[libcxx-commits] [libcxx] [libc++] Make std::pair trivially copyable if its members are (PR #89652)

[Louis Dionne via libcxx-commits]

================
@@ -74,6 +75,31 @@ struct _LIBCPP_TEMPLATE_VIS pair
   _LIBCPP_HIDE_FROM_ABI pair(pair const&) = default;
   _LIBCPP_HIDE_FROM_ABI pair(pair&&)      = default;
 
+  // Make pair trivially copyable if we have a way to do it
+  static const bool __enable_defaulted_assignment_operators =
+      !is_reference<first_type>::value && !is_reference<second_type>::value;
+#if _LIBCPP_STD_VER >= 20 && defined(_LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR)
+  static const bool __has_defaulted_members = __enable_defaulted_assignment_operators;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(const pair&)
+    requires __enable_defaulted_assignment_operators
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(pair&&)
+    requires __enable_defaulted_assignment_operators
+  = default;
+#elif __has_attribute(__enable_if__) && defined(_LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR)
----------------
ldionne wrote:

```suggestion
#elif defined(_LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR) && __has_attribute(__enable_if__)
```

https://github.com/llvm/llvm-project/pull/89652