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diff --git a/third_party/abseil_cpp/absl/hash/hash_test.cc b/third_party/abseil_cpp/absl/hash/hash_test.cc
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+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/hash/hash.h"
+
+#include <array>
+#include <bitset>
+#include <cstring>
+#include <deque>
+#include <forward_list>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <list>
+#include <map>
+#include <memory>
+#include <numeric>
+#include <random>
+#include <set>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/hash/internal/spy_hash_state.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/int128.h"
+#include "absl/strings/cord_test_helpers.h"
+
+namespace {
+
+using absl::Hash;
+using absl::hash_internal::SpyHashState;
+
+template <typename T>
+class HashValueIntTest : public testing::Test {
+};
+TYPED_TEST_SUITE_P(HashValueIntTest);
+
+template <typename T>
+SpyHashState SpyHash(const T& value) {
+  return SpyHashState::combine(SpyHashState(), value);
+}
+
+// Helper trait to verify if T is hashable. We use absl::Hash's poison status to
+// detect it.
+template <typename T>
+using is_hashable = std::is_default_constructible<absl::Hash<T>>;
+
+TYPED_TEST_P(HashValueIntTest, BasicUsage) {
+  EXPECT_TRUE((is_hashable<TypeParam>::value));
+
+  TypeParam n = 42;
+  EXPECT_EQ(SpyHash(n), SpyHash(TypeParam{42}));
+  EXPECT_NE(SpyHash(n), SpyHash(TypeParam{0}));
+  EXPECT_NE(SpyHash(std::numeric_limits<TypeParam>::max()),
+            SpyHash(std::numeric_limits<TypeParam>::min()));
+}
+
+TYPED_TEST_P(HashValueIntTest, FastPath) {
+  // Test the fast-path to make sure the values are the same.
+  TypeParam n = 42;
+  EXPECT_EQ(absl::Hash<TypeParam>{}(n),
+            absl::Hash<std::tuple<TypeParam>>{}(std::tuple<TypeParam>(n)));
+}
+
+REGISTER_TYPED_TEST_CASE_P(HashValueIntTest, BasicUsage, FastPath);
+using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t, uint32_t,
+                                uint64_t, size_t>;
+INSTANTIATE_TYPED_TEST_CASE_P(My, HashValueIntTest, IntTypes);
+
+enum LegacyEnum { kValue1, kValue2, kValue3 };
+
+enum class EnumClass { kValue4, kValue5, kValue6 };
+
+TEST(HashValueTest, EnumAndBool) {
+  EXPECT_TRUE((is_hashable<LegacyEnum>::value));
+  EXPECT_TRUE((is_hashable<EnumClass>::value));
+  EXPECT_TRUE((is_hashable<bool>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      LegacyEnum::kValue1, LegacyEnum::kValue2, LegacyEnum::kValue3)));
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      EnumClass::kValue4, EnumClass::kValue5, EnumClass::kValue6)));
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(true, false)));
+}
+
+TEST(HashValueTest, FloatingPoint) {
+  EXPECT_TRUE((is_hashable<float>::value));
+  EXPECT_TRUE((is_hashable<double>::value));
+  EXPECT_TRUE((is_hashable<long double>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(42.f, 0.f, -0.f, std::numeric_limits<float>::infinity(),
+                      -std::numeric_limits<float>::infinity())));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(42., 0., -0., std::numeric_limits<double>::infinity(),
+                      -std::numeric_limits<double>::infinity())));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      // Add some values with small exponent to test that NORMAL values also
+      // append their category.
+      .5L, 1.L, 2.L, 4.L, 42.L, 0.L, -0.L,
+      17 * static_cast<long double>(std::numeric_limits<double>::max()),
+      std::numeric_limits<long double>::infinity(),
+      -std::numeric_limits<long double>::infinity())));
+}
+
+TEST(HashValueTest, Pointer) {
+  EXPECT_TRUE((is_hashable<int*>::value));
+
+  int i;
+  int* ptr = &i;
+  int* n = nullptr;
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(&i, ptr, nullptr, ptr + 1, n)));
+}
+
+TEST(HashValueTest, PointerAlignment) {
+  // We want to make sure that pointer alignment will not cause bits to be
+  // stuck.
+
+  constexpr size_t kTotalSize = 1 << 20;
+  std::unique_ptr<char[]> data(new char[kTotalSize]);
+  constexpr size_t kLog2NumValues = 5;
+  constexpr size_t kNumValues = 1 << kLog2NumValues;
+
+  for (size_t align = 1; align < kTotalSize / kNumValues;
+       align < 8 ? align += 1 : align < 1024 ? align += 8 : align += 32) {
+    SCOPED_TRACE(align);
+    ASSERT_LE(align * kNumValues, kTotalSize);
+
+    size_t bits_or = 0;
+    size_t bits_and = ~size_t{};
+
+    for (size_t i = 0; i < kNumValues; ++i) {
+      size_t hash = absl::Hash<void*>()(data.get() + i * align);
+      bits_or |= hash;
+      bits_and &= hash;
+    }
+
+    // Limit the scope to the bits we would be using for Swisstable.
