone = std::true_type;
+
+ public:
+  // Probe each implementation in order.
+  // disjunction provides short circuting wrt instantiation.
+  static constexpr InvokeHashTag value = absl::disjunction<
+      Probe<ProbeUniquelyRepresented, InvokeHashTag::kUniquelyRepresented>,
+      Probe<ProbeHashValue, InvokeHashTag::kHashValue>,
+#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+      Probe<ProbeLegacyHash, InvokeHashTag::kLegacyHash>,
+#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+      Probe<ProbeStdHash, InvokeHashTag::kStdHash>,
+      Probe<ProbeNone, InvokeHashTag::kNone>>::kTag;
+};
+
+template <typename T>
+struct is_hashable : std::integral_constant<bool, HashSelect<T>::value !=
+                                                      InvokeHashTag::kNone> {};
+
+template <typename H, typename T>
+absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kUniquelyRepresented,
+                  H>
+InvokeHash(H state, const T& value) {
+  return hash_internal::hash_bytes(std::move(state), value);
+}
+
+template <typename H, typename T>
+absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kHashValue, H>
+InvokeHash(H state, const T& value) {
+  return AbslHashValue(std::move(state), value);
+}
+
+#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+template <typename H, typename T>
+absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kLegacyHash, H>
+InvokeHash(H state, const T& value) {
+  return hash_internal::hash_bytes(
+      std::move(state), ABSL_INTERNAL_LEGACY_HASH_NAMESPACE::hash<T>{}(value));
+}
+#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+
+template <typename H, typename T>
+absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kStdHash, H>
+InvokeHash(H state, const T& value) {
+  return hash_internal::hash_bytes(std::move(state), std::hash<T>{}(value));
+}
+
+// CityHashState
+class CityHashState : public HashStateBase<CityHashState> {
+  // absl::uint128 is not an alias or a thin wrapper around the intrinsic.
+  // We use the intrinsic when available to improve performance.
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  using uint128 = __uint128_t;
+#else   // ABSL_HAVE_INTRINSIC_INT128
+  using uint128 = absl::uint128;
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+  static constexpr uint64_t kMul =
+      sizeof(size_t) == 4 ? uint64_t{0xcc9e2d51} : uint64_t{0x9ddfea08eb382d69};
+
+  template <typename T>
+  using IntegralFastPath =
+      conjunction<std::is_integral<T>, is_uniquely_represented<T>>;
+
+ public:
+  // Move only
+  CityHashState(CityHashState&&) = default;
+  CityHashState& operator=(CityHashState&&) = default;
+
+  // CityHashState::combine_contiguous()
+  //
+  // Fundamental base case for hash recursion: mixes the given range of bytes
+  // into the hash state.
+  static CityHashState combine_contiguous(CityHashState hash_state,
+                                          const unsigned char* first,
+                                          size_t size) {
+    return CityHashState(
+        CombineContiguousImpl(hash_state.state_, first, size,
+                              std::integral_constant<int, sizeof(size_t)>{}));
+  }
+  using CityHashState::HashStateBase::combine_contiguous;
+
+  // CityHashState::hash()
+  //
+  // For performance reasons in non-opt mode, we specialize this for
+  // integral types.
+  // Otherwise we would be instantiating and calling dozens of functions for
+  // something that is just one multiplication and a couple xor's.
+  // The result should be the same as running the whole algorithm, but faster.
+  template <typename T, absl::enable_if_t<IntegralFastPath<T>::value, int> = 0>
+  static size_t hash(T value) {
+    return static_cast<size_t>(Mix(Seed(), static_cast<uint64_t>(value)));
+  }
+
+  // Overload of CityHashState::hash()
+  template <typename T, absl::enable_if_t<!IntegralFastPath<T>::value, int> = 0>
+  static size_t hash(const T& value) {
+    return static_cast<size_t>(combine(CityHashState{}, value).state_);
+  }
+
+ private:
+  // Invoked only once for a given argument; that plus the fact that this is
+  // move-only ensures that there is only one non-moved-from object.
+  CityHashState() : state_(Seed()) {}
+
+  // Workaround for MSVC bug.
+  // We make the type copyable to fix the calling convention, even though we
+  // never actually copy it. Keep it private to not affect the public API of the
+  // type.
+  CityHashState(const CityHashState&) = default;
+
+  explicit CityHashState(uint64_t state) : state_(state) {}
+
+  // Implementation of the base case for combine_contiguous where we actually
+  // mix the bytes into the state.
+  // Dispatch to different implementations of the combine_contiguous depending
+  // on the value of `sizeof(size_t)`.
