Initial Commit

12 files changed, 5769 insertions, 0 deletions

diff --git a/absl/types/BUILD.bazel b/absl/types/BUILD.bazel
new file mode 100644
index 000000000000..8d09440ef104
--- /dev/null
+++ b/absl/types/BUILD.bazel
@@ -0,0 +1,178 @@
+#
+# Copyright 2017 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",
+    "ABSL_EXCEPTIONS_FLAG",
+)
+load(
+    "//absl:test_dependencies.bzl",
+    "GUNIT_MAIN_DEPS_SELECTOR",
+)
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+cc_library(
+    name = "any",
+    hdrs = ["any.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":bad_any_cast",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/meta:type_traits",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "bad_any_cast",
+    srcs = ["bad_any_cast.cc"],
+    hdrs = ["bad_any_cast.h"],
+    copts = ABSL_EXCEPTIONS_FLAG + ABSL_DEFAULT_COPTS,
+    features = [
+        "-use_header_modules",  # b/33207452
+    ],
+    deps = [
+        "//absl/base",
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "any_test",
+    size = "small",
+    srcs = [
+        "any_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
+    deps = [
+        ":any",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:exception_testing",
+        "//absl/container:test_instance_tracker",
+    ] + select(GUNIT_MAIN_DEPS_SELECTOR),
+)
+
+cc_test(
+    name = "any_test_noexceptions",
+    size = "small",
+    srcs = [
+        "any_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":any",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:exception_testing",
+        "//absl/container:test_instance_tracker",
+    ] + select(GUNIT_MAIN_DEPS_SELECTOR),
+)
+
+cc_library(
+    name = "span",
+    hdrs = ["span.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        "//absl/algorithm",
+        "//absl/base:core_headers",
+        "//absl/base:throw_delegate",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+    ],
+)
+
+cc_test(
+    name = "span_test",
+    size = "small",
+    srcs = ["span_test.cc"],
+    copts = ABSL_TEST_COPTS + ["-fexceptions"],
+    deps = [
+        ":span",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:exception_testing",
+        "//absl/container:fixed_array",
+        "//absl/container:inlined_vector",
+        "//absl/strings",
+    ] + select(GUNIT_MAIN_DEPS_SELECTOR),
+)
+
+cc_test(
+    name = "span_test_noexceptions",
+    size = "small",
+    srcs = ["span_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":span",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:exception_testing",
+        "//absl/container:fixed_array",
+        "//absl/container:inlined_vector",
+        "//absl/strings",
+    ] + select(GUNIT_MAIN_DEPS_SELECTOR),
+)
+
+cc_library(
+    name = "optional",
+    srcs = ["optional.cc"],
+    hdrs = ["optional.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":bad_optional_access",
+        "//absl/base:config",
+        "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "bad_optional_access",
+    srcs = ["bad_optional_access.cc"],
+    hdrs = ["bad_optional_access.h"],
+    copts = ABSL_DEFAULT_COPTS + ABSL_EXCEPTIONS_FLAG,
+    features = [
+        "-use_header_modules",  # b/33207452
+    ],
+    deps = [
+        "//absl/base",
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "optional_test",
+    size = "small",
+    srcs = [
+        "optional_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
+    deps = [
+        ":optional",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+    ] + select(GUNIT_MAIN_DEPS_SELECTOR),
+)
diff --git a/absl/types/any.h b/absl/types/any.h
new file mode 100644
index 000000000000..a51dea110d0f
--- /dev/null
+++ b/absl/types/any.h
@@ -0,0 +1,539 @@
+//
+// Copyright 2017 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.
+//
+// -----------------------------------------------------------------------------
+// any.h
+// -----------------------------------------------------------------------------
+//
+// This header file define the `absl::any` type for holding a type-safe value
+// of any type. The 'absl::any` type is useful for providing a way to hold
+// something that is, as yet, unspecified. Such unspecified types
+// traditionally are passed between API boundaries until they are later cast to
+// their "destination" types. To cast to such a destination type, use
+// `absl::any_cast()`. Note that when casting an `absl::any`, you must cast it
+// to an explicit type; implicit conversions will throw.
+//
+// Example:
+//
+//   auto a = absl::any(65);
+//   absl::any_cast<int>(a);         // 65
+//   absl::any_cast<char>(a);        // throws absl::bad_any_cast
+//   absl::any_cast<std::string>(a); // throws absl::bad_any_cast
+//
+// `absl::any` is a C++11 compatible version of the C++17 `std::any` abstraction
+// and is designed to be a drop-in replacement for code compliant with C++17.
+//
+// Traditionally, the behavior of casting to a temporary unspecified type has
+// been accomplished with the `void *` paradigm, where the pointer was to some
+// other unspecified type. `absl::any` provides an "owning" version of `void *`
+// that avoids issues of pointer management.
+//
+// Note: just as in the case of `void *`, use of `absl::any` (and its C++17
+// version `std::any`) is a code smell indicating that your API might not be
+// constructed correctly. We have seen that most uses of `any` are unwarranted,
+// and `absl::any`, like `std::any`, is difficult to use properly. Before using
+// this abstraction, make sure that you should not instead be rewriting your
+// code to be more specific.
+//
+// Abseil expects to release an `absl::variant` type shortly (a C++11 compatible
+// version of the C++17 `std::variant), which is generally preferred for use
+// over `absl::any`.
+#ifndef ABSL_TYPES_ANY_H_
+#define ABSL_TYPES_ANY_H_
+
+#include "absl/base/config.h"
+#include "absl/utility/utility.h"
+
+#ifdef ABSL_HAVE_STD_ANY
+
+#include <any>
+
+namespace absl {
+using std::any;
+using std::any_cast;
+using std::bad_any_cast;
+using std::make_any;
+}  // namespace absl
+
+#else  // ABSL_HAVE_STD_ANY
+
+#include <algorithm>
+#include <cstddef>
+#include <initializer_list>
+#include <memory>
+#include <stdexcept>
+#include <type_traits>
+#include <typeinfo>
+#include <utility>
+
+#include "absl/base/macros.h"
+#include "absl/meta/type_traits.h"
+#include "absl/types/bad_any_cast.h"
+
+// NOTE: This macro is an implementation detail that is undefined at the bottom
+// of the file. It is not intended for expansion directly from user code.
+#ifdef ABSL_ANY_DETAIL_HAS_RTTI
+#error ABSL_ANY_DETAIL_HAS_RTTI cannot be directly set
+#elif !defined(__GNUC__) || defined(__GXX_RTTI)
+#define ABSL_ANY_DETAIL_HAS_RTTI 1
+#endif  // !defined(__GNUC__) || defined(__GXX_RTTI)
+
+namespace absl {
+
+namespace any_internal {
+
+// FastTypeId<Type>() evaluates at compile/link-time to a unique integer for the
+// passed in type. Their values are neither contiguous nor small, making them
+// unfit for using as an index into a vector, but a good match for keys into
+// maps or straight up comparisons.
+// Note that on 64-bit (unix) systems size_t is 64-bit while int is 32-bit and
+// the compiler will happily and quietly assign such a 64-bit value to a
+// 32-bit integer. While a client should never do that it SHOULD still be safe,
+// assuming the BSS segment doesn't span more than 4GiB.
+template<typename Type>
+inline size_t FastTypeId() {
+  static_assert(sizeof(char*) <= sizeof(size_t),
+                "ptr size too large for size_t");
+
+  // This static variable isn't actually used, only its address, so there are
+  // no concurrency issues.
+  static char dummy_var;
+  return reinterpret_cast<size_t>(&dummy_var);
+}
+
+}  // namespace any_internal
+
+class any;
+
+// swap()
+//
+// Swaps two `absl::any` values. Equivalent to `x.swap(y) where `x` and `y` are
+// `absl::any` types.
+void swap(any& x, any& y) noexcept;
+
+// make_any()
+//
+// Constructs an `absl::any` of type `T` with the given arguments.
+template <typename T, typename... Args>
+any make_any(Args&&... args);
+
+// Overload of `absl::make_any()` for constructing an `absl::any` type from an
+// initializer list.
+template <typename T, typename U, typename... Args>
+any make_any(std::initializer_list<U> il, Args&&... args);
+
+// any_cast()
+//
+// Statically casts the value of a `const absl::any` type to the given type.
+// This function will throw `absl::bad_any_cast` if the stored value type of the
+// `absl::any` does not match the cast.
+//
+// `any_cast()` can also be used to get a reference to the internal storage iff
+// a reference type is passed as its `ValueType`:
+//
+// Example:
+//
+//   absl::any my_any = std::vector<int>();
+//   absl::any_cast<std::vector<int>&>(my_any).push_back(42);
+template <typename ValueType>
+ValueType any_cast(const any& operand);
+
+// Overload of `any_cast()` to statically cast the value of a non-const
+// `absl::any` type to the given type. This function will throw
+// `absl::bad_any_cast` if the stored value type of the `absl::any` does not
+// match the cast.
+template <typename ValueType>
+ValueType any_cast(any& operand);  // NOLINT(runtime/references)
+
+// Overload of `any_cast()` to statically cast the rvalue of an `absl::any`
+// type. This function will throw `absl::bad_any_cast` if the stored value type
+// of the `absl::any` does not match the cast.
+template <typename ValueType>
+ValueType any_cast(any&& operand);
+
+// Overload of `any_cast()` to statically cast the value of a const pointer
+// `absl::any` type to the given pointer type, or `nullptr` if the stored value
+// type of the `absl::any` does not match the cast.
+template <typename ValueType>
+const ValueType* any_cast(const any* operand) noexcept;
+
+// Overload of `any_cast()` to statically cast the value of a pointer
+// `absl::any` type to the given pointer type, or `nullptr` if the stored value
+// type of the `absl::any` does not match the cast.
+template <typename ValueType>
+ValueType* any_cast(any* operand) noexcept;
+
+// any
+//
+// An `absl::any` object provides the facility to either store an instance of a
+// type, known as the "contained object", or no value. An `absl::any` is used to
+// store values of types that are unknown at compile time. The `absl::any`
+// object, when containing a value, must contain a value type; storing a
+// reference type is neither desired nor supported.
+//
+// An `absl::any` can only store a type that is copy-constructable; move-only
+// types are not allowed within an `any` object.
+//
+// Example:
+//
+//   auto a = absl::any(65);                 // Literal, copyable
+//   auto b = absl::any(std::vector<int>()); // Default-initialized, copyable
+//   std::unique_ptr<Foo> my_foo;
+//   auto c = absl::any(std::move(my_foo));  // Error, not copy-constructable
+//
+// Note that `absl::any` makes use of decayed types (`absl::decay_t` in this
+// context) to remove const-volative qualifiers (known as "cv qualifiers"),
+// decay functions to function pointers, etc. We essentially "decay" a given
+// type into its essential type.
+//
+// `absl::any` makes use of decayed types when determing the basic type `T` of
+// the value to store in the any's contained object. In the documentation below,
+// we explcitly denote this by using the phrase "a decayed type of `T`".
+//
+// Example:
+//
+//   const int a = 4;
+//   absl::any foo(a);  // Decay ensures we store an "int", not a "const int&".
+//
+//   void my_function() {}
+//   absl::any bar(my_function);  // Decay ensures we store a function pointer.
+//
+// `absl::any` is a C++11 compatible version of the C++17 `std::any` abstraction
+// and is designed to be a drop-in replacement for code compliant with C++17.
+class any {
+ private:
+  template <typename T>
+  struct IsInPlaceType;
+
+ public:
+  // Constructors
+
+  // Constructs an empty `absl::any` object (`any::has_value()` will return
+  // `false`).
+  constexpr any() noexcept;
+
+  // Copy constructs an `absl::any` object with a "contained object" of the
+  // passed type of `other` (or an empty `absl::any` if `other.has_value()` is
+  // `false`.
+  any(const any& other)
+      : obj_(other.has_value() ? other.obj_->Clone()
+                               : std::unique_ptr<ObjInterface>()) {}
+
+  // Move constructs an `absl::any` object with a "contained object" of the
+  // passed type of `other` (or an empty `absl::any` if `other.has_value()` is
+  // `false`).
+  any(any&& other) noexcept = default;
+
+  // Constructs an `absl::any` object with a "contained object" of the decayed
+  // type of `T`, which is initialized via `std::forward<T>(value)`.
+  //
+  // This constructor will not participate in overload resolution if the
+  // decayed type of `T` is not copy-constructible.
+  template <
+      typename T, typename VT = absl::decay_t<T>,
+      absl::enable_if_t<!absl::disjunction<
+          std::is_same<any, VT>, IsInPlaceType<VT>,
+          absl::negation<std::is_copy_constructible<VT> > >::value>* = nullptr>
+  any(T&& value) : obj_(new Obj<VT>(in_place, std::forward<T>(value))) {}
+
+  // Constructs an `absl::any` object with a "contained object" of the decayed
+  // type of `T`, which is initialized via `std::forward<T>(value)`.
+  template <typename T, typename... Args, typename VT = absl::decay_t<T>,
+            absl::enable_if_t<absl::conjunction<
+                std::is_copy_constructible<VT>,
+                std::is_constructible<VT, Args...>>::value>* = nullptr>
+  explicit any(in_place_type_t<T> /*tag*/, Args&&... args)
+      : obj_(new Obj<VT>(in_place, std::forward<Args>(args)...)) {}
+
+  // Constructs an `absl::any` object with a "contained object" of the passed
+  // type `VT` as a decayed type of `T`. `VT` is initialized as if
+  // direct-non-list-initializing an object of type `VT` with the arguments
+  // `initializer_list, std::forward<Args>(args)...`.
+  template <
+      typename T, typename U, typename... Args, typename VT = absl::decay_t<T>,
+      absl::enable_if_t<
+          absl::conjunction<std::is_copy_constructible<VT>,
+                            std::is_constructible<VT, std::initializer_list<U>&,
+                                                  Args...>>::value>* = nullptr>
+  explicit any(in_place_type_t<T> /*tag*/, std::initializer_list<U> ilist,
+               Args&&... args)
+      : obj_(new Obj<VT>(in_place, ilist, std::forward<Args>(args)...)) {}
+
+  // Assignment operators
+
+  // Copy assigns an `absl::any` object with a "contained object" of the
+  // passed type.
+  any& operator=(const any& rhs) {
+    any(rhs).swap(*this);
+    return *this;
+  }
+
+  // Move assigns an `absl::any` object with a "contained object" of the
+  // passed type. `rhs` is left in a valid but otherwise unspecified state.
+  any& operator=(any&& rhs) noexcept {
+    any(std::move(rhs)).swap(*this);
+    return *this;
+  }
+
+  // Assigns an `absl::any` object with a "contained object" of the passed type.
+  template <typename T, typename VT = absl::decay_t<T>,
+            absl::enable_if_t<absl::conjunction<
+                absl::negation<std::is_same<VT, any>>,
+                std::is_copy_constructible<VT>>::value>* = nullptr>
+  any& operator=(T&& rhs) {
+    any tmp(in_place_type_t<VT>(), std::forward<T>(rhs));
+    tmp.swap(*this);
+    return *this;
+  }
+
+  // Modifiers
+
+  // any::emplace()
+  //
+  // Emplaces a value within an `absl::any` object by calling `any::reset()`,
+  // initializing the contained value as if direct-non-list-initializing an
+  // object of type `VT` with the arguments `std::forward<Args>(args)...`, and
+  // returning a reference to the new contained value.
+  //
+  // Note: If an exception is thrown during the call to `VT`’s constructor,
+  // `*this` does not contain a value, and any previously contained value has
+  // been destroyed.
+  template <
+      typename T, typename... Args, typename VT = absl::decay_t<T>,
+      absl::enable_if_t<std::is_copy_constructible<VT>::value &&
+                        std::is_constructible<VT, Args...>::value>* = nullptr>
+  VT& emplace(Args&&... args) {
+    reset();  // NOTE: reset() is required here even in the world of exceptions.
+    Obj<VT>* const object_ptr =
+        new Obj<VT>(in_place, std::forward<Args>(args)...);
+    obj_ = std::unique_ptr<ObjInterface>(object_ptr);
+    return object_ptr->value;
+  }
+
+  // Overload of `any::emplace()` to emplace a value within an `absl::any`
+  // object by calling `any::reset()`, initializing the contained value as if
+  // direct-non-list-initializing an object of type `VT` with the arguments
+  // `initilizer_list, std::forward<Args>(args)...`, and returning a reference
+  // to the new contained value.
+  //
+  // Note: If an exception is thrown during the call to `VT`’s constructor,
+  // `*this` does not contain a value, and any previously contained value has
+  // been destroyed. The function shall not participate in overload resolution
+  // unless `is_copy_constructible_v<VT>` is `true` and
+  // `is_constructible_v<VT, initializer_list<U>&, Args...>` is `true`.
+  template <
+      typename T, typename U, typename... Args, typename VT = absl::decay_t<T>,
+      absl::enable_if_t<std::is_copy_constructible<VT>::value &&
+                        std::is_constructible<VT, std::initializer_list<U>&,
+                                              Args...>::value>* = nullptr>
+  VT& emplace(std::initializer_list<U> ilist, Args&&... args) {
+    reset();  // NOTE: reset() is required here even in the world of exceptions.
+    Obj<VT>* const object_ptr =
+        new Obj<VT>(in_place, ilist, std::forward<Args>(args)...);
+    obj_ = std::unique_ptr<ObjInterface>(object_ptr);
+    return object_ptr->value;
+  }
+
+  // any::reset()
+  //
+  // Resets the state of the `absl::any` object, destroying the contained object
+  // if present.
+  void reset() noexcept { obj_ = nullptr; }
+
+  // any::swap()
+  //
+  // Swaps the passed value and the value of this `absl::any` object.
+  void swap(any& other) noexcept { obj_.swap(other.obj_); }
+
+  // Observors
+
+  // any::has_value()
+  //
+  // Returns `true` if the `any` object has a contained value, otherwise
+  // returns `false`.
+  bool has_value() const noexcept { return obj_ != nullptr; }
+
+#if ABSL_ANY_DETAIL_HAS_RTTI
+  // Returns: typeid(T) if *this has a contained object of type T, otherwise
+  // typeid(void).
+  const std::type_info& type() const noexcept {
+    if (has_value()) {
+      return obj_->Type();
+    }
+
+    return typeid(void);
+  }
+#endif  // ABSL_ANY_DETAIL_HAS_RTTI
+ private:
+  // Tagged type-erased abstraction for holding a cloneable object.
+  class ObjInterface {
+   public:
+    virtual ~ObjInterface() = default;
+    virtual std::unique_ptr<ObjInterface> Clone() const = 0;
+    virtual size_t type_id() const noexcept = 0;
+#if ABSL_ANY_DETAIL_HAS_RTTI
+    virtual const std::type_info& Type() const noexcept = 0;
+#endif  // ABSL_ANY_DETAIL_HAS_RTTI
+  };
+
+  // Hold a value of some queryable type, with an ability to Clone it.
+  template <typename T>
+  class Obj : public ObjInterface {
+   public:
+    template <typename... Args>
+    explicit Obj(in_place_t /*tag*/, Args&&... args)
+        : value(std::forward<Args>(args)...) {}
+
+    std::unique_ptr<ObjInterface> Clone() const final {
+      return std::unique_ptr<ObjInterface>(new Obj(in_place, value));
+    }
+
+    size_t type_id() const noexcept final { return IdForType<T>(); }
+
+#if ABSL_ANY_DETAIL_HAS_RTTI
+    const std::type_info& Type() const noexcept final { return typeid(T); }
+#endif  // ABSL_ANY_DETAIL_HAS_RTTI
+
+    T value;
+  };
+
+  std::unique_ptr<ObjInterface> CloneObj() const {
+    if (!obj_) return nullptr;
+    return obj_->Clone();
+  }
+
+  template <typename T>
+  static size_t IdForType() {
+    // Note: This type dance is to make the behavior consistent with typeid.
+    using NormalizedType =
+        typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+
+    return any_internal::FastTypeId<NormalizedType>();
+  }
+
+  size_t GetObjTypeId() const {
+    return obj_ == nullptr ? any_internal::FastTypeId<void>() : obj_->type_id();
+  }
+
+  // `absl::any` nonmember functions //
+
+  // Description at the declaration site (top of file).
+  template <typename ValueType>
+  friend ValueType any_cast(const any& operand);
+
+  // Description at the declaration site (top of file).
+  template <typename ValueType>
+  friend ValueType any_cast(any& operand);  // NOLINT(runtime/references)
+
+  // Description at the declaration site (top of file).
+  template <typename T>
+  friend const T* any_cast(const any* operand) noexcept;
+
+  // Description at the declaration site (top of file).
+  template <typename T>
+  friend T* any_cast(any* operand) noexcept;
+
+  std::unique_ptr<ObjInterface> obj_;
+};
+
+// -----------------------------------------------------------------------------
+// Implementation Details
+// -----------------------------------------------------------------------------
+
+constexpr any::any() noexcept = default;
+
+template <typename T>
+struct any::IsInPlaceType : std::false_type {};
+
+template <typename T>
+struct any::IsInPlaceType<in_place_type_t<T>> : std::true_type {};
+
+inline void swap(any& x, any& y) noexcept { x.swap(y); }
+
+// Description at the declaration site (top of file).
+template <typename T, typename... Args>
+any make_any(Args&&... args) {
+  return any(in_place_type_t<T>(), std::forward<Args>(args)...);
+}
+
+// Description at the declaration site (top of file).
+template <typename T, typename U, typename... Args>
+any make_any(std::initializer_list<U> il, Args&&... args) {
+  return any(in_place_type_t<T>(), il, std::forward<Args>(args)...);
+}
+
+// Description at the declaration site (top of file).
+template <typename ValueType>
+ValueType any_cast(const any& operand) {
+  using U = typename std::remove_cv<
+      typename std::remove_reference<ValueType>::type>::type;
+  static_assert(std::is_constructible<ValueType, const U&>::value,
+                "Invalid ValueType");
+  auto* const result = (any_cast<U>)(&operand);
+  if (result == nullptr) {
+    any_internal::ThrowBadAnyCast();
+  }
+  return static_cast<ValueType>(*result);
+}
+
+// Description at the declaration site (top of file).
+template <typename ValueType>
+ValueType any_cast(any& operand) {  // NOLINT(runtime/references)
+  using U = typename std::remove_cv<
+      typename std::remove_reference<ValueType>::type>::type;
+  static_assert(std::is_constructible<ValueType, U&>::value,
+                "Invalid ValueType");
+  auto* result = (any_cast<U>)(&operand);
+  if (result == nullptr) {
+    any_internal::ThrowBadAnyCast();
+  }
+  return static_cast<ValueType>(*result);
+}
+
+// Description at the declaration site (top of file).
+template <typename ValueType>
+ValueType any_cast(any&& operand) {
+  using U = typename std::remove_cv<
+      typename std::remove_reference<ValueType>::type>::type;
+  static_assert(std::is_constructible<ValueType, U>::value,
+                "Invalid ValueType");
+  return static_cast<ValueType>(std::move((any_cast<U&>)(operand)));
+}
+
+// Description at the declaration site (top of file).
+template <typename T>
+const T* any_cast(const any* operand) noexcept {
+  return operand && operand->GetObjTypeId() == any::IdForType<T>()
+             ? std::addressof(
+                   static_cast<const any::Obj<T>*>(operand->obj_.get())->value)
+             : nullptr;
+}
+
+// Description at the declaration site (top of file).
+template <typename T>
+T* any_cast(any* operand) noexcept {
+  return operand && operand->GetObjTypeId() == any::IdForType<T>()
+             ? std::addressof(
+                   static_cast<any::Obj<T>*>(operand->obj_.get())->value)
+             : nullptr;
+}
+
+}  // namespace absl
+
+#undef ABSL_ANY_DETAIL_HAS_RTTI
+
+#endif  // ABSL_HAVE_STD_ANY
+
+#endif  // ABSL_TYPES_ANY_H_
diff --git a/absl/types/any_test.cc b/absl/types/any_test.cc
new file mode 100644
index 000000000000..ab04bf5aac60
--- /dev/null
+++ b/absl/types/any_test.cc
@@ -0,0 +1,713 @@
+// Copyright 2017 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/types/any.h"
+
+#include <initializer_list>
+#include <type_traits>
+#include <typeinfo>
+#include <utility>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/exception_testing.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/internal/test_instance_tracker.h"
+
+namespace {
+using absl::test_internal::CopyableOnlyInstance;
+using absl::test_internal::InstanceTracker;
+
+template <typename T>
+const T& AsConst(const T& t) {
+  return t;
+}
+
+struct MoveOnly {
+  MoveOnly() = default;
+  explicit MoveOnly(int value) : value(value) {}
+  MoveOnly(MoveOnly&&) = default;
+  MoveOnly& operator=(MoveOnly&&) = default;
+
+  int value = 0;
+};
+
+struct CopyOnly {
+  CopyOnly() = default;
+  explicit CopyOnly(int value) : value(value) {}
+  CopyOnly(CopyOnly&&) = delete;
+  CopyOnly& operator=(CopyOnly&&) = delete;
+  CopyOnly(const CopyOnly&) = default;
+  CopyOnly& operator=(const CopyOnly&) = default;
+
+  int value = 0;
+};
+
+struct MoveOnlyWithListConstructor {
+  MoveOnlyWithListConstructor() = default;
+  explicit MoveOnlyWithListConstructor(std::initializer_list<int> /*ilist*/,
+                                       int value)
+      : value(value) {}
+  MoveOnlyWithListConstructor(MoveOnlyWithListConstructor&&) = default;
+  MoveOnlyWithListConstructor& operator=(MoveOnlyWithListConstructor&&) =
+      default;
+
+  int value = 0;
+};
+
+struct IntMoveOnlyCopyOnly {
+  IntMoveOnlyCopyOnly(int value, MoveOnly /*move_only*/, CopyOnly /*copy_only*/)
+      : value(value) {}
+
+  int value;
+};
+
+struct ListMoveOnlyCopyOnly {
+  ListMoveOnlyCopyOnly(std::initializer_list<int> ilist, MoveOnly /*move_only*/,
+                       CopyOnly /*copy_only*/)
+      : values(ilist) {}
+
+  std::vector<int> values;
+};
+
+using FunctionType = void();
+void FunctionToEmplace() {}
+
+using ArrayType = int[2];
+using DecayedArray = absl::decay_t<ArrayType>;
+
+TEST(AnyTest, Noexcept) {
+  static_assert(std::is_nothrow_default_constructible<absl::any>(), "");
+  static_assert(std::is_nothrow_move_constructible<absl::any>(), "");
+  static_assert(std::is_nothrow_move_assignable<absl::any>(), "");
+  static_assert(noexcept(std::declval<absl::any&>().has_value()), "");
+  static_assert(noexcept(std::declval<absl::any&>().type()), "");
+  static_assert(noexcept(absl::any_cast<int>(std::declval<absl::any*>())), "");
+  static_assert(
+      noexcept(std::declval<absl::any&>().swap(std::declval<absl::any&>())),
+      "");
+
+  using std::swap;
+  static_assert(
+      noexcept(swap(std::declval<absl::any&>(), std::declval<absl::any&>())),
+      "");
+}
+
+TEST(AnyTest, HasValue) {
+  absl::any o;
+  EXPECT_FALSE(o.has_value());
+  o.emplace<int>();
+  EXPECT_TRUE(o.has_value());
+  o.reset();
+  EXPECT_FALSE(o.has_value());
+}
+
+TEST(AnyTest, Type) {
+  absl::any o;
+  EXPECT_EQ(typeid(void), o.type());
+  o.emplace<int>(5);
+  EXPECT_EQ(typeid(int), o.type());
+  o.emplace<float>(5.f);
+  EXPECT_EQ(typeid(float), o.type());
+  o.reset();
+  EXPECT_EQ(typeid(void), o.type());
+}
+
+TEST(AnyTest, EmptyPointerCast) {
+  // pointer-to-unqualified overload
+  {
+    absl::any o;
+    EXPECT_EQ(nullptr, absl::any_cast<int>(&o));
+    o.emplace<int>();
+    EXPECT_NE(nullptr, absl::any_cast<int>(&o));
+    o.reset();
+    EXPECT_EQ(nullptr, absl::any_cast<int>(&o));
+  }
+
+  // pointer-to-const overload
+  {
+    absl::any o;
+    EXPECT_EQ(nullptr, absl::any_cast<int>(&AsConst(o)));
+    o.emplace<int>();
+    EXPECT_NE(nullptr, absl::any_cast<int>(&AsConst(o)));
+    o.reset();
+    EXPECT_EQ(nullptr, absl::any_cast<int>(&AsConst(o)));
+  }
+}
+
+TEST(AnyTest, InPlaceConstruction) {
+  const CopyOnly copy_only{};
+  absl::any o(absl::in_place_type_t<IntMoveOnlyCopyOnly>(), 5, MoveOnly(),
+              copy_only);
+  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
+  EXPECT_EQ(5, v.value);
+}
+
+TEST(AnyTest, InPlaceConstructionWithCV) {
+  const CopyOnly copy_only{};
+  absl::any o(absl::in_place_type_t<const volatile IntMoveOnlyCopyOnly>(), 5,
+              MoveOnly(), copy_only);
+  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
+  EXPECT_EQ(5, v.value);
+}
+
+TEST(AnyTest, InPlaceConstructionWithFunction) {
+  absl::any o(absl::in_place_type_t<FunctionType>(), FunctionToEmplace);
+  FunctionType*& construction_result = absl::any_cast<FunctionType*&>(o);
+  EXPECT_EQ(&FunctionToEmplace, construction_result);
+}
+
+TEST(AnyTest, InPlaceConstructionWithArray) {
+  ArrayType ar = {5, 42};
+  absl::any o(absl::in_place_type_t<ArrayType>(), ar);
+  DecayedArray& construction_result = absl::any_cast<DecayedArray&>(o);
+  EXPECT_EQ(&ar[0], construction_result);
+}
+
+TEST(AnyTest, InPlaceConstructionIlist) {
+  const CopyOnly copy_only{};
+  absl::any o(absl::in_place_type_t<ListMoveOnlyCopyOnly>(), {1, 2, 3, 4},
+              MoveOnly(), copy_only);
+  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
+  std::vector<int> expected_values = {1, 2, 3, 4};
+  EXPECT_EQ(expected_values, v.values);
+}
+
+TEST(AnyTest, InPlaceConstructionIlistWithCV) {
+  const CopyOnly copy_only{};
+  absl::any o(absl::in_place_type_t<const volatile ListMoveOnlyCopyOnly>(),
+              {1, 2, 3, 4}, MoveOnly(), copy_only);
+  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
+  std::vector<int> expected_values = {1, 2, 3, 4};
+  EXPECT_EQ(expected_values, v.values);
+}
+
+TEST(AnyTest, InPlaceNoArgs) {
+  absl::any o(absl::in_place_type_t<int>{});
+  EXPECT_EQ(0, absl::any_cast<int&>(o));
+}
+
+template <typename Enabler, typename T, typename... Args>
+struct CanEmplaceAnyImpl : std::false_type {};
+
+template <typename T, typename... Args>
+struct CanEmplaceAnyImpl<
+    absl::void_t<decltype(
+        std::declval<absl::any&>().emplace<T>(std::declval<Args>()...))>,
+    T, Args...> : std::true_type {};
+
+template <typename T, typename... Args>
+using CanEmplaceAny = CanEmplaceAnyImpl<void, T, Args...>;
+
+TEST(AnyTest, Emplace) {
+  const CopyOnly copy_only{};
+  absl::any o;
+  EXPECT_TRUE((std::is_same<decltype(o.emplace<IntMoveOnlyCopyOnly>(
+                                5, MoveOnly(), copy_only)),
+                            IntMoveOnlyCopyOnly&>::value));
+  IntMoveOnlyCopyOnly& emplace_result =
+      o.emplace<IntMoveOnlyCopyOnly>(5, MoveOnly(), copy_only);
+  EXPECT_EQ(5, emplace_result.value);
+  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
+  EXPECT_EQ(5, v.value);
+  EXPECT_EQ(&emplace_result, &v);
+
+  static_assert(!CanEmplaceAny<int, int, int>::value, "");
+  static_assert(!CanEmplaceAny<MoveOnly, MoveOnly>::value, "");
+}
+
+TEST(AnyTest, EmplaceWithCV) {
+  const CopyOnly copy_only{};
+  absl::any o;
+  EXPECT_TRUE(
+      (std::is_same<decltype(o.emplace<const volatile IntMoveOnlyCopyOnly>(
+                        5, MoveOnly(), copy_only)),
+                    IntMoveOnlyCopyOnly&>::value));
+  IntMoveOnlyCopyOnly& emplace_result =
+      o.emplace<const volatile IntMoveOnlyCopyOnly>(5, MoveOnly(), copy_only);
+  EXPECT_EQ(5, emplace_result.value);
+  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
+  EXPECT_EQ(5, v.value);
+  EXPECT_EQ(&emplace_result, &v);
+}
+
+TEST(AnyTest, EmplaceWithFunction) {
+  absl::any o;
+  EXPECT_TRUE(
+      (std::is_same<decltype(o.emplace<FunctionType>(FunctionToEmplace)),
+                    FunctionType*&>::value));
+  FunctionType*& emplace_result = o.emplace<FunctionType>(FunctionToEmplace);
+  EXPECT_EQ(&FunctionToEmplace, emplace_result);
+}
+
+TEST(AnyTest, EmplaceWithArray) {
+  absl::any o;
+  ArrayType ar = {5, 42};
+  EXPECT_TRUE(
+      (std::is_same<decltype(o.emplace<ArrayType>(ar)), DecayedArray&>::value));
+  DecayedArray& emplace_result = o.emplace<ArrayType>(ar);
+  EXPECT_EQ(&ar[0], emplace_result);
+}
+
+TEST(AnyTest, EmplaceIlist) {
+  const CopyOnly copy_only{};
+  absl::any o;
+  EXPECT_TRUE((std::is_same<decltype(o.emplace<ListMoveOnlyCopyOnly>(
+                                {1, 2, 3, 4}, MoveOnly(), copy_only)),
+                            ListMoveOnlyCopyOnly&>::value));
+  ListMoveOnlyCopyOnly& emplace_result =
+      o.emplace<ListMoveOnlyCopyOnly>({1, 2, 3, 4}, MoveOnly(), copy_only);
+  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
+  EXPECT_EQ(&v, &emplace_result);
+  std::vector<int> expected_values = {1, 2, 3, 4};
+  EXPECT_EQ(expected_values, v.values);
+
+  static_assert(!CanEmplaceAny<int, std::initializer_list<int>>::value, "");
+  static_assert(!CanEmplaceAny<MoveOnlyWithListConstructor,
+                               std::initializer_list<int>, int>::value,
+                "");
+}
+
+TEST(AnyTest, EmplaceIlistWithCV) {
+  const CopyOnly copy_only{};
+  absl::any o;
+  EXPECT_TRUE(
+      (std::is_same<decltype(o.emplace<const volatile ListMoveOnlyCopyOnly>(
+                        {1, 2, 3, 4}, MoveOnly(), copy_only)),
+                    ListMoveOnlyCopyOnly&>::value));
+  ListMoveOnlyCopyOnly& emplace_result =
+      o.emplace<const volatile ListMoveOnlyCopyOnly>({1, 2, 3, 4}, MoveOnly(),
+                                                     copy_only);
+  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
+  EXPECT_EQ(&v, &emplace_result);
+  std::vector<int> expected_values = {1, 2, 3, 4};
+  EXPECT_EQ(expected_values, v.values);
+}
+
+TEST(AnyTest, EmplaceNoArgs) {
+  absl::any o;
+  o.emplace<int>();
+  EXPECT_EQ(0, absl::any_cast<int>(o));
+}
+
+TEST(AnyTest, ConversionConstruction) {
+  {
+    absl::any o = 5;
+    EXPECT_EQ(5, absl::any_cast<int>(o));
+  }
+
+  {
+    const CopyOnly copy_only(5);
+    absl::any o = copy_only;
+    EXPECT_EQ(5, absl::any_cast<CopyOnly&>(o).value);
+  }
+
+  static_assert(!std::is_convertible<MoveOnly, absl::any>::value, "");
+}
+
+TEST(AnyTest, ConversionAssignment) {
+  {
+    absl::any o;
+    o = 5;
+    EXPECT_EQ(5, absl::any_cast<int>(o));
+  }
+
+  {
+    const CopyOnly copy_only(5);
+    absl::any o;
+    o = copy_only;
+    EXPECT_EQ(5, absl::any_cast<CopyOnly&>(o).value);
+  }
+
+  static_assert(!std::is_assignable<MoveOnly, absl::any>::value, "");
+}
+
+// Suppress MSVC warnings.
+// 4521: multiple copy constructors specified
+// We wrote multiple of them to test that the correct overloads are selected.
+#ifdef _MSC_VER
+#pragma warning( push )
+#pragma warning( disable : 4521)
+#endif
+
+// Weird type for testing, only used to make sure we "properly" perfect-forward
+// when being placed into an absl::any (use the l-value constructor if given an
+// l-value rather than use the copy constructor).
+struct WeirdConstructor42 {
+  explicit WeirdConstructor42(int value) : value(value) {}
+
+  // Copy-constructor
+  WeirdConstructor42(const WeirdConstructor42& other) : value(other.value) {}
+
+  // L-value "weird" constructor (used when given an l-value)
+  WeirdConstructor42(
+      WeirdConstructor42& /*other*/)  // NOLINT(runtime/references)
+      : value(42) {}
+
+  int value;
+};
+#ifdef _MSC_VER
+#pragma warning( pop )
+#endif
+
+TEST(AnyTest, WeirdConversionConstruction) {
+  {
+    const WeirdConstructor42 source(5);
+    absl::any o = source;  // Actual copy
+    EXPECT_EQ(5, absl::any_cast<WeirdConstructor42&>(o).value);
+  }
+
+  {
+    WeirdConstructor42 source(5);
+    absl::any o = source;  // Weird "conversion"
+    EXPECT_EQ(42, absl::any_cast<WeirdConstructor42&>(o).value);
+  }
+}
+
+TEST(AnyTest, WeirdConversionAssignment) {
+  {
+    const WeirdConstructor42 source(5);
+    absl::any o;
+    o = source;  // Actual copy
+    EXPECT_EQ(5, absl::any_cast<WeirdConstructor42&>(o).value);
+  }
+
+  {
+    WeirdConstructor42 source(5);
+    absl::any o;
+    o = source;  // Weird "conversion"
+    EXPECT_EQ(42, absl::any_cast<WeirdConstructor42&>(o).value);
+  }
+}
+
+struct Value {};
+
+TEST(AnyTest, AnyCastValue) {
+  {
+    absl::any o;
+    o.emplace<int>(5);
+    EXPECT_EQ(5, absl::any_cast<int>(o));
+    EXPECT_EQ(5, absl::any_cast<int>(AsConst(o)));
+    static_assert(
+        std::is_same<decltype(absl::any_cast<Value>(o)), Value>::value, "");
+  }
+
+  {
+    absl::any o;
+    o.emplace<int>(5);
+    EXPECT_EQ(5, absl::any_cast<const int>(o));
+    EXPECT_EQ(5, absl::any_cast<const int>(AsConst(o)));
+    static_assert(std::is_same<decltype(absl::any_cast<const Value>(o)),
+                               const Value>::value,
+                  "");
+  }
+}
+
+TEST(AnyTest, AnyCastReference) {
+  {
+    absl::any o;
+    o.emplace<int>(5);
+    EXPECT_EQ(5, absl::any_cast<int&>(o));
+    EXPECT_EQ(5, absl::any_cast<const int&>(AsConst(o)));
+    static_assert(
+        std::is_same<decltype(absl::any_cast<Value&>(o)), Value&>::value, "");
+  }
+
+  {
+    absl::any o;
+    o.emplace<int>(5);
+    EXPECT_EQ(5, absl::any_cast<const int>(o));
+    EXPECT_EQ(5, absl::any_cast<const int>(AsConst(o)));
+    static_assert(std::is_same<decltype(absl::any_cast<const Value&>(o)),
+                               const Value&>::value,
+                  "");
+  }
+
+  {
+    absl::any o;
+    o.emplace<int>(5);
+    EXPECT_EQ(5, absl::any_cast<int&&>(std::move(o)));
+    static_assert(std::is_same<decltype(absl::any_cast<Value&&>(std::move(o))),
+                               Value&&>::value,
+                  "");
+  }
+
+  {
+    absl::any o;
+    o.emplace<int>(5);
+    EXPECT_EQ(5, absl::any_cast<const int>(std::move(o)));
+    static_assert(
+        std::is_same<decltype(absl::any_cast<const Value&&>(std::move(o))),
+                     const Value&&>::value,
+        "");
+  }
+}
+
+TEST(AnyTest, AnyCastPointer) {
+  {
+    absl::any o;
+    EXPECT_EQ(nullptr, absl::any_cast<char>(&o));
+    o.emplace<int>(5);
+    EXPECT_EQ(nullptr, absl::any_cast<char>(&o));
+    o.emplace<char>('a');
+    EXPECT_EQ('a', *absl::any_cast<char>(&o));
+    static_assert(
+        std::is_same<decltype(absl::any_cast<Value>(&o)), Value*>::value, "");
+  }
+
+  {
+    absl::any o;
+    EXPECT_EQ(nullptr, absl::any_cast<const char>(&o));
+    o.emplace<int>(5);
+    EXPECT_EQ(nullptr, absl::any_cast<const char>(&o));
+    o.emplace<char>('a');
+    EXPECT_EQ('a', *absl::any_cast<const char>(&o));
+    static_assert(std::is_same<decltype(absl::any_cast<const Value>(&o)),
+                               const Value*>::value,
+                  "");
+  }
+}
+
+TEST(AnyTest, MakeAny) {
+  const CopyOnly copy_only{};
+  auto o = absl::make_any<IntMoveOnlyCopyOnly>(5, MoveOnly(), copy_only);
+  static_assert(std::is_same<decltype(o), absl::any>::value, "");
+  EXPECT_EQ(5, absl::any_cast<IntMoveOnlyCopyOnly&>(o).value);
+}
+
+TEST(AnyTest, MakeAnyIList) {
+  const CopyOnly copy_only{};
+  auto o =
+      absl::make_any<ListMoveOnlyCopyOnly>({1, 2, 3}, MoveOnly(), copy_only);
+  static_assert(std::is_same<decltype(o), absl::any>::value, "");
+  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
+  std::vector<int> expected_values = {1, 2, 3};
+  EXPECT_EQ(expected_values, v.values);
+}
+
+// Test the use of copy constructor and operator=
+TEST(AnyTest, Copy) {
+  InstanceTracker tracker_raii;
+
+  {
+    absl::any o(absl::in_place_type_t<CopyableOnlyInstance>{}, 123);
+    CopyableOnlyInstance* f1 = absl::any_cast<CopyableOnlyInstance>(&o);
+
+    absl::any o2(o);
+    const CopyableOnlyInstance* f2 = absl::any_cast<CopyableOnlyInstance>(&o2);
+    EXPECT_EQ(123, f2->value());
+    EXPECT_NE(f1, f2);
+
+    absl::any o3;
+    o3 = o2;
+    const CopyableOnlyInstance* f3 = absl::any_cast<CopyableOnlyInstance>(&o3);
+    EXPECT_EQ(123, f3->value());
+    EXPECT_NE(f2, f3);
+
+    const absl::any o4(4);
+    // copy construct from const lvalue ref.
+    absl::any o5 = o4;
+    EXPECT_EQ(4, absl::any_cast<int>(o4));
+    EXPECT_EQ(4, absl::any_cast<int>(o5));
+
+    // Copy construct from const rvalue ref.
+    absl::any o6 = std::move(o4);  // NOLINT
+    EXPECT_EQ(4, absl::any_cast<int>(o4));
+    EXPECT_EQ(4, absl::any_cast<int>(o6));
+  }
+}
+
+TEST(AnyTest, Move) {
+  InstanceTracker tracker_raii;
+
+  absl::any any1;
+  any1.emplace<CopyableOnlyInstance>(5);
+
+  // This is a copy, so copy count increases to 1.
+  absl::any any2 = any1;
+  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any1).value());
+  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any2).value());
+  EXPECT_EQ(1, tracker_raii.copies());
+
+  // This isn't a copy, so copy count doesn't increase.
+  absl::any any3 = std::move(any2);
+  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any3).value());
+  EXPECT_EQ(1, tracker_raii.copies());
+
+  absl::any any4;
+  any4 = std::move(any3);
+  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any4).value());
+  EXPECT_EQ(1, tracker_raii.copies());
+
+  absl::any tmp4(4);
+  absl::any o4(std::move(tmp4));  // move construct
+  EXPECT_EQ(4, absl::any_cast<int>(o4));
+  o4 = o4;  // self assign
+  EXPECT_EQ(4, absl::any_cast<int>(o4));
+  EXPECT_TRUE(o4.has_value());
+
+  absl::any o5;
+  absl::any tmp5(5);
+  o5 = std::move(tmp5);  // move assign
+  EXPECT_EQ(5, absl::any_cast<int>(o5));
+}
+
+// Reset the ObjectOwner with an object of a different type
+TEST(AnyTest, Reset) {
+  absl::any o;
+  o.emplace<int>();
+
+  o.reset();
+  EXPECT_FALSE(o.has_value());
+
+  o.emplace<char>();
+  EXPECT_TRUE(o.has_value());
+}
+
+TEST(AnyTest, ConversionConstructionCausesOneCopy) {
+  InstanceTracker tracker_raii;
+  CopyableOnlyInstance counter(5);
+  absl::any o(counter);
+  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(o).value());
+  EXPECT_EQ(1, tracker_raii.copies());
+}
+
+//////////////////////////////////
+// Tests for Exception Behavior //
+//////////////////////////////////
+
+#define ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(...)                      \
+  ABSL_BASE_INTERNAL_EXPECT_FAIL((__VA_ARGS__), absl::bad_any_cast, \
+                                 "Bad any cast")
+
+TEST(AnyTest, ThrowBadAlloc) {
+  {
+    absl::any a;
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int&>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int&>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int&&>(absl::any{}));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int&&>(absl::any{}));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int>(absl::any{}));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int>(absl::any{}));
+
+    // const absl::any operand
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int&>(AsConst(a)));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int>(AsConst(a)));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int>(AsConst(a)));
+  }
+
+  {
+    absl::any a(absl::in_place_type_t<int>{});
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float&>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float&>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float&&>(absl::any{}));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(
+        absl::any_cast<const float&&>(absl::any{}));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float>(a));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float>(absl::any{}));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float>(absl::any{}));
+
+    // const absl::any operand
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float&>(AsConst(a)));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float>(AsConst(a)));
+    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float>(AsConst(a)));
+  }
+}
+
+class BadCopy {};
+
+struct BadCopyable {
+  BadCopyable() = default;
+  BadCopyable(BadCopyable&&) = default;
+  BadCopyable(const BadCopyable&) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+    throw BadCopy();
+#else
+    ABSL_RAW_LOG(FATAL, "Bad copy");
+#endif
+  }
+};
+
+#define ABSL_ANY_TEST_EXPECT_BAD_COPY(...) \
+  ABSL_BASE_INTERNAL_EXPECT_FAIL((__VA_ARGS__), BadCopy, "Bad copy")
+
+// Test the guarantees regarding exceptions in copy/assign.
+TEST(AnyTest, FailedCopy) {
+  {
+    const BadCopyable bad{};
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(absl::any{bad});
+  }
+
+  {
+    absl::any src(absl::in_place_type_t<BadCopyable>{});
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(absl::any{src});
+  }
+
+  {
+    BadCopyable bad;
+    absl::any target;
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = bad);
+  }
+
+  {
+    BadCopyable bad;
+    absl::any target(absl::in_place_type_t<BadCopyable>{});
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = bad);
+    EXPECT_TRUE(target.has_value());
+  }
+
+  {
+    absl::any src(absl::in_place_type_t<BadCopyable>{});
+    absl::any target;
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = src);
+    EXPECT_FALSE(target.has_value());
+  }
+
+  {
+    absl::any src(absl::in_place_type_t<BadCopyable>{});
+    absl::any target(absl::in_place_type_t<BadCopyable>{});
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = src);
+    EXPECT_TRUE(target.has_value());
+  }
+}
+
+// Test the guarantees regarding exceptions in emplace.
+TEST(AnyTest, FailedEmplace) {
+  {
+    BadCopyable bad;
+    absl::any target;
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(target.emplace<BadCopyable>(bad));
+  }
+
+  {
+    BadCopyable bad;
+    absl::any target(absl::in_place_type_t<int>{});
+    ABSL_ANY_TEST_EXPECT_BAD_COPY(target.emplace<BadCopyable>(bad));
+#if defined(ABSL_HAVE_STD_ANY) && defined(__GLIBCXX__)
+    // libstdc++ std::any::emplace() implementation (as of 7.2) has a bug: if an
+    // exception is thrown, *this contains a value.
+#define ABSL_GLIBCXX_ANY_EMPLACE_EXCEPTION_BUG 1
+#endif
+#if defined(ABSL_HAVE_EXCEPTIONS) && \
+    !defined(ABSL_GLIBCXX_ANY_EMPLACE_EXCEPTION_BUG)
+    EXPECT_FALSE(target.has_value());
+#endif
+  }
+}
+
+}  // namespace
diff --git a/absl/types/bad_any_cast.cc b/absl/types/bad_any_cast.cc
new file mode 100644
index 000000000000..c9b73300690f
--- /dev/null
+++ b/absl/types/bad_any_cast.cc
@@ -0,0 +1,40 @@
+// Copyright 2017 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/types/bad_any_cast.h"
+
+#include <cstdlib>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+
+bad_any_cast::~bad_any_cast() = default;
+
+const char* bad_any_cast::what() const noexcept { return "Bad any cast"; }
+
+namespace any_internal {
+
+void ThrowBadAnyCast() {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw bad_any_cast();
+#else
+  ABSL_RAW_LOG(FATAL, "Bad any cast");
+  std::abort();
+#endif
+}
+
+}  // namespace any_internal
+}  // namespace absl
diff --git a/absl/types/bad_any_cast.h b/absl/types/bad_any_cast.h
new file mode 100644
index 000000000000..8ffbe4bff473
--- /dev/null
+++ b/absl/types/bad_any_cast.h
@@ -0,0 +1,44 @@
+// Copyright 2017 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_TYPES_BAD_ANY_CAST_H_
+#define ABSL_TYPES_BAD_ANY_CAST_H_
+
+#include <typeinfo>
+
+namespace absl {
+
+////////////////////////
+// [any.bad_any_cast] //
+////////////////////////
+
+// Objects of type bad_any_cast are thrown by a failed any_cast.
+class bad_any_cast : public std::bad_cast {
+ public:
+  ~bad_any_cast() override;
+  const char* what() const noexcept override;
+};
+
+//////////////////////////////////////////////
+// Implementation-details beyond this point //
+//////////////////////////////////////////////
+
+namespace any_internal {
+
+[[noreturn]] void ThrowBadAnyCast();
+
+}  // namespace any_internal
+}  // namespace absl
+
+#endif  // ABSL_TYPES_BAD_ANY_CAST_H_
diff --git a/absl/types/bad_optional_access.cc b/absl/types/bad_optional_access.cc
new file mode 100644
index 000000000000..6bc67df77c9a
--- /dev/null
+++ b/absl/types/bad_optional_access.cc
@@ -0,0 +1,42 @@
+// Copyright 2017 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/types/bad_optional_access.h"
+
+#include <cstdlib>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+
+bad_optional_access::~bad_optional_access() = default;
+
+const char* bad_optional_access::what() const noexcept {
+  return "optional has no value";
+}
+
+namespace optional_internal {
+
+void throw_bad_optional_access() {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw bad_optional_access();
+#else
+  ABSL_RAW_LOG(FATAL, "Bad optional access");
+  abort();
+#endif
+}
+
+}  // namespace optional_internal
+}  // namespace absl
diff --git a/absl/types/bad_optional_access.h b/absl/types/bad_optional_access.h
new file mode 100644
index 000000000000..c4c74447d493
--- /dev/null
+++ b/absl/types/bad_optional_access.h
@@ -0,0 +1,37 @@
+// Copyright 2017 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_TYPES_BAD_OPTIONAL_ACCESS_H_
+#define ABSL_TYPES_BAD_OPTIONAL_ACCESS_H_
+
+#include <stdexcept>
+
+namespace absl {
+
+class bad_optional_access : public std::exception {
+ public:
+  bad_optional_access() = default;
+  ~bad_optional_access() override;
+  const char* what() const noexcept override;
+};
+
+namespace optional_internal {
+
+// throw delegator
+[[noreturn]] void throw_bad_optional_access();
+
+}  // namespace optional_internal
+}  // namespace absl
+
+#endif  // ABSL_TYPES_BAD_OPTIONAL_ACCESS_H_
diff --git a/absl/types/optional.cc b/absl/types/optional.cc
new file mode 100644
index 000000000000..ef2729041990
--- /dev/null
+++ b/absl/types/optional.cc
@@ -0,0 +1,24 @@
+// Copyright 2017 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/types/optional.h"
+
+#ifndef ABSL_HAVE_STD_OPTIONAL
+namespace absl {
+
+nullopt_t::init_t nullopt_t::init;
+extern const nullopt_t nullopt{nullopt_t::init};
+
+}  // namespace absl
+#endif  // ABSL_HAVE_STD_OPTIONAL
diff --git a/absl/types/optional.h b/absl/types/optional.h
new file mode 100644
index 000000000000..5099d4899d6d
--- /dev/null
+++ b/absl/types/optional.h
@@ -0,0 +1,1092 @@
+//
+// Copyright 2017 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.
+//
+// -----------------------------------------------------------------------------
+// optional.h
+// -----------------------------------------------------------------------------
+//
+// This header file define the `absl::optional` type for holding a value which
+// may or may not be present. This type is useful for providing value semantics
+// for operations that may either wish to return or hold "something-or-nothing".
+//
+// Example:
+//
+//   // A common way to signal operation failure is to provide an output
+//   // parameter and a bool return type:
+//   bool AcquireResource(const Input&, Resource * out);
+//
+//   // Providing an absl::optional return type provides a cleaner API:
+//   absl::optional<Resource> AcquireResource(const Input&);
+//
+// `absl::optional` is a C++11 compatible version of the C++17 `std::optional`
+// abstraction and is designed to be a drop-in replacement for code compliant
+// with C++17.
+#ifndef ABSL_TYPES_OPTIONAL_H_
+#define ABSL_TYPES_OPTIONAL_H_
+
+#include "absl/base/config.h"
+#include "absl/utility/utility.h"
+
+#ifdef ABSL_HAVE_STD_OPTIONAL
+
+#include <optional>
+
+namespace absl {
+using std::bad_optional_access;
+using std::optional;
+using std::make_optional;
+using std::nullopt_t;
+using std::nullopt;
+}
+
+#else  // ABSL_HAVE_STD_OPTIONAL
+
+#include <cassert>
+#include <functional>
+#include <initializer_list>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
+#include "absl/types/bad_optional_access.h"
+
+// ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+//
+// Inheriting constructors is supported in GCC 4.8+, Clang 3.3+ and MSVC 2015.
+// __cpp_inheriting_constructors is a predefined macro and a recommended way to
+// check for this language feature, but GCC doesn't support it until 5.0 and
+// Clang doesn't support it until 3.6.
+// Also, MSVC 2015 has a bug: it doesn't inherit the constexpr template
+// constructor. For example, the following code won't work on MSVC 2015 Update3:
+// struct Base {
+//   int t;
+//   template <typename T>
+//   constexpr Base(T t_) : t(t_) {}
+// };
+// struct Foo : Base {
+//   using Base::Base;
+// }
+// constexpr Foo foo(0);  // doesn't work on MSVC 2015
+#if defined(__clang__)
+#if __has_feature(cxx_inheriting_constructors)
+#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
+#endif
+#elif (defined(__GNUC__) &&                                       \
+       (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 8)) || \
+    (__cpp_inheriting_constructors >= 200802) ||                  \
+    (defined(_MSC_VER) && _MSC_VER >= 1910)
+#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
+#endif
+
+namespace absl {
+
+// optional
+//
+// A value of type `absl::optional<T>` holds either a value of `T` or an
+// "empty" value.  When it holds a value of `T`, it stores it as a direct
+// sub-object, so `sizeof(optional<T>)` is approximately
+// `sizeof(T) + sizeof(bool)`.
+//
+// This implementation is based on the specification in the latest draft of the
+// C++17 `std::optional` specification as of May 2017, section 20.6.
+//
+// Differences between `absl::optional<T>` and `std::optional<T>` include:
+//
+//    * `constexpr` is not used for non-const member functions.
+//      (dependency on some differences between C++11 and C++14.)
+//    * `absl::nullopt` and `absl::in_place` are not declared `constexpr`. We
+//      need the inline variable support in C++17 for external linkage.
+//    * Throws `absl::bad_optional_access` instead of
+//      `std::bad_optional_access`.
+//    * `optional::swap()` and `absl::swap()` relies on
+//      `std::is_(nothrow_)swappable()`, which has been introduced in C++17.
+//      As a workaround, we assume `is_swappable()` is always `true`
+//      and `is_nothrow_swappable()` is the same as `std::is_trivial()`.
+//    * `make_optional()` cannot be declared `constexpr` due to the absence of
+//      guaranteed copy elision.
+template <typename T>
+class optional;
+
+// nullopt_t
+//
+// Class type for `absl::nullopt` used to indicate an `absl::optional<T>` type
+// that does not contain a value.
+struct nullopt_t {
+  struct init_t {};
+  static init_t init;
+
+  // It must not be default-constructible to avoid ambiguity for opt = {}.
+  // Note the non-const reference, which is to eliminate ambiguity for code
+  // like:
+  //
+  // struct S { int value; };
+  //
+  // void Test() {
+  //   optional<S> opt;
+  //   opt = {{}};
+  // }
+  explicit constexpr nullopt_t(init_t& /*unused*/) {}
+};
+
+// nullopt
+//
+// A tag constant of type `absl::nullopt_t` used to indicate an empty
+// `absl::optional` in certain functions, such as construction or assignment.
+extern const nullopt_t nullopt;
+
+namespace optional_internal {
+
+struct empty_struct {};
+// This class stores the data in optional<T>.
+// It is specialized based on whether T is trivially destructible.
+// This is the specialization for non trivially destructible type.
+template <typename T, bool = std::is_trivially_destructible<T>::value>
+class optional_data_dtor_base {
+  struct dummy_type {
+    static_assert(sizeof(T) % sizeof(empty_struct) == 0, "");
+    // Use an array to avoid GCC 6 placement-new warning.
+    empty_struct data[sizeof(T) / sizeof(empty_struct)];
+  };
+
+ protected:
+  // Whether there is data or not.
+  bool engaged_;
+  // Data storage
+  union {
+    dummy_type dummy_;
+    T data_;
+  };
+
+  void destruct() noexcept {
+    if (engaged_) {
+      data_.~T();
+      engaged_ = false;
+    }
+  }
+
+  // dummy_ must be initialized for constexpr constructor.
+  constexpr optional_data_dtor_base() noexcept : engaged_(false), dummy_{{}} {}
+
+  template <typename... Args>
+  constexpr explicit optional_data_dtor_base(in_place_t, Args&&... args)
+      : engaged_(true), data_(absl::forward<Args>(args)...) {}
+
+  ~optional_data_dtor_base() { destruct(); }
+};
+
+// Specialization for trivially destructible type.
+template <typename T>
+class optional_data_dtor_base<T, true> {
+  struct dummy_type {
+    static_assert(sizeof(T) % sizeof(empty_struct) == 0, "");
+    // Use array to avoid GCC 6 placement-new warning.
+    empty_struct data[sizeof(T) / sizeof(empty_struct)];
+  };
+
+ protected:
+  // Whether there is data or not.
+  bool engaged_;
+  // Data storage
+  union {
+    dummy_type dummy_;
+    T data_;
+  };
+  void destruct() noexcept { engaged_ = false; }
+
+  // dummy_ must be initialized for constexpr constructor.
+  constexpr optional_data_dtor_base() noexcept : engaged_(false), dummy_{{}} {}
+
+  template <typename... Args>
+  constexpr explicit optional_data_dtor_base(in_place_t, Args&&... args)
+      : engaged_(true), data_(absl::forward<Args>(args)...) {}
+};
+
+template <typename T>
+class optional_data_base : public optional_data_dtor_base<T> {
+ protected:
+  using base = optional_data_dtor_base<T>;
+#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+  using base::base;
+#else
+  optional_data_base() = default;
+
+  template <typename... Args>
+  constexpr explicit optional_data_base(in_place_t t, Args&&... args)
+      : base(t, absl::forward<Args>(args)...) {}
+#endif
+
+  template <typename... Args>
+  void construct(Args&&... args) {
+    // Use dummy_'s address to work around casting cv-qualified T* to void*.
+    ::new (static_cast<void*>(&this->dummy_)) T(std::forward<Args>(args)...);
+    this->engaged_ = true;
+  }
+
+  template <typename U>
+  void assign(U&& u) {
+    if (this->engaged_) {
+      this->data_ = std::forward<U>(u);
+    } else {
+      construct(std::forward<U>(u));
+    }
+  }
+};
+
+// TODO(b/34201852): Add another base class using
+// std::is_trivially_move_constructible trait when available to match
+// http://cplusplus.github.io/LWG/lwg-defects.html#2900, for types that
+// have trivial move but nontrivial copy.
+// Also, we should be checking is_trivially_copyable here, which is not
+// supported now, so we use is_trivially_* traits instead.
+template <typename T, bool = absl::is_trivially_copy_constructible<T>::value&&
+                          absl::is_trivially_copy_assignable<
+                              typename std::remove_cv<T>::type>::value&&
+                              std::is_trivially_destructible<T>::value>
+class optional_data;
+
+// Trivially copyable types
+template <typename T>
+class optional_data<T, true> : public optional_data_base<T> {
+ protected:
+#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+  using optional_data_base<T>::optional_data_base;
+#else
+  optional_data() = default;
+
+  template <typename... Args>
+  constexpr explicit optional_data(in_place_t t, Args&&... args)
+      : optional_data_base<T>(t, absl::forward<Args>(args)...) {}
+#endif
+};
+
+template <typename T>
+class optional_data<T, false> : public optional_data_base<T> {
+ protected:
+#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+  using optional_data_base<T>::optional_data_base;
+#else
+  template <typename... Args>
+  constexpr explicit optional_data(in_place_t t, Args&&... args)
+      : optional_data_base<T>(t, absl::forward<Args>(args)...) {}
+#endif
+
+  optional_data() = default;
+
+  optional_data(const optional_data& rhs) {
+    if (rhs.engaged_) {
+      this->construct(rhs.data_);
+    }
+  }
+
+  optional_data(optional_data&& rhs) noexcept(
+      absl::default_allocator_is_nothrow::value ||
+      std::is_nothrow_move_constructible<T>::value) {
+    if (rhs.engaged_) {
+      this->construct(std::move(rhs.data_));
+    }
+  }
+
+  optional_data& operator=(const optional_data& rhs) {
+    if (rhs.engaged_) {
+      this->assign(rhs.data_);
+    } else {
+      this->destruct();
+    }
+    return *this;
+  }
+
+  optional_data& operator=(optional_data&& rhs) noexcept(
+      std::is_nothrow_move_assignable<T>::value&&
+          std::is_nothrow_move_constructible<T>::value) {
+    if (rhs.engaged_) {
+      this->assign(std::move(rhs.data_));
+    } else {
+      this->destruct();
+    }
+    return *this;
+  }
+};
+
+// Ordered by level of restriction, from low to high.
+// Copyable implies movable.
+enum class copy_traits { copyable = 0, movable = 1, non_movable = 2 };
+
+// Base class for enabling/disabling copy/move constructor.
+template <copy_traits>
+class optional_ctor_base;
+
+template <>
+class optional_ctor_base<copy_traits::copyable> {
+ public:
+  constexpr optional_ctor_base() = default;
+  optional_ctor_base(const optional_ctor_base&) = default;
+  optional_ctor_base(optional_ctor_base&&) = default;
+  optional_ctor_base& operator=(const optional_ctor_base&) = default;
+  optional_ctor_base& operator=(optional_ctor_base&&) = default;
+};
+
+template <>
+class optional_ctor_base<copy_traits::movable> {
+ public:
+  constexpr optional_ctor_base() = default;
+  optional_ctor_base(const optional_ctor_base&) = delete;
+  optional_ctor_base(optional_ctor_base&&) = default;
+  optional_ctor_base& operator=(const optional_ctor_base&) = default;
+  optional_ctor_base& operator=(optional_ctor_base&&) = default;
+};
+
+template <>
+class optional_ctor_base<copy_traits::non_movable> {
+ public:
+  constexpr optional_ctor_base() = default;
+  optional_ctor_base(const optional_ctor_base&) = delete;
+  optional_ctor_base(optional_ctor_base&&) = delete;
+  optional_ctor_base& operator=(const optional_ctor_base&) = default;
+  optional_ctor_base& operator=(optional_ctor_base&&) = default;
+};
+
+// Base class for enabling/disabling copy/move assignment.
+template <copy_traits>
+class optional_assign_base;
+
+template <>
+class optional_assign_base<copy_traits::copyable> {
+ public:
+  constexpr optional_assign_base() = default;
+  optional_assign_base(const optional_assign_base&) = default;
+  optional_assign_base(optional_assign_base&&) = default;
+  optional_assign_base& operator=(const optional_assign_base&) = default;
+  optional_assign_base& operator=(optional_assign_base&&) = default;
+};
+
+template <>
+class optional_assign_base<copy_traits::movable> {
+ public:
+  constexpr optional_assign_base() = default;
+  optional_assign_base(const optional_assign_base&) = default;
+  optional_assign_base(optional_assign_base&&) = default;
+  optional_assign_base& operator=(const optional_assign_base&) = delete;
+  optional_assign_base& operator=(optional_assign_base&&) = default;
+};
+
+template <>
+class optional_assign_base<copy_traits::non_movable> {
+ public:
+  constexpr optional_assign_base() = default;
+  optional_assign_base(const optional_assign_base&) = default;
+  optional_assign_base(optional_assign_base&&) = default;
+  optional_assign_base& operator=(const optional_assign_base&) = delete;
+  optional_assign_base& operator=(optional_assign_base&&) = delete;
+};
+
+template <typename T>
+constexpr copy_traits get_ctor_copy_traits() {
+  return std::is_copy_constructible<T>::value
+             ? copy_traits::copyable
+             : std::is_move_constructible<T>::value ? copy_traits::movable
+                                                    : copy_traits::non_movable;
+}
+
+template <typename T>
+constexpr copy_traits get_assign_copy_traits() {
+  return std::is_copy_assignable<T>::value &&
+                 std::is_copy_constructible<T>::value
+             ? copy_traits::copyable
+             : std::is_move_assignable<T>::value &&
+                       std::is_move_constructible<T>::value
+                   ? copy_traits::movable
+                   : copy_traits::non_movable;
+}
+
+// Whether T is constructible or convertible from optional<U>.
+template <typename T, typename U>
+struct is_constructible_convertible_from_optional
+    : std::integral_constant<
+          bool, std::is_constructible<T, optional<U>&>::value ||
+                    std::is_constructible<T, optional<U>&&>::value ||
+                    std::is_constructible<T, const optional<U>&>::value ||
+                    std::is_constructible<T, const optional<U>&&>::value ||
+                    std::is_convertible<optional<U>&, T>::value ||
+                    std::is_convertible<optional<U>&&, T>::value ||
+                    std::is_convertible<const optional<U>&, T>::value ||
+                    std::is_convertible<const optional<U>&&, T>::value> {};
+
+// Whether T is constructible or convertible or assignable from optional<U>.
+template <typename T, typename U>
+struct is_constructible_convertible_assignable_from_optional
+    : std::integral_constant<
+          bool, is_constructible_convertible_from_optional<T, U>::value ||
+                    std::is_assignable<T&, optional<U>&>::value ||
+                    std::is_assignable<T&, optional<U>&&>::value ||
+                    std::is_assignable<T&, const optional<U>&>::value ||
+                    std::is_assignable<T&, const optional<U>&&>::value> {};
+
+// Helper function used by [optional.relops], [optional.comp_with_t],
+// for checking whether an expression is convertible to bool.
+bool convertible_to_bool(bool);
+
+}  // namespace optional_internal
+
+// -----------------------------------------------------------------------------
+// absl::optional class definition
+// -----------------------------------------------------------------------------
+
+template <typename T>
+class optional : private optional_internal::optional_data<T>,
+                 private optional_internal::optional_ctor_base<
+                     optional_internal::get_ctor_copy_traits<T>()>,
+                 private optional_internal::optional_assign_base<
+                     optional_internal::get_assign_copy_traits<T>()> {
+  using data_base = optional_internal::optional_data<T>;
+
+ public:
+  typedef T value_type;
+
+  // Constructors
+
+  // Constructs a default-constructed `optional` holding the empty value, NOT a
+  // default constructed `T`.
+  constexpr optional() noexcept {}
+
+  // Construct an` optional` initialized with `nullopt` to hold an empty value.
+  constexpr optional(nullopt_t) noexcept {}  // NOLINT(runtime/explicit)
+
+  // Copy constructor, standard semantics
+  optional(const optional& src) = default;
+
+  // Move constructor, standard semantics
+  optional(optional&& src) = default;
+
+  // Constructs a non-empty `optional` direct-initialized value of type `T` from
+  // the arguments `std::forward<Args>(args)...`  within the `optional`.
+  // (The `in_place_t` is a tag used to indicate that the contained object
+  // should be constructed in-place.)
+  // TODO(b/34201852): Add std::is_constructible<T, Args&&...> SFINAE.
+  template <typename... Args>
+  constexpr explicit optional(in_place_t, Args&&... args)
+      : data_base(in_place_t(), absl::forward<Args>(args)...) {}
+
+  // Constructs a non-empty `optional' direct-initialized value of type `T` from
+  // the arguments of an initializer_list and `std::forward<Args>(args)...`.
+  // (The `in_place_t` is a tag used to indicate that the contained object
+  // should be constructed in-place.)
+  template <typename U, typename... Args,
+            typename = typename std::enable_if<std::is_constructible<
+                T, std::initializer_list<U>&, Args&&...>::value>::type>
+  constexpr explicit optional(in_place_t, std::initializer_list<U> il,
+                              Args&&... args)
+      : data_base(in_place_t(), il, absl::forward<Args>(args)...) {
+  }
+
+  // Value constructor (implicit)
+  template <
+      typename U = T,
+      typename std::enable_if<
+          absl::conjunction<absl::negation<std::is_same<
+                                in_place_t, typename std::decay<U>::type> >,
+                            absl::negation<std::is_same<
+                                optional<T>, typename std::decay<U>::type> >,
+                            std::is_convertible<U&&, T>,
+                            std::is_constructible<T, U&&> >::value,
+          bool>::type = false>
+  constexpr optional(U&& v) : data_base(in_place_t(), absl::forward<U>(v)) {}
+
+  // Value constructor (explicit)
+  template <
+      typename U = T,
+      typename std::enable_if<
+          absl::conjunction<absl::negation<std::is_same<
+                                in_place_t, typename std::decay<U>::type>>,
+                            absl::negation<std::is_same<
+                                optional<T>, typename std::decay<U>::type>>,
+                            absl::negation<std::is_convertible<U&&, T>>,
+                            std::is_constructible<T, U&&>>::value,
+          bool>::type = false>
+  explicit constexpr optional(U&& v)
+      : data_base(in_place_t(), absl::forward<U>(v)) {}
+
+  // Converting copy constructor (implicit)
+  template <typename U,
+            typename std::enable_if<
+                absl::conjunction<
+                    absl::negation<std::is_same<T, U> >,
+                    std::is_constructible<T, const U&>,
+                    absl::negation<
+                        optional_internal::
+                            is_constructible_convertible_from_optional<T, U> >,
+                    std::is_convertible<const U&, T> >::value,
+                bool>::type = false>
+  optional(const optional<U>& rhs) {
+    if (rhs) {
+      this->construct(*rhs);
+    }
+  }
+
+  // Converting copy constructor (explicit)
+  template <typename U,
+            typename std::enable_if<
+                absl::conjunction<
+                    absl::negation<std::is_same<T, U>>,
+                    std::is_constructible<T, const U&>,
+                    absl::negation<
+                        optional_internal::
+                            is_constructible_convertible_from_optional<T, U>>,
+                    absl::negation<std::is_convertible<const U&, T>>>::value,
+                bool>::type = false>
+  explicit optional(const optional<U>& rhs) {
+    if (rhs) {
+      this->construct(*rhs);
+    }
+  }
+
+  // Converting move constructor (implicit)
+  template <typename U,
+            typename std::enable_if<
+                absl::conjunction<
+                    absl::negation<std::is_same<T, U> >,
+                    std::is_constructible<T, U&&>,
+                    absl::negation<
+                        optional_internal::
+                            is_constructible_convertible_from_optional<T, U> >,
+                    std::is_convertible<U&&, T> >::value,
+                bool>::type = false>
+  optional(optional<U>&& rhs) {
+    if (rhs) {
+      this->construct(std::move(*rhs));
+    }
+  }
+
+  // Converting move constructor (explicit)
+  template <
+      typename U,
+      typename std::enable_if<
+          absl::conjunction<
+              absl::negation<std::is_same<T, U>>, std::is_constructible<T, U&&>,
+              absl::negation<
+                  optional_internal::is_constructible_convertible_from_optional<
+                      T, U>>,
+              absl::negation<std::is_convertible<U&&, T>>>::value,
+          bool>::type = false>
+  explicit optional(optional<U>&& rhs) {
+    if (rhs) {
+      this->construct(std::move(*rhs));
+    }
+  }
+
+  // Destructor. Trivial if `T` is trivially destructible.
+  ~optional() = default;
+
+  // Assignment Operators
+
+  // Assignment from `nullopt`
+  //
+  // Example:
+  //
+  //   struct S { int value; };
+  //   optional<S> opt = absl::nullopt;  // Could also use opt = { };
+  optional& operator=(nullopt_t) noexcept {
+    this->destruct();
+    return *this;
+  }
+
+  // Copy assignment operator, standard semantics
+  optional& operator=(const optional& src) = default;
+
+  // Move assignment operator, standard semantics
+  optional& operator=(optional&& src) = default;
+
+  // Value assignment operators
+  template <
+      typename U = T,
+      typename = typename std::enable_if<absl::conjunction<
+          absl::negation<
+              std::is_same<optional<T>, typename std::decay<U>::type>>,
+          absl::negation<
+              absl::conjunction<std::is_scalar<T>,
+                                std::is_same<T, typename std::decay<U>::type>>>,
+          std::is_constructible<T, U>, std::is_assignable<T&, U>>::value>::type>
+  optional& operator=(U&& v) {
+    this->assign(std::forward<U>(v));
+    return *this;
+  }
+
+  template <
+      typename U,
+      typename = typename std::enable_if<absl::conjunction<
+          absl::negation<std::is_same<T, U>>,
+          std::is_constructible<T, const U&>, std::is_assignable<T&, const U&>,
+          absl::negation<
+              optional_internal::
+                  is_constructible_convertible_assignable_from_optional<
+                      T, U>>>::value>::type>
+  optional& operator=(const optional<U>& rhs) {
+    if (rhs) {
+      this->assign(*rhs);
+    } else {
+      this->destruct();
+    }
+    return *this;
+  }
+
+  template <typename U,
+            typename = typename std::enable_if<absl::conjunction<
+                absl::negation<std::is_same<T, U>>, std::is_constructible<T, U>,
+                std::is_assignable<T&, U>,
+                absl::negation<
+                    optional_internal::
+                        is_constructible_convertible_assignable_from_optional<
+                            T, U>>>::value>::type>
+  optional& operator=(optional<U>&& rhs) {
+    if (rhs) {
+      this->assign(std::move(*rhs));
+    } else {
+      this->destruct();
+    }
+    return *this;
+  }
+
+  // Modifiers
+
+  // optional::reset()
+  //
+  // Destroys the inner `T` value of an `absl::optional` if one is present.
+  void reset() noexcept { this->destruct(); }
+
+  // optional::emplace()
+  //
+  // (Re)constructs the underlying `T` in-place with the given forwarded
+  // arguments.
+  //
+  // Example:
+  //
+  //   optional<Foo> opt;
+  //   opt.emplace(arg1,arg2,arg3);  // Constructs Foo(arg1,arg2,arg3)
+  //
+  // If the optional is non-empty, and the `args` refer to subobjects of the
+  // current object, then behaviour is undefined, because the current object
+  // will be destructed before the new object is constructed with `args`.
+  template <typename... Args,
+            typename = typename std::enable_if<
+                std::is_constructible<T, Args&&...>::value>::type>
+  T& emplace(Args&&... args) {
+    this->destruct();
+    this->construct(std::forward<Args>(args)...);
+    return reference();
+  }
+
+  // Emplace reconstruction overload for an initializer list and the given
+  // forwarded arguments.
+  //
+  // Example:
+  //
+  //   struct Foo {
+  //     Foo(std::initializer_list<int>);
+  //   };
+  //
+  //   optional<Foo> opt;
+  //   opt.emplace({1,2,3});  // Constructs Foo({1,2,3})
+  template <typename U, typename... Args,
+            typename = typename std::enable_if<std::is_constructible<
+                T, std::initializer_list<U>&, Args&&...>::value>::type>
+  T& emplace(std::initializer_list<U> il, Args&&... args) {
+    this->destruct();
+    this->construct(il, std::forward<Args>(args)...);
+    return reference();
+  }
+
+  // Swaps
+
+  // Swap, standard semantics
+  void swap(optional& rhs) noexcept(
+      std::is_nothrow_move_constructible<T>::value&&
+          std::is_trivial<T>::value) {
+    if (*this) {
+      if (rhs) {
+        using std::swap;
+        swap(**this, *rhs);
+      } else {
+        rhs.construct(std::move(**this));
+        this->destruct();
+      }
+    } else {
+      if (rhs) {
+        this->construct(std::move(*rhs));
+        rhs.destruct();
+      } else {
+        // No effect (swap(disengaged, disengaged)).
+      }
+    }
+  }
+
+  // Observers
+
+  // optional::operator->()
+  //
+  // Accesses the underlying `T` value's member `m` of an `optional`. If the
+  // `optional` is empty, behavior is undefined.
+  constexpr const T* operator->() const { return this->pointer(); }
+  T* operator->() {
+    assert(this->engaged_);
+    return this->pointer();
+  }
+
+  // optional::operator*()
+  //
+  // Accesses the underlying `T `value of an `optional`. If the `optional` is
+  // empty, behavior is undefined.
+  constexpr const T& operator*() const & { return reference(); }
+  T& operator*() & {
+    assert(this->engaged_);
+    return reference();
+  }
+  constexpr const T&& operator*() const && {
+    return absl::move(reference());
+  }
+  T&& operator*() && {
+    assert(this->engaged_);
+    return std::move(reference());
+  }
+
+  // optional::operator bool()
+  //
+  // Returns false if and only if the `optional` is empty.
+  //
+  //   if (opt) {
+  //     // do something with opt.value();
+  //   } else {
+  //     // opt is empty.
+  //   }
+  //
+  constexpr explicit operator bool() const noexcept { return this->engaged_; }
+
+  // optional::has_value()
+  //
+  // Determines whether the `optional` contains a value. Returns `false` if and
+  // only if `*this` is empty.
+  constexpr bool has_value() const noexcept { return this->engaged_; }
+
+  // optional::value()
+  //
+  // Returns a reference to an `optional`s underlying value. The constness
+  // and lvalue/rvalue-ness of the `optional` is preserved to the view of
+  // the `T` sub-object. Throws `absl::bad_optional_access` when the `optional`
+  // is empty.
+  constexpr const T& value() const & {
+    return static_cast<bool>(*this)
+               ? reference()
+               : (optional_internal::throw_bad_optional_access(), reference());
+  }
+  T& value() & {
+    return static_cast<bool>(*this)
+               ? reference()
+               : (optional_internal::throw_bad_optional_access(), reference());
+  }
+  T&& value() && {  // NOLINT(build/c++11)
+    return std::move(
+        static_cast<bool>(*this)
+            ? reference()
+            : (optional_internal::throw_bad_optional_access(), reference()));
+  }
+  constexpr const T&& value() const && {  // NOLINT(build/c++11)
+    return absl::move(
+        static_cast<bool>(*this)
+            ? reference()
+            : (optional_internal::throw_bad_optional_access(), reference()));
+  }
+
+  // optional::value_or()
+  //
+  // Returns either the value of `T` or a passed default `val` if the `optional`
+  // is empty.
+  template <typename U>
+  constexpr T value_or(U&& v) const& {
+    return static_cast<bool>(*this)
+               ? **this
+               : static_cast<T>(absl::forward<U>(v));
+  }
+  template <typename U>
+  T value_or(U&& v) && {  // NOLINT(build/c++11)
+    return static_cast<bool>(*this) ? std::move(**this)
+                                    : static_cast<T>(std::forward<U>(v));
+  }
+
+ private:
+  // Private accessors for internal storage viewed as pointer to T.
+  constexpr const T* pointer() const { return &this->data_; }
+  T* pointer() { return &this->data_; }
+
+  // Private accessors for internal storage viewed as reference to T.
+  constexpr const T& reference() const { return *this->pointer(); }
+  T& reference() { return *(this->pointer()); }
+
+  // T constraint checks.  You can't have an optional of nullopt_t, in_place_t
+  // or a reference.
+  static_assert(
+      !std::is_same<nullopt_t, typename std::remove_cv<T>::type>::value,
+      "optional<nullopt_t> is not allowed.");
+  static_assert(
+      !std::is_same<in_place_t, typename std::remove_cv<T>::type>::value,
+      "optional<in_place_t> is not allowed.");
+  static_assert(!std::is_reference<T>::value,
+                "optional<reference> is not allowed.");
+};
+
+// Non-member functions
+
+// swap()
+//
+// Performs a swap between two `absl::optional` objects, using standard
+// semantics.
+//
+// NOTE: we assume `is_swappable()` is always `true`. A compile error will
+// result if this is not the case.
+template <typename T,
+          typename std::enable_if<std::is_move_constructible<T>::value,
+                                  bool>::type = false>
+void swap(optional<T>& a, optional<T>& b) noexcept(noexcept(a.swap(b))) {
+  a.swap(b);
+}
+
+// make_optional()
+//
+// Creates a non-empty `optional<T>` where the type of `T` is deduced. An
+// `absl::optional` can also be explicitly instantiated with
+// `make_optional<T>(v)`.
+//
+// Note: `make_optional()` constructions may be declared `constexpr` for
+// trivially copyable types `T`. Non-trivial types require copy elision
+// support in C++17 for `make_optional` to support `constexpr` on such
+// non-trivial types.
+//
+// Example:
+//
+//   constexpr absl::optional<int> opt = absl::make_optional(1);
+//   static_assert(opt.value() == 1, "");
+template <typename T>
+constexpr optional<typename std::decay<T>::type> make_optional(T&& v) {
+  return optional<typename std::decay<T>::type>(absl::forward<T>(v));
+}
+
+template <typename T, typename... Args>
+constexpr optional<T> make_optional(Args&&... args) {
+  return optional<T>(in_place_t(), absl::forward<Args>(args)...);
+}
+
+template <typename T, typename U, typename... Args>
+constexpr optional<T> make_optional(std::initializer_list<U> il,
+                                    Args&&... args) {
+  return optional<T>(in_place_t(), il,
+                     absl::forward<Args>(args)...);
+}
+
+// Relational operators [optional.relops]
+
+// Empty optionals are considered equal to each other and less than non-empty
+// optionals. Supports relations between optional<T> and optional<U>, between
+// optional<T> and U, and between optional<T> and nullopt.
+//
+// Note: We're careful to support T having non-bool relationals.
+
+// Requires: The expression, e.g. "*x == *y" shall be well-formed and its result
+// shall be convertible to bool.
+// The C++17 (N4606) "Returns:" statements are translated into
+// code in an obvious way here, and the original text retained as function docs.
+// Returns: If bool(x) != bool(y), false; otherwise if bool(x) == false, true;
+// otherwise *x == *y.
+template <typename T, typename U>
+constexpr auto operator==(const optional<T>& x, const optional<U>& y)
+    -> decltype(optional_internal::convertible_to_bool(*x == *y)) {
+  return static_cast<bool>(x) != static_cast<bool>(y)
+             ? false
+             : static_cast<bool>(x) == false ? true : *x == *y;
+}
+
+// Returns: If bool(x) != bool(y), true; otherwise, if bool(x) == false, false;
+// otherwise *x != *y.
+template <typename T, typename U>
+constexpr auto operator!=(const optional<T>& x, const optional<U>& y)
+    -> decltype(optional_internal::convertible_to_bool(*x != *y)) {
+  return static_cast<bool>(x) != static_cast<bool>(y)
+             ? true
+             : static_cast<bool>(x) == false ? false : *x != *y;
+}
+// Returns: If !y, false; otherwise, if !x, true; otherwise *x < *y.
+template <typename T, typename U>
+constexpr auto operator<(const optional<T>& x, const optional<U>& y)
+    -> decltype(optional_internal::convertible_to_bool(*x < *y)) {
+  return !y ? false : !x ? true : *x < *y;
+}
+// Returns: If !x, false; otherwise, if !y, true; otherwise *x > *y.
+template <typename T, typename U>
+constexpr auto operator>(const optional<T>& x, const optional<U>& y)
+    -> decltype(optional_internal::convertible_to_bool(*x > *y)) {
+  return !x ? false : !y ? true : *x > *y;
+}
+// Returns: If !x, true; otherwise, if !y, false; otherwise *x <= *y.
+template <typename T, typename U>
+constexpr auto operator<=(const optional<T>& x, const optional<U>& y)
+    -> decltype(optional_internal::convertible_to_bool(*x <= *y)) {
+  return !x ? true : !y ? false : *x <= *y;
+}
+// Returns: If !y, true; otherwise, if !x, false; otherwise *x >= *y.
+template <typename T, typename U>
+constexpr auto operator>=(const optional<T>& x, const optional<U>& y)
+    -> decltype(optional_internal::convertible_to_bool(*x >= *y)) {
+  return !y ? true : !x ? false : *x >= *y;
+}
+
+// Comparison with nullopt [optional.nullops]
+// The C++17 (N4606) "Returns:" statements are used directly here.
+template <typename T>
+constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept {
+  return !x;
+}
+template <typename T>
+constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept {
+  return !x;
+}
+template <typename T>
+constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept {
+  return static_cast<bool>(x);
+}
+template <typename T>
+constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept {
+  return static_cast<bool>(x);
+}
+template <typename T>
+constexpr bool operator<(const optional<T>&, nullopt_t) noexcept {
+  return false;
+}
+template <typename T>
+constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept {
+  return static_cast<bool>(x);
+}
+template <typename T>
+constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept {
+  return !x;
+}
+template <typename T>
+constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept {
+  return true;
+}
+template <typename T>
+constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept {
+  return static_cast<bool>(x);
+}
+template <typename T>
+constexpr bool operator>(nullopt_t, const optional<T>&) noexcept {
+  return false;
+}
+template <typename T>
+constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept {
+  return true;
+}
+template <typename T>
+constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept {
+  return !x;
+}
+
+// Comparison with T [optional.comp_with_t]
+
+// Requires: The expression, e.g. "*x == v" shall be well-formed and its result
+// shall be convertible to bool.
+// The C++17 (N4606) "Equivalent to:" statements are used directly here.
+template <typename T, typename U>
+constexpr auto operator==(const optional<T>& x, const U& v)
+    -> decltype(optional_internal::convertible_to_bool(*x == v)) {
+  return static_cast<bool>(x) ? *x == v : false;
+}
+template <typename T, typename U>
+constexpr auto operator==(const U& v, const optional<T>& x)
+    -> decltype(optional_internal::convertible_to_bool(v == *x)) {
+  return static_cast<bool>(x) ? v == *x : false;
+}
+template <typename T, typename U>
+constexpr auto operator!=(const optional<T>& x, const U& v)
+    -> decltype(optional_internal::convertible_to_bool(*x != v)) {
+  return static_cast<bool>(x) ? *x != v : true;
+}
+template <typename T, typename U>
+constexpr auto operator!=(const U& v, const optional<T>& x)
+    -> decltype(optional_internal::convertible_to_bool(v != *x)) {
+  return static_cast<bool>(x) ? v != *x : true;
+}
+template <typename T, typename U>
+constexpr auto operator<(const optional<T>& x, const U& v)
+    -> decltype(optional_internal::convertible_to_bool(*x < v)) {
+  return static_cast<bool>(x) ? *x < v : true;
+}
+template <typename T, typename U>
+constexpr auto operator<(const U& v, const optional<T>& x)
+    -> decltype(optional_internal::convertible_to_bool(v < *x)) {
+  return static_cast<bool>(x) ? v < *x : false;
+}
+template <typename T, typename U>
+constexpr auto operator<=(const optional<T>& x, const U& v)
+    -> decltype(optional_internal::convertible_to_bool(*x <= v)) {
+  return static_cast<bool>(x) ? *x <= v : true;
+}
+template <typename T, typename U>
+constexpr auto operator<=(const U& v, const optional<T>& x)
+    -> decltype(optional_internal::convertible_to_bool(v <= *x)) {
+  return static_cast<bool>(x) ? v <= *x : false;
+}
+template <typename T, typename U>
+constexpr auto operator>(const optional<T>& x, const U& v)
+    -> decltype(optional_internal::convertible_to_bool(*x > v)) {
+  return static_cast<bool>(x) ? *x > v : false;
+}
+template <typename T, typename U>
+constexpr auto operator>(const U& v, const optional<T>& x)
+    -> decltype(optional_internal::convertible_to_bool(v > *x)) {
+  return static_cast<bool>(x) ? v > *x : true;
+}
+template <typename T, typename U>
+constexpr auto operator>=(const optional<T>& x, const U& v)
+    -> decltype(optional_internal::convertible_to_bool(*x >= v)) {
+  return static_cast<bool>(x) ? *x >= v : false;
+}
+template <typename T, typename U>
+constexpr auto operator>=(const U& v, const optional<T>& x)
+    -> decltype(optional_internal::convertible_to_bool(v >= *x)) {
+  return static_cast<bool>(x) ? v >= *x : true;
+}
+
+}  // namespace absl
+
+namespace std {
+
+// std::hash specialization for absl::optional.
+template <typename T>
+struct hash<absl::optional<T>> {
+  size_t operator()(const absl::optional<T>& opt) const {
+    if (opt) {
+      return hash<T>()(*opt);
+    } else {
+      return static_cast<size_t>(0x297814aaad196e6dULL);
+    }
+  }
+};
+
+}  // namespace std
+
+#undef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+#undef ABSL_MSVC_CONSTEXPR_BUG_IN_UNION_LIKE_CLASS
+
+#endif  // ABSL_HAVE_STD_OPTIONAL
+
+#endif  // ABSL_TYPES_OPTIONAL_H_
diff --git a/absl/types/optional_test.cc b/absl/types/optional_test.cc
new file mode 100644
index 000000000000..25b44b171c9d
--- /dev/null
+++ b/absl/types/optional_test.cc
@@ -0,0 +1,1539 @@
+// Copyright 2017 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/types/optional.h"
+
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+std::string TypeQuals(std::string&) { return "&"; }
+std::string TypeQuals(std::string&&) { return "&&"; }
+std::string TypeQuals(const std::string&) { return "c&"; }
+std::string TypeQuals(const std::string&&) { return "c&&"; }
+
+struct StructorListener {
+  int construct0 = 0;
+  int construct1 = 0;
+  int construct2 = 0;
+  int listinit = 0;
+  int copy = 0;
+  int move = 0;
+  int copy_assign = 0;
+  int move_assign = 0;
+  int destruct = 0;
+  int volatile_copy = 0;
+  int volatile_move = 0;
+  int volatile_copy_assign = 0;
+  int volatile_move_assign = 0;
+};
+
+// Suppress MSVC warnings.
+// 4521: multiple copy constructors specified
+// 4522: multiple assignment operators specified
+// We wrote multiple of them to test that the correct overloads are selected.
+#ifdef _MSC_VER
+#pragma warning( push )
+#pragma warning( disable : 4521)
+#pragma warning( disable : 4522)
+#endif
+struct Listenable {
+  static StructorListener* listener;
+
+  Listenable() { ++listener->construct0; }
+  explicit Listenable(int /*unused*/) { ++listener->construct1; }
+  Listenable(int /*unused*/, int /*unused*/) { ++listener->construct2; }
+  Listenable(std::initializer_list<int> /*unused*/) { ++listener->listinit; }
+  Listenable(const Listenable& /*unused*/) { ++listener->copy; }
+  Listenable(const volatile Listenable& /*unused*/) {
+    ++listener->volatile_copy;
+  }
+  Listenable(volatile Listenable&& /*unused*/) { ++listener->volatile_move; }
+  Listenable(Listenable&& /*unused*/) { ++listener->move; }
+  Listenable& operator=(const Listenable& /*unused*/) {
+    ++listener->copy_assign;
+    return *this;
+  }
+  Listenable& operator=(Listenable&& /*unused*/) {
+    ++listener->move_assign;
+    return *this;
+  }
+  // use void return type instead of volatile T& to work around GCC warning
+  // when the assignment's returned reference is ignored.
+  void operator=(const volatile Listenable& /*unused*/) volatile {
+    ++listener->volatile_copy_assign;
+  }
+  void operator=(volatile Listenable&& /*unused*/) volatile {
+    ++listener->volatile_move_assign;
+  }
+  ~Listenable() { ++listener->destruct; }
+};
+#ifdef _MSC_VER
+#pragma warning( pop )
+#endif
+
+StructorListener* Listenable::listener = nullptr;
+
+// ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST is defined to 1 when the standard
+// library implementation doesn't marked initializer_list's default constructor
+// constexpr. The C++11 standard doesn't specify constexpr on it, but C++14
+// added it. However, libstdc++ 4.7 marked it constexpr.
+#if defined(_LIBCPP_VERSION) && \
+    (_LIBCPP_STD_VER <= 11 || defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR))
+#define ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST 1
+#endif
+
+struct ConstexprType {
+  enum CtorTypes {
+    kCtorDefault,
+    kCtorInt,
+    kCtorInitializerList,
+    kCtorConstChar
+  };
+  constexpr ConstexprType() : x(kCtorDefault) {}
+  constexpr explicit ConstexprType(int i) : x(kCtorInt) {}
+#ifndef ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST
+  constexpr ConstexprType(std::initializer_list<int> il)
+      : x(kCtorInitializerList) {}
+#endif
+  constexpr ConstexprType(const char*)  // NOLINT(runtime/explicit)
+      : x(kCtorConstChar) {}
+  int x;
+};
+
+struct Copyable {
+  Copyable() {}
+  Copyable(const Copyable&) {}
+  Copyable& operator=(const Copyable&) { return *this; }
+};
+
+struct MoveableThrow {
+  MoveableThrow() {}
+  MoveableThrow(MoveableThrow&&) {}
+  MoveableThrow& operator=(MoveableThrow&&) { return *this; }
+};
+
+struct MoveableNoThrow {
+  MoveableNoThrow() {}
+  MoveableNoThrow(MoveableNoThrow&&) noexcept {}
+  MoveableNoThrow& operator=(MoveableNoThrow&&) noexcept { return *this; }
+};
+
+struct NonMovable {
+  NonMovable() {}
+  NonMovable(const NonMovable&) = delete;
+  NonMovable& operator=(const NonMovable&) = delete;
+  NonMovable(NonMovable&&) = delete;
+  NonMovable& operator=(NonMovable&&) = delete;
+};
+
+TEST(optionalTest, DefaultConstructor) {
+  absl::optional<int> empty;
+  EXPECT_FALSE(empty);
+  constexpr absl::optional<int> cempty;
+  static_assert(!cempty.has_value(), "");
+  EXPECT_TRUE(
+      std::is_nothrow_default_constructible<absl::optional<int>>::value);
+}
+
+TEST(optionalTest, nulloptConstructor) {
+  absl::optional<int> empty(absl::nullopt);
+  EXPECT_FALSE(empty);
+
+#ifdef ABSL_HAVE_STD_OPTIONAL
+  constexpr absl::optional<int> cempty{absl::nullopt};
+#else
+  // Creating a temporary absl::nullopt_t object instead of using absl::nullopt
+  // because absl::nullopt cannot be constexpr and have external linkage at the
+  // same time.
+  constexpr absl::optional<int> cempty{absl::nullopt_t(absl::nullopt_t::init)};
+#endif
+  static_assert(!cempty.has_value(), "");
+  EXPECT_TRUE((std::is_nothrow_constructible<absl::optional<int>,
+                                             absl::nullopt_t>::value));
+}
+
+TEST(optionalTest, CopyConstructor) {
+  {
+    absl::optional<int> empty, opt42 = 42;
+    absl::optional<int> empty_copy(empty);
+    EXPECT_FALSE(empty_copy);
+    absl::optional<int> opt42_copy(opt42);
+    EXPECT_TRUE(opt42_copy);
+    EXPECT_EQ(42, *opt42_copy);
+  }
+  {
+    absl::optional<const int> empty, opt42 = 42;
+    absl::optional<const int> empty_copy(empty);
+    EXPECT_FALSE(empty_copy);
+    absl::optional<const int> opt42_copy(opt42);
+    EXPECT_TRUE(opt42_copy);
+    EXPECT_EQ(42, *opt42_copy);
+  }
+  {
+    absl::optional<volatile int> empty, opt42 = 42;
+    absl::optional<volatile int> empty_copy(empty);
+    EXPECT_FALSE(empty_copy);
+    absl::optional<volatile int> opt42_copy(opt42);
+    EXPECT_TRUE(opt42_copy);
+    EXPECT_EQ(42, *opt42_copy);
+  }
+  // test copyablility
+  EXPECT_TRUE(std::is_copy_constructible<absl::optional<int>>::value);
+  EXPECT_TRUE(std::is_copy_constructible<absl::optional<Copyable>>::value);
+  EXPECT_FALSE(
+      std::is_copy_constructible<absl::optional<MoveableThrow>>::value);
+  EXPECT_FALSE(
+      std::is_copy_constructible<absl::optional<MoveableNoThrow>>::value);
+  EXPECT_FALSE(std::is_copy_constructible<absl::optional<NonMovable>>::value);
+
+  EXPECT_FALSE(
+      absl::is_trivially_copy_constructible<absl::optional<Copyable>>::value);
+#if defined(ABSL_HAVE_STD_OPTIONAL) && defined(__GLIBCXX__)
+  // libstdc++ std::optional implementation (as of 7.2) has a bug: when T is
+  // trivially copyable, optional<T> is not trivially copyable (due to one of
+  // its base class is unconditionally nontrivial).
+#define ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG 1
+#endif
+#ifndef ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG
+  EXPECT_TRUE(
+      absl::is_trivially_copy_constructible<absl::optional<int>>::value);
+  EXPECT_TRUE(
+      absl::is_trivially_copy_constructible<absl::optional<const int>>::value);
+#ifndef _MSC_VER
+  // See defect report "Trivial copy/move constructor for class with volatile
+  // member" at
+  // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2094
+  // A class with non-static data member of volatile-qualified type should still
+  // have a trivial copy constructor if the data member is trivial.
+  // Also a cv-qualified scalar type should be trivially copyable.
+  EXPECT_TRUE(absl::is_trivially_copy_constructible<
+              absl::optional<volatile int>>::value);
+#endif  // _MSC_VER
+#endif  // ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG
+
+  // constexpr copy constructor for trivially copyable types
+  {
+    constexpr absl::optional<int> o1;
+    constexpr absl::optional<int> o2 = o1;
+    static_assert(!o2, "");
+  }
+  {
+    constexpr absl::optional<int> o1 = 42;
+    constexpr absl::optional<int> o2 = o1;
+    static_assert(o2, "");
+    static_assert(*o2 == 42, "");
+  }
+  {
+    struct TrivialCopyable {
+      constexpr TrivialCopyable() : x(0) {}
+      constexpr explicit TrivialCopyable(int i) : x(i) {}
+      int x;
+    };
+    constexpr absl::optional<TrivialCopyable> o1(42);
+    constexpr absl::optional<TrivialCopyable> o2 = o1;
+    static_assert(o2, "");
+    static_assert(o2->x == 42, "");
+#ifndef ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG
+    EXPECT_TRUE(absl::is_trivially_copy_constructible<
+                absl::optional<TrivialCopyable>>::value);
+    EXPECT_TRUE(absl::is_trivially_copy_constructible<
+                absl::optional<const TrivialCopyable>>::value);
+#endif
+    EXPECT_FALSE(std::is_copy_constructible<
+                 absl::optional<volatile TrivialCopyable>>::value);
+  }
+}
+
+TEST(optionalTest, MoveConstructor) {
+  absl::optional<int> empty, opt42 = 42;
+  absl::optional<int> empty_move(std::move(empty));
+  EXPECT_FALSE(empty_move);
+  absl::optional<int> opt42_move(std::move(opt42));
+  EXPECT_TRUE(opt42_move);
+  EXPECT_EQ(42, opt42_move);
+  // test movability
+  EXPECT_TRUE(std::is_move_constructible<absl::optional<int>>::value);
+  EXPECT_TRUE(std::is_move_constructible<absl::optional<Copyable>>::value);
+  EXPECT_TRUE(std::is_move_constructible<absl::optional<MoveableThrow>>::value);
+  EXPECT_TRUE(
+      std::is_move_constructible<absl::optional<MoveableNoThrow>>::value);
+  EXPECT_FALSE(std::is_move_constructible<absl::optional<NonMovable>>::value);
+  // test noexcept
+  EXPECT_TRUE(std::is_nothrow_move_constructible<absl::optional<int>>::value);
+#ifndef ABSL_HAVE_STD_OPTIONAL
+  EXPECT_EQ(
+      absl::default_allocator_is_nothrow::value,
+      std::is_nothrow_move_constructible<absl::optional<MoveableThrow>>::value);
+#endif
+  EXPECT_TRUE(std::is_nothrow_move_constructible<
+              absl::optional<MoveableNoThrow>>::value);
+}
+
+TEST(optionalTest, Destructor) {
+  struct Trivial {};
+
+  struct NonTrivial {
+    NonTrivial(const NonTrivial&) {}
+    NonTrivial& operator=(const NonTrivial&) { return *this; }
+    ~NonTrivial() {}
+  };
+
+  EXPECT_TRUE(std::is_trivially_destructible<absl::optional<int>>::value);
+  EXPECT_TRUE(std::is_trivially_destructible<absl::optional<Trivial>>::value);
+  EXPECT_FALSE(
+      std::is_trivially_destructible<absl::optional<NonTrivial>>::value);
+}
+
+TEST(optionalTest, InPlaceConstructor) {
+  constexpr absl::optional<ConstexprType> opt0{absl::in_place_t()};
+  static_assert(opt0, "");
+  static_assert(opt0->x == ConstexprType::kCtorDefault, "");
+  constexpr absl::optional<ConstexprType> opt1{absl::in_place_t(), 1};
+  static_assert(opt1, "");
+  static_assert(opt1->x == ConstexprType::kCtorInt, "");
+#ifndef ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST
+  constexpr absl::optional<ConstexprType> opt2{absl::in_place_t(), {1, 2}};
+  static_assert(opt2, "");
+  static_assert(opt2->x == ConstexprType::kCtorInitializerList, "");
+#endif
+
+  // TODO(b/34201852): uncomment these when std::is_constructible<T, Args&&...>
+  // SFINAE is added to optional::optional(absl::in_place_t, Args&&...).
+  // struct I {
+  //   I(absl::in_place_t);
+  // };
+
+  // EXPECT_FALSE((std::is_constructible<absl::optional<I>,
+  // absl::in_place_t>::value));
+  // EXPECT_FALSE((std::is_constructible<absl::optional<I>, const
+  // absl::in_place_t&>::value));
+}
+
+// template<U=T> optional(U&&);
+TEST(optionalTest, ValueConstructor) {
+  constexpr absl::optional<int> opt0(0);
+  static_assert(opt0, "");
+  static_assert(*opt0 == 0, "");
+  EXPECT_TRUE((std::is_convertible<int, absl::optional<int>>::value));
+  // Copy initialization ( = "abc") won't work due to optional(optional&&)
+  // is not constexpr. Use list initialization instead. This invokes
+  // absl::optional<ConstexprType>::absl::optional<U>(U&&), with U = const char
+  // (&) [4], which direct-initializes the ConstexprType value held by the
+  // optional via ConstexprType::ConstexprType(const char*).
+  constexpr absl::optional<ConstexprType> opt1 = {"abc"};
+  static_assert(opt1, "");
+  static_assert(ConstexprType::kCtorConstChar == opt1->x, "");
+  EXPECT_TRUE(
+      (std::is_convertible<const char*, absl::optional<ConstexprType>>::value));
+  // direct initialization
+  constexpr absl::optional<ConstexprType> opt2{2};
+  static_assert(opt2, "");
+  static_assert(ConstexprType::kCtorInt == opt2->x, "");
+  EXPECT_FALSE(
+      (std::is_convertible<int, absl::optional<ConstexprType>>::value));
+
+  // this invokes absl::optional<int>::optional(int&&)
+  // NOTE: this has different behavior than assignment, e.g.
+  // "opt3 = {};" clears the optional rather than setting the value to 0
+  // According to C++17 standard N4659 [over.ics.list] 16.3.3.1.5, (9.2)- "if
+  // the initializer list has no elements, the implicit conversion is the
+  // identity conversion", so `optional(int&&)` should be a better match than
+  // `optional(optional&&)` which is a user-defined conversion.
+  // Note: GCC 7 has a bug with this overload selection when compiled with
+  // `-std=c++17`.
+#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ == 7 && \
+    __cplusplus == 201703L
+#define ABSL_GCC7_OVER_ICS_LIST_BUG 1
+#endif
+#ifndef ABSL_GCC7_OVER_ICS_LIST_BUG
+  constexpr absl::optional<int> opt3({});
+  static_assert(opt3, "");
+  static_assert(*opt3 == 0, "");
+#endif
+
+  // this invokes the move constructor with a default constructed optional
+  // because non-template function is a better match than template function.