+    constexpr size_t kMask = (1 << (kLog2NumValues + 7)) - 1;
+    size_t stuck_bits = (~bits_or | bits_and) & kMask;
+    EXPECT_EQ(stuck_bits, 0) << "0x" << std::hex << stuck_bits;
+  }
+}
+
+TEST(HashValueTest, PairAndTuple) {
+  EXPECT_TRUE((is_hashable<std::pair<int, int>>::value));
+  EXPECT_TRUE((is_hashable<std::pair<const int&, const int&>>::value));
+  EXPECT_TRUE((is_hashable<std::tuple<int&, int&>>::value));
+  EXPECT_TRUE((is_hashable<std::tuple<int&&, int&&>>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::make_pair(0, 42), std::make_pair(0, 42), std::make_pair(42, 0),
+      std::make_pair(0, 0), std::make_pair(42, 42), std::make_pair(1, 42))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(std::make_tuple(0, 0, 0), std::make_tuple(0, 0, 42),
+                      std::make_tuple(0, 23, 0), std::make_tuple(17, 0, 0),
+                      std::make_tuple(42, 0, 0), std::make_tuple(3, 9, 9),
+                      std::make_tuple(0, 0, -42))));
+
+  // Test that tuples of lvalue references work (so we need a few lvalues):
+  int a = 0, b = 1, c = 17, d = 23;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::tie(a, a), std::tie(a, b), std::tie(b, c), std::tie(c, d))));
+
+  // Test that tuples of rvalue references work:
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::forward_as_tuple(0, 0, 0), std::forward_as_tuple(0, 0, 42),
+      std::forward_as_tuple(0, 23, 0), std::forward_as_tuple(17, 0, 0),
+      std::forward_as_tuple(42, 0, 0), std::forward_as_tuple(3, 9, 9),
+      std::forward_as_tuple(0, 0, -42))));
+}
+
+TEST(HashValueTest, CombineContiguousWorks) {
+  std::vector<std::tuple<int>> v1 = {std::make_tuple(1), std::make_tuple(3)};
+  std::vector<std::tuple<int>> v2 = {std::make_tuple(1), std::make_tuple(2)};
+
+  auto vh1 = SpyHash(v1);
+  auto vh2 = SpyHash(v2);
+  EXPECT_NE(vh1, vh2);
+}
+
+struct DummyDeleter {
+  template <typename T>
+  void operator() (T* ptr) {}
+};
+
+struct SmartPointerEq {
+  template <typename T, typename U>
+  bool operator()(const T& t, const U& u) const {
+    return GetPtr(t) == GetPtr(u);
+  }
+
+  template <typename T>
+  static auto GetPtr(const T& t) -> decltype(&*t) {
+    return t ? &*t : nullptr;
+  }
+
+  static std::nullptr_t GetPtr(std::nullptr_t) { return nullptr; }
+};
+
+TEST(HashValueTest, SmartPointers) {
+  EXPECT_TRUE((is_hashable<std::unique_ptr<int>>::value));
+  EXPECT_TRUE((is_hashable<std::unique_ptr<int, DummyDeleter>>::value));
+  EXPECT_TRUE((is_hashable<std::shared_ptr<int>>::value));
+
+  int i, j;
+  std::unique_ptr<int, DummyDeleter> unique1(&i);
+  std::unique_ptr<int, DummyDeleter> unique2(&i);
+  std::unique_ptr<int, DummyDeleter> unique_other(&j);
+  std::unique_ptr<int, DummyDeleter> unique_null;
+
+  std::shared_ptr<int> shared1(&i, DummyDeleter());
+  std::shared_ptr<int> shared2(&i, DummyDeleter());
+  std::shared_ptr<int> shared_other(&j, DummyDeleter());
+  std::shared_ptr<int> shared_null;
+
+  // Sanity check of the Eq function.
