Short live q23::invoke_r!

Add an implementation of C++23's std::invoke_r. Will need it in our
implementation of function_ref.

Differences to std::invoke_r:

- only constexpr in C++20, not in C++17 (to fix, we'd need to
  implement our own q20::invoke just to add the constexpr keyword;
  let's first see whether we need this).

Task-number: QTBUG-103739
Change-Id: Id864f83f27ed499849b6bde0c34353127e37e7ed
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/corelib/CMakeLists.txt

@@ -72,6 +72,7 @@ qt_internal_add_module(Core
         global/qvolatile_p.h
         global/q20algorithm.h
         global/q20functional.h
+        global/q23functional.h
         global/q20iterator.h
         io/qabstractfileengine.cpp io/qabstractfileengine_p.h
         io/qbuffer.cpp io/qbuffer.h

src/corelib/global/q23functional.h

@@ -0,0 +1,49 @@
+// Copyright (C) 2022 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 Q23FUNCTIONAL_H
+#define Q23FUNCTIONAL_H
+
+#include <QtCore/qglobal.h>
+#include <QtCore/q20functional.h>
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API. Types and functions defined
+// in this file will behave exactly as their std counterparts. 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 q23 {
+// like std::invoke_r
+#ifdef __cpp_lib_invoke_r
+using std::invoke_r;
+#else
+template <typename R, typename F, typename...Args>
+constexpr
+std::enable_if_t<std::is_invocable_r_v<R, F, Args...>, R>
+invoke_r(F&& f, Args&&... args)
+    noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>)
+{
+    // ### use q20::invoke for a constexpr std::invoke
+    if constexpr (std::is_void_v<R>)
+        std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
+    else
+        return std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
+}
+#endif // __cpp_lib_invoke_r
+} // namespace q23
+
+QT_END_NAMESPACE
+
+#endif /* Q23FUNCTIONAL_H */