+  absl::optional<ConstexprType> opt4({});
+  EXPECT_FALSE(opt4);
+}
+
+struct Implicit {};
+
+struct Explicit {};
+
+struct Convert {
+  Convert(const Implicit&)  // NOLINT(runtime/explicit)
+      : implicit(true), move(false) {}
+  Convert(Implicit&&)  // NOLINT(runtime/explicit)
+      : implicit(true), move(true) {}
+  explicit Convert(const Explicit&) : implicit(false), move(false) {}
+  explicit Convert(Explicit&&) : implicit(false), move(true) {}
+
+  bool implicit;
+  bool move;
+};
+
+struct ConvertFromOptional {
+  ConvertFromOptional(const Implicit&)  // NOLINT(runtime/explicit)
+      : implicit(true), move(false), from_optional(false) {}
+  ConvertFromOptional(Implicit&&)  // NOLINT(runtime/explicit)
+      : implicit(true), move(true), from_optional(false) {}
+  ConvertFromOptional(
+      const absl::optional<Implicit>&)  // NOLINT(runtime/explicit)
+      : implicit(true), move(false), from_optional(true) {}
+  ConvertFromOptional(absl::optional<Implicit>&&)  // NOLINT(runtime/explicit)
+      : implicit(true), move(true), from_optional(true) {}
+  explicit ConvertFromOptional(const Explicit&)
+      : implicit(false), move(false), from_optional(false) {}
+  explicit ConvertFromOptional(Explicit&&)
+      : implicit(false), move(true), from_optional(false) {}
+  explicit ConvertFromOptional(const absl::optional<Explicit>&)
+      : implicit(false), move(false), from_optional(true) {}
+  explicit ConvertFromOptional(absl::optional<Explicit>&&)
+      : implicit(false), move(true), from_optional(true) {}
+
+  bool implicit;
+  bool move;
+  bool from_optional;
+};
+
+TEST(optionalTest, ConvertingConstructor) {
+  absl::optional<Implicit> i_empty;
+  absl::optional<Implicit> i(absl::in_place);
+  absl::optional<Explicit> e_empty;
+  absl::optional<Explicit> e(absl::in_place);
+  {
+    // implicitly constructing absl::optional<Convert> from
+    // absl::optional<Implicit>
+    absl::optional<Convert> empty = i_empty;
+    EXPECT_FALSE(empty);
+    absl::optional<Convert> opt_copy = i;
+    EXPECT_TRUE(opt_copy);
+    EXPECT_TRUE(opt_copy->implicit);
+    EXPECT_FALSE(opt_copy->move);
+    absl::optional<Convert> opt_move = absl::optional<Implicit>(absl::in_place);
+    EXPECT_TRUE(opt_move);
+    EXPECT_TRUE(opt_move->implicit);
+    EXPECT_TRUE(opt_move->move);
+  }
+  {
+    // explicitly constructing absl::optional<Convert> from
+    // absl::optional<Explicit>
+    absl::optional<Convert> empty(e_empty);
+    EXPECT_FALSE(empty);
+    absl::optional<Convert> opt_copy(e);
+    EXPECT_TRUE(opt_copy);
+    EXPECT_FALSE(opt_copy->implicit);
+    EXPECT_FALSE(opt_copy->move);
+    EXPECT_FALSE((std::is_convertible<const absl::optional<Explicit>&,
+                                      absl::optional<Convert>>::value));
+    absl::optional<Convert> opt_move{absl::optional<Explicit>(absl::in_place)};
+    EXPECT_TRUE(opt_move);
+    EXPECT_FALSE(opt_move->implicit);
+    EXPECT_TRUE(opt_move->move);
+    EXPECT_FALSE((std::is_convertible<absl::optional<Explicit>&&,
+                                      absl::optional<Convert>>::value));
+  }
+  {
+    // implicitly constructing absl::optional<ConvertFromOptional> from
+    // absl::optional<Implicit> via
+    // ConvertFromOptional(absl::optional<Implicit>&&) check that
+    // ConvertFromOptional(Implicit&&) is NOT called
+    static_assert(
+        std::is_convertible<absl::optional<Implicit>,
+                            absl::optional<ConvertFromOptional>>::value,
+        "");
+    absl::optional<ConvertFromOptional> opt0 = i_empty;
+    EXPECT_TRUE(opt0);
+    EXPECT_TRUE(opt0->implicit);
+    EXPECT_FALSE(opt0->move);
+    EXPECT_TRUE(opt0->from_optional);
+    absl::optional<ConvertFromOptional> opt1 = absl::optional<Implicit>();
+    EXPECT_TRUE(opt1);
+    EXPECT_TRUE(opt1->implicit);
+    EXPECT_TRUE(opt1->move);
+    EXPECT_TRUE(opt1->from_optional);
+  }
+  {
+    // implicitly constructing absl::optional<ConvertFromOptional> from
+    // absl::optional<Explicit> via
+    // ConvertFromOptional(absl::optional<Explicit>&&) check that
+    // ConvertFromOptional(Explicit&&) is NOT called
+    absl::optional<ConvertFromOptional> opt0(e_empty);
+    EXPECT_TRUE(opt0);
+    EXPECT_FALSE(opt0->implicit);
+    EXPECT_FALSE(opt0->move);
+    EXPECT_TRUE(opt0->from_optional);
+    EXPECT_FALSE(
+        (std::is_convertible<const absl::optional<Explicit>&,
+                             absl::optional<ConvertFromOptional>>::value));
+    absl::optional<ConvertFromOptional> opt1{absl::optional<Explicit>()};
+    EXPECT_TRUE(opt1);
+    EXPECT_FALSE(opt1->implicit);
+    EXPECT_TRUE(opt1->move);
+    EXPECT_TRUE(opt1->from_optional);
+    EXPECT_FALSE(
+        (std::is_convertible<absl::optional<Explicit>&&,
+                             absl::optional<ConvertFromOptional>>::value));
+  }
+}
+
+TEST(optionalTest, StructorBasic) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+  {
+    absl::optional<Listenable> empty;
+    EXPECT_FALSE(empty);
+    absl::optional<Listenable> opt0(absl::in_place);
+    EXPECT_TRUE(opt0);
+    absl::optional<Listenable> opt1(absl::in_place, 1);
+    EXPECT_TRUE(opt1);
+    absl::optional<Listenable> opt2(absl::in_place, 1, 2);
+    EXPECT_TRUE(opt2);
+  }
+  EXPECT_EQ(1, listener.construct0);
+  EXPECT_EQ(1, listener.construct1);
+  EXPECT_EQ(1, listener.construct2);
+  EXPECT_EQ(3, listener.destruct);
+}
+
+TEST(optionalTest, CopyMoveStructor) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+  absl::optional<Listenable> original(absl::in_place);
+  EXPECT_EQ(1, listener.construct0);
+  EXPECT_EQ(0, listener.copy);
+  EXPECT_EQ(0, listener.move);
+  absl::optional<Listenable> copy(original);
+  EXPECT_EQ(1, listener.construct0);
+  EXPECT_EQ(1, listener.copy);
+  EXPECT_EQ(0, listener.move);
+  absl::optional<Listenable> move(std::move(original));
+  EXPECT_EQ(1, listener.construct0);
+  EXPECT_EQ(1, listener.copy);
+  EXPECT_EQ(1, listener.move);
+}
+
+TEST(optionalTest, ListInit) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+  absl::optional<Listenable> listinit1(absl::in_place, {1});
+  absl::optional<Listenable> listinit2(absl::in_place, {1, 2});
+  EXPECT_EQ(2, listener.listinit);
+}
+
+TEST(optionalTest, AssignFromNullopt) {
+  absl::optional<int> opt(1);
+  opt = absl::nullopt;
+  EXPECT_FALSE(opt);
+
+  StructorListener listener;
+  Listenable::listener = &listener;
+  absl::optional<Listenable> opt1(absl::in_place);
+  opt1 = absl::nullopt;
+  EXPECT_FALSE(opt1);
+  EXPECT_EQ(1, listener.construct0);
+  EXPECT_EQ(1, listener.destruct);
+
+  EXPECT_TRUE((
+      std::is_nothrow_assignable<absl::optional<int>, absl::nullopt_t>::value));
+  EXPECT_TRUE((std::is_nothrow_assignable<absl::optional<Listenable>,
+                                          absl::nullopt_t>::value));
+}
+
+TEST(optionalTest, CopyAssignment) {
+  const absl::optional<int> empty, opt1 = 1, opt2 = 2;
+  absl::optional<int> empty_to_opt1, opt1_to_opt2, opt2_to_empty;
+
+  EXPECT_FALSE(empty_to_opt1);
+  empty_to_opt1 = empty;
+  EXPECT_FALSE(empty_to_opt1);
+  empty_to_opt1 = opt1;
+  EXPECT_TRUE(empty_to_opt1);
+  EXPECT_EQ(1, empty_to_opt1.value());
+
+  EXPECT_FALSE(opt1_to_opt2);
+  opt1_to_opt2 = opt1;
+  EXPECT_TRUE(opt1_to_opt2);
+  EXPECT_EQ(1, opt1_to_opt2.value());
+  opt1_to_opt2 = opt2;
+  EXPECT_TRUE(opt1_to_opt2);
+  EXPECT_EQ(2, opt1_to_opt2.value());
+
+  EXPECT_FALSE(opt2_to_empty);
+  opt2_to_empty = opt2;
+  EXPECT_TRUE(opt2_to_empty);
+  EXPECT_EQ(2, opt2_to_empty.value());
+  opt2_to_empty = empty;
+  EXPECT_FALSE(opt2_to_empty);
+
+  EXPECT_FALSE(std::is_copy_assignable<absl::optional<const int>>::value);
+  EXPECT_TRUE(std::is_copy_assignable<absl::optional<Copyable>>::value);
+  EXPECT_FALSE(std::is_copy_assignable<absl::optional<MoveableThrow>>::value);
+  EXPECT_FALSE(std::is_copy_assignable<absl::optional<MoveableNoThrow>>::value);
+  EXPECT_FALSE(std::is_copy_assignable<absl::optional<NonMovable>>::value);
+
+  EXPECT_TRUE(absl::is_trivially_copy_assignable<int>::value);
+  EXPECT_TRUE(absl::is_trivially_copy_assignable<volatile int>::value);
+
+  struct Trivial {
+    int i;
+  };
+  struct NonTrivial {
+    NonTrivial& operator=(const NonTrivial&) { return *this; }
+    int i;
+  };
+
+  EXPECT_TRUE(absl::is_trivially_copy_assignable<Trivial>::value);
+  EXPECT_FALSE(std::is_copy_assignable<const Trivial>::value);
+  EXPECT_FALSE(std::is_copy_assignable<volatile Trivial>::value);
+  EXPECT_TRUE(std::is_copy_assignable<NonTrivial>::value);
+  EXPECT_FALSE(absl::is_trivially_copy_assignable<NonTrivial>::value);
+
+  // std::optional doesn't support volatile nontrivial types.
+#ifndef ABSL_HAVE_STD_OPTIONAL
+  {
+    StructorListener listener;
+    Listenable::listener = &listener;
+
+    absl::optional<volatile Listenable> empty, set(absl::in_place);
+    EXPECT_EQ(1, listener.construct0);
+    absl::optional<volatile Listenable> empty_to_empty, empty_to_set,
+        set_to_empty(absl::in_place), set_to_set(absl::in_place);
+    EXPECT_EQ(3, listener.construct0);
+    empty_to_empty = empty;  // no effect
+    empty_to_set = set;      // copy construct
+    set_to_empty = empty;    // destruct
+    set_to_set = set;        // copy assign
+    EXPECT_EQ(1, listener.volatile_copy);
+    EXPECT_EQ(0, listener.volatile_move);
+    EXPECT_EQ(1, listener.destruct);
+    EXPECT_EQ(1, listener.volatile_copy_assign);
+  }
+#endif  // ABSL_HAVE_STD_OPTIONAL
+}
+
+TEST(optionalTest, MoveAssignment) {
+  {
+    StructorListener listener;
+    Listenable::listener = &listener;
+
+    absl::optional<Listenable> empty1, empty2, set1(absl::in_place),
+        set2(absl::in_place);
+    EXPECT_EQ(2, listener.construct0);
+    absl::optional<Listenable> empty_to_empty, empty_to_set,
+        set_to_empty(absl::in_place), set_to_set(absl::in_place);
+    EXPECT_EQ(4, listener.construct0);
+    empty_to_empty = std::move(empty1);
+    empty_to_set = std::move(set1);
+    set_to_empty = std::move(empty2);
+    set_to_set = std::move(set2);
+    EXPECT_EQ(0, listener.copy);
+    EXPECT_EQ(1, listener.move);
+    EXPECT_EQ(1, listener.destruct);
+    EXPECT_EQ(1, listener.move_assign);
+  }
+  // std::optional doesn't support volatile nontrivial types.
+#ifndef ABSL_HAVE_STD_OPTIONAL
+  {
+    StructorListener listener;
+    Listenable::listener = &listener;
+
+    absl::optional<volatile Listenable> empty1, empty2, set1(absl::in_place),
+        set2(absl::in_place);
+    EXPECT_EQ(2, listener.construct0);
+    absl::optional<volatile Listenable> empty_to_empty, empty_to_set,
+        set_to_empty(absl::in_place), set_to_set(absl::in_place);
+    EXPECT_EQ(4, listener.construct0);
+    empty_to_empty = std::move(empty1);  // no effect
+    empty_to_set = std::move(set1);      // move construct
+    set_to_empty = std::move(empty2);    // destruct
+    set_to_set = std::move(set2);        // move assign
+    EXPECT_EQ(0, listener.volatile_copy);
+    EXPECT_EQ(1, listener.volatile_move);
+    EXPECT_EQ(1, listener.destruct);
+    EXPECT_EQ(1, listener.volatile_move_assign);
+  }
+#endif  // ABSL_HAVE_STD_OPTIONAL
+  EXPECT_FALSE(std::is_move_assignable<absl::optional<const int>>::value);
+  EXPECT_TRUE(std::is_move_assignable<absl::optional<Copyable>>::value);
+  EXPECT_TRUE(std::is_move_assignable<absl::optional<MoveableThrow>>::value);
+  EXPECT_TRUE(std::is_move_assignable<absl::optional<MoveableNoThrow>>::value);
+  EXPECT_FALSE(std::is_move_assignable<absl::optional<NonMovable>>::value);
+
+  EXPECT_FALSE(
+      std::is_nothrow_move_assignable<absl::optional<MoveableThrow>>::value);
+  EXPECT_TRUE(
+      std::is_nothrow_move_assignable<absl::optional<MoveableNoThrow>>::value);
+}
+
+struct NoConvertToOptional {
+  // disable implicit conversion from const NoConvertToOptional&
+  // to absl::optional<NoConvertToOptional>.
+  NoConvertToOptional(const NoConvertToOptional&) = delete;
+};
+
+struct CopyConvert {
+  CopyConvert(const NoConvertToOptional&);
+  CopyConvert& operator=(const CopyConvert&) = delete;
+  CopyConvert& operator=(const NoConvertToOptional&);
+};
+
+struct CopyConvertFromOptional {
+  CopyConvertFromOptional(const NoConvertToOptional&);
+  CopyConvertFromOptional(const absl::optional<NoConvertToOptional>&);
+  CopyConvertFromOptional& operator=(const CopyConvertFromOptional&) = delete;
+  CopyConvertFromOptional& operator=(const NoConvertToOptional&);
+  CopyConvertFromOptional& operator=(
+      const absl::optional<NoConvertToOptional>&);
+};
+
+struct MoveConvert {
+  MoveConvert(NoConvertToOptional&&);
+  MoveConvert& operator=(const MoveConvert&) = delete;
+  MoveConvert& operator=(NoConvertToOptional&&);
+};
+
+struct MoveConvertFromOptional {
+  MoveConvertFromOptional(NoConvertToOptional&&);
+  MoveConvertFromOptional(absl::optional<NoConvertToOptional>&&);
+  MoveConvertFromOptional& operator=(const MoveConvertFromOptional&) = delete;
+  MoveConvertFromOptional& operator=(NoConvertToOptional&&);
+  MoveConvertFromOptional& operator=(absl::optional<NoConvertToOptional>&&);
+};
+
+// template <typename U = T> absl::optional<T>& operator=(U&& v);
+TEST(optionalTest, ValueAssignment) {
+  absl::optional<int> opt;
+  EXPECT_FALSE(opt);
+  opt = 42;
+  EXPECT_TRUE(opt);
+  EXPECT_EQ(42, opt.value());
+  opt = absl::nullopt;
+  EXPECT_FALSE(opt);
+  opt = 42;
+  EXPECT_TRUE(opt);
+  EXPECT_EQ(42, opt.value());
+  opt = 43;
+  EXPECT_TRUE(opt);
+  EXPECT_EQ(43, opt.value());
+  opt = {};  // this should clear optional
+  EXPECT_FALSE(opt);
+
+  opt = {44};
+  EXPECT_TRUE(opt);
+  EXPECT_EQ(44, opt.value());
+
+  // U = const NoConvertToOptional&
+  EXPECT_TRUE((std::is_assignable<absl::optional<CopyConvert>&,
+                                  const NoConvertToOptional&>::value));
+  // U = const absl::optional<NoConvertToOptional>&
+  EXPECT_TRUE((std::is_assignable<absl::optional<CopyConvertFromOptional>&,
+                                  const NoConvertToOptional&>::value));
+  // U = const NoConvertToOptional& triggers SFINAE because
+  // std::is_constructible_v<MoveConvert, const NoConvertToOptional&> is false
+  EXPECT_FALSE((std::is_assignable<absl::optional<MoveConvert>&,
+                                   const NoConvertToOptional&>::value));
+  // U = NoConvertToOptional
+  EXPECT_TRUE((std::is_assignable<absl::optional<MoveConvert>&,
+                                  NoConvertToOptional&&>::value));
+  // U = const NoConvertToOptional& triggers SFINAE because
+  // std::is_constructible_v<MoveConvertFromOptional, const
+  // NoConvertToOptional&> is false
+  EXPECT_FALSE((std::is_assignable<absl::optional<MoveConvertFromOptional>&,
+                                   const NoConvertToOptional&>::value));
+  // U = NoConvertToOptional
+  EXPECT_TRUE((std::is_assignable<absl::optional<MoveConvertFromOptional>&,
+                                  NoConvertToOptional&&>::value));
+  // U = const absl::optional<NoConvertToOptional>&
+  EXPECT_TRUE(
+      (std::is_assignable<absl::optional<CopyConvertFromOptional>&,
+                          const absl::optional<NoConvertToOptional>&>::value));
+  // U = absl::optional<NoConvertToOptional>
+  EXPECT_TRUE(
+      (std::is_assignable<absl::optional<MoveConvertFromOptional>&,
+                          absl::optional<NoConvertToOptional>&&>::value));
+}
+
+// template <typename U> absl::optional<T>& operator=(const absl::optional<U>&
+// rhs); template <typename U> absl::optional<T>& operator=(absl::optional<U>&&
+// rhs);
+TEST(optionalTest, ConvertingAssignment) {
+  absl::optional<int> opt_i;
+  absl::optional<char> opt_c('c');
+  opt_i = opt_c;
+  EXPECT_TRUE(opt_i);
+  EXPECT_EQ(*opt_c, *opt_i);
+  opt_i = absl::optional<char>();
+  EXPECT_FALSE(opt_i);
+  opt_i = absl::optional<char>('d');
+  EXPECT_TRUE(opt_i);
+  EXPECT_EQ('d', *opt_i);
+
+  absl::optional<std::string> opt_str;
+  absl::optional<const char*> opt_cstr("abc");
+  opt_str = opt_cstr;
+  EXPECT_TRUE(opt_str);
+  EXPECT_EQ(std::string("abc"), *opt_str);
+  opt_str = absl::optional<const char*>();
+  EXPECT_FALSE(opt_str);
+  opt_str = absl::optional<const char*>("def");
+  EXPECT_TRUE(opt_str);
+  EXPECT_EQ(std::string("def"), *opt_str);
+
+  // operator=(const absl::optional<U>&) with U = NoConvertToOptional
+  EXPECT_TRUE(
+      (std::is_assignable<absl::optional<CopyConvert>,
+                          const absl::optional<NoConvertToOptional>&>::value));
+  // operator=(const absl::optional<U>&) with U = NoConvertToOptional
+  // triggers SFINAE because
+  // std::is_constructible_v<MoveConvert, const NoConvertToOptional&> is false
+  EXPECT_FALSE(
+      (std::is_assignable<absl::optional<MoveConvert>&,
+                          const absl::optional<NoConvertToOptional>&>::value));
+  // operator=(absl::optional<U>&&) with U = NoConvertToOptional
+  EXPECT_TRUE(
+      (std::is_assignable<absl::optional<MoveConvert>&,
+                          absl::optional<NoConvertToOptional>&&>::value));
+  // operator=(const absl::optional<U>&) with U = NoConvertToOptional triggers
+  // SFINAE because std::is_constructible_v<MoveConvertFromOptional, const
+  // NoConvertToOptional&> is false. operator=(U&&) with U = const
+  // absl::optional<NoConverToOptional>& triggers SFINAE because
+  // std::is_constructible<MoveConvertFromOptional,
+  // absl::optional<NoConvertToOptional>&&> is true.
+  EXPECT_FALSE(
+      (std::is_assignable<absl::optional<MoveConvertFromOptional>&,
+                          const absl::optional<NoConvertToOptional>&>::value));
+}
+
+TEST(optionalTest, ResetAndHasValue) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+  absl::optional<Listenable> opt;
+  EXPECT_FALSE(opt);
+  EXPECT_FALSE(opt.has_value());
+  opt.emplace();
+  EXPECT_TRUE(opt);
+  EXPECT_TRUE(opt.has_value());
+  opt.reset();
+  EXPECT_FALSE(opt);
+  EXPECT_FALSE(opt.has_value());
+  EXPECT_EQ(1, listener.destruct);
+  opt.reset();
+  EXPECT_FALSE(opt);
+  EXPECT_FALSE(opt.has_value());
+
+  constexpr absl::optional<int> empty;
+  static_assert(!empty.has_value(), "");
+  constexpr absl::optional<int> nonempty(1);
+  static_assert(nonempty.has_value(), "");
+}
+
+TEST(optionalTest, Emplace) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+  absl::optional<Listenable> opt;
+  EXPECT_FALSE(opt);
+  opt.emplace(1);
+  EXPECT_TRUE(opt);
+  opt.emplace(1, 2);
+  EXPECT_EQ(1, listener.construct1);
+  EXPECT_EQ(1, listener.construct2);
+  EXPECT_EQ(1, listener.destruct);
+
+  absl::optional<std::string> o;
+  EXPECT_TRUE((std::is_same<std::string&, decltype(o.emplace("abc"))>::value));
+  std::string& ref = o.emplace("abc");
+  EXPECT_EQ(&ref, &o.value());
+}
+
+TEST(optionalTest, ListEmplace) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+  absl::optional<Listenable> opt;
+  EXPECT_FALSE(opt);
+  opt.emplace({1});
+  EXPECT_TRUE(opt);
+  opt.emplace({1, 2});
+  EXPECT_EQ(2, listener.listinit);
+  EXPECT_EQ(1, listener.destruct);
+
+  absl::optional<Listenable> o;
+  EXPECT_TRUE((std::is_same<Listenable&, decltype(o.emplace({1}))>::value));
+  Listenable& ref = o.emplace({1});
+  EXPECT_EQ(&ref, &o.value());
+}
+
+TEST(optionalTest, Swap) {
+  absl::optional<int> opt_empty, opt1 = 1, opt2 = 2;
+  EXPECT_FALSE(opt_empty);
+  EXPECT_TRUE(opt1);
+  EXPECT_EQ(1, opt1.value());
+  EXPECT_TRUE(opt2);
+  EXPECT_EQ(2, opt2.value());
+  swap(opt_empty, opt1);
+  EXPECT_FALSE(opt1);
+  EXPECT_TRUE(opt_empty);
+  EXPECT_EQ(1, opt_empty.value());
+  EXPECT_TRUE(opt2);
+  EXPECT_EQ(2, opt2.value());
+  swap(opt_empty, opt1);
+  EXPECT_FALSE(opt_empty);
+  EXPECT_TRUE(opt1);
+  EXPECT_EQ(1, opt1.value());
+  EXPECT_TRUE(opt2);
+  EXPECT_EQ(2, opt2.value());
+  swap(opt1, opt2);
+  EXPECT_FALSE(opt_empty);
+  EXPECT_TRUE(opt1);
+  EXPECT_EQ(2, opt1.value());
+  EXPECT_TRUE(opt2);
+  EXPECT_EQ(1, opt2.value());
+
+  EXPECT_TRUE(noexcept(opt1.swap(opt2)));
+  EXPECT_TRUE(noexcept(swap(opt1, opt2)));
+}
+
+TEST(optionalTest, PointerStuff) {
+  absl::optional<std::string> opt(absl::in_place, "foo");
+  EXPECT_EQ("foo", *opt);
+  const auto& opt_const = opt;
+  EXPECT_EQ("foo", *opt_const);
+  EXPECT_EQ(opt->size(), 3);
+  EXPECT_EQ(opt_const->size(), 3);
+
+  constexpr absl::optional<ConstexprType> opt1(1);
+  static_assert(opt1->x == ConstexprType::kCtorInt, "");
+}
+
+// gcc has a bug pre 4.9.1 where it doesn't do correct overload resolution
+// when overloads are const-qualified and *this is an raluve.
+// Skip that test to make the build green again when using the old compiler.
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59296 is fixed in 4.9.1.
+#if defined(__GNUC__) && !defined(__clang__)
+#define GCC_VERSION (__GNUC__ * 10000 \
+                     + __GNUC_MINOR__ * 100 \
+                     + __GNUC_PATCHLEVEL__)
+#if GCC_VERSION < 40901
+#define ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG
+#endif
+#endif
+
+// MSVC has a bug with "cv-qualifiers in class construction", fixed in 2017. See
+// https://docs.microsoft.com/en-us/cpp/cpp-conformance-improvements-2017#bug-fixes
+// The compiler some incorrectly ingores the cv-qualifier when generating a
+// class object via a constructor call. For example:
+//
+// class optional {
+//   constexpr T&& value() &&;
+//   constexpr const T&& value() const &&;
+// }
+//
+// using COI = const absl::optional<int>;
+// static_assert(2 == COI(2).value(), "");  // const &&
+//
+// This should invoke the "const &&" overload but since it ignores the const
+// qualifier it finds the "&&" overload the best candidate.
+#if defined(_MSC_VER) && _MSC_VER < 1910
+#define ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG
+#endif
+
+TEST(optionalTest, Value) {
+  using O = absl::optional<std::string>;
+  using CO = const absl::optional<std::string>;
+  using OC = absl::optional<const std::string>;
+  O lvalue(absl::in_place, "lvalue");
+  CO clvalue(absl::in_place, "clvalue");
+  OC lvalue_c(absl::in_place, "lvalue_c");
+  EXPECT_EQ("lvalue", lvalue.value());
+  EXPECT_EQ("clvalue", clvalue.value());
+  EXPECT_EQ("lvalue_c", lvalue_c.value());
+  EXPECT_EQ("xvalue", O(absl::in_place, "xvalue").value());
+  EXPECT_EQ("xvalue_c", OC(absl::in_place, "xvalue_c").value());
+#ifndef ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG
+  EXPECT_EQ("cxvalue", CO(absl::in_place, "cxvalue").value());
+#endif
+  EXPECT_EQ("&", TypeQuals(lvalue.value()));
+  EXPECT_EQ("c&", TypeQuals(clvalue.value()));
+  EXPECT_EQ("c&", TypeQuals(lvalue_c.value()));
+  EXPECT_EQ("&&", TypeQuals(O(absl::in_place, "xvalue").value()));
+#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
+    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
+  EXPECT_EQ("c&&", TypeQuals(CO(absl::in_place, "cxvalue").value()));
+#endif
+  EXPECT_EQ("c&&", TypeQuals(OC(absl::in_place, "xvalue_c").value()));
+
+  // test on volatile type
+  using OV = absl::optional<volatile int>;
+  OV lvalue_v(absl::in_place, 42);
+  EXPECT_EQ(42, lvalue_v.value());
+  EXPECT_EQ(42, OV(42).value());
+  EXPECT_TRUE((std::is_same<volatile int&, decltype(lvalue_v.value())>::value));
+  EXPECT_TRUE((std::is_same<volatile int&&, decltype(OV(42).value())>::value));
+
+  // test exception throw on value()
+  absl::optional<int> empty;
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(empty.value(), absl::bad_optional_access);
+#else
+  EXPECT_DEATH(empty.value(), "Bad optional access");
+#endif
+
+  // test constexpr value()
+  constexpr absl::optional<int> o1(1);
+  static_assert(1 == o1.value(), "");  // const &
+#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
+    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
+  using COI = const absl::optional<int>;
+  static_assert(2 == COI(2).value(), "");  // const &&
+#endif
+}
+
+TEST(optionalTest, DerefOperator) {
+  using O = absl::optional<std::string>;
+  using CO = const absl::optional<std::string>;
+  using OC = absl::optional<const std::string>;
+  O lvalue(absl::in_place, "lvalue");
+  CO clvalue(absl::in_place, "clvalue");
+  OC lvalue_c(absl::in_place, "lvalue_c");
+  EXPECT_EQ("lvalue", *lvalue);
+  EXPECT_EQ("clvalue", *clvalue);
+  EXPECT_EQ("lvalue_c", *lvalue_c);
+  EXPECT_EQ("xvalue", *O(absl::in_place, "xvalue"));
+  EXPECT_EQ("xvalue_c", *OC(absl::in_place, "xvalue_c"));
+#ifndef ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG
+  EXPECT_EQ("cxvalue", *CO(absl::in_place, "cxvalue"));
+#endif
+  EXPECT_EQ("&", TypeQuals(*lvalue));
+  EXPECT_EQ("c&", TypeQuals(*clvalue));
+  EXPECT_EQ("&&", TypeQuals(*O(absl::in_place, "xvalue")));
+#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
+    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
+  EXPECT_EQ("c&&", TypeQuals(*CO(absl::in_place, "cxvalue")));
+#endif
+  EXPECT_EQ("c&&", TypeQuals(*OC(absl::in_place, "xvalue_c")));
+
+  // test on volatile type
+  using OV = absl::optional<volatile int>;
+  OV lvalue_v(absl::in_place, 42);
+  EXPECT_EQ(42, *lvalue_v);
+  EXPECT_EQ(42, *OV(42));
+  EXPECT_TRUE((std::is_same<volatile int&, decltype(*lvalue_v)>::value));
+  EXPECT_TRUE((std::is_same<volatile int&&, decltype(*OV(42))>::value));
+
+  constexpr absl::optional<int> opt1(1);
+  static_assert(*opt1 == 1, "");
+#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
+    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
+  using COI = const absl::optional<int>;
+  static_assert(*COI(2) == 2, "");
+#endif
+}
+
+TEST(optionalTest, ValueOr) {
+  absl::optional<double> opt_empty, opt_set = 1.2;
+  EXPECT_EQ(42.0, opt_empty.value_or(42));
+  EXPECT_EQ(1.2, opt_set.value_or(42));
+  EXPECT_EQ(42.0, absl::optional<double>().value_or(42));
+  EXPECT_EQ(1.2, absl::optional<double>(1.2).value_or(42));
+
+  constexpr absl::optional<double> copt_empty, copt_set = {1.2};
+  static_assert(42.0 == copt_empty.value_or(42), "");
+  static_assert(1.2 == copt_set.value_or(42), "");
+#ifndef ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG
+  using COD = const absl::optional<double>;
+  static_assert(42.0 == COD().value_or(42), "");
+  static_assert(1.2 == COD(1.2).value_or(42), "");
+#endif
+}
+
+// make_optional cannot be constexpr until C++17
+TEST(optionalTest, make_optional) {
+  auto opt_int = absl::make_optional(42);
+  EXPECT_TRUE((std::is_same<decltype(opt_int), absl::optional<int>>::value));
+  EXPECT_EQ(42, opt_int);
+
+  StructorListener listener;
+  Listenable::listener = &listener;
+
+  absl::optional<Listenable> opt0 = absl::make_optional<Listenable>();
+  EXPECT_EQ(1, listener.construct0);
+  absl::optional<Listenable> opt1 = absl::make_optional<Listenable>(1);
+  EXPECT_EQ(1, listener.construct1);
+  absl::optional<Listenable> opt2 = absl::make_optional<Listenable>(1, 2);
+  EXPECT_EQ(1, listener.construct2);
+  absl::optional<Listenable> opt3 = absl::make_optional<Listenable>({1});
+  absl::optional<Listenable> opt4 = absl::make_optional<Listenable>({1, 2});
+  EXPECT_EQ(2, listener.listinit);
+
+  // Constexpr tests on trivially copyable types
+  // optional<T> has trivial copy/move ctors when T is trivially copyable.
+  // For nontrivial types with constexpr constructors, we need copy elision in
+  // C++17 for make_optional to be constexpr.
+  {
+    constexpr absl::optional<int> c_opt = absl::make_optional(42);
+    static_assert(c_opt.value() == 42, "");
+  }
+  {
+    struct TrivialCopyable {
+      constexpr TrivialCopyable() : x(0) {}
+      constexpr explicit TrivialCopyable(int i) : x(i) {}
+      int x;
+    };
+
+    constexpr TrivialCopyable v;
+    constexpr absl::optional<TrivialCopyable> c_opt0 = absl::make_optional(v);
+    static_assert(c_opt0->x == 0, "");
+    constexpr absl::optional<TrivialCopyable> c_opt1 =
+        absl::make_optional<TrivialCopyable>();
+    static_assert(c_opt1->x == 0, "");
+    constexpr absl::optional<TrivialCopyable> c_opt2 =
+        absl::make_optional<TrivialCopyable>(42);
+    static_assert(c_opt2->x == 42, "");
+  }
+}
+
+template <typename T, typename U>
+void optionalTest_Comparisons_EXPECT_LESS(T x, U y) {
+  EXPECT_FALSE(x == y);
+  EXPECT_TRUE(x != y);
+  EXPECT_TRUE(x < y);
+  EXPECT_FALSE(x > y);
+  EXPECT_TRUE(x <= y);
+  EXPECT_FALSE(x >= y);
+}
+
+template <typename T, typename U>
+void optionalTest_Comparisons_EXPECT_SAME(T x, U y) {
+  EXPECT_TRUE(x == y);
+  EXPECT_FALSE(x != y);
+  EXPECT_FALSE(x < y);
+  EXPECT_FALSE(x > y);
+  EXPECT_TRUE(x <= y);
+  EXPECT_TRUE(x >= y);
+}
+
+template <typename T, typename U>
+void optionalTest_Comparisons_EXPECT_GREATER(T x, U y) {
+  EXPECT_FALSE(x == y);
+  EXPECT_TRUE(x != y);
+  EXPECT_FALSE(x < y);
+  EXPECT_TRUE(x > y);
+  EXPECT_FALSE(x <= y);
+  EXPECT_TRUE(x >= y);
+}
+
+
+template <typename T, typename U, typename V>
+void TestComparisons() {
+  absl::optional<T> ae, a2{2}, a4{4};
+  absl::optional<U> be, b2{2}, b4{4};
+  V v3 = 3;
+
+  // LHS: absl::nullopt, ae, a2, v3, a4
+  // RHS: absl::nullopt, be, b2, v3, b4
+
+  // optionalTest_Comparisons_EXPECT_NOT_TO_WORK(absl::nullopt,absl::nullopt);
+  optionalTest_Comparisons_EXPECT_SAME(absl::nullopt, be);
+  optionalTest_Comparisons_EXPECT_LESS(absl::nullopt, b2);
+  // optionalTest_Comparisons_EXPECT_NOT_TO_WORK(absl::nullopt,v3);
+  optionalTest_Comparisons_EXPECT_LESS(absl::nullopt, b4);
+
+  optionalTest_Comparisons_EXPECT_SAME(ae, absl::nullopt);
+  optionalTest_Comparisons_EXPECT_SAME(ae, be);
+  optionalTest_Comparisons_EXPECT_LESS(ae, b2);
+  optionalTest_Comparisons_EXPECT_LESS(ae, v3);
+  optionalTest_Comparisons_EXPECT_LESS(ae, b4);
+
+  optionalTest_Comparisons_EXPECT_GREATER(a2, absl::nullopt);
+  optionalTest_Comparisons_EXPECT_GREATER(a2, be);
+  optionalTest_Comparisons_EXPECT_SAME(a2, b2);
+  optionalTest_Comparisons_EXPECT_LESS(a2, v3);
+  optionalTest_Comparisons_EXPECT_LESS(a2, b4);
+
+  // optionalTest_Comparisons_EXPECT_NOT_TO_WORK(v3,absl::nullopt);
+  optionalTest_Comparisons_EXPECT_GREATER(v3, be);
+  optionalTest_Comparisons_EXPECT_GREATER(v3, b2);
+  optionalTest_Comparisons_EXPECT_SAME(v3, v3);
+  optionalTest_Comparisons_EXPECT_LESS(v3, b4);
+
+  optionalTest_Comparisons_EXPECT_GREATER(a4, absl::nullopt);
+  optionalTest_Comparisons_EXPECT_GREATER(a4, be);
+  optionalTest_Comparisons_EXPECT_GREATER(a4, b2);
+  optionalTest_Comparisons_EXPECT_GREATER(a4, v3);
+  optionalTest_Comparisons_EXPECT_SAME(a4, b4);
+}
+
+struct Int1 {
+  Int1() = default;
+  Int1(int i) : i(i) {}  // NOLINT(runtime/explicit)
+  int i;
+};
+
+struct Int2 {
+  Int2() = default;
+  Int2(int i) : i(i) {}  // NOLINT(runtime/explicit)
+  int i;
+};
+
+// comparison between Int1 and Int2
+constexpr bool operator==(const Int1& lhs, const Int2& rhs) {
+  return lhs.i == rhs.i;
+}
+constexpr bool operator!=(const Int1& lhs, const Int2& rhs) {
+  return !(lhs == rhs);
+}
+constexpr bool operator<(const Int1& lhs, const Int2& rhs) {
+  return lhs.i < rhs.i;
+}
+constexpr bool operator<=(const Int1& lhs, const Int2& rhs) {
+  return lhs < rhs || lhs == rhs;
+}
+constexpr bool operator>(const Int1& lhs, const Int2& rhs) {
+  return !(lhs <= rhs);
+}
+constexpr bool operator>=(const Int1& lhs, const Int2& rhs) {
+  return !(lhs < rhs);
+}
+
+TEST(optionalTest, Comparisons) {
+  TestComparisons<int, int, int>();
+  TestComparisons<const int, int, int>();
+  TestComparisons<Int1, int, int>();
+  TestComparisons<int, Int2, int>();
+  TestComparisons<Int1, Int2, int>();
+
+  // compare absl::optional<std::string> with const char*
+  absl::optional<std::string> opt_str = "abc";
+  const char* cstr = "abc";
+  EXPECT_TRUE(opt_str == cstr);
+  // compare absl::optional<std::string> with absl::optional<const char*>
+  absl::optional<const char*> opt_cstr = cstr;
+  EXPECT_TRUE(opt_str == opt_cstr);
+  // compare absl::optional<std::string> with absl::optional<absl::string_view>
+  absl::optional<absl::string_view> e1;
+  absl::optional<std::string> e2;
+  EXPECT_TRUE(e1 == e2);
+}
+
+
+TEST(optionalTest, SwapRegression) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+
+  {
+    absl::optional<Listenable> a;
+    absl::optional<Listenable> b(absl::in_place);
+    a.swap(b);
+  }
+
+  EXPECT_EQ(1, listener.construct0);
+  EXPECT_EQ(1, listener.move);
+  EXPECT_EQ(2, listener.destruct);
+
+  {
+    absl::optional<Listenable> a(absl::in_place);
+    absl::optional<Listenable> b;
+    a.swap(b);
+  }
+
+  EXPECT_EQ(2, listener.construct0);
+  EXPECT_EQ(2, listener.move);
+  EXPECT_EQ(4, listener.destruct);
+}
+
+TEST(optionalTest, BigStringLeakCheck) {
+  constexpr size_t n = 1 << 16;
+
+  using OS = absl::optional<std::string>;
+
+  OS a;
+  OS b = absl::nullopt;
+  OS c = std::string(n, 'c');
+  std::string sd(n, 'd');
+  OS d = sd;
+  OS e(absl::in_place, n, 'e');
+  OS f;
+  f.emplace(n, 'f');
+
+  OS ca(a);
+  OS cb(b);
+  OS cc(c);
+  OS cd(d);
+  OS ce(e);
+
+  OS oa;
+  OS ob = absl::nullopt;
+  OS oc = std::string(n, 'c');
+  std::string sod(n, 'd');
+  OS od = sod;
+  OS oe(absl::in_place, n, 'e');
+  OS of;
+  of.emplace(n, 'f');
+
+  OS ma(std::move(oa));
+  OS mb(std::move(ob));
+  OS mc(std::move(oc));
+  OS md(std::move(od));
+  OS me(std::move(oe));
+  OS mf(std::move(of));
+
+  OS aa1;
+  OS ab1 = absl::nullopt;
+  OS ac1 = std::string(n, 'c');
+  std::string sad1(n, 'd');
+  OS ad1 = sad1;
+  OS ae1(absl::in_place, n, 'e');
+  OS af1;
+  af1.emplace(n, 'f');
+
+  OS aa2;
+  OS ab2 = absl::nullopt;
+  OS ac2 = std::string(n, 'c');
+  std::string sad2(n, 'd');
+  OS ad2 = sad2;
+  OS ae2(absl::in_place, n, 'e');
+  OS af2;
+  af2.emplace(n, 'f');
+
+  aa1 = af2;
+  ab1 = ae2;
+  ac1 = ad2;
+  ad1 = ac2;
+  ae1 = ab2;
+  af1 = aa2;
+
+  OS aa3;
+  OS ab3 = absl::nullopt;
+  OS ac3 = std::string(n, 'c');
+  std::string sad3(n, 'd');
+  OS ad3 = sad3;
+  OS ae3(absl::in_place, n, 'e');
+  OS af3;
+  af3.emplace(n, 'f');
+
+  aa3 = absl::nullopt;
+  ab3 = absl::nullopt;
+  ac3 = absl::nullopt;
+  ad3 = absl::nullopt;
+  ae3 = absl::nullopt;
+  af3 = absl::nullopt;
+
+  OS aa4;
+  OS ab4 = absl::nullopt;
+  OS ac4 = std::string(n, 'c');
+  std::string sad4(n, 'd');
+  OS ad4 = sad4;
+  OS ae4(absl::in_place, n, 'e');
+  OS af4;
+  af4.emplace(n, 'f');
+
+  aa4 = OS(absl::in_place, n, 'a');
+  ab4 = OS(absl::in_place, n, 'b');
+  ac4 = OS(absl::in_place, n, 'c');
+  ad4 = OS(absl::in_place, n, 'd');
+  ae4 = OS(absl::in_place, n, 'e');
+  af4 = OS(absl::in_place, n, 'f');
+
+  OS aa5;
+  OS ab5 = absl::nullopt;
+  OS ac5 = std::string(n, 'c');
+  std::string sad5(n, 'd');
+  OS ad5 = sad5;
+  OS ae5(absl::in_place, n, 'e');
+  OS af5;
+  af5.emplace(n, 'f');
+
+  std::string saa5(n, 'a');
+  std::string sab5(n, 'a');
+  std::string sac5(n, 'a');
+  std::string sad52(n, 'a');
+  std::string sae5(n, 'a');
+  std::string saf5(n, 'a');
+
+  aa5 = saa5;
+  ab5 = sab5;
+  ac5 = sac5;
+  ad5 = sad52;
+  ae5 = sae5;
+  af5 = saf5;
+
+  OS aa6;
+  OS ab6 = absl::nullopt;
+  OS ac6 = std::string(n, 'c');
+  std::string sad6(n, 'd');
+  OS ad6 = sad6;
+  OS ae6(absl::in_place, n, 'e');
+  OS af6;
+  af6.emplace(n, 'f');
+
+  aa6 = std::string(n, 'a');
+  ab6 = std::string(n, 'b');
+  ac6 = std::string(n, 'c');
+  ad6 = std::string(n, 'd');
+  ae6 = std::string(n, 'e');
+  af6 = std::string(n, 'f');
+
+  OS aa7;
+  OS ab7 = absl::nullopt;
+  OS ac7 = std::string(n, 'c');
+  std::string sad7(n, 'd');
+  OS ad7 = sad7;
+  OS ae7(absl::in_place, n, 'e');
+  OS af7;
+  af7.emplace(n, 'f');
+
+  aa7.emplace(n, 'A');
+  ab7.emplace(n, 'B');
+  ac7.emplace(n, 'C');
+  ad7.emplace(n, 'D');
+  ae7.emplace(n, 'E');
+  af7.emplace(n, 'F');
+}
+
+TEST(optionalTest, MoveAssignRegression) {
+  StructorListener listener;
+  Listenable::listener = &listener;
+
+  {
+    absl::optional<Listenable> a;
+    Listenable b;
+    a = std::move(b);
+  }
+
+  EXPECT_EQ(1, listener.construct0);
+  EXPECT_EQ(1, listener.move);
+  EXPECT_EQ(2, listener.destruct);
+}
+
+TEST(optionalTest, ValueType) {
+  EXPECT_TRUE((std::is_same<absl::optional<int>::value_type, int>::value));
+  EXPECT_TRUE(
+      (std::is_same<absl::optional<std::string>::value_type, std::string>::value));
+  EXPECT_FALSE(
+      (std::is_same<absl::optional<int>::value_type, absl::nullopt_t>::value));
+}
+
+TEST(optionalTest, Hash) {
+  std::hash<absl::optional<int>> hash;
+  std::set<size_t> hashcodes;
+  hashcodes.insert(hash(absl::nullopt));
+  for (int i = 0; i < 100; ++i) {
+    hashcodes.insert(hash(i));
+  }
+  EXPECT_GT(hashcodes.size(), 90);
+}
+
+struct MoveMeNoThrow {
+  MoveMeNoThrow() : x(0) {}
+  [[noreturn]] MoveMeNoThrow(const MoveMeNoThrow& other) : x(other.x) {
+    ABSL_RAW_LOG(FATAL, "Should not be called.");
+    abort();
+  }
+  MoveMeNoThrow(MoveMeNoThrow&& other) noexcept : x(other.x) {}
+  int x;
+};
+
+struct MoveMeThrow {
+  MoveMeThrow() : x(0) {}
+  MoveMeThrow(const MoveMeThrow& other) : x(other.x) {}
+  MoveMeThrow(MoveMeThrow&& other) : x(other.x) {}
+  int x;
+};
+
+TEST(optionalTest, NoExcept) {
+  static_assert(
+      std::is_nothrow_move_constructible<absl::optional<MoveMeNoThrow>>::value,
+      "");
+#ifndef ABSL_HAVE_STD_OPTIONAL
+  static_assert(absl::default_allocator_is_nothrow::value ==
+                    std::is_nothrow_move_constructible<
+                        absl::optional<MoveMeThrow>>::value,
+                "");
+#endif
+  std::vector<absl::optional<MoveMeNoThrow>> v;
+  for (int i = 0; i < 10; ++i) v.emplace_back();
+}
+
+struct AnyLike {
+  AnyLike(AnyLike&&) = default;
+  AnyLike(const AnyLike&) = default;
+
+  template <typename ValueType,
+            typename T = typename std::decay<ValueType>::type,
+            typename std::enable_if<
+                !absl::disjunction<
+                    std::is_same<AnyLike, T>,
+                    absl::negation<std::is_copy_constructible<T>>>::value,
+                int>::type = 0>
+  AnyLike(ValueType&&) {}  // NOLINT(runtime/explicit)
+
+  AnyLike& operator=(AnyLike&&) = default;
+  AnyLike& operator=(const AnyLike&) = default;
+
+  template <typename ValueType,
+            typename T = typename std::decay<ValueType>::type>
+  typename std::enable_if<
+      absl::conjunction<absl::negation<std::is_same<AnyLike, T>>,
+                        std::is_copy_constructible<T>>::value,
+      AnyLike&>::type
+  operator=(ValueType&& /* rhs */) {
+    return *this;
+  }
+};
+
+TEST(optionalTest, ConstructionConstraints) {
+  EXPECT_TRUE((std::is_constructible<AnyLike, absl::optional<AnyLike>>::value));
+
+  EXPECT_TRUE(
+      (std::is_constructible<AnyLike, const absl::optional<AnyLike>&>::value));
+
+  EXPECT_TRUE((std::is_constructible<absl::optional<AnyLike>, AnyLike>::value));
+  EXPECT_TRUE(
+      (std::is_constructible<absl::optional<AnyLike>, const AnyLike&>::value));
+
+  EXPECT_TRUE((std::is_convertible<absl::optional<AnyLike>, AnyLike>::value));
+
+  EXPECT_TRUE(
+      (std::is_convertible<const absl::optional<AnyLike>&, AnyLike>::value));
+
+  EXPECT_TRUE((std::is_convertible<AnyLike, absl::optional<AnyLike>>::value));
+  EXPECT_TRUE(
+      (std::is_convertible<const AnyLike&, absl::optional<AnyLike>>::value));
+
+  EXPECT_TRUE(std::is_move_constructible<absl::optional<AnyLike>>::value);
+  EXPECT_TRUE(std::is_copy_constructible<absl::optional<AnyLike>>::value);
+}
+
+TEST(optionalTest, AssignmentConstraints) {
+  EXPECT_TRUE((std::is_assignable<AnyLike&, absl::optional<AnyLike>>::value));
+  EXPECT_TRUE(
+      (std::is_assignable<AnyLike&, const absl::optional<AnyLike>&>::value));
+  EXPECT_TRUE((std::is_assignable<absl::optional<AnyLike>&, AnyLike>::value));
+  EXPECT_TRUE(
+      (std::is_assignable<absl::optional<AnyLike>&, const AnyLike&>::value));
+  EXPECT_TRUE(std::is_move_assignable<absl::optional<AnyLike>>::value);
+  EXPECT_TRUE(std::is_copy_assignable<absl::optional<AnyLike>>::value);
+}
+
+}  // namespace
diff --git a/absl/types/span.h b/absl/types/span.h
new file mode 100644
index 000000000000..0e26fd4ded17
--- /dev/null
+++ b/absl/types/span.h
@@ -0,0 +1,738 @@
+//
+// Copyright 2017 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.
+//
+// -----------------------------------------------------------------------------
+// span.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines a `Span<T>` type for holding a view of an existing
+// array of data. The `Span` object, much like the `absl::string_view` object,
+// does not own such data itself. A span provides a lightweight way to pass
+// around view of such data.
+//
+// Additionally, this header file defines `MakeSpan()` and `MakeConstSpan()`
+// factory functions, for clearly creating spans of type `Span<T>` or read-only
+// `Span<const T>` when such types may be difficult to identify due to issues
+// with implicit conversion.
+//
+// The C++ standards committee currently has a proposal for a `std::span` type,
+// (http://wg21.link/p0122), which is not yet part of the standard (though may
+// become part of C++20). As of August 2017, the differences between
+// `absl::Span` and this proposal are:
+//    * `absl::Span` uses `size_t` for `size_type`
+//    * `absl::Span` has no `operator()`
+//    * `absl::Span` has no constructors for `std::unique_ptr` or
+//      `std::shared_ptr`
+//    * `absl::span` has the factory functions `MakeSpan()` and
+//      `MakeConstSpan()`
+//    * `absl::Span` has `front()` and `back()` methods
+//    * bounds-checked access to `absl::Span` is accomplished with `at()`
+//    * `absl::Span` has compiler-provided move and copy constructors and
+//      assignment. This is due to them being specified as `constexpr`, but that
+//      implies const in C++11.
+//    * `absl::Span` has no `element_type` or `index_type` typedefs
+//    * A read-only `absl::Span<const T>` can be implicitly constructed from an
+//      initializer list.
+//    * `absl::Span` has no `bytes()`, `size_bytes()`, `as_bytes()`, or
+//      `as_mutable_bytes()` methods
+//    * `absl::Span` has no static extent template parameter, nor constructors
+//      which exist only because of the static extent parameter.
+//    * `absl::Span` has an explicit mutable-reference constructor
+//
+// For more information, see the class comments below.
+#ifndef ABSL_TYPES_SPAN_H_
+#define ABSL_TYPES_SPAN_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <initializer_list>
+#include <iterator>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/algorithm/algorithm.h"
+#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+#include "absl/base/port.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+
+template <typename T>
+class Span;
+
+namespace span_internal {
+// A constexpr min function
+constexpr size_t Min(size_t a, size_t b) noexcept { return a < b ? a : b; }
+
+// Wrappers for access to container data pointers.
+template <typename C>
+constexpr auto GetDataImpl(C& c, char) noexcept  // NOLINT(runtime/references)
+    -> decltype(c.data()) {
+  return c.data();
+}
+
+// Before C++17, std::string::data returns a const char* in all cases.
+inline char* GetDataImpl(std::string& s,  // NOLINT(runtime/references)
+                         int) noexcept {
+  return &s[0];
+}
+
+template <typename C>
+constexpr auto GetData(C& c) noexcept  // NOLINT(runtime/references)
+    -> decltype(GetDataImpl(c, 0)) {
+  return GetDataImpl(c, 0);
+}
+
+// Detection idioms for size() and data().
+template <typename C>
+using HasSize =
+    std::is_integral<absl::decay_t<decltype(std::declval<C&>().size())>>;
+
+// We want to enable conversion from vector<T*> to Span<const T* const> but
+// disable conversion from vector<Derived> to Span<Base>. Here we use
+// the fact that U** is convertible to Q* const* if and only if Q is the same
+// type or a more cv-qualified version of U.  We also decay the result type of
+// data() to avoid problems with classes which have a member function data()
+// which returns a reference.
+template <typename T, typename C>
+using HasData =
+    std::is_convertible<absl::decay_t<decltype(GetData(std::declval<C&>()))>*,
+                        T* const*>;
+
+// Extracts value type from a Container
+template <typename C>
+struct ElementType {
+  using type = typename absl::remove_reference_t<C>::value_type;
+};
+
+template <typename T, size_t N>
+struct ElementType<T (&)[N]> {
+  using type = T;
+};
+
+template <typename C>
+using ElementT = typename ElementType<C>::type;
+
+template <typename T>
+using EnableIfMutable =
+    typename std::enable_if<!std::is_const<T>::value, int>::type;
+
+template <typename T>
+bool EqualImpl(Span<T> a, Span<T> b) {
+  static_assert(std::is_const<T>::value, "");
+  return absl::equal(a.begin(), a.end(), b.begin(), b.end());
+}
+
+template <typename T>
+bool LessThanImpl(Span<T> a, Span<T> b) {
+  static_assert(std::is_const<T>::value, "");
+  return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end());
+}
+
+// The `IsConvertible` classes here are needed because of the
+// `std::is_convertible` bug in libcxx when compiled with GCC. This build
+// configuration is used by Android NDK toolchain. Reference link:
+// https://bugs.llvm.org/show_bug.cgi?id=27538.