+  ASSERT_TRUE(SmartPointerEq{}(unique1, shared1));
+  ASSERT_FALSE(SmartPointerEq{}(unique1, shared_other));
+  ASSERT_TRUE(SmartPointerEq{}(unique_null, nullptr));
+  ASSERT_FALSE(SmartPointerEq{}(shared2, nullptr));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::forward_as_tuple(&i, nullptr,                    //
+                            unique1, unique2, unique_null,  //
+                            absl::make_unique<int>(),       //
+                            shared1, shared2, shared_null,  //
+                            std::make_shared<int>()),
+      SmartPointerEq{}));
+}
+
+TEST(HashValueTest, FunctionPointer) {
+  using Func = int (*)();
+  EXPECT_TRUE(is_hashable<Func>::value);
+
+  Func p1 = [] { return 2; }, p2 = [] { return 1; };
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(p1, p2, nullptr)));
+}
+
+struct WrapInTuple {
+  template <typename T>
+  std::tuple<int, T, size_t> operator()(const T& t) const {
+    return std::make_tuple(7, t, 0xdeadbeef);
+  }
+};
+
+absl::Cord FlatCord(absl::string_view sv) {
+  absl::Cord c(sv);
+  c.Flatten();
+  return c;
+}
+
+absl::Cord FragmentedCord(absl::string_view sv) {
+  if (sv.size() < 2) {
+    return absl::Cord(sv);
+  }
+  size_t halfway = sv.size() / 2;
+  std::vector<absl::string_view> parts = {sv.substr(0, halfway),
+                                          sv.substr(halfway)};
+  return absl::MakeFragmentedCord(parts);
+}
+
+TEST(HashValueTest, Strings) {
+  EXPECT_TRUE((is_hashable<std::string>::value));
+
+  const std::string small = "foo";
+  const std::string dup = "foofoo";
+  const std::string large = std::string(2048, 'x');  // multiple of chunk size
+  const std::string huge = std::string(5000, 'a');   // not a multiple
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(  //
+      std::string(), absl::string_view(), absl::Cord(),                     //
+      std::string(""), absl::string_view(""), absl::Cord(""),               //
+      std::string(small), absl::string_view(small), absl::Cord(small),      //
+      std::string(dup), absl::string_view(dup), absl::Cord(dup),            //
+      std::string(large), absl::string_view(large), absl::Cord(large),      //
+      std::string(huge), absl::string_view(huge), FlatCord(huge),           //
+      FragmentedCord(huge))));
+
+  // Also check that nested types maintain the same hash.
+  const WrapInTuple t{};
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(  //
+      t(std::string()), t(absl::string_view()), t(absl::Cord()),            //
+      t(std::string("")), t(absl::string_view("")), t(absl::Cord("")),      //
+      t(std::string(small)), t(absl::string_view(small)),                   //
+          t(absl::Cord(small)),                                             //
+      t(std::string(dup)), t(absl::string_view(dup)), t(absl::Cord(dup)),   //
+      t(std::string(large)), t(absl::string_view(large)),                   //
+          t(absl::Cord(large)),                                             //
+      t(std::string(huge)), t(absl::string_view(huge)),                     //
+          t(FlatCord(huge)), t(FragmentedCord(huge)))));
+
+  // Make sure that hashing a `const char*` does not use its string-value.
+  EXPECT_NE(SpyHash(static_cast<const char*>("ABC")),
+            SpyHash(absl::string_view("ABC")));
+}
+
+TEST(HashValueTest, WString) {
+  EXPECT_TRUE((is_hashable<std::wstring>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::wstring(), std::wstring(L"ABC"), std::wstring(L"ABC"),
+      std::wstring(L"Some other different string"),
+      std::wstring(L"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, U16String) {
+  EXPECT_TRUE((is_hashable<std::u16string>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::u16string(), std::u16string(u"ABC"), std::u16string(u"ABC"),
+      std::u16string(u"Some other different string"),
+      std::u16string(u"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, U32String) {
+  EXPECT_TRUE((is_hashable<std::u32string>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::u32string(), std::u32string(U"ABC"), std::u32string(U"ABC"),
+      std::u32string(U"Some other different string"),
+      std::u32string(U"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, StdArray) {
+  EXPECT_TRUE((is_hashable<std::array<int, 3>>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(std::array<int, 3>{}, std::array<int, 3>{{0, 23, 42}})));
+}
+
+TEST(HashValueTest, StdBitset) {
+  EXPECT_TRUE((is_hashable<std::bitset<257>>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      {std::bitset<2>("00"), std::bitset<2>("01"), std::bitset<2>("10"),
+       std::bitset<2>("11")}));
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      {std::bitset<5>("10101"), std::bitset<5>("10001"), std::bitset<5>()}));
+
+  constexpr int kNumBits = 256;
+  std::array<std::string, 6> bit_strings;
+  bit_strings.fill(std::string(kNumBits, '1'));
+  bit_strings[1][0] = '0';
+  bit_strings[2][1] = '0';
+  bit_strings[3][kNumBits / 3] = '0';
+  bit_strings[4][kNumBits - 2] = '0';
+  bit_strings[5][kNumBits - 1] = '0';
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      {std::bitset<kNumBits>(bit_strings[0].c_str()),
+       std::bitset<kNumBits>(bit_strings[1].c_str()),
+       std::bitset<kNumBits>(bit_strings[2].c_str()),
+       std::bitset<kNumBits>(bit_strings[3].c_str()),
+       std::bitset<kNumBits>(bit_strings[4].c_str()),
+       std::bitset<kNumBits>(bit_strings[5].c_str())}));
+}  // namespace
+
+template <typename T>
+class HashValueSequenceTest : public testing::Test {
+};
+TYPED_TEST_SUITE_P(HashValueSequenceTest);
+
+TYPED_TEST_P(HashValueSequenceTest, BasicUsage) {
+  EXPECT_TRUE((is_hashable<TypeParam>::value));
+
+  using ValueType = typename TypeParam::value_type;
+  auto a = static_cast<ValueType>(0);
+  auto b = static_cast<ValueType>(23);
+  auto c = static_cast<ValueType>(42);
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(TypeParam(), TypeParam{}, TypeParam{a, b, c},
+                      TypeParam{a, b}, TypeParam{b, c})));
+}
+
+REGISTER_TYPED_TEST_CASE_P(HashValueSequenceTest, BasicUsage);
+using IntSequenceTypes =
+    testing::Types<std::deque<int>, std::forward_list<int>, std::list<int>,
+                   std::vector<int>, std::vector<bool>, std::set<int>,
+                   std::multiset<int>>;
+INSTANTIATE_TYPED_TEST_CASE_P(My, HashValueSequenceTest, IntSequenceTypes);
+
+// Private type that only supports AbslHashValue to make sure our chosen hash
+// implementation is recursive within absl::Hash.
