<memory> construct_at()

stl/inc/memory

@@ -308,10 +308,24 @@ _NoThrowFwdIt uninitialized_fill_n(_NoThrowFwdIt _First, const _Diff _Count_raw,
 }
 #endif // _HAS_IF_CONSTEXPR
 
+#if _HAS_CXX20
+template <class _Ty, class... _Types>
+auto construct_at(_Ty* const _Location, _Types&&... _Args) noexcept(noexcept(::new (const_cast<void*>(
+    static_cast<const volatile void*>(_Location))) _Ty(_STD forward<_Types>(_Args)...))) // strengthened
+    -> decltype(
+        ::new (const_cast<void*>(static_cast<const volatile void*>(_Location))) _Ty(_STD forward<_Types>(_Args)...)) {
+    return ::new (const_cast<void*>(static_cast<const volatile void*>(_Location))) _Ty(_STD forward<_Types>(_Args)...);
+}
+
+namespace ranges {
+    using _STD construct_at;
+} // namespace ranges
+#endif // _HAS_CXX20
+
 #if _HAS_CXX17
 // FUNCTION TEMPLATE destroy_at
 template <class _Ty>
-void destroy_at(_Ty* const _Location) { // destroy _Ty at memory address _Location
+void destroy_at(_Ty* const _Location) noexcept /* strengthened */ {
     _Location->~_Ty();
 }
 

tests/std/test.lst

@@ -221,6 +221,7 @@ tests\P0674R1_make_shared_for_arrays
 tests\P0758R1_is_nothrow_convertible
 tests\P0768R1_spaceship_operator
 tests\P0769R2_shift_left_shift_right
+tests\P0784R7_library_support_for_more_constexpr_containers
 tests\P0811R3_midpoint_lerp
 tests\P0896R4_P1614R2_comparisons
 tests\P0896R4_ranges_iterator_machinery

tests/std/tests/P0784R7_library_support_for_more_constexpr_containers/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/P0784R7_library_support_for_more_constexpr_containers/test.cpp

@@ -0,0 +1,131 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <assert.h>
+#include <memory>
+#include <stddef.h>
+#include <string.h>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+using namespace std;
+
+template <class Void, class Ty, class... Types>
+inline constexpr bool can_construct_at_impl = false;
+
+template <class Ty, class... Types>
+inline constexpr bool
+    can_construct_at_impl<void_t<decltype(construct_at(declval<Ty*>(), declval<Types>()...))>, Ty, Types...> = true;
+
+template <class Ty, class... Types>
+inline constexpr bool can_construct_at = can_construct_at_impl<void, Ty, Types...>;
+
+static_assert(can_construct_at<int>);
+static_assert(can_construct_at<const int>);
+static_assert(can_construct_at<volatile int>);
+static_assert(can_construct_at<const volatile int>);
+static_assert(can_construct_at<int, int>);
+static_assert(can_construct_at<const int, int>);
+static_assert(can_construct_at<volatile int, int>);
+static_assert(can_construct_at<const volatile int, int>);
+static_assert(can_construct_at<int, int&>);
+static_assert(can_construct_at<const int, int&>);
+static_assert(can_construct_at<volatile int, int&>);
+static_assert(can_construct_at<const volatile int, int&>);
+
+struct X {};
+
+#ifndef __EDG__ // TRANSITION, VSO-1075296
+static_assert(!can_construct_at<int, X>);
+static_assert(!can_construct_at<X, int>);
+#endif // __EDG__
+
+// note that indestructible isn't constructible but is construct_at-ible:
+struct indestructible {
+    void destroy() {
+        this->~indestructible();
+    };
+
+private:
+    ~indestructible() = default;
+};
+
+static_assert(can_construct_at<indestructible>);
+static_assert(can_construct_at<const indestructible>);
+static_assert(can_construct_at<volatile indestructible>);
+static_assert(can_construct_at<const volatile indestructible>);
+
+static_assert(can_construct_at<X>);
+static_assert(can_construct_at<X, X>);
+static_assert(can_construct_at<X, const X>);
+static_assert(can_construct_at<X, const X&>);
+static_assert(can_construct_at<X, X&>);
+
+static_assert(can_construct_at<string>);
+static_assert(can_construct_at<string, size_t, char>);
+static_assert(!can_construct_at<string, size_t, char, char>);
+static_assert(!can_construct_at<string, X>);
+
+// The following static_asserts test our strengthening of noexcept
+
+#ifndef __EDG__ // TRANSITION, VSO-1075296
+static_assert(noexcept(construct_at(declval<int*>(), 42)));
+static_assert(noexcept(construct_at(declval<const int*>(), 42)));
+static_assert(noexcept(construct_at(declval<volatile int*>(), 42)));
+static_assert(noexcept(construct_at(declval<const volatile int*>(), 42)));
+#endif // __EDG__
+
+static_assert(!noexcept(construct_at(declval<string*>(), "hello")));
+static_assert(!noexcept(construct_at(declval<const string*>(), "hello")));
+static_assert(!noexcept(construct_at(declval<volatile string*>(), "hello")));
+static_assert(!noexcept(construct_at(declval<const volatile string*>(), "hello")));
+
+static_assert(noexcept(destroy_at(declval<int*>())));
+static_assert(noexcept(destroy_at(declval<string*>())));
+static_assert(noexcept(destroy_at(declval<const int*>())));
+static_assert(noexcept(destroy_at(declval<const string*>())));
+static_assert(noexcept(destroy_at(declval<volatile int*>())));
+static_assert(noexcept(destroy_at(declval<volatile string*>())));
+static_assert(noexcept(destroy_at(declval<const volatile int*>())));
+static_assert(noexcept(destroy_at(declval<const volatile string*>())));
+
+struct throwing_dtor {
+    ~throwing_dtor() noexcept(false) {}
+};
+
+static_assert(noexcept(destroy_at(declval<throwing_dtor*>())));
+
+template <class Ty>
+void test_runtime(const Ty& val) {
+    alignas(Ty) unsigned char storage[sizeof(Ty)];
+    memset(storage, 42, sizeof(Ty));
+    const auto asPtrTy = reinterpret_cast<Ty*>(&storage);
+    assert(asPtrTy == construct_at(asPtrTy, val));
+    assert(*asPtrTy == val);
+    destroy_at(asPtrTy);
+
+    // test ranges:
+    assert(asPtrTy == ranges::construct_at(asPtrTy, val));
+    assert(*asPtrTy == val);
+    destroy_at(asPtrTy);
+
+    // test voidify:
+    const auto asCv = static_cast<const volatile Ty*>(asPtrTy);
+    assert(asPtrTy == construct_at(asCv, val));
+    assert(const_cast<const Ty&>(*asCv) == val);
+    destroy_at(asCv);
+}
+
+int main() {
+    test_runtime(1234);
+    test_runtime(string("hello world"));
+    test_runtime(string("hello to some really long world that certainly doesn't fit in SSO"));
+
+    {
+        alignas(indestructible) unsigned char storage[sizeof(indestructible)];
+        const auto ptr = reinterpret_cast<indestructible*>(storage);
+        construct_at(ptr);
+        ptr->destroy();
+    }
+}