Short live q20::exchange()!

We have weaned ourselves off of qExchange(), which, however, has C++23
semantics (is constexpr and noexcept), while we only require C++17 atm.
At the same time, we have more and more uses of a constexpr exchange(),
iow: the C++20 extension.

We have the qNN namespaces for this, so let's use it: Add
q20::exchange(), with C++20 semantics (constexpr, not noexcept), and
use it to port the only remaining qExchange() user (since ported to
2×std::move() to make the header compatible with QT_NO_QEXCHANGE),
QScopedValueRollback.

No user requires the C++23 noexcept on the function (compilers will
figure it out, because the std::exchange is fully inline), and we
can't provide a q23::exchange() even if we needed to, because the
author of P2401 forgot to ask to update the value of
__cpp_lib_exchange_function...

Conflict resolution for 6.8:
- worked around missing a313caca239638b384018aec18906de44aab2171,
  by back-porting the relevant subset of it

Fixes: QTBUG-133038
Change-Id: I003df445001d1c5ab02402119c5e5106aa156263
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit b5ed7fb203335cef7b7b4f70f4130b07166cecf2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit cebb2dd0681f7e4f8a838730a2b15e0c7384c711)
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/CMakeLists.txt

@@ -96,6 +96,7 @@ qt_internal_add_module(Core
         global/q20map.h
         global/q20memory.h
         global/q20type_traits.h
+        global/q20utility.h
         global/q20vector.h
         global/q23functional.h
         global/q23utility.cpp # remove once we have a user that tests this

src/corelib/global/q20utility.h

@@ -0,0 +1,46 @@
+// Copyright (C) 2024 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 Q20UTILITY_H
+#define Q20UTILITY_H
+
+#include <QtCore/qtconfigmacros.h>
+
+#include <utility>
+
+//
+//  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
+
+// like C++20 std::exchange (ie. constexpr, not yet noexcept)
+namespace q20 {
+#ifdef __cpp_lib_constexpr_algorithms
+using std::exchange;
+#else
+template <typename T, typename U = T>
+constexpr T exchange(T& obj, U&& newValue)
+{
+    T old = std::move(obj);
+    obj = std::forward<U>(newValue);
+    return old;
+}
+#endif
+}
+
+QT_END_NAMESPACE
+
+#endif /* Q20UTILITY_H */

src/corelib/global/q23utility.h

@@ -5,7 +5,7 @@
 
 #include <QtCore/qtconfigmacros.h>
 
-#include <utility>
+#include <QtCore/q20utility.h>
 
 //
 //  W A R N I N G

src/corelib/tools/qscopedvaluerollback.h

@@ -6,6 +6,8 @@
 
 #include <QtCore/qglobal.h>
 
+#include <QtCore/q20utility.h>
+
 QT_BEGIN_NAMESPACE
 
 template <typename T>
@@ -20,9 +22,8 @@ public:
 
     Q_NODISCARD_CTOR
     explicit constexpr QScopedValueRollback(T &var, T value)
-        : varRef(var), oldValue(std::move(var)) // ### C++20: std::exchange(var, std::move(value))
+        : varRef(var), oldValue(q20::exchange(var, std::move(value)))
     {
-        var = std::move(value);
     }
 
 #if __cpp_constexpr >= 201907L