+// It uses std::abs() on the value to provide different bitwise representations
+// of the same logical value.
+struct Private {
+  int i;
+  template <typename H>
+  friend H AbslHashValue(H h, Private p) {
+    return H::combine(std::move(h), std::abs(p.i));
+  }
+
+  friend bool operator==(Private a, Private b) {
+    return std::abs(a.i) == std::abs(b.i);
+  }
+
+  friend std::ostream& operator<<(std::ostream& o, Private p) {
+    return o << p.i;
+  }
+};
+
+// Test helper for combine_piecewise_buffer.  It holds a string_view to the
+// buffer-to-be-hashed.  Its AbslHashValue specialization will split up its
+// contents at the character offsets requested.
+class PiecewiseHashTester {
+ public:
+  // Create a hash view of a buffer to be hashed contiguously.
+  explicit PiecewiseHashTester(absl::string_view buf)
+      : buf_(buf), piecewise_(false), split_locations_() {}
+
+  // Create a hash view of a buffer to be hashed piecewise, with breaks at the
+  // given locations.
+  PiecewiseHashTester(absl::string_view buf, std::set<size_t> split_locations)
+      : buf_(buf),
+        piecewise_(true),
+        split_locations_(std::move(split_locations)) {}
+
+  template <typename H>
+  friend H AbslHashValue(H h, const PiecewiseHashTester& p) {
+    if (!p.piecewise_) {
+      return H::combine_contiguous(std::move(h), p.buf_.data(), p.buf_.size());
+    }
+    absl::hash_internal::PiecewiseCombiner combiner;
+    if (p.split_locations_.empty()) {
+      h = combiner.add_buffer(std::move(h), p.buf_.data(), p.buf_.size());
+      return combiner.finalize(std::move(h));
+    }
+    size_t begin = 0;
+    for (size_t next : p.split_locations_) {
+      absl::string_view chunk = p.buf_.substr(begin, next - begin);
+      h = combiner.add_buffer(std::move(h), chunk.data(), chunk.size());
+      begin = next;
+    }
+    absl::string_view last_chunk = p.buf_.substr(begin);
+    if (!last_chunk.empty()) {
+      h = combiner.add_buffer(std::move(h), last_chunk.data(),
+                              last_chunk.size());
+    }
+    return combiner.finalize(std::move(h));
+  }
+
+ private:
+  absl::string_view buf_;
+  bool piecewise_;
+  std::set<size_t> split_locations_;
+};
+
+// Dummy object that hashes as two distinct contiguous buffers, "foo" followed
+// by "bar"
+struct DummyFooBar {
+  template <typename H>
+  friend H AbslHashValue(H h, const DummyFooBar&) {
+    const char* foo = "foo";
+    const char* bar = "bar";
+    h = H::combine_contiguous(std::move(h), foo, 3);
+    h = H::combine_contiguous(std::move(h), bar, 3);
+    return h;
+  }
+};
+
+TEST(HashValueTest, CombinePiecewiseBuffer) {
+  absl::Hash<PiecewiseHashTester> hash;
+
+  // Check that hashing an empty buffer through the piecewise API works.
+  EXPECT_EQ(hash(PiecewiseHashTester("")), hash(PiecewiseHashTester("", {})));
+
+  // Similarly, small buffers should give consistent results
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {})));
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {3})));
+
+  // But hashing "foobar" in pieces gives a different answer than hashing "foo"
+  // contiguously, then "bar" contiguously.