+  static uint64_t CombineContiguousImpl(uint64_t state,
+                                        const unsigned char* first, size_t len,
+                                        std::integral_constant<int, 4>
+                                        /* sizeof_size_t */);
+  static uint64_t CombineContiguousImpl(uint64_t state,
+                                        const unsigned char* first, size_t len,
+                                        std::integral_constant<int, 8>
+                                        /* sizeof_size_t*/);
+
+  // Reads 9 to 16 bytes from p.
+  // The first 8 bytes are in .first, the rest (zero padded) bytes are in
+  // .second.
+  static std::pair<uint64_t, uint64_t> Read9To16(const unsigned char* p,
+                                                 size_t len) {
+    uint64_t high = little_endian::Load64(p + len - 8);
+    return {little_endian::Load64(p), high >> (128 - len * 8)};
+  }
+
+  // Reads 4 to 8 bytes from p. Zero pads to fill uint64_t.
+  static uint64_t Read4To8(const unsigned char* p, size_t len) {
+    return (static_cast<uint64_t>(little_endian::Load32(p + len - 4))
+            << (len - 4) * 8) |
+           little_endian::Load32(p);
+  }
+
+  // Reads 1 to 3 bytes from p. Zero pads to fill uint32_t.
+  static uint32_t Read1To3(const unsigned char* p, size_t len) {
+    return static_cast<uint32_t>((p[0]) |                         //
+                                 (p[len / 2] << (len / 2 * 8)) |  //
+                                 (p[len - 1] << ((len - 1) * 8)));
+  }
+
+  ABSL_ATTRIBUTE_ALWAYS_INLINE static uint64_t Mix(uint64_t state, uint64_t v) {
+    using MultType =
+        absl::conditional_t<sizeof(size_t) == 4, uint64_t, uint128>;
+    // We do the addition in 64-bit space to make sure the 128-bit
+    // multiplication is fast. If we were to do it as MultType the compiler has
+    // to assume that the high word is non-zero and needs to perform 2
+    // multiplications instead of one.
+    MultType m = state + v;
+    m *= kMul;
+    return static_cast<uint64_t>(m ^ (m >> (sizeof(m) * 8 / 2)));
+  }
+
+  // Seed()
+  //
+  // A non-deterministic seed.
+  //
+  // The current purpose of this seed is to generate non-deterministic results
+  // and prevent having users depend on the particular hash values.
+  // It is not meant as a security feature right now, but it leaves the door
+  // open to upgrade it to a true per-process random seed. A true random seed
+  // costs more and we don't need to pay for that right now.
+  //
+  // On platforms with ASLR, we take advantage of it to make a per-process
+  // random value.
+  // See https://en.wikipedia.org/wiki/Address_space_layout_randomization
+  //
+  // On other platforms this is still going to be non-deterministic but most
+  // probably per-build and not per-process.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE static uint64_t Seed() {
+    return static_cast<uint64_t>(reinterpret_cast<uintptr_t>(kSeed));
+  }
+  static const void* const kSeed;
+
+  uint64_t state_;
+};
+
+// CityHashState::CombineContiguousImpl()
+inline uint64_t CityHashState::CombineContiguousImpl(
+    uint64_t state, const unsigned char* first, size_t len,
+    std::integral_constant<int, 4> /* sizeof_size_t */) {
+  // For large values we use CityHash, for small ones we just use a
+  // multiplicative hash.
+  uint64_t v;
+  if (len > 8) {
+    v = absl::hash_internal::CityHash32(reinterpret_cast<const char*>(first), len);
+  } else if (len >= 4) {
+    v = Read4To8(first, len);
+  } else if (len > 0) {
+    v = Read1To3(first, len);
+  } else {
+    // Empty ranges have no effect.
+    return state;
+  }
+  return Mix(state, v);
+}
+
+// Overload of CityHashState::CombineContiguousImpl()
+inline uint64_t CityHashState::CombineContiguousImpl(
+    uint64_t state, const unsigned char* first, size_t len,
+    std::integral_constant<int, 8> /* sizeof_size_t */) {
+  // For large values we use CityHash, for small ones we just use a
+  // multiplicative hash.
+  uint64_t v;
+  if (len > 16) {
+    v = absl::hash_internal::CityHash64(reinterpret_cast<const char*>(first), len);
+  } else if (len > 8) {
+    auto p = Read9To16(first, len);
+    state = Mix(state, p.first);
+    v = p.second;
+  } else if (len >= 4) {
+    v = Read4To8(first, len);
+  } else if (len > 0) {
+    v = Read1To3(first, len);
+  } else {
+    // Empty ranges have no effect.
+    return state;
+  }
+  return Mix(state, v);
+}
+
+
+struct AggregateBarrier {};
+
+// HashImpl
+
+// Add a private base class to make sure this type is not an aggregate.
