P2590R2 Explicit Lifetime Management

stl/inc/memory

@@ -4529,6 +4529,63 @@ _NODISCARD auto inout_ptr(_SmartPtr& _Smart_ptr, _ArgsT&&... _Args) {
         return inout_ptr_t<_SmartPtr, _Pointer, _ArgsT&&...>(_Smart_ptr, _STD forward<_ArgsT>(_Args)...);
     }
 }
+
+#ifdef __cpp_lib_start_lifetime_as // TRANSITION
+template <class _Ty>
+_Ty* start_lifetime_as(void* const _Ptr) noexcept {
+#ifdef __cpp_lib_is_implicit_lifetime // TRANSITION
+    static_assert(is_implicit_lifetime_v<_Ty>, "T must be an implicit-lifetime type. (N5032 [obj.lifetime]/1)");
+#endif // ^^^ no workaround ^^^
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/1)");
+    return __builtin_start_lifetime_as<_Ty>(_Ptr);
+}
+template <class _Ty>
+const _Ty* start_lifetime_as(const void* const _Ptr) noexcept {
+#ifdef __cpp_lib_is_implicit_lifetime // TRANSITION
+    static_assert(is_implicit_lifetime_v<_Ty>, "T must be an implicit-lifetime type. (N5032 [obj.lifetime]/1)");
+#endif // ^^^ no workaround ^^^
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/1)");
+    return __builtin_start_lifetime_as<_Ty>(_Ptr);
+}
+template <class _Ty>
+volatile _Ty* start_lifetime_as(volatile void* const _Ptr) noexcept {
+#ifdef __cpp_lib_is_implicit_lifetime // TRANSITION
+    static_assert(is_implicit_lifetime_v<_Ty>, "T must be an implicit-lifetime type. (N5032 [obj.lifetime]/1)");
+#endif // ^^^ no workaround ^^^
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/1)");
+    return __builtin_start_lifetime_as<_Ty>(_Ptr);
+}
+template <class _Ty>
+const volatile _Ty* start_lifetime_as(const volatile void* const _Ptr) noexcept {
+#ifdef __cpp_lib_is_implicit_lifetime // TRANSITION
+    static_assert(is_implicit_lifetime_v<_Ty>, "T must be an implicit-lifetime type. (N5032 [obj.lifetime]/1)");
+#endif // ^^^ no workaround ^^^
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/1)");
+    return __builtin_start_lifetime_as<_Ty>(_Ptr);
+}
+
+template <class _Ty>
+_Ty* start_lifetime_as_array(void* const _Ptr, const size_t _Nx) noexcept {
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/5)");
+    return __builtin_start_lifetime_as_array<_Ty>(_Ptr, _Nx);
+}
+template <class _Ty>
+const _Ty* start_lifetime_as_array(const void* const _Ptr, const size_t _Nx) noexcept {
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/5)");
+    return __builtin_start_lifetime_as_array<_Ty>(_Ptr, _Nx);
+}
+template <class _Ty>
+volatile _Ty* start_lifetime_as_array(volatile void* const _Ptr, const size_t _Nx) noexcept {
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/5)");
+    return __builtin_start_lifetime_as_array<_Ty>(_Ptr, _Nx);
+}
+template <class _Ty>
+const volatile _Ty* start_lifetime_as_array(const volatile void* const _Ptr, const size_t _Nx) noexcept {
+    static_assert(sizeof(_Ty) > 0, "T must be a complete type. (N5032 [obj.lifetime]/5)");
+    return __builtin_start_lifetime_as_array<_Ty>(_Ptr, _Nx);
+}
+#endif // ^^^ defined(__cpp_lib_start_lifetime_as) ^^^
+
 #endif // _HAS_CXX23
 _STD_END
 

stl/inc/yvals_core.h

@@ -389,12 +389,14 @@
 // P2540R1 Empty Product For Certain Views
 // P2549R1 unexpected<E>::error()
 // P2585R1 Improve Default Container Formatting
+// P2590R2 Explicit Lifetime Management
 // P2599R2 mdspan: index_type, size_type
 // P2604R0 mdspan: data_handle_type, data_handle(), exhaustive
 // P2613R1 mdspan: empty()
 // P2614R2 Deprecating float_denorm_style, numeric_limits::has_denorm, numeric_limits::has_denorm_loss
 // P2652R2 Disallowing User Specialization Of allocator_traits
 // P2674R1 is_implicit_lifetime
+// P2679R2 Fixing start_lifetime_as And start_lifetime_as_array
 // P2693R1 Formatting thread::id And stacktrace
 // P2713R1 Escaping Improvements In std::format
 // P2763R1 Fixing layout_stride's Default Constructor For Fully Static Extents
@@ -1788,8 +1790,13 @@ _EMIT_STL_ERROR(STL1004, "C++98 unexpected() is incompatible with C++23 unexpect
 #define __cpp_lib_reference_from_temporary 202202L
 #endif // ^^^ no workaround ^^^
 
-#define __cpp_lib_spanstream                  202106L
-#define __cpp_lib_stacktrace                  202011L
+#define __cpp_lib_spanstream 202106L
+#define __cpp_lib_stacktrace 202011L
+
+#if !defined(__clang__) && !defined(__EDG__) && _MSC_VER >= 1951 // TRANSITION, GH-6169, toolset update
+#define __cpp_lib_start_lifetime_as 202207L
+#endif // ^^^ no workaround ^^^
+
 #define __cpp_lib_stdatomic_h                 202011L
 #define __cpp_lib_string_contains             202011L
 #define __cpp_lib_string_resize_and_overwrite 202110L

tests/std/test.lst

@@ -726,6 +726,7 @@ tests\P2505R5_monadic_functions_for_std_expected
 tests\P2510R3_text_formatting_pointers
 tests\P2517R1_apply_conditional_noexcept
 tests\P2538R1_adl_proof_std_projected
+tests\P2590R2_explicit_lifetime_management
 tests\P2609R3_relaxing_ranges_just_a_smidge
 tests\P2674R1_is_implicit_lifetime
 tests\P2693R1_ostream_and_thread_id

tests/std/tests/P2590R2_explicit_lifetime_management/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

tests/std/tests/P2590R2_explicit_lifetime_management/test.cpp

@@ -0,0 +1,66 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <concepts>
+#include <memory>
+using namespace std;
+
+#ifdef __cpp_lib_start_lifetime_as // TRANSITION
+struct Point {
+    int x;
+    int y;
+};
+static_assert(alignof(Point) == alignof(int));
+static_assert(sizeof(Point) == 2 * sizeof(int));
+
+template <class CvVoidPtr, class CvPointPtr>
+void test_scalar() {
+    int arr[2]{17, 29};
+
+    const auto cv_void_ptr = static_cast<CvVoidPtr>(arr);
+
+    static_assert(noexcept(start_lifetime_as<Point>(cv_void_ptr)));
+    const same_as<CvPointPtr> auto cv_point_ptr = start_lifetime_as<Point>(cv_void_ptr);
+
+    assert(cv_point_ptr->x == 17);
+    assert(cv_point_ptr->y == 29);
+}
+
+template <class CvVoidPtr, class CvPointPtr>
+void test_array() {
+    {
+        // N5032 [obj.lifetime]/6: "Preconditions: p is suitably aligned for an array of T or is null."
+        // N5032 [obj.lifetime]/7: "Effects: If n > 0 is true, [...]. Otherwise, there are no effects."
+        const same_as<CvPointPtr> auto nil = start_lifetime_as_array<Point>(static_cast<CvVoidPtr>(nullptr), 0);
+        assert(nil == nullptr); // N5032 [obj.lifetime]/8: "otherwise, a pointer that compares equal to p"
+    }
+
+    int arr[4]{17, 29, 123, 456};
+
+    const auto cv_void_ptr = static_cast<CvVoidPtr>(arr);
+
+    static_assert(noexcept(start_lifetime_as_array<Point>(cv_void_ptr, 2)));
+    const same_as<CvPointPtr> auto cv_point_ptr = start_lifetime_as_array<Point>(cv_void_ptr, 2);
+
+    assert(cv_point_ptr[0].x == 17);
+    assert(cv_point_ptr[0].y == 29);
+    assert(cv_point_ptr[1].x == 123);
+    assert(cv_point_ptr[1].y == 456);
+}
+
+template <class CvVoidPtr, class CvPointPtr>
+void test() {
+    test_scalar<CvVoidPtr, CvPointPtr>();
+    test_array<CvVoidPtr, CvPointPtr>();
+}
+
+int main() {
+    test<void*, Point*>();
+    test<const void*, const Point*>();
+    test<volatile void*, volatile Point*>();
+    test<const volatile void*, const volatile Point*>();
+}
+#else // ^^^ no workaround / workaround vvv
+int main() {}
+#endif // ^^^ workaround ^^^

tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp

@@ -945,6 +945,12 @@ STATIC_ASSERT(__cpp_lib_stacktrace == 202011L);
 #error __cpp_lib_stacktrace is defined
 #endif
 
+#if _HAS_CXX23 && !defined(__clang__) && !defined(__EDG__) // TRANSITION, GH-6169 tracking LLVM-105234 and VSO-2846756
+STATIC_ASSERT(__cpp_lib_start_lifetime_as == 202207L);
+#elif defined(__cpp_lib_start_lifetime_as)
+#error __cpp_lib_start_lifetime_as is defined
+#endif
+
 #if _HAS_CXX20
 STATIC_ASSERT(__cpp_lib_starts_ends_with == 201711L);
 #elif defined(__cpp_lib_starts_ends_with)