+template <typename From, typename To>
+struct IsConvertibleHelper {
+ private:
+  static std::true_type test(To);
+  static std::false_type test(...);
+
+ public:
+  using type = decltype(test(std::declval<From>()));
+};
+
+template <typename From, typename To>
+struct IsConvertible : IsConvertibleHelper<From, To>::type {};
+
+// TODO(zhangxy): replace `IsConvertible` with `std::is_convertible` once the
+// older version of libcxx is not supported.
+template <typename From, typename To>
+using EnableIfConvertibleToSpanConst =
+    typename std::enable_if<IsConvertible<From, Span<const To>>::value>::type;
+}  // namespace span_internal
+
+//------------------------------------------------------------------------------
+// Span
+//------------------------------------------------------------------------------
+//
+// A `Span` is an "array view" type for holding a view of a contiguous data
+// array; the `Span` object does not and cannot own such data itself. A span
+// provides an easy way to provide overloads for anything operating on
+// contiguous sequences without needing to manage pointers and array lengths
+// manually.
+
+// A span is conceptually a pointer (ptr) and a length (size) into an already
+// existing array of contiguous memory; the array it represents references the
+// elements "ptr[0] .. ptr[size-1]". Passing a properly-constructed `Span`
+// instead of raw pointers avoids many issues related to index out of bounds
+// errors.
+//
+// Spans may also be constructed from containers holding contiguous sequences.
+// Such containers must supply `data()` and `size() const` methods (e.g
+// `std::vector<T>`, `absl::InlinedVector<T, N>`). All implicit conversions to
+// `absl::Span` from such containers will create spans of type `const T`;
+// spans which can mutate their values (of type `T`) must use explicit
+// constructors.
+//
+// A `Span<T>` is somewhat analogous to an `absl::string_view`, but for an array
+// of elements of type `T`. A user of `Span` must ensure that the data being
+// pointed to outlives the `Span` itself.
+//
+// You can construct a `Span<T>` in several ways:
+//
+//   * Explicitly from a reference to a container type
+//   * Explicitly from a pointer and size
+//   * Implicitly from a container type (but only for spans of type `const T`)
+//   * Using the `MakeSpan()` or `MakeConstSpan()` factory functions.
+//
+// Examples:
+//
+//   // Construct a Span explicitly from a container:
+//   std::vector<int> v = {1, 2, 3, 4, 5};
+//   auto span = absl::Span<const int>(v);
+//
+//   // Construct a Span explicitly from a C-style array:
+//   int a[5] =  {1, 2, 3, 4, 5};
+//   auto span = absl::Span<const int>(a);
+//
+//   // Construct a Span implicitly from a container
+//   void MyRoutine(absl::Span<const int> a) {
+//     ...
+//   };
+//   std::vector v = {1,2,3,4,5};
+//   MyRoutine(v)                     // convert to Span<const T>
+//
+// Note that `Span` objects, in addition to requiring that the memory they
+// point to remains alive, must also ensure that such memory does not get
+// reallocated. Therefore, to avoid undefined behavior, containers with
+// associated span views should not invoke operations that may reallocate memory
+// (such as resizing) or invalidate iterarors into the container.
+//
+// One common use for a `Span` is when passing arguments to a routine that can
+// accept a variety of array types (e.g. a `std::vector`, `absl::InlinedVector`,
+// a C-style array, etc.). Instead of creating overloads for each case, you
+// can simply specify a `Span` as the argument to such a routine.
+//
+// Example:
+//
+//   void MyRoutine(absl::Span<const int> a) {
+//     ...
+//   };
+//
+//   std::vector v = {1,2,3,4,5};
+//   MyRoutine(v);
+//
+//   absl::InlinedVector<int, 4> my_inline_vector;
+//   MyRoutine(my_inline_vector);
+//
+//   // Explicit constructor from pointer,size
+//   int* my_array = new int[10];
+//   MyRoutine(absl::Span<const int>(my_array, 10));
+template <typename T>
+class Span {
+ private:
+  // Used to determine whether a Span can be constructed from a container of
+  // type C.
+  template <typename C>
+  using EnableIfConvertibleFrom =
+      typename std::enable_if<span_internal::HasData<T, C>::value &&
+                              span_internal::HasSize<C>::value>::type;
+
+  // Used to SFINAE-enable a function when the slice elements are const.
+  template <typename U>
+  using EnableIfConstView =
+      typename std::enable_if<std::is_const<T>::value, U>::type;
+
+  // Used to SFINAE-enable a function when the slice elements are mutable.
+  template <typename U>
+  using EnableIfMutableView =
+      typename std::enable_if<!std::is_const<T>::value, U>::type;
+
+ public:
+  using value_type = absl::remove_cv_t<T>;
+  using pointer = T*;
+  using const_pointer = const T*;
+  using reference = T&;
+  using const_reference = const T&;
+  using iterator = pointer;
+  using const_iterator = const_pointer;
+  using reverse_iterator = std::reverse_iterator<iterator>;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+  using size_type = size_t;
+  using difference_type = ptrdiff_t;
+
+  static const size_type npos = -1;
+
+  constexpr Span() noexcept : Span(nullptr, 0) {}
+  constexpr Span(pointer array, size_type length) noexcept
+      : ptr_(array), len_(length) {}
+
+  // Implicit conversion constructors
+  template <size_t N>
+  constexpr Span(T (&a)[N]) noexcept  // NOLINT(runtime/explicit)
+      : Span(a, N) {}
+
+  // Explicit reference constructor for a mutable `Span<T>` type
+  template <typename V, typename = EnableIfConvertibleFrom<V>,
+            typename = EnableIfMutableView<V>>
+  explicit Span(V& v) noexcept  // NOLINT(runtime/references)
+      : Span(span_internal::GetData(v), v.size()) {}
+
+  // Implicit reference constructor for a read-only `Span<const T>` type
+  template <typename V, typename = EnableIfConvertibleFrom<V>,
+            typename = EnableIfConstView<V>>
+  constexpr Span(const V& v) noexcept  // NOLINT(runtime/explicit)
+      : Span(span_internal::GetData(v), v.size()) {}
+
+  // Implicit constructor from an initializer list, making it possible to pass a
+  // brace-enclosed initializer list to a function expecting a `Span`. Such
+  // spans constructed from an initializer list must be of type `Span<const T>`.
+  //
+  //   void Process(absl::Span<const int> x);
+  //   Process({1, 2, 3});
+  //
+  // Note that as always the array referenced by the span must outlive the span.
+  // Since an initializer list constructor acts as if it is fed a temporary
+  // array (cf. C++ standard [dcl.init.list]/5), it's safe to use this
+  // constructor only when the `std::initializer_list` itself outlives the span.
+  // In order to meet this requirement it's sufficient to ensure that neither
+  // the span nor a copy of it is used outside of the expression in which it's
+  // created:
+  //
+  //   // Assume that this function uses the array directly, not retaining any
+  //   // copy of the span or pointer to any of its elements.
+  //   void Process(absl::Span<const int> ints);
+  //
+  //   // Okay: the std::initializer_list<int> will reference a temporary array
+  //   // that isn't destroyed until after the call to Process returns.
+  //   Process({ 17, 19 });
+  //
+  //   // Not okay: the storage used by the std::initializer_list<int> is not
+  //   // allowed to be referenced after the first line.
+  //   absl::Span<const int> ints = { 17, 19 };
+  //   Process(ints);
+  //
+  //   // Not okay for the same reason as above: even when the elements of the
+  //   // initializer list expression are not temporaries the underlying array
+  //   // is, so the initializer list must still outlive the span.
+  //   const int foo = 17;
+  //   absl::Span<const int> ints = { foo };
+  //   Process(ints);
+  //
+  template <typename LazyT = T,
+            typename = EnableIfConstView<LazyT>>
+  Span(
+      std::initializer_list<value_type> v) noexcept  // NOLINT(runtime/explicit)
+      : Span(v.begin(), v.size()) {}
+
+  // Accessors
+
+  // Span::data()
+  //
+  // Returns a pointer to the span's underlying array of data (which is held
+  // outside the span).
+  constexpr pointer data() const noexcept { return ptr_; }
+
+  // Span::size()
+  //
+  // Returns the size of this span.
+  constexpr size_type size() const noexcept { return len_; }
+
+  // Span::length()
+  //
+  // Returns the length (size) of this span.
+  constexpr size_type length() const noexcept { return size(); }
+
+  // Span::empty()
+  //
+  // Returns a boolean indicating whether or not this span is considered empty.
+  constexpr bool empty() const noexcept { return size() == 0; }
+
+  // Span::operator[]
+  //
+  // Returns a reference to the i'th element of this span.
+  constexpr reference operator[](size_type i) const noexcept {
+    // MSVC 2015 accepts this as constexpr, but not ptr_[i]
+    return *(data() + i);
+  }
+
+  // Span::at()
+  //
+  // Returns a reference to the i'th element of this span.
+  constexpr reference at(size_type i) const {
+    return ABSL_PREDICT_FALSE(i < size())
+               ? ptr_[i]
+               : (base_internal::ThrowStdOutOfRange(
+                      "Span::at failed bounds check"),
+                  ptr_[i]);
+  }
+
+  // Span::front()
+  //
+  // Returns a reference to the first element of this span.
+  reference front() const noexcept { return ABSL_ASSERT(size() > 0), ptr_[0]; }
+
+  // Span::back()
+  //
+  // Returns a reference to the last element of this span.
+  reference back() const noexcept {
+    return ABSL_ASSERT(size() > 0), ptr_[size() - 1];
+  }
+
+  // Span::begin()
+  //
+  // Returns an iterator to the first element of this span.
+  constexpr iterator begin() const noexcept { return ptr_; }
+
+  // Span::cbegin()
+  //
+  // Returns a const iterator to the first element of this span.
+  constexpr const_iterator cbegin() const noexcept { return ptr_; }
+
+  // Span::end()
+  //
+  // Returns an iterator to the last element of this span.
+  iterator end() const noexcept { return ptr_ + len_; }
+
+  // Span::cend()
+  //
+  // Returns a const iterator to the last element of this span.
+  const_iterator cend() const noexcept { return end(); }
+
+  // Span::rbegin()
+  //
+  // Returns a reverse iterator starting at the last element of this span.
+  reverse_iterator rbegin() const noexcept { return reverse_iterator(end()); }
+
+  // Span::crbegin()
+  //
+  // Returns a reverse const iterator starting at the last element of this span.
+  const_reverse_iterator crbegin() const noexcept { return rbegin(); }
+
+  // Span::rend()
+  //
+  // Returns a reverse iterator starting at the first element of this span.
+  reverse_iterator rend() const noexcept { return reverse_iterator(begin()); }
+
+  // Span::crend()
+  //
+  // Returns a reverse iterator starting at the first element of this span.
+  const_reverse_iterator crend() const noexcept { return rend(); }
+
+  // Span mutations
+
+  // Span::remove_prefix()
+  //
+  // Removes the first `n` elements from the span.
+  void remove_prefix(size_type n) noexcept {
+    assert(len_ >= n);
+    ptr_ += n;
+    len_ -= n;
+  }
+
+  // Span::remove_suffix()
+  //
+  // Removes the last `n` elements from the span.
+  void remove_suffix(size_type n) noexcept {
+    assert(len_ >= n);
+    len_ -= n;
+  }
+
+  // Span::subspan()
+  //
+  // Returns a `Span` starting at element `pos` and of length `len`, with
+  // proper bounds checking to ensure `len` does not exceed the ptr+size of the
+  // original array. (Spans whose `len` would point past the end of the array
+  // will throw a `std::out_of_range`.)
+  constexpr Span subspan(size_type pos = 0, size_type len = npos) const {
+    return (pos <= len_)
+               ? Span(ptr_ + pos, span_internal::Min(len_ - pos, len))
+               : (base_internal::ThrowStdOutOfRange("pos > size()"), Span());
+  }
+
+ private:
+  pointer ptr_;
+  size_type len_;
+};
+
+template <typename T>
+const typename Span<T>::size_type Span<T>::npos;
+
+// Span relationals
+
+// Equality is compared element-by-element, while ordering is lexicographical.
+// We provide three overloads for each operator to cover any combination on the
+// left or right hand side of mutable Span<T>, read-only Span<const T>, and
+// convertible-to-read-only Span<T>.
+// TODO(zhangxy): Due to MSVC overload resolution bug with partial ordering
+// template functions, 5 overloads per operator is needed as a workaround. We
+// should update them to 3 overloads per operator using non-deduced context like
+// string_view, i.e.
+// - (Span<T>, Span<T>)
+// - (Span<T>, non_deduced<Span<const T>>)
+// - (non_deduced<Span<const T>>, Span<T>)
+
+// operator==
+template <typename T>
+bool operator==(Span<T> a, Span<T> b) {
+  return span_internal::EqualImpl<const T>(a, b);
+}
+template <typename T>
+bool operator==(Span<const T> a, Span<T> b) {
+  return span_internal::EqualImpl<const T>(a, b);
+}
+template <typename T>
+bool operator==(Span<T> a, Span<const T> b) {
+  return span_internal::EqualImpl<const T>(a, b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator==(const U& a, Span<T> b) {
+  return span_internal::EqualImpl<const T>(a, b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator==(Span<T> a, const U& b) {
+  return span_internal::EqualImpl<const T>(a, b);
+}
+
+// operator!=
+template <typename T>
+bool operator!=(Span<T> a, Span<T> b) {
+  return !(a == b);
+}
+template <typename T>
+bool operator!=(Span<const T> a, Span<T> b) {
+  return !(a == b);
+}
+template <typename T>
+bool operator!=(Span<T> a, Span<const T> b) {
+  return !(a == b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator!=(const U& a, Span<T> b) {
+  return !(a == b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator!=(Span<T> a, const U& b) {
+  return !(a == b);
+}
+
+// operator<
+template <typename T>
+bool operator<(Span<T> a, Span<T> b) {
+  return span_internal::LessThanImpl<const T>(a, b);
+}
+template <typename T>
+bool operator<(Span<const T> a, Span<T> b) {
+  return span_internal::LessThanImpl<const T>(a, b);
+}
+template <typename T>
+bool operator<(Span<T> a, Span<const T> b) {
+  return span_internal::LessThanImpl<const T>(a, b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator<(const U& a, Span<T> b) {
+  return span_internal::LessThanImpl<const T>(a, b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator<(Span<T> a, const U& b) {
+  return span_internal::LessThanImpl<const T>(a, b);
+}
+
+// operator>
+template <typename T>
+bool operator>(Span<T> a, Span<T> b) {
+  return b < a;
+}
+template <typename T>
+bool operator>(Span<const T> a, Span<T> b) {
+  return b < a;
+}
+template <typename T>
+bool operator>(Span<T> a, Span<const T> b) {
+  return b < a;
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator>(const U& a, Span<T> b) {
+  return b < a;
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator>(Span<T> a, const U& b) {
+  return b < a;
+}
+
+// operator<=
+template <typename T>
+bool operator<=(Span<T> a, Span<T> b) {
+  return !(b < a);
+}
+template <typename T>
+bool operator<=(Span<const T> a, Span<T> b) {
+  return !(b < a);
+}
+template <typename T>
+bool operator<=(Span<T> a, Span<const T> b) {
+  return !(b < a);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator<=(const U& a, Span<T> b) {
+  return !(b < a);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator<=(Span<T> a, const U& b) {
+  return !(b < a);
+}
+
+// operator>=
+template <typename T>
+bool operator>=(Span<T> a, Span<T> b) {
+  return !(a < b);
+}
+template <typename T>
+bool operator>=(Span<const T> a, Span<T> b) {
+  return !(a < b);
+}
+template <typename T>
+bool operator>=(Span<T> a, Span<const T> b) {
+  return !(a < b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator>=(const U& a, Span<T> b) {
+  return !(a < b);
+}
+template <typename T, typename U,
+          typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
+bool operator>=(Span<T> a, const U& b) {
+  return !(a < b);
+}
+
+// MakeSpan()
+//
+// Constructs a mutable `Span<T>`, deducing `T` automatically from either a
+// container or pointer+size.
+//
+// Because a read-only `Span<const T>` is implicitly constructed from container
+// types regardless of whether the container itself is a const container,
+// constructing mutable spans of type `Span<T>` from containers requires
+// explicit constructors. The container-accepting version of `MakeSpan()`
+// deduces the type of `T` by the constness of the pointer received from the
+// container's `data()` member. Similarly, the pointer-accepting version returns
+// a `Span<const T>` if `T` is `const`, and a `Span<T>` otherwise.
+//
+// Examples:
+//
+//   void MyRoutine(absl::Span<MyComplicatedType> a) {
+//     ...
+//   };
+//   // my_vector is a container of non-const types
+//   std::vector<MyComplicatedType> my_vector;
+//
+//   // Constructing a Span implicitly attempts to create a Span of type
+//   // `Span<const T>`
+//   MyRoutine(my_vector);                // error, type mismatch
+//
+//   // Explicitly constructing the Span is verbose
+//   MyRoutine(absl::Span<MyComplicatedType>(my_vector);
+//
+//   // Use MakeSpan() to make an absl::Span<T>
+//   MyRoutine(absl::MakeSpan(my_vector));
+//
+//   // Construct a span from an array ptr+size
+//   absl::Span<T> my_span() {
+//     return absl::MakeSpan(&array[0], num_elements_);
+//   }
+//
+template <int&... ExplicitArgumentBarrier, typename T>
+constexpr Span<T> MakeSpan(T* ptr, size_t size) noexcept {
+  return Span<T>(ptr, size);
+}
+
+template <int&... ExplicitArgumentBarrier, typename T>
+Span<T> MakeSpan(T* begin, T* end) noexcept {
+  return ABSL_ASSERT(begin <= end), Span<T>(begin, end - begin);
+}
+
+template <int&... ExplicitArgumentBarrier, typename C>
+constexpr auto MakeSpan(C& c) noexcept  // NOLINT(runtime/references)
+    -> decltype(absl::MakeSpan(span_internal::GetData(c), c.size())) {
+  return MakeSpan(span_internal::GetData(c), c.size());
+}
+
+template <int&... ExplicitArgumentBarrier, typename T, size_t N>
+constexpr Span<T> MakeSpan(T (&array)[N]) noexcept {
+  return Span<T>(array, N);
+}
+
+// MakeConstSpan()
+//
+// Constructs a `Span<const T>` as with `MakeSpan`, deducing `T` automatically,
+// but always returning a `Span<const T>`.
+//
+// Examples:
+//
+//   void ProcessInts(absl::Span<const int> some_ints);
+//
+//   // Call with a pointer and size.
+//   int array[3] = { 0, 0, 0 };
+//   ProcessInts(absl::MakeConstSpan(&array[0], 3));
+//
+//   // Call with a [begin, end) pair.
+//   ProcessInts(absl::MakeConstSpan(&array[0], &array[3]));
+//
+//   // Call directly with an array.
+//   ProcessInts(absl::MakeConstSpan(array));
+//
+//   // Call with a contiguous container.
+//   std::vector<int> some_ints = ...;
+//   ProcessInts(absl::MakeConstSpan(some_ints));
+//   ProcessInts(absl::MakeConstSpan(std::vector<int>{ 0, 0, 0 }));
+//
+template <int&... ExplicitArgumentBarrier, typename T>
+constexpr Span<const T> MakeConstSpan(T* ptr, size_t size) noexcept {
+  return Span<const T>(ptr, size);
+}
+
+template <int&... ExplicitArgumentBarrier, typename T>
+Span<const T> MakeConstSpan(T* begin, T* end) noexcept {
+  return ABSL_ASSERT(begin <= end), Span<const T>(begin, end - begin);
+}
+
+template <int&... ExplicitArgumentBarrier, typename C>
+constexpr auto MakeConstSpan(const C& c) noexcept -> decltype(MakeSpan(c)) {
+  return MakeSpan(c);
+}
+
+template <int&... ExplicitArgumentBarrier, typename T, size_t N>
+constexpr Span<const T> MakeConstSpan(const T (&array)[N]) noexcept {
+  return Span<const T>(array, N);
+}
+}  // namespace absl
+#endif  // ABSL_TYPES_SPAN_H_
diff --git a/absl/types/span_test.cc b/absl/types/span_test.cc
new file mode 100644
index 000000000000..22ea33e0a278
--- /dev/null
+++ b/absl/types/span_test.cc
@@ -0,0 +1,783 @@
+// Copyright 2017 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/types/span.h"
+
+#include <algorithm>
+#include <array>
+#include <initializer_list>
+#include <numeric>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/exception_testing.h"
+#include "absl/base/macros.h"
+#include "absl/base/port.h"
+#include "absl/container/fixed_array.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/strings/str_cat.h"
+
+namespace {
+
+MATCHER_P(DataIs, data,
+          absl::StrCat("data() is ", negation ? "is " : "isn't ",
+                       testing::PrintToString(data))) {
+  return arg.data() == data;
+}
+
+template <typename T>
+auto SpanIs(T data, size_t size)
+    -> decltype(testing::AllOf(DataIs(data), testing::SizeIs(size))) {
+  return testing::AllOf(DataIs(data), testing::SizeIs(size));
+}
+
+template <typename Container>
+auto SpanIs(const Container& c) -> decltype(SpanIs(c.data(), c.size())) {
+  return SpanIs(c.data(), c.size());
+}
+
+std::vector<int> MakeRamp(int len, int offset = 0) {
+  std::vector<int> v(len);
+  std::iota(v.begin(), v.end(), offset);
+  return v;
+}
+
+TEST(IntSpan, EmptyCtors) {
+  absl::Span<int> s;
+  EXPECT_THAT(s, SpanIs(nullptr, 0));
+}
+
+TEST(IntSpan, PtrLenCtor) {
+  int a[] = {1, 2, 3};
+  absl::Span<int> s(&a[0], 2);
+  EXPECT_THAT(s, SpanIs(a, 2));
+}
+
+TEST(IntSpan, ArrayCtor) {
+  int a[] = {1, 2, 3};
+  absl::Span<int> s(a);
+  EXPECT_THAT(s, SpanIs(a, 3));
+
+  EXPECT_TRUE((std::is_constructible<absl::Span<const int>, int[3]>::value));
+  EXPECT_TRUE(
+      (std::is_constructible<absl::Span<const int>, const int[3]>::value));
+  EXPECT_FALSE((std::is_constructible<absl::Span<int>, const int[3]>::value));
+  EXPECT_TRUE((std::is_convertible<int[3], absl::Span<const int>>::value));
+  EXPECT_TRUE(
+      (std::is_convertible<const int[3], absl::Span<const int>>::value));
+}
+
+template <typename T>
+void TakesGenericSpan(absl::Span<T>) {}
+
+TEST(IntSpan, ContainerCtor) {
+  std::vector<int> empty;
+  absl::Span<int> s_empty(empty);
+  EXPECT_THAT(s_empty, SpanIs(empty));
+
+  std::vector<int> filled{1, 2, 3};
+  absl::Span<int> s_filled(filled);
+  EXPECT_THAT(s_filled, SpanIs(filled));
+
+  absl::Span<int> s_from_span(filled);
+  EXPECT_THAT(s_from_span, SpanIs(s_filled));
+
+  absl::Span<const int> const_filled = filled;
+  EXPECT_THAT(const_filled, SpanIs(filled));
+
+  absl::Span<const int> const_from_span = s_filled;
+  EXPECT_THAT(const_from_span, SpanIs(s_filled));
+
+  EXPECT_TRUE(
+      (std::is_convertible<std::vector<int>&, absl::Span<const int>>::value));
+  EXPECT_TRUE(
+      (std::is_convertible<absl::Span<int>&, absl::Span<const int>>::value));
+
+  TakesGenericSpan(absl::Span<int>(filled));
+}
+
+// A struct supplying shallow data() const.
+struct ContainerWithShallowConstData {
+  std::vector<int> storage;
+  int* data() const { return const_cast<int*>(storage.data()); }
+  int size() const { return storage.size(); }
+};
+
+TEST(IntSpan, ShallowConstness) {
+  const ContainerWithShallowConstData c{MakeRamp(20)};
+  absl::Span<int> s(
+      c);  // We should be able to do this even though data() is const.