+// Aggregates can be aggregate initialized even if the default constructor is
+// deleted.
+struct PoisonedHash : private AggregateBarrier {
+  PoisonedHash() = delete;
+  PoisonedHash(const PoisonedHash&) = delete;
+  PoisonedHash& operator=(const PoisonedHash&) = delete;
+};
+
+template <typename T>
+struct HashImpl {
+  size_t operator()(const T& value) const { return CityHashState::hash(value); }
+};
+
+template <typename T>
+struct Hash
+    : absl::conditional_t<is_hashable<T>::value, HashImpl<T>, PoisonedHash> {};
+
+template <typename H>
+template <typename T, typename... Ts>
+H HashStateBase<H>::combine(H state, const T& value, const Ts&... values) {
+  return H::combine(hash_internal::InvokeHash(std::move(state), value),
+                    values...);
+}
+
+// HashStateBase::combine_contiguous()
+template <typename H>
+template <typename T>
+H HashStateBase<H>::combine_contiguous(H state, const T* data, size_t size) {
+  return hash_internal::hash_range_or_bytes(std::move(state), data, size);
+}
+}  // namespace hash_internal
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_HASH_H_
diff --git a/absl/hash/internal/print_hash_of.cc b/absl/hash/internal/print_hash_of.cc
new file mode 100644
index 000000000000..b6df31cc5e85
--- /dev/null
+++ b/absl/hash/internal/print_hash_of.cc
@@ -0,0 +1,23 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstdlib>
+
+#include "absl/hash/hash.h"
+
+// Prints the hash of argv[1].
+int main(int argc, char** argv) {
+  if (argc < 2) return 1;
+  printf("%zu\n", absl::Hash<int>{}(std::atoi(argv[1])));  // NOLINT
+}
diff --git a/absl/hash/internal/spy_hash_state.h b/absl/hash/internal/spy_hash_state.h
new file mode 100644
index 000000000000..03d795b09001
--- /dev/null
+++ b/absl/hash/internal/spy_hash_state.h
@@ -0,0 +1,218 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
+#define ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
+
+#include <ostream>
+#include <string>
+#include <vector>
+
+#include "absl/hash/hash.h"
+#include "absl/strings/match.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_join.h"
+
+namespace absl {
+namespace hash_internal {
+
+// SpyHashState is an implementation of the HashState API that simply
+// accumulates all input bytes in an internal buffer. This makes it useful
+// for testing AbslHashValue overloads (so long as they are templated on the
+// HashState parameter), since it can report the exact hash representation
+// that the AbslHashValue overload produces.
+//
+// Sample usage:
+// EXPECT_EQ(SpyHashState::combine(SpyHashState(), foo),
+//           SpyHashState::combine(SpyHashState(), bar));
+template <typename T>
+class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
+ public:
+  SpyHashStateImpl()
+      : error_(std::make_shared<absl::optional<std::string>>()) {
+    static_assert(std::is_void<T>::value, "");
+  }
+
+  // Move-only
+  SpyHashStateImpl(const SpyHashStateImpl&) = delete;
+  SpyHashStateImpl& operator=(const SpyHashStateImpl&) = delete;
+
+  SpyHashStateImpl(SpyHashStateImpl&& other) noexcept {
+    *this = std::move(other);
+  }
+
+  SpyHashStateImpl& operator=(SpyHashStateImpl&& other) noexcept {
+    hash_representation_ = std::move(other.hash_representation_);
+    error_ = other.error_;
+    moved_from_ = other.moved_from_;
+    other.moved_from_ = true;
+    return *this;
+  }
+
+  template <typename U>
+  SpyHashStateImpl(SpyHashStateImpl<U>&& other) {  // NOLINT
+    hash_representation_ = std::move(other.hash_representation_);
+    error_ = other.error_;
+    moved_from_ = other.moved_from_;
+    other.moved_from_ = true;
+  }
+
+  template <typename A, typename... Args>
+  static SpyHashStateImpl combine(SpyHashStateImpl s, const A& a,
+                                  const Args&... args) {
+    // Pass an instance of SpyHashStateImpl<A> when trying to combine `A`. This
+    // allows us to test that the user only uses this instance for combine calls
+    // and does not call AbslHashValue directly.
+    // See AbslHashValue implementation at the bottom.
+    s = SpyHashStateImpl<A>::HashStateBase::combine(std::move(s), a);
+    return SpyHashStateImpl::combine(std::move(s), args...);
+  }
+  static SpyHashStateImpl combine(SpyHashStateImpl s) {
+    if (direct_absl_hash_value_error_) {
+      *s.error_ = "AbslHashValue should not be invoked directly.";
+    } else if (s.moved_from_) {
+      *s.error_ = "Used moved-from instance of the hash state object.";
+    }
+    return s;
+  }
+
+  static void SetDirectAbslHashValueError() {
+    direct_absl_hash_value_error_ = true;
+  }
+
+  // Two SpyHashStateImpl objects are equal if they hold equal hash
+  // representations.
