Remove the last in-tree user of qExchange() and mark all of Qt free of it

We've ported all qExchange() to std::exchange by now, across all
modules, but the one in QScopedValueRollback was left behind, because
it requires C++20's version of std::exchange (constexpr).

Since q20::exchange was not approved, replace the qExchange() here
with two moves and add a comment to port to std::exchange() once we
can depend on C++20.

Then add QT_NO_QEXCHANGE to avoid new uses from creeping in.

Change-Id: I488e252433e78fb2766639dbe77a22a55196cfd1
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

cmake/QtInternalTargets.cmake

@@ -151,6 +151,7 @@ qt_internal_add_target_aliases(PlatformToolInternal)
 target_link_libraries(PlatformToolInternal INTERFACE PlatformAppInternal)
 
 qt_internal_add_global_definition(QT_NO_JAVA_STYLE_ITERATORS)
+qt_internal_add_global_definition(QT_NO_QEXCHANGE)
 qt_internal_add_global_definition(QT_NO_NARROWING_CONVERSIONS_IN_CONNECT)
 qt_internal_add_global_definition(QT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH)
 

src/corelib/global/qttypetraits.h

@@ -34,6 +34,8 @@ void qAsConst(const T &&) = delete;
 
 #endif // QT_NO_AS_CONST
 
+#ifndef QT_NO_QEXCHANGE
+
 // like std::exchange
 template <typename T, typename U = T>
 constexpr T qExchange(T &t, U &&newValue)
@@ -45,6 +47,8 @@ noexcept(std::conjunction_v<std::is_nothrow_move_constructible<T>,
     return old;
 }
 
+#endif // QT_NO_QEXCHANGE
+
 namespace QtPrivate {
 // helper to be used to trigger a "dependent static_assert(false)"
 // (for instance, in a final `else` branch of a `if constexpr`.)

src/corelib/tools/qscopedvaluerollback.h

@@ -18,8 +18,9 @@ public:
     }
 
     explicit constexpr QScopedValueRollback(T &var, T value)
-        : varRef(var), oldValue(qExchange(var, std::move(value)))
+        : varRef(var), oldValue(std::move(var)) // ### C++20: std::exchange(var, std::move(value))
     {
+        var = std::move(value);
     }
 
 #if __cpp_constexpr >= 201907L