+  s[0] = -1;
+  EXPECT_EQ(c.storage[0], -1);
+}
+
+TEST(CharSpan, StringCtor) {
+  std::string empty = "";
+  absl::Span<char> s_empty(empty);
+  EXPECT_THAT(s_empty, SpanIs(empty));
+
+  std::string abc = "abc";
+  absl::Span<char> s_abc(abc);
+  EXPECT_THAT(s_abc, SpanIs(abc));
+
+  absl::Span<const char> s_const_abc = abc;
+  EXPECT_THAT(s_const_abc, SpanIs(abc));
+
+  EXPECT_FALSE((std::is_constructible<absl::Span<int>, std::string>::value));
+  EXPECT_FALSE((std::is_constructible<absl::Span<const int>, std::string>::value));
+  EXPECT_TRUE((std::is_convertible<std::string, absl::Span<const char>>::value));
+}
+
+TEST(IntSpan, FromConstPointer) {
+  EXPECT_TRUE((std::is_constructible<absl::Span<const int* const>,
+                                     std::vector<int*>>::value));
+  EXPECT_TRUE((std::is_constructible<absl::Span<const int* const>,
+                                     std::vector<const int*>>::value));
+  EXPECT_FALSE((
+      std::is_constructible<absl::Span<const int*>, std::vector<int*>>::value));
+  EXPECT_FALSE((
+      std::is_constructible<absl::Span<int*>, std::vector<const int*>>::value));
+}
+
+struct TypeWithMisleadingData {
+  int& data() { return i; }
+  int size() { return 1; }
+  int i;
+};
+
+struct TypeWithMisleadingSize {
+  int* data() { return &i; }
+  const char* size() { return "1"; }
+  int i;
+};
+
+TEST(IntSpan, EvilTypes) {
+  EXPECT_FALSE(
+      (std::is_constructible<absl::Span<int>, TypeWithMisleadingData&>::value));
+  EXPECT_FALSE(
+      (std::is_constructible<absl::Span<int>, TypeWithMisleadingSize&>::value));
+}
+
+struct Base {
+  int* data() { return &i; }
+  int size() { return 1; }
+  int i;
+};
+struct Derived : Base {};
+
+TEST(IntSpan, SpanOfDerived) {
+  EXPECT_TRUE((std::is_constructible<absl::Span<int>, Base&>::value));
+  EXPECT_TRUE((std::is_constructible<absl::Span<int>, Derived&>::value));
+  EXPECT_FALSE(
+      (std::is_constructible<absl::Span<Base>, std::vector<Derived>>::value));
+}
+
+void TestInitializerList(absl::Span<const int> s, const std::vector<int>& v) {
+  EXPECT_TRUE(absl::equal(s.begin(), s.end(), v.begin(), v.end()));
+}
+
+TEST(ConstIntSpan, InitializerListConversion) {
+  TestInitializerList({}, {});
+  TestInitializerList({1}, {1});
+  TestInitializerList({1, 2, 3}, {1, 2, 3});
+
+  EXPECT_FALSE((std::is_constructible<absl::Span<int>,
+                                      std::initializer_list<int>>::value));
+  EXPECT_FALSE((
+      std::is_convertible<absl::Span<int>, std::initializer_list<int>>::value));
+}
+
+TEST(IntSpan, Data) {
+  int i;
+  absl::Span<int> s(&i, 1);
+  EXPECT_EQ(&i, s.data());
+}
+
+TEST(IntSpan, SizeLengthEmpty) {
+  absl::Span<int> empty;
+  EXPECT_EQ(empty.size(), 0);
+  EXPECT_TRUE(empty.empty());
+  EXPECT_EQ(empty.size(), empty.length());
+
+  auto v = MakeRamp(10);
+  absl::Span<int> s(v);
+  EXPECT_EQ(s.size(), 10);
+  EXPECT_FALSE(s.empty());
+  EXPECT_EQ(s.size(), s.length());
+}
+
+TEST(IntSpan, ElementAccess) {
+  auto v = MakeRamp(10);
+  absl::Span<int> s(v);
+  for (int i = 0; i < s.size(); ++i) {
+    EXPECT_EQ(s[i], s.at(i));
+  }
+
+  EXPECT_EQ(s.front(), s[0]);
+  EXPECT_EQ(s.back(), s[9]);
+}
+
+TEST(IntSpan, AtThrows) {
+  auto v = MakeRamp(10);
+  absl::Span<int> s(v);
+
+  EXPECT_EQ(s.at(9), 9);
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(s.at(10), std::out_of_range,
+                                 "failed bounds check");
+}
+
+TEST(IntSpan, RemovePrefixAndSuffix) {
+  auto v = MakeRamp(20, 1);
+  absl::Span<int> s(v);
+  EXPECT_EQ(s.size(), 20);
+
+  s.remove_suffix(0);
+  s.remove_prefix(0);
+  EXPECT_EQ(s.size(), 20);
+
+  s.remove_prefix(1);
+  EXPECT_EQ(s.size(), 19);
+  EXPECT_EQ(s[0], 2);
+
+  s.remove_suffix(1);
+  EXPECT_EQ(s.size(), 18);
+  EXPECT_EQ(s.back(), 19);
+
+  s.remove_prefix(7);
+  EXPECT_EQ(s.size(), 11);
+  EXPECT_EQ(s[0], 9);
+
+  s.remove_suffix(11);
+  EXPECT_EQ(s.size(), 0);
+
+  EXPECT_EQ(v, MakeRamp(20, 1));
+}
+
+TEST(IntSpan, Subspan) {
+  std::vector<int> empty;
+  EXPECT_EQ(absl::MakeSpan(empty).subspan(), empty);
+  EXPECT_THAT(absl::MakeSpan(empty).subspan(0, 0), SpanIs(empty));
+  EXPECT_THAT(absl::MakeSpan(empty).subspan(0, absl::Span<const int>::npos),
+              SpanIs(empty));
+
+  auto ramp = MakeRamp(10);
+  EXPECT_THAT(absl::MakeSpan(ramp).subspan(), SpanIs(ramp));
+  EXPECT_THAT(absl::MakeSpan(ramp).subspan(0, 10), SpanIs(ramp));
+  EXPECT_THAT(absl::MakeSpan(ramp).subspan(0, absl::Span<const int>::npos),
+              SpanIs(ramp));
+  EXPECT_THAT(absl::MakeSpan(ramp).subspan(0, 3), SpanIs(ramp.data(), 3));
+  EXPECT_THAT(absl::MakeSpan(ramp).subspan(5, absl::Span<const int>::npos),
+              SpanIs(ramp.data() + 5, 5));
+  EXPECT_THAT(absl::MakeSpan(ramp).subspan(3, 3), SpanIs(ramp.data() + 3, 3));
+  EXPECT_THAT(absl::MakeSpan(ramp).subspan(10, 5), SpanIs(ramp.data() + 10, 0));
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(absl::MakeSpan(ramp).subspan(11, 5), std::out_of_range);
+#else
+  EXPECT_DEATH(absl::MakeSpan(ramp).subspan(11, 5), "");
+#endif
+}
+
+TEST(IntSpan, MakeSpanPtrLength) {
+  std::vector<int> empty;
+  auto s_empty = absl::MakeSpan(empty.data(), empty.size());
+  EXPECT_THAT(s_empty, SpanIs(empty));
+
+  std::array<int, 3> a{{1, 2, 3}};
+  auto s = absl::MakeSpan(a.data(), a.size());
+  EXPECT_THAT(s, SpanIs(a));
+
+  EXPECT_THAT(absl::MakeConstSpan(empty.data(), empty.size()), SpanIs(s_empty));
+  EXPECT_THAT(absl::MakeConstSpan(a.data(), a.size()), SpanIs(s));
+}
+
+TEST(IntSpan, MakeSpanTwoPtrs) {
+  std::vector<int> empty;
+  auto s_empty = absl::MakeSpan(empty.data(), empty.data());
+  EXPECT_THAT(s_empty, SpanIs(empty));
+
+  std::vector<int> v{1, 2, 3};
+  auto s = absl::MakeSpan(v.data(), v.data() + 1);
+  EXPECT_THAT(s, SpanIs(v.data(), 1));
+
+  EXPECT_THAT(absl::MakeConstSpan(empty.data(), empty.data()), SpanIs(s_empty));
+  EXPECT_THAT(absl::MakeConstSpan(v.data(), v.data() + 1), SpanIs(s));
+}
+
+TEST(IntSpan, MakeSpanContainer) {
+  std::vector<int> empty;
+  auto s_empty = absl::MakeSpan(empty);
+  EXPECT_THAT(s_empty, SpanIs(empty));
+
+  std::vector<int> v{1, 2, 3};
+  auto s = absl::MakeSpan(v);
+  EXPECT_THAT(s, SpanIs(v));
+
+  EXPECT_THAT(absl::MakeConstSpan(empty), SpanIs(s_empty));
+  EXPECT_THAT(absl::MakeConstSpan(v), SpanIs(s));
+
+  EXPECT_THAT(absl::MakeSpan(s), SpanIs(s));
+  EXPECT_THAT(absl::MakeConstSpan(s), SpanIs(s));
+}
+
+TEST(CharSpan, MakeSpanString) {
+  std::string empty = "";
+  auto s_empty = absl::MakeSpan(empty);
+  EXPECT_THAT(s_empty, SpanIs(empty));
+
+  std::string str = "abc";
+  auto s_str = absl::MakeSpan(str);
+  EXPECT_THAT(s_str, SpanIs(str));
+
+  EXPECT_THAT(absl::MakeConstSpan(empty), SpanIs(s_empty));
+  EXPECT_THAT(absl::MakeConstSpan(str), SpanIs(s_str));
+}
+
+TEST(IntSpan, MakeSpanArray) {
+  int a[] = {1, 2, 3};
+  auto s = absl::MakeSpan(a);
+  EXPECT_THAT(s, SpanIs(a, 3));
+
+  const int ca[] = {1, 2, 3};
+  auto s_ca = absl::MakeSpan(ca);
+  EXPECT_THAT(s_ca, SpanIs(ca, 3));
+
+  EXPECT_THAT(absl::MakeConstSpan(a), SpanIs(s));
+  EXPECT_THAT(absl::MakeConstSpan(ca), SpanIs(s_ca));
+}
+
+// Compile-asserts that the argument has the expected decayed type.
+template <typename Expected, typename T>
+void CheckType(const T& /* value */) {
+  testing::StaticAssertTypeEq<Expected, T>();
+}
+
+TEST(IntSpan, MakeSpanTypes) {
+  std::vector<int> vec;
+  const std::vector<int> cvec;
+  int a[1];
+  const int ca[] = {1};
+  int* ip = a;
+  const int* cip = ca;
+  std::string s = "";
+  const std::string cs = "";
+  CheckType<absl::Span<int>>(absl::MakeSpan(vec));
+  CheckType<absl::Span<const int>>(absl::MakeSpan(cvec));
+  CheckType<absl::Span<int>>(absl::MakeSpan(ip, ip + 1));
+  CheckType<absl::Span<int>>(absl::MakeSpan(ip, 1));
+  CheckType<absl::Span<const int>>(absl::MakeSpan(cip, cip + 1));
+  CheckType<absl::Span<const int>>(absl::MakeSpan(cip, 1));
+  CheckType<absl::Span<int>>(absl::MakeSpan(a));
+  CheckType<absl::Span<int>>(absl::MakeSpan(a, a + 1));
+  CheckType<absl::Span<int>>(absl::MakeSpan(a, 1));
+  CheckType<absl::Span<const int>>(absl::MakeSpan(ca));
+  CheckType<absl::Span<const int>>(absl::MakeSpan(ca, ca + 1));
+  CheckType<absl::Span<const int>>(absl::MakeSpan(ca, 1));
+  CheckType<absl::Span<char>>(absl::MakeSpan(s));
+  CheckType<absl::Span<const char>>(absl::MakeSpan(cs));
+}
+
+TEST(ConstIntSpan, MakeConstSpanTypes) {
+  std::vector<int> vec;
+  const std::vector<int> cvec;
+  int array[1];
+  const int carray[] = {0};
+  int* ptr = array;
+  const int* cptr = carray;
+  std::string s = "";
+  std::string cs = "";
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(vec));
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(cvec));
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(ptr, ptr + 1));
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(ptr, 1));
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(cptr, cptr + 1));
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(cptr, 1));
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(array));
+  CheckType<absl::Span<const int>>(absl::MakeConstSpan(carray));
+  CheckType<absl::Span<const char>>(absl::MakeConstSpan(s));
+  CheckType<absl::Span<const char>>(absl::MakeConstSpan(cs));
+}
+
+TEST(IntSpan, Equality) {
+  const int arr1[] = {1, 2, 3, 4, 5};
+  int arr2[] = {1, 2, 3, 4, 5};
+  std::vector<int> vec1(std::begin(arr1), std::end(arr1));
+  std::vector<int> vec2 = vec1;
+  std::vector<int> other_vec = {2, 4, 6, 8, 10};
+  // These two slices are from different vectors, but have the same size and
+  // have the same elements (right now).  They should compare equal. Test both
+  // == and !=.
+  const absl::Span<const int> from1 = vec1;
+  const absl::Span<const int> from2 = vec2;
+  EXPECT_EQ(from1, from1);
+  EXPECT_FALSE(from1 != from1);
+  EXPECT_EQ(from1, from2);
+  EXPECT_FALSE(from1 != from2);
+
+  // These two slices have different underlying vector values. They should be
+  // considered not equal. Test both == and !=.
+  const absl::Span<const int> from_other = other_vec;
+  EXPECT_NE(from1, from_other);
+  EXPECT_FALSE(from1 == from_other);
+
+  // Comparison between a vector and its slice should be equal. And vice-versa.
+  // This ensures implicit conversion to Span works on both sides of ==.
+  EXPECT_EQ(vec1, from1);
+  EXPECT_FALSE(vec1 != from1);
+  EXPECT_EQ(from1, vec1);
+  EXPECT_FALSE(from1 != vec1);
+
+  // This verifies that absl::Span<T> can be compared freely with
+  // absl::Span<const T>.
+  const absl::Span<int> mutable_from1(vec1);
+  const absl::Span<int> mutable_from2(vec2);
+  EXPECT_EQ(from1, mutable_from1);
+  EXPECT_EQ(mutable_from1, from1);
+  EXPECT_EQ(mutable_from1, mutable_from2);
+  EXPECT_EQ(mutable_from2, mutable_from1);
+
+  // Comparison between a vector and its slice should be equal for mutable
+  // Spans as well.
+  EXPECT_EQ(vec1, mutable_from1);
+  EXPECT_FALSE(vec1 != mutable_from1);
+  EXPECT_EQ(mutable_from1, vec1);
+  EXPECT_FALSE(mutable_from1 != vec1);
+
+  // Comparison between convertible-to-Span-of-const and Span-of-mutable. Arrays
+  // are used because they're the only value type which converts to a
+  // Span-of-mutable. EXPECT_TRUE is used instead of EXPECT_EQ to avoid
+  // array-to-pointer decay.
+  EXPECT_TRUE(arr1 == mutable_from1);
+  EXPECT_FALSE(arr1 != mutable_from1);
+  EXPECT_TRUE(mutable_from1 == arr1);
+  EXPECT_FALSE(mutable_from1 != arr1);
+
+  // Comparison between convertible-to-Span-of-mutable and Span-of-const
+  EXPECT_TRUE(arr2 == from1);
+  EXPECT_FALSE(arr2 != from1);
+  EXPECT_TRUE(from1 == arr2);
+  EXPECT_FALSE(from1 != arr2);
+
+  // With a different size, the array slices should not be equal.
+  EXPECT_NE(from1, absl::Span<const int>(from1).subspan(0, from1.size() - 1));
+
+  // With different contents, the array slices should not be equal.
+  ++vec2.back();
+  EXPECT_NE(from1, from2);
+}
+
+class IntSpanOrderComparisonTest : public testing::Test {
+ public:
+  IntSpanOrderComparisonTest()
+      : arr_before_{1, 2, 3},
+        arr_after_{1, 2, 4},
+        carr_after_{1, 2, 4},
+        vec_before_(std::begin(arr_before_), std::end(arr_before_)),
+        vec_after_(std::begin(arr_after_), std::end(arr_after_)),
+        before_(vec_before_),
+        after_(vec_after_),
+        cbefore_(vec_before_),
+        cafter_(vec_after_) {}
+
+ protected:
+  int arr_before_[3], arr_after_[3];
+  const int carr_after_[3];
+  std::vector<int> vec_before_, vec_after_;
+  absl::Span<int> before_, after_;
+  absl::Span<const int> cbefore_, cafter_;
+};
+
+TEST_F(IntSpanOrderComparisonTest, CompareSpans) {
+  EXPECT_TRUE(cbefore_ < cafter_);
+  EXPECT_TRUE(cbefore_ <= cafter_);
+  EXPECT_TRUE(cafter_ > cbefore_);
+  EXPECT_TRUE(cafter_ >= cbefore_);
+
+  EXPECT_FALSE(cbefore_ > cafter_);
+  EXPECT_FALSE(cafter_ < cbefore_);
+
+  EXPECT_TRUE(before_ < after_);
+  EXPECT_TRUE(before_ <= after_);
+  EXPECT_TRUE(after_ > before_);
+  EXPECT_TRUE(after_ >= before_);
+
+  EXPECT_FALSE(before_ > after_);
+  EXPECT_FALSE(after_ < before_);
+
+  EXPECT_TRUE(cbefore_ < after_);
+  EXPECT_TRUE(cbefore_ <= after_);
+  EXPECT_TRUE(after_ > cbefore_);
+  EXPECT_TRUE(after_ >= cbefore_);
+
+  EXPECT_FALSE(cbefore_ > after_);
+  EXPECT_FALSE(after_ < cbefore_);
+}
+
+TEST_F(IntSpanOrderComparisonTest, SpanOfConstAndContainer) {
+  EXPECT_TRUE(cbefore_ < vec_after_);
+  EXPECT_TRUE(cbefore_ <= vec_after_);
+  EXPECT_TRUE(vec_after_ > cbefore_);
+  EXPECT_TRUE(vec_after_ >= cbefore_);
+
+  EXPECT_FALSE(cbefore_ > vec_after_);
+  EXPECT_FALSE(vec_after_ < cbefore_);
+
+  EXPECT_TRUE(arr_before_ < cafter_);
+  EXPECT_TRUE(arr_before_ <= cafter_);
+  EXPECT_TRUE(cafter_ > arr_before_);
+  EXPECT_TRUE(cafter_ >= arr_before_);
+
+  EXPECT_FALSE(arr_before_ > cafter_);
+  EXPECT_FALSE(cafter_ < arr_before_);
+}
+
+TEST_F(IntSpanOrderComparisonTest, SpanOfMutableAndContainer) {
+  EXPECT_TRUE(vec_before_ < after_);
+  EXPECT_TRUE(vec_before_ <= after_);
+  EXPECT_TRUE(after_ > vec_before_);
+  EXPECT_TRUE(after_ >= vec_before_);
+
+  EXPECT_FALSE(vec_before_ > after_);
+  EXPECT_FALSE(after_ < vec_before_);
+
+  EXPECT_TRUE(before_ < carr_after_);
+  EXPECT_TRUE(before_ <= carr_after_);
+  EXPECT_TRUE(carr_after_ > before_);
+  EXPECT_TRUE(carr_after_ >= before_);
+
+  EXPECT_FALSE(before_ > carr_after_);
+  EXPECT_FALSE(carr_after_ < before_);
+}
+
+TEST_F(IntSpanOrderComparisonTest, EqualSpans) {
+  EXPECT_FALSE(before_ < before_);
+  EXPECT_TRUE(before_ <= before_);
+  EXPECT_FALSE(before_ > before_);
+  EXPECT_TRUE(before_ >= before_);
+}
+
+TEST_F(IntSpanOrderComparisonTest, Subspans) {
+  auto subspan = before_.subspan(0, 1);
+  EXPECT_TRUE(subspan < before_);
+  EXPECT_TRUE(subspan <= before_);
+  EXPECT_TRUE(before_ > subspan);
+  EXPECT_TRUE(before_ >= subspan);
+
+  EXPECT_FALSE(subspan > before_);
+  EXPECT_FALSE(before_ < subspan);
+}
+
+TEST_F(IntSpanOrderComparisonTest, EmptySpans) {
+  absl::Span<int> empty;
+  EXPECT_FALSE(empty < empty);
+  EXPECT_TRUE(empty <= empty);
+  EXPECT_FALSE(empty > empty);
+  EXPECT_TRUE(empty >= empty);
+
+  EXPECT_TRUE(empty < before_);
+  EXPECT_TRUE(empty <= before_);
+  EXPECT_TRUE(before_ > empty);
+  EXPECT_TRUE(before_ >= empty);
+
+  EXPECT_FALSE(empty > before_);
+  EXPECT_FALSE(before_ < empty);
+}
+
+TEST(IntSpan, ExposesContainerTypesAndConsts) {
+  absl::Span<int> slice;
+  CheckType<absl::Span<int>::iterator>(slice.begin());
+  EXPECT_TRUE((std::is_convertible<decltype(slice.begin()),
+                                   absl::Span<int>::const_iterator>::value));
+  CheckType<absl::Span<int>::const_iterator>(slice.cbegin());
+  EXPECT_TRUE((std::is_convertible<decltype(slice.end()),
+                                   absl::Span<int>::const_iterator>::value));
+  CheckType<absl::Span<int>::const_iterator>(slice.cend());
+  CheckType<absl::Span<int>::reverse_iterator>(slice.rend());
+  EXPECT_TRUE(
+      (std::is_convertible<decltype(slice.rend()),
+                           absl::Span<int>::const_reverse_iterator>::value));
+  CheckType<absl::Span<int>::const_reverse_iterator>(slice.crend());
+  testing::StaticAssertTypeEq<int, absl::Span<int>::value_type>();
+  testing::StaticAssertTypeEq<int, absl::Span<const int>::value_type>();
+  testing::StaticAssertTypeEq<int*, absl::Span<int>::pointer>();
+  testing::StaticAssertTypeEq<const int*, absl::Span<const int>::pointer>();
+  testing::StaticAssertTypeEq<int&, absl::Span<int>::reference>();
+  testing::StaticAssertTypeEq<const int&, absl::Span<const int>::reference>();
+  testing::StaticAssertTypeEq<const int&, absl::Span<int>::const_reference>();
+  testing::StaticAssertTypeEq<const int&,
+                              absl::Span<const int>::const_reference>();
+  EXPECT_EQ(static_cast<absl::Span<int>::size_type>(-1), absl::Span<int>::npos);
+}
+
+TEST(IntSpan, IteratorsAndReferences) {
+  auto accept_pointer = [](int*) {};
+  auto accept_reference = [](int&) {};
+  auto accept_iterator = [](absl::Span<int>::iterator) {};
+  auto accept_const_iterator = [](absl::Span<int>::const_iterator) {};
+  auto accept_reverse_iterator = [](absl::Span<int>::reverse_iterator) {};
+  auto accept_const_reverse_iterator =
+      [](absl::Span<int>::const_reverse_iterator) {};
+
+  int a[1];
+  absl::Span<int> s = a;
+
+  accept_pointer(s.data());
+  accept_iterator(s.begin());
+  accept_const_iterator(s.begin());
+  accept_const_iterator(s.cbegin());
+  accept_iterator(s.end());
+  accept_const_iterator(s.end());
+  accept_const_iterator(s.cend());
+  accept_reverse_iterator(s.rbegin());
+  accept_const_reverse_iterator(s.rbegin());
+  accept_const_reverse_iterator(s.crbegin());
+  accept_reverse_iterator(s.rend());
+  accept_const_reverse_iterator(s.rend());
+  accept_const_reverse_iterator(s.crend());
+
+  accept_reference(s[0]);
+  accept_reference(s.at(0));
+  accept_reference(s.front());
+  accept_reference(s.back());
+}
+
+TEST(IntSpan, IteratorsAndReferences_Const) {
+  auto accept_pointer = [](int*) {};
+  auto accept_reference = [](int&) {};
+  auto accept_iterator = [](absl::Span<int>::iterator) {};
+  auto accept_const_iterator = [](absl::Span<int>::const_iterator) {};
+  auto accept_reverse_iterator = [](absl::Span<int>::reverse_iterator) {};
+  auto accept_const_reverse_iterator =
+      [](absl::Span<int>::const_reverse_iterator) {};
+
+  int a[1];
+  const absl::Span<int> s = a;
+
+  accept_pointer(s.data());
+  accept_iterator(s.begin());
+  accept_const_iterator(s.begin());
+  accept_const_iterator(s.cbegin());
+  accept_iterator(s.end());
+  accept_const_iterator(s.end());
+  accept_const_iterator(s.cend());
+  accept_reverse_iterator(s.rbegin());
+  accept_const_reverse_iterator(s.rbegin());
+  accept_const_reverse_iterator(s.crbegin());
+  accept_reverse_iterator(s.rend());
+  accept_const_reverse_iterator(s.rend());
+  accept_const_reverse_iterator(s.crend());
+
+  accept_reference(s[0]);
+  accept_reference(s.at(0));
+  accept_reference(s.front());
+  accept_reference(s.back());
+}
+
+TEST(IntSpan, NoexceptTest) {
+  int a[] = {1, 2, 3};
+  std::vector<int> v;
+  EXPECT_TRUE(noexcept(absl::Span<const int>()));
+  EXPECT_TRUE(noexcept(absl::Span<const int>(a, 2)));
+  EXPECT_TRUE(noexcept(absl::Span<const int>(a)));
+  EXPECT_TRUE(noexcept(absl::Span<const int>(v)));
+  EXPECT_TRUE(noexcept(absl::Span<int>(v)));
+  EXPECT_TRUE(noexcept(absl::Span<const int>({1, 2, 3})));
+  EXPECT_TRUE(noexcept(absl::MakeSpan(v)));
+  EXPECT_TRUE(noexcept(absl::MakeSpan(a)));
+  EXPECT_TRUE(noexcept(absl::MakeSpan(a, 2)));
+  EXPECT_TRUE(noexcept(absl::MakeSpan(a, a + 1)));
+  EXPECT_TRUE(noexcept(absl::MakeConstSpan(v)));
+  EXPECT_TRUE(noexcept(absl::MakeConstSpan(a)));
+  EXPECT_TRUE(noexcept(absl::MakeConstSpan(a, 2)));
+  EXPECT_TRUE(noexcept(absl::MakeConstSpan(a, a + 1)));
+
+  absl::Span<int> s(v);
+  EXPECT_TRUE(noexcept(s.data()));
+  EXPECT_TRUE(noexcept(s.size()));
+  EXPECT_TRUE(noexcept(s.length()));
+  EXPECT_TRUE(noexcept(s.empty()));
+  EXPECT_TRUE(noexcept(s[0]));
+  EXPECT_TRUE(noexcept(s.front()));
+  EXPECT_TRUE(noexcept(s.back()));
+  EXPECT_TRUE(noexcept(s.begin()));
+  EXPECT_TRUE(noexcept(s.cbegin()));
+  EXPECT_TRUE(noexcept(s.end()));
+  EXPECT_TRUE(noexcept(s.cend()));
+  EXPECT_TRUE(noexcept(s.rbegin()));
+  EXPECT_TRUE(noexcept(s.crbegin()));
+  EXPECT_TRUE(noexcept(s.rend()));
+  EXPECT_TRUE(noexcept(s.crend()));
+  EXPECT_TRUE(noexcept(s.remove_prefix(0)));
+  EXPECT_TRUE(noexcept(s.remove_suffix(0)));
+}
+
+// ConstexprTester exercises expressions in a constexpr context. Simply placing
+// the expression in a constexpr function is not enough, as some compilers will
+// simply compile the constexpr function as runtime code. Using template
+// parameters forces compile-time execution.
+template <int i>
+struct ConstexprTester {};
+
+#define ABSL_TEST_CONSTEXPR(expr)                       \
+  do {                                                  \
+    ABSL_ATTRIBUTE_UNUSED ConstexprTester<(expr, 1)> t; \
+  } while (0)
+
+struct ContainerWithConstexprMethods {
+  constexpr int size() const { return 1; }
+  constexpr const int* data() const { return &i; }
+  const int i;
+};
+
+TEST(ConstIntSpan, ConstexprTest) {
+  static constexpr int a[] = {1, 2, 3};
+  static constexpr int sized_arr[2] = {1, 2};
+  static constexpr ContainerWithConstexprMethods c{1};
+  ABSL_TEST_CONSTEXPR(absl::Span<const int>());
+  ABSL_TEST_CONSTEXPR(absl::Span<const int>(a, 2));
+  ABSL_TEST_CONSTEXPR(absl::Span<const int>(sized_arr));
+  ABSL_TEST_CONSTEXPR(absl::Span<const int>(c));
+  ABSL_TEST_CONSTEXPR(absl::MakeSpan(&a[0], 1));
+  ABSL_TEST_CONSTEXPR(absl::MakeSpan(c));
+  ABSL_TEST_CONSTEXPR(absl::MakeSpan(a));
+  ABSL_TEST_CONSTEXPR(absl::MakeConstSpan(&a[0], 1));
+  ABSL_TEST_CONSTEXPR(absl::MakeConstSpan(c));
+  ABSL_TEST_CONSTEXPR(absl::MakeConstSpan(a));
+
+  constexpr absl::Span<const int> span = c;
+  ABSL_TEST_CONSTEXPR(span.data());
+  ABSL_TEST_CONSTEXPR(span.size());
+  ABSL_TEST_CONSTEXPR(span.length());
+  ABSL_TEST_CONSTEXPR(span.empty());
+  ABSL_TEST_CONSTEXPR(span.begin());
+  ABSL_TEST_CONSTEXPR(span.cbegin());
+  ABSL_TEST_CONSTEXPR(span.subspan(0, 0));
+  ABSL_TEST_CONSTEXPR(span[0]);
+}
+
+struct BigStruct {
+  char bytes[10000];
+};
+
+TEST(Span, SpanSize) {
+  EXPECT_LE(sizeof(absl::Span<int>), 2 * sizeof(void*));
+  EXPECT_LE(sizeof(absl::Span<BigStruct>), 2 * sizeof(void*));
+}
+
+}  // namespace