Short live q20::construct_at()!

Move the helper from qsystemsemaphore.cpp to q20memory.h
to prevent clashes in CMake Unity (Jumbo) builds.

Pick-to: 6.5
Task-number: QTBUG-109394
Change-Id: Id0127af1f0d51c87a5887090cc90ab232eff8093
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/CMakeLists.txt

@@ -104,6 +104,7 @@ qt_internal_add_module(Core
         global/q20algorithm.h
         global/q20functional.h
         global/q20iterator.h
+        global/q20memory.h
         global/q20type_traits.h
         global/q23functional.h
         global/qxpfunctional.h

src/corelib/global/q20memory.h

@@ -0,0 +1,46 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#ifndef Q20MEMORY_H
+#define Q20MEMORY_H
+
+#include <QtCore/qxptype_traits.h>
+
+#include <memory>
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API. Types and functions defined in this
+// file can reliably be replaced by their std counterparts, once available.
+// You may use these definitions in your own code, but be aware that we
+// will remove them once Qt depends on the C++ version that supports
+// them in namespace std. There will be NO deprecation warning, the
+// definitions will JUST go away.
+//
+// If you can't agree to these terms, don't use these definitions!
+//
+// We mean it.
+//
+
+QT_BEGIN_NAMESPACE
+
+namespace q20 {
+
+#if __cplusplus >= 202002L
+using std::construct_at;
+#else
+template <typename T,
+          typename... Args,
+          typename Enable = std::void_t<decltype(::new (std::declval<void *>()) T(std::declval<Args>()...))> >
+         static void construct_at(T *ptr, Args && ... args)
+{
+    ::new (const_cast<void*>(static_cast<const volatile void*>(ptr))) T(std::forward<Args>(args)...);
+}
+#endif
+} // namespace q20
+
+QT_END_NAMESPACE
+
+#endif /* Q20MEMORY_H */

src/corelib/ipc/qsystemsemaphore.cpp

@@ -5,25 +5,16 @@
 #include "qsystemsemaphore_p.h"
 
 #if QT_CONFIG(systemsemaphore)
-#include <memory>
+#include <QtCore/q20memory.h>
 
 QT_BEGIN_NAMESPACE
 
 using namespace QtIpcCommon;
 using namespace Qt::StringLiterals;
 
-#if __cplusplus >= 202002L
-using std::construct_at;
-#else
-template <typename T> static void construct_at(T *ptr)
-{
-    new (ptr) T;
-}
-#endif
-
 inline void QSystemSemaphorePrivate::constructBackend()
 {
-    visit([](auto p) { construct_at(p); });
+    visit([](auto p) { q20::construct_at(p); });
 }
 
 inline void QSystemSemaphorePrivate::destructBackend()