Pacify MSVC compiler incorrectly warning about unused variable

Under some circumstances, MSVC seems to complain about SlotArgumentCount
being unused

  qobject.h(210): warning C4189: 'SlotArgumentCount': local variable is
  initialized but not referenced
  note: see reference to function template instantiation
  'QMetaObject::Connection QObject::connect<void(__cdecl QAction::* )(bool),
  main::<lambda_1>>(const QAction *,Func1,
    const QtPrivate::ContextTypeForFunctor<main::