Define combine_weakly_mixed_integer in HashSelect::State in order to allow friend auto AbslHashValue instead of friend H AbslHashValue.

PiperOrigin-RevId: 750259237
Change-Id: Icb1c9427537dbabb55f971d3f8acdeb0be37b42c

Author: goldvitaly

absl/hash/hash_test.cc

@@ -1223,4 +1223,20 @@ TEST(HashOf, MatchesTypeErasedHashState) {
                                     TypeErasedHashStateUser{1, s}));
 }
 
+struct AutoReturnTypeUser {
+  int a;
+  std::string b;
+
+  template <typename H>
+  friend auto AbslHashValue(H state, const AutoReturnTypeUser& value) {
+    return H::combine(std::move(state), value.a, value.b);
+  }
+};
+
+TEST(HashOf, AutoReturnTypeUser) {
+  std::string s = "s";
+  EXPECT_EQ(absl::HashOf(1, s),
+            absl::Hash<AutoReturnTypeUser>{}(AutoReturnTypeUser{1, s}));
+}
+
 }  // namespace

absl/hash/internal/hash.h

@@ -988,6 +988,8 @@ struct HashSelect {
                                     size_t);
     using State::HashStateBase::combine_contiguous;
     static State combine_raw(State state, uint64_t value);
+    static State combine_weakly_mixed_integer(State hash_state,
+                                              WeaklyMixedInteger value);
   };
 
   struct UniquelyRepresentedProbe {