Export of internal Abseil changes

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dc36565216b3022abb5a3c92f46fb4079bda869e by Samuel Benzaquen <sbenza@google.com>:

Add support for std::reference_wrapper to absl::Hash.

PiperOrigin-RevId: 311733171
GitOrigin-RevId: dc36565216b3022abb5a3c92f46fb4079bda869e
Change-Id: I74543390446c2b7f6d856935aa36f28539810eef

absl/hash/hash_test.cc

@@ -581,6 +581,24 @@ TEST(HashValueTest, Maps) {
       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 {};
 

absl/hash/internal/hash.h

@@ -551,6 +551,13 @@ typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
 // AbslHashValue for Wrapper Types
 // -----------------------------------------------------------------------------
 
+// AbslHashValue for hashing std::reference_wrapper
+template <typename H, typename T>
+typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+    H hash_state, std::reference_wrapper<T> opt) {
+  return H::combine(std::move(hash_state), opt.get());
+}
+
 // AbslHashValue for hashing absl::optional
 template <typename H, typename T>
 typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(