+  friend bool operator==(const SpyHashStateImpl& lhs,
+                         const SpyHashStateImpl& rhs) {
+    return lhs.hash_representation_ == rhs.hash_representation_;
+  }
+
+  friend bool operator!=(const SpyHashStateImpl& lhs,
+                         const SpyHashStateImpl& rhs) {
+    return !(lhs == rhs);
+  }
+
+  enum class CompareResult {
+    kEqual,
+    kASuffixB,
+    kBSuffixA,
+    kUnequal,
+  };
+
+  static CompareResult Compare(const SpyHashStateImpl& a,
+                               const SpyHashStateImpl& b) {
+    const std::string a_flat = absl::StrJoin(a.hash_representation_, "");
+    const std::string b_flat = absl::StrJoin(b.hash_representation_, "");
+    if (a_flat == b_flat) return CompareResult::kEqual;
+    if (absl::EndsWith(a_flat, b_flat)) return CompareResult::kBSuffixA;
+    if (absl::EndsWith(b_flat, a_flat)) return CompareResult::kASuffixB;
+    return CompareResult::kUnequal;
+  }
+
+  // operator<< prints the hash representation as a hex and ASCII dump, to
+  // facilitate debugging.
+  friend std::ostream& operator<<(std::ostream& out,
+                                  const SpyHashStateImpl& hash_state) {
+    out << "[\n";
+    for (auto& s : hash_state.hash_representation_) {
+      size_t offset = 0;
+      for (char c : s) {
+        if (offset % 16 == 0) {
+          out << absl::StreamFormat("\n0x%04x: ", offset);
+        }
+        if (offset % 2 == 0) {
+          out << " ";
+        }
+        out << absl::StreamFormat("%02x", c);
+        ++offset;
+      }
+      out << "\n";
+    }
+    return out << "]";
+  }
+
+  // The base case of the combine recursion, which writes raw bytes into the
+  // internal buffer.
+  static SpyHashStateImpl combine_contiguous(SpyHashStateImpl hash_state,
+                                             const unsigned char* begin,
+                                             size_t size) {
+    hash_state.hash_representation_.emplace_back(
+        reinterpret_cast<const char*>(begin), size);
+    return hash_state;
+  }
+
+  using SpyHashStateImpl::HashStateBase::combine_contiguous;
+
+  absl::optional<std::string> error() const {
+    if (moved_from_) {
+      return "Returned a moved-from instance of the hash state object.";
+    }
+    return *error_;
+  }
+
+ private:
+  template <typename U>
+  friend class SpyHashStateImpl;
+
+  // This is true if SpyHashStateImpl<T> has been passed to a call of
+  // AbslHashValue with the wrong type. This detects that the user called
+  // AbslHashValue directly (because the hash state type does not match).
+  static bool direct_absl_hash_value_error_;
+
+
+  std::vector<std::string> hash_representation_;
+  // This is a shared_ptr because we want all instances of the particular
+  // SpyHashState run to share the field. This way we can set the error for
+  // use-after-move and all the copies will see it.
+  std::shared_ptr<absl::optional<std::string>> error_;
+  bool moved_from_ = false;
+};
+
+template <typename T>
+bool SpyHashStateImpl<T>::direct_absl_hash_value_error_;
+
+template <bool& B>
+struct OdrUse {
+  constexpr OdrUse() {}
+  bool& b = B;
+};
+
+template <void (*)()>
+struct RunOnStartup {
+  static bool run;
+  static constexpr OdrUse<run> kOdrUse{};
+};
+
+template <void (*f)()>
+bool RunOnStartup<f>::run = (f(), true);
+
+template <
+    typename T, typename U,
+    // Only trigger for when (T != U),
+    absl::enable_if_t<!std::is_same<T, U>::value, int> = 0,
+    // This statement works in two ways:
+    //  - First, it instantiates RunOnStartup and forces the initialization of
+    //    `run`, which set the global variable.
+    //  - Second, it triggers a SFINAE error disabling the overload to prevent
+    //    compile time errors. If we didn't disable the overload we would get
+    //    ambiguous overload errors, which we don't want.
+    int = RunOnStartup<SpyHashStateImpl<T>::SetDirectAbslHashValueError>::run>
+void AbslHashValue(SpyHashStateImpl<T>, const U&);
+
+using SpyHashState = SpyHashStateImpl<void>;
+
+}  // namespace hash_internal
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_