Add a workaround for std::pair not being trivially copyable in C++23
in some standard library versions

PiperOrigin-RevId: 736979189
Change-Id: I652fa421bfa6c63f37b22dd0e1e09cce6809075a

absl/container/flat_hash_map_test.cc

diff --git a/absl/container/flat_hash_map_test.cc b/absl/container/flat_hash_map_test.cc
index 10cddcf..5c83c94 100644
--- a/absl/container/flat_hash_map_test.cc
+++ b/absl/container/flat_hash_map_test.cc
@@ -115,23 +115,29 @@
 }
 
 TEST(FlatHashMap, Relocatability) {
-  static_assert(absl::is_trivially_relocatable<int>::value, "");
+  static_assert(absl::is_trivially_relocatable<int>::value);
+#if ABSL_INTERNAL_CPLUSPLUS_LANG <= 202002L
+  // std::pair is not trivially copyable in C++23 in some standard
+  // library versions.
+  // See https://github.com/llvm/llvm-project/pull/95444 for instance.
+  // container_memory.h contains a workaround so what really matters
+  // is the transfer test below.
   static_assert(
-      absl::is_trivially_relocatable<std::pair<const int, int>>::value, "");
+      absl::is_trivially_relocatable<std::pair<const int, int>>::value);
+#endif
   static_assert(
       std::is_same<decltype(absl::container_internal::FlatHashMapPolicy<
                             int, int>::transfer<std::allocator<char>>(nullptr,
                                                                       nullptr,
                                                                       nullptr)),
-                   std::true_type>::value,
-      "");
+                   std::true_type>::value);
 
-    struct NonRelocatable {
-      NonRelocatable() = default;
-      NonRelocatable(NonRelocatable&&) {}
-      NonRelocatable& operator=(NonRelocatable&&) { return *this; }
-      void* self = nullptr;
-    };
+  struct NonRelocatable {
+    NonRelocatable() = default;
+    NonRelocatable(NonRelocatable&&) {}
+    NonRelocatable& operator=(NonRelocatable&&) { return *this; }
+    void* self = nullptr;
+  };
 
   EXPECT_FALSE(absl::is_trivially_relocatable<NonRelocatable>::value);
   EXPECT_TRUE(

absl/container/internal/container_memory.h

diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
index 8df5c8e..e7ac1db 100644
--- a/absl/container/internal/container_memory.h
+++ b/absl/container/internal/container_memory.h
@@ -433,8 +433,15 @@
   template <class Allocator>
   static auto transfer(Allocator* alloc, slot_type* new_slot,
                        slot_type* old_slot) {
-    auto is_relocatable =
-        typename absl::is_trivially_relocatable<value_type>::type();
+    // This should really just be
+    // typename absl::is_trivially_relocatable<value_type>::type()
+    // but std::pair is not trivially copyable in C++23 in some standard
+    // library versions.
+    // See https://github.com/llvm/llvm-project/pull/95444 for instance.
+    auto is_relocatable = typename std::conjunction<
+        absl::is_trivially_relocatable<typename value_type::first_type>,
+        absl::is_trivially_relocatable<typename value_type::second_type>>::
+        type();
 
     emplace(new_slot);
     if (is_relocatable) {