[libcxx-commits] [libcxx] [libc++] LWG 3821 uses_allocator_construction_args should have overload for pair-like (PR #66939)

[Louis Dionne via libcxx-commits]

================
@@ -139,23 +157,33 @@ template <class _Tp>
 inline constexpr bool __convertible_to_const_pair_ref =
     decltype(__uses_allocator_detail::__convertible_to_const_pair_ref_impl<_Tp>(0))::value;
 
+#  if _LIBCPP_STD_VER >= 23
+template <class _Tp, class _Up>
+inline constexpr bool __uses_allocator_constraints =
+    __is_cv_std_pair<_Tp> &&
+    (__is_specialization_of_subrange<remove_cvref_t<_Up>>::value ||
+     (!__pair_like<_Up> && !__convertible_to_const_pair_ref<_Up>));
+#  else
+template <class _Tp, class _Up>
+inline constexpr bool __uses_allocator_constraints = __is_cv_std_pair<_Tp> && !__convertible_to_const_pair_ref<_Up>;
+#  endif
+
 } // namespace __uses_allocator_detail
 
-template <
-    class _Pair,
-    class _Alloc,
-    class _Type,
-    __enable_if_t<__is_std_pair<_Pair> && !__uses_allocator_detail::__convertible_to_const_pair_ref<_Type>, int> = 0>
+template < class _Pair,
+           class _Alloc,
+           class _Type,
+           __enable_if_t<__uses_allocator_detail::__uses_allocator_constraints<_Pair, _Type>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI constexpr auto
 __uses_allocator_construction_args(const _Alloc& __alloc, _Type&& __value) noexcept;
 
 template <class _Type, class _Alloc, class... _Args>
 _LIBCPP_HIDE_FROM_ABI constexpr _Type __make_obj_using_allocator(const _Alloc& __alloc, _Args&&... __args);
 
-template <class _Pair,
-          class _Alloc,
-          class _Type,
-          __enable_if_t<__is_std_pair<_Pair> && !__uses_allocator_detail::__convertible_to_const_pair_ref<_Type>, int>>
+template < class _Pair,
+           class _Alloc,
+           class _Type,
+           __enable_if_t< __uses_allocator_detail::__uses_allocator_constraints<_Pair, _Type>, int>>
----------------
ldionne wrote:

```suggestion
           __enable_if_t< __uses_allocator_detail::__uses_allocator_constraints<_Pair, _Type>, int> = 0>
```

https://github.com/llvm/llvm-project/pull/66939