Forward declare `uses_allocator_construction_args` (#2023)

Co-authored-by: Casey Carter <cartec69@gmail.com>

stl/inc/xmemory

@@ -2115,6 +2115,18 @@ _NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al, _T
     }
 }
 
+template <class _Ty, class _Alloc, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+_NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al) noexcept;
+
+template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+_NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al, _Uty1&& _Val1, _Uty2&& _Val2) noexcept;
+
+template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+_NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al, const pair<_Uty1, _Uty2>& _Pair) noexcept;
+
+template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+_NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al, pair<_Uty1, _Uty2>&& _Pair) noexcept;
+
 template <class _Ty, class _Alloc, class _Tuple1, class _Tuple2, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
 _NODISCARD constexpr auto uses_allocator_construction_args(
     const _Alloc& _Al, piecewise_construct_t, _Tuple1&& _Tup1, _Tuple2&& _Tup2) noexcept {
@@ -2133,7 +2145,7 @@ _NODISCARD constexpr auto uses_allocator_construction_args(
             _STD forward<_Tuple2>(_Tup2)));
 }
 
-template <class _Ty, class _Alloc, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+template <class _Ty, class _Alloc, enable_if_t<_Is_specialization_v<_Ty, pair>, int>>
 _NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al) noexcept {
     // equivalent to
     // return _STD uses_allocator_construction_args<_Ty>(_Al, piecewise_construct, tuple<>{}, tuple<>{});
@@ -2141,7 +2153,7 @@ _NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al) no
         _STD uses_allocator_construction_args<typename _Ty::second_type>(_Al));
 }
 
-template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int>>
 _NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al, _Uty1&& _Val1, _Uty2&& _Val2) noexcept {
     // equivalent to
     // return _STD uses_allocator_construction_args<_Ty>(_Al, piecewise_construct,
@@ -2151,7 +2163,7 @@ _NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al, _U
         _STD uses_allocator_construction_args<typename _Ty::second_type>(_Al, _STD forward<_Uty2>(_Val2)));
 }
 
-template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int>>
 _NODISCARD constexpr auto uses_allocator_construction_args(
     const _Alloc& _Al, const pair<_Uty1, _Uty2>& _Pair) noexcept {
     // equivalent to
@@ -2162,7 +2174,7 @@ _NODISCARD constexpr auto uses_allocator_construction_args(
         _STD uses_allocator_construction_args<typename _Ty::second_type>(_Al, _Pair.second));
 }
 
-template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int> = 0>
+template <class _Ty, class _Alloc, class _Uty1, class _Uty2, enable_if_t<_Is_specialization_v<_Ty, pair>, int>>
 _NODISCARD constexpr auto uses_allocator_construction_args(const _Alloc& _Al, pair<_Uty1, _Uty2>&& _Pair) noexcept {
     // equivalent to
     // return _STD uses_allocator_construction_args<_Ty>(_Al, piecewise_construct,

tests/std/tests/P0475R1_P0591R4_uses_allocator_construction/test.cpp

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <cassert>
+#include <map>
 #include <memory>
 #include <scoped_allocator>
 #include <tuple>
@@ -173,6 +174,11 @@ constexpr bool test_P0591R4() {
     return true;
 }
 
+void test_GH_2021() { // COMPILE-ONLY
+    pmr::map<int, pair<int, int>> tags;
+    tags[0];
+}
+
 int main() {
     test_P0475R1();