+  EXPECT_NE(hash(PiecewiseHashTester("foobar", {3})),
+            absl::Hash<DummyFooBar>()(DummyFooBar{}));
+
+  // Test hashing a large buffer incrementally, broken up in several different
+  // ways.  Arrange for breaks on and near the stride boundaries to look for
+  // off-by-one errors in the implementation.
+  //
+  // This test is run on a buffer that is a multiple of the stride size, and one
+  // that isn't.
+  for (size_t big_buffer_size : {1024 * 2 + 512, 1024 * 3}) {
+    SCOPED_TRACE(big_buffer_size);
+    std::string big_buffer;
+    for (int i = 0; i < big_buffer_size; ++i) {
+      // Arbitrary string
+      big_buffer.push_back(32 + (i * (i / 3)) % 64);
+    }
+    auto big_buffer_hash = hash(PiecewiseHashTester(big_buffer));
+
+    const int possible_breaks = 9;
+    size_t breaks[possible_breaks] = {1,    512,  1023, 1024, 1025,
+                                      1536, 2047, 2048, 2049};
+    for (unsigned test_mask = 0; test_mask < (1u << possible_breaks);
+         ++test_mask) {
+      SCOPED_TRACE(test_mask);
+      std::set<size_t> break_locations;
+      for (int j = 0; j < possible_breaks; ++j) {
+        if (test_mask & (1u << j)) {
+          break_locations.insert(breaks[j]);
+        }
+      }
+      EXPECT_EQ(
+          hash(PiecewiseHashTester(big_buffer, std::move(break_locations))),
+          big_buffer_hash);
+    }
+  }
+}
+
+TEST(HashValueTest, PrivateSanity) {
+  // Sanity check that Private is working as the tests below expect it to work.
+  EXPECT_TRUE(is_hashable<Private>::value);
+  EXPECT_NE(SpyHash(Private{0}), SpyHash(Private{1}));
+  EXPECT_EQ(SpyHash(Private{1}), SpyHash(Private{1}));
+}
+
+TEST(HashValueTest, Optional) {
+  EXPECT_TRUE(is_hashable<absl::optional<Private>>::value);
+
+  using O = absl::optional<Private>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(O{}, O{{1}}, O{{-1}}, O{{10}})));
+}
+
+TEST(HashValueTest, Variant) {
+  using V = absl::variant<Private, std::string>;
+  EXPECT_TRUE(is_hashable<V>::value);
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      V(Private{1}), V(Private{-1}), V(Private{2}), V("ABC"), V("BCD"))));
+
+#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+  struct S {};
+  EXPECT_FALSE(is_hashable<absl::variant<S>>::value);
+#endif
+}
+
+TEST(HashValueTest, Maps) {
+  EXPECT_TRUE((is_hashable<std::map<int, std::string>>::value));
+
+  using M = std::map<int, std::string>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      M{}, M{{0, "foo"}}, M{{1, "foo"}}, M{{0, "bar"}}, M{{1, "bar"}},
+      M{{0, "foo"}, {42, "bar"}}, M{{1, "foo"}, {42, "bar"}},
+      M{{1, "foo"}, {43, "bar"}}, M{{1, "foo"}, {43, "baz"}})));
+
+  using MM = std::multimap<int, std::string>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      MM{}, MM{{0, "foo"}}, MM{{1, "foo"}}, MM{{0, "bar"}}, MM{{1, "bar"}},
+      MM{{0, "foo"}, {0, "bar"}}, MM{{0, "bar"}, {0, "foo"}},
+      MM{{0, "foo"}, {42, "bar"}}, MM{{1, "foo"}, {42, "bar"}},
+      MM{{1, "foo"}, {1, "foo"}, {43, "bar"}}, MM{{1, "foo"}, {43, "baz"}})));
+}
+
+TEST(HashValueTest, ReferenceWrapper) {
+  EXPECT_TRUE(is_hashable<std::reference_wrapper<Private>>::value);
+
+  Private p1{1}, p10{10};
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      p1, p10, std::ref(p1), std::ref(p10), std::cref(p1), std::cref(p10))));
+
+  EXPECT_TRUE(is_hashable<std::reference_wrapper<int>>::value);
+  int one = 1, ten = 10;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      one, ten, std::ref(one), std::ref(ten), std::cref(one), std::cref(ten))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(std::tuple<std::reference_wrapper<int>>(std::ref(one)),
+                      std::tuple<std::reference_wrapper<int>>(std::ref(ten)),
+                      std::tuple<int>(one), std::tuple<int>(ten))));
+}
+
+template <typename T, typename = void>
+struct IsHashCallable : std::false_type {};
+
+template <typename T>
+struct IsHashCallable<T, absl::void_t<decltype(std::declval<absl::Hash<T>>()(
+                            std::declval<const T&>()))>> : std::true_type {};
+
+template <typename T, typename = void>
+struct IsAggregateInitializable : std::false_type {};
