Implement Smart Pointer Creation With Default Initialization

stl/inc/xutility

@@ -119,11 +119,29 @@ struct _Get_rebind_alias<_Ty, _Other, void_t<typename _Ty::template rebind<_Othe
     using type = typename _Ty::template rebind<_Other>;
 };
 
+// FUNCTION TEMPLATE _Voidify_iter
+template <class _Iter>
+_NODISCARD void* _Voidify_iter(_Iter _It) noexcept {
+#if _HAS_IF_CONSTEXPR
+    if constexpr (is_pointer_v<_Iter>) {
+        return const_cast<void*>(static_cast<const volatile void*>(_It));
+    } else
+#endif // _HAS_IF_CONSTEXPR
+    {
+        return const_cast<void*>(static_cast<const volatile void*>(_STD addressof(*_It)));
+    }
+}
+
 // FUNCTION TEMPLATE _Construct_in_place
 template <class _Ty, class... _Types>
 void _Construct_in_place(_Ty& _Obj, _Types&&... _Args) noexcept(is_nothrow_constructible_v<_Ty, _Types...>) {
-    ::new (const_cast<void*>(static_cast<const volatile void*>(_STD addressof(_Obj))))
-        _Ty(_STD forward<_Types>(_Args)...);
+    ::new (_Voidify_iter(_STD addressof(_Obj))) _Ty(_STD forward<_Types>(_Args)...);
+}
+
+// FUNCTION TEMPLATE _Default_construct_in_place
+template <class _Ty>
+void _Default_construct_in_place(_Ty& _Obj) noexcept(is_nothrow_default_constructible_v<_Ty>) {
+    ::new (_Voidify_iter(_STD addressof(_Obj))) _Ty;
 }
 
 // STRUCT TEMPLATE pointer_traits

stl/inc/yvals_core.h

@@ -179,6 +179,7 @@
 // P1001R2 execution::unseq
 // P1006R1 constexpr For pointer_traits<T*>::pointer_to()
 // P1007R3 assume_aligned()
+// P1020R1 Smart Pointer Creation With Default Initialization
 // P1023R0 constexpr For std::array Comparisons
 // P1024R3 Enhancing span Usability
 // P1032R1 Miscellaneous constexpr
@@ -219,6 +220,7 @@
 // P1959R0 Removing weak_equality And strong_equality
 // P1960R0 atomic_ref Cleanup
 // P1964R2 Replacing boolean With boolean-testable
+// P1973R1 Renaming default_init To for_overwrite
 // P1976R2 Explicit Constructors For Fixed-Extent span From Dynamic-Extent Ranges
 // P2091R0 Fixing Issues With Range Access CPOs
 // P2102R0 Making "Implicit Expression Variations" More Explicit
@@ -1208,6 +1210,7 @@
 #define __cpp_lib_remove_cvref                 201711L
 #define __cpp_lib_semaphore                    201907L
 #define __cpp_lib_shift                        201806L
+#define __cpp_lib_smart_ptr_for_overwrite      202002L
 #define __cpp_lib_span                         202002L
 #define __cpp_lib_ssize                        201902L
 #define __cpp_lib_starts_ends_with             201711L

tests/std/test.lst

@@ -354,6 +354,7 @@ tests\P0912R5_coroutine
 tests\P0919R3_heterogeneous_unordered_lookup
 tests\P0966R1_string_reserve_should_not_shrink
 tests\P1007R3_assume_aligned
+tests\P1020R1_smart_pointer_for_overwrite
 tests\P1023R0_constexpr_for_array_comparisons
 tests\P1032R1_miscellaneous_constexpr
 tests\P1135R6_atomic_flag_test

tests/std/tests/P1020R1_smart_pointer_for_overwrite/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst