change SFINAE constraint on construct_at (MSVC workaround)

stl/inc/xutility

@@ -218,8 +218,13 @@ _NODISCARD constexpr void* _Voidify_iter(_Iter _It) noexcept {
 }
 
 #if _HAS_CXX20
+#ifdef __EDG__ // TRANSITION, DevCom-1691516
 template <class _Ty, class... _Types,
     class = void_t<decltype(::new (_STD declval<void*>()) _Ty(_STD declval<_Types>()...))>>
+#else // ^^^ no workaround // workaround vvv
+template <class _Ty, class... _Types,
+    void_t<decltype(::new (_STD declval<void*>()) _Ty(_STD declval<_Types>()...))>* = nullptr>
+#endif // TRANSITION, DevCom-1691516
 constexpr _Ty* construct_at(_Ty* const _Location, _Types&&... _Args) noexcept(
     noexcept(::new (_Voidify_iter(_Location)) _Ty(_STD forward<_Types>(_Args)...))) /* strengthened */ {
     _MSVC_CONSTEXPR return ::new (_Voidify_iter(_Location)) _Ty(_STD forward<_Types>(_Args)...);

tests/std/test.lst

@@ -202,6 +202,7 @@ tests\GH_002431_byte_range_find_with_unreachable_sentinel
 tests\GH_002488_promise_not_default_constructible_types
 tests\GH_002558_format_presetPadding
 tests\GH_002581_common_reference_workaround
+tests\GH_002620_construct_at_workaround
 tests\GH_002655_alternate_name_broke_linker
 tests\GH_002711_Zc_alignedNew-
 tests\GH_002760_syncstream_memory_leak

tests/std/tests/GH_002620_construct_at_workaround/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_20_matrix.lst

tests/std/tests/GH_002620_construct_at_workaround/test.cpp

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <memory>
+
+using namespace std;
+
+struct S {
+    int v;
+    explicit S(int v_) : v(v_) {}
+    S(const S&) = delete;
+};
+
+union U {
+    char c;
+    S s;
+    U() : c{} {}
+    ~U() noexcept {}
+};
+
+struct copy_elider {
+    operator S() const {
+        return S{42};
+    }
+};
+
+int main() {
+    U u;
+    // GH-2620: <xutility>: SFINAE constraint on construct_at prevents emplacing immovable objects with copy elision
+#ifndef __EDG__ // TRANSITION, DevCom-10000388
+    construct_at(&u.s, copy_elider{});
+#endif // TRANSITION, DevCom-10000388
+}