+
+template <typename T>
+struct IsAggregateInitializable<T, absl::void_t<decltype(T{})>>
+    : std::true_type {};
+
+TEST(IsHashableTest, ValidHash) {
+  EXPECT_TRUE((is_hashable<int>::value));
+  EXPECT_TRUE(std::is_default_constructible<absl::Hash<int>>::value);
+  EXPECT_TRUE(std::is_copy_constructible<absl::Hash<int>>::value);
+  EXPECT_TRUE(std::is_move_constructible<absl::Hash<int>>::value);
+  EXPECT_TRUE(absl::is_copy_assignable<absl::Hash<int>>::value);
+  EXPECT_TRUE(absl::is_move_assignable<absl::Hash<int>>::value);
+  EXPECT_TRUE(IsHashCallable<int>::value);
+  EXPECT_TRUE(IsAggregateInitializable<absl::Hash<int>>::value);
+}
+
+#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+TEST(IsHashableTest, PoisonHash) {
+  struct X {};
+  EXPECT_FALSE((is_hashable<X>::value));
+  EXPECT_FALSE(std::is_default_constructible<absl::Hash<X>>::value);
+  EXPECT_FALSE(std::is_copy_constructible<absl::Hash<X>>::value);
+  EXPECT_FALSE(std::is_move_constructible<absl::Hash<X>>::value);
+  EXPECT_FALSE(absl::is_copy_assignable<absl::Hash<X>>::value);
+  EXPECT_FALSE(absl::is_move_assignable<absl::Hash<X>>::value);
+  EXPECT_FALSE(IsHashCallable<X>::value);
+#if !defined(__GNUC__) || __GNUC__ < 9
+  // This doesn't compile on GCC 9.
+  EXPECT_FALSE(IsAggregateInitializable<absl::Hash<X>>::value);
+#endif
+}
+#endif  // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+
+// Hashable types
+//
+// These types exist simply to exercise various AbslHashValue behaviors, so
+// they are named by what their AbslHashValue overload does.
+struct NoOp {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, NoOp n) {
+    return h;
+  }
+};
+
+struct EmptyCombine {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, EmptyCombine e) {
+    return HashCode::combine(std::move(h));
+  }
+};
+
+template <typename Int>
+struct CombineIterative {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, CombineIterative c) {
+    for (int i = 0; i < 5; ++i) {
+      h = HashCode::combine(std::move(h), Int(i));
+    }
+    return h;
+  }
+};
+
+template <typename Int>
+struct CombineVariadic {
+  template <typename HashCode>
+  friend HashCode AbslHashValue(HashCode h, CombineVariadic c) {
+    return HashCode::combine(std::move(h), Int(0), Int(1), Int(2), Int(3),
+                             Int(4));
+  }
+};
+enum class InvokeTag {
+  kUniquelyRepresented,
+  kHashValue,
+#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+  kLegacyHash,
+#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+  kStdHash,
+  kNone
+};
+
+template <InvokeTag T>
+using InvokeTagConstant = std::integral_constant<InvokeTag, T>;
+
+template <InvokeTag... Tags>
+struct MinTag;
+
+template <InvokeTag a, InvokeTag b, InvokeTag... Tags>
+struct MinTag<a, b, Tags...> : MinTag<(a < b ? a : b), Tags...> {};
+
+template <InvokeTag a>
+struct MinTag<a> : InvokeTagConstant<a> {};
+
+template <InvokeTag... Tags>
+struct CustomHashType {
+  explicit CustomHashType(size_t val) : value(val) {}
+  size_t value;
+};
+
+template <InvokeTag allowed, InvokeTag... tags>
+struct EnableIfContained
+    : std::enable_if<absl::disjunction<
+          std::integral_constant<bool, allowed == tags>...>::value> {};
+
+template <
+    typename H, InvokeTag... Tags,
+    typename = typename EnableIfContained<InvokeTag::kHashValue, Tags...>::type>
+H AbslHashValue(H state, CustomHashType<Tags...> t) {
+  static_assert(MinTag<Tags...>::value == InvokeTag::kHashValue, "");
+  return H::combine(std::move(state),
+                    t.value + static_cast<int>(InvokeTag::kHashValue));
+}
+
+}  // namespace
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace hash_internal {
+template <InvokeTag... Tags>
+struct is_uniquely_represented<
+    CustomHashType<Tags...>,
+    typename EnableIfContained<InvokeTag::kUniquelyRepresented, Tags...>::type>
+    : std::true_type {};
+}  // namespace hash_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+namespace ABSL_INTERNAL_LEGACY_HASH_NAMESPACE {
+template <InvokeTag... Tags>
+struct hash<CustomHashType<Tags...>> {
+  template <InvokeTag... TagsIn, typename = typename EnableIfContained<
+                                     InvokeTag::kLegacyHash, TagsIn...>::type>
+  size_t operator()(CustomHashType<TagsIn...> t) const {
+    static_assert(MinTag<Tags...>::value == InvokeTag::kLegacyHash, "");
+    return t.value + static_cast<int>(InvokeTag::kLegacyHash);
+  }
+};
+}  // namespace ABSL_INTERNAL_LEGACY_HASH_NAMESPACE
+#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+
+namespace std {
+template <InvokeTag... Tags>  // NOLINT
+struct hash<CustomHashType<Tags...>> {
+  template <InvokeTag... TagsIn, typename = typename EnableIfContained<
+                                     InvokeTag::kStdHash, TagsIn...>::type>
+  size_t operator()(CustomHashType<TagsIn...> t) const {
+    static_assert(MinTag<Tags...>::value == InvokeTag::kStdHash, "");
+    return t.value + static_cast<int>(InvokeTag::kStdHash);
+  }
+};
+}  // namespace std
+
+namespace {
+
+template <typename... T>
+void TestCustomHashType(InvokeTagConstant<InvokeTag::kNone>, T...) {
+  using type = CustomHashType<T::value...>;
+  SCOPED_TRACE(testing::PrintToString(std::vector<InvokeTag>{T::value...}));
+  EXPECT_TRUE(is_hashable<type>());
+  EXPECT_TRUE(is_hashable<const type>());
+  EXPECT_TRUE(is_hashable<const type&>());
+
+  const size_t offset = static_cast<int>(std::min({T::value...}));
+  EXPECT_EQ(SpyHash(type(7)), SpyHash(size_t{7 + offset}));
+}
+
+void TestCustomHashType(InvokeTagConstant<InvokeTag::kNone>) {
+#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+  // is_hashable is false if we don't support any of the hooks.
+  using type = CustomHashType<>;
+  EXPECT_FALSE(is_hashable<type>());
+  EXPECT_FALSE(is_hashable<const type>());
+  EXPECT_FALSE(is_hashable<const type&>());
+#endif  // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+}
+
+template <InvokeTag Tag, typename... T>
+void TestCustomHashType(InvokeTagConstant<Tag> tag, T... t) {
+  constexpr auto next = static_cast<InvokeTag>(static_cast<int>(Tag) + 1);
+  TestCustomHashType(InvokeTagConstant<next>(), tag, t...);
+  TestCustomHashType(InvokeTagConstant<next>(), t...);
+}
+
+TEST(HashTest, CustomHashType) {
+  TestCustomHashType(InvokeTagConstant<InvokeTag{}>());
+}
+
+TEST(HashTest, NoOpsAreEquivalent) {
+  EXPECT_EQ(Hash<NoOp>()({}), Hash<NoOp>()({}));
+  EXPECT_EQ(Hash<NoOp>()({}), Hash<EmptyCombine>()({}));
+}
+
+template <typename T>
+class HashIntTest : public testing::Test {
+};
+TYPED_TEST_SUITE_P(HashIntTest);
+
+TYPED_TEST_P(HashIntTest, BasicUsage) {
+  EXPECT_NE(Hash<NoOp>()({}), Hash<TypeParam>()(0));
+  EXPECT_NE(Hash<NoOp>()({}),
+            Hash<TypeParam>()(std::numeric_limits<TypeParam>::max()));
+  if (std::numeric_limits<TypeParam>::min() != 0) {
+    EXPECT_NE(Hash<NoOp>()({}),
+              Hash<TypeParam>()(std::numeric_limits<TypeParam>::min()));
+  }
+
+  EXPECT_EQ(Hash<CombineIterative<TypeParam>>()({}),
+            Hash<CombineVariadic<TypeParam>>()({}));
+}
+
+REGISTER_TYPED_TEST_CASE_P(HashIntTest, BasicUsage);
+using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t, uint32_t,
+                                uint64_t, size_t>;
+INSTANTIATE_TYPED_TEST_CASE_P(My, HashIntTest, IntTypes);
+
+struct StructWithPadding {
+  char c;
+  int i;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const StructWithPadding& s) {
+    return H::combine(std::move(hash_state), s.c, s.i);
+  }
+};
+
+static_assert(sizeof(StructWithPadding) > sizeof(char) + sizeof(int),
+              "StructWithPadding doesn't have padding");
+static_assert(std::is_standard_layout<StructWithPadding>::value, "");
+
+// This check has to be disabled because libstdc++ doesn't support it.
+// static_assert(std::is_trivially_constructible<StructWithPadding>::value, "");
+
+template <typename T>
+struct ArraySlice {
+  T* begin;
+  T* end;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const ArraySlice& slice) {
+    for (auto t = slice.begin; t != slice.end; ++t) {
+      hash_state = H::combine(std::move(hash_state), *t);
+    }
+    return hash_state;
+  }
+};
+
+TEST(HashTest, HashNonUniquelyRepresentedType) {
+  // Create equal StructWithPadding objects that are known to have non-equal
+  // padding bytes.
+  static const size_t kNumStructs = 10;
+  unsigned char buffer1[kNumStructs * sizeof(StructWithPadding)];
+  std::memset(buffer1, 0, sizeof(buffer1));
+  auto* s1 = reinterpret_cast<StructWithPadding*>(buffer1);
+
+  unsigned char buffer2[kNumStructs * sizeof(StructWithPadding)];
+  std::memset(buffer2, 255, sizeof(buffer2));
+  auto* s2 = reinterpret_cast<StructWithPadding*>(buffer2);
+  for (int i = 0; i < kNumStructs; ++i) {
+    SCOPED_TRACE(i);
+    s1[i].c = s2[i].c = '0' + i;
+    s1[i].i = s2[i].i = i;
+    ASSERT_FALSE(memcmp(buffer1 + i * sizeof(StructWithPadding),
+                        buffer2 + i * sizeof(StructWithPadding),
+                        sizeof(StructWithPadding)) == 0)
+        << "Bug in test code: objects do not have unequal"
+        << " object representations";
+  }
+
+  EXPECT_EQ(Hash<StructWithPadding>()(s1[0]), Hash<StructWithPadding>()(s2[0]));
+  EXPECT_EQ(Hash<ArraySlice<StructWithPadding>>()({s1, s1 + kNumStructs}),
+            Hash<ArraySlice<StructWithPadding>>()({s2, s2 + kNumStructs}));
+}
+
+TEST(HashTest, StandardHashContainerUsage) {
+  std::unordered_map<int, std::string, Hash<int>> map = {{0, "foo"},
+                                                         {42, "bar"}};
+
+  EXPECT_NE(map.find(0), map.end());
+  EXPECT_EQ(map.find(1), map.end());
+  EXPECT_NE(map.find(0u), map.end());
+}
+
+struct ConvertibleFromNoOp {
+  ConvertibleFromNoOp(NoOp) {}  // NOLINT(runtime/explicit)
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, ConvertibleFromNoOp) {
+    return H::combine(std::move(hash_state), 1);
+  }
+};
+
+TEST(HashTest, HeterogeneousCall) {
+  EXPECT_NE(Hash<ConvertibleFromNoOp>()(NoOp()),
+            Hash<NoOp>()(NoOp()));
+}
+
+TEST(IsUniquelyRepresentedTest, SanityTest) {
+  using absl::hash_internal::is_uniquely_represented;
+
+  EXPECT_TRUE(is_uniquely_represented<unsigned char>::value);
+  EXPECT_TRUE(is_uniquely_represented<int>::value);
+  EXPECT_FALSE(is_uniquely_represented<bool>::value);
+  EXPECT_FALSE(is_uniquely_represented<int*>::value);
+}
+
+struct IntAndString {
+  int i;
+  std::string s;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, IntAndString int_and_string) {
+    return H::combine(std::move(hash_state), int_and_string.s,
+                      int_and_string.i);
+  }
+};
+
+TEST(HashTest, SmallValueOn64ByteBoundary) {
+  Hash<IntAndString>()(IntAndString{0, std::string(63, '0')});
+}
+
+struct TypeErased {
+  size_t n;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const TypeErased& v) {
+    v.HashValue(absl::HashState::Create(&hash_state));
+    return hash_state;
+  }
+
+  void HashValue(absl::HashState state) const {
+    absl::HashState::combine(std::move(state), n);
+  }
+};
+
+TEST(HashTest, TypeErased) {
+  EXPECT_TRUE((is_hashable<TypeErased>::value));
+  EXPECT_TRUE((is_hashable<std::pair<TypeErased, int>>::value));
+
+  EXPECT_EQ(SpyHash(TypeErased{7}), SpyHash(size_t{7}));
+  EXPECT_NE(SpyHash(TypeErased{7}), SpyHash(size_t{13}));
+
+  EXPECT_EQ(SpyHash(std::make_pair(TypeErased{7}, 17)),
+            SpyHash(std::make_pair(size_t{7}, 17)));
+}
+
+struct ValueWithBoolConversion {
+  operator bool() const { return false; }
+  int i;
+};
+
+}  // namespace
+namespace std {
+template <>
+struct hash<ValueWithBoolConversion> {
+  size_t operator()(ValueWithBoolConversion v) { return v.i; }
+};
+}  // namespace std
+
+namespace {
+
+TEST(HashTest, DoesNotUseImplicitConversionsToBool) {
+  EXPECT_NE(absl::Hash<ValueWithBoolConversion>()(ValueWithBoolConversion{0}),
+            absl::Hash<ValueWithBoolConversion>()(ValueWithBoolConversion{1}));
+}
+
+}  // namespace