q20::construct_at(): fix various issues

Fix several issues in 72c2cdbc572f8b8b45a57a451e2bc19bb1c53b0c,
which I was too slow to review before it went in:

- use the correct feature macro, not __cplusplus
- use the correct signature (return T*, not void)
- don't make the function static
- add a comment mentioning the material difference to std::construct_at
- drop unneeded <qxptype_traits.h> include

Task-number: QTBUG-109394
Change-Id: I39d1908f565b1c1a31d5741924ac173447ec9057
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 67ed712235375fa1d1b9ea0e5c71755886f65f83)
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/global/q20memory.h

@@ -4,9 +4,12 @@
 #ifndef Q20MEMORY_H
 #define Q20MEMORY_H
 
-#include <QtCore/qxptype_traits.h>
+#include <QtCore/qtconfigmacros.h>
 
 #include <memory>
+#include <utility>
+
+#include <type_traits>
 
 //
 //  W A R N I N G
@@ -26,19 +29,20 @@
 
 QT_BEGIN_NAMESPACE
 
+// like std::construct_at (but not whitelisted for constexpr)
 namespace q20 {
-
-#if __cplusplus >= 202002L
+#ifdef __cpp_lib_constexpr_dynamic_alloc
 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)
+T *construct_at(T *ptr, Args && ... args)
 {
-    ::new (const_cast<void*>(static_cast<const volatile void*>(ptr))) T(std::forward<Args>(args)...);
+    return ::new (const_cast<void *>(static_cast<const volatile void *>(ptr)))
+                                                                T(std::forward<Args>(args)...);
 }
-#endif
+#endif // __cpp_lib_constexpr_dynamic_alloc
 } // namespace q20
 
 QT_END_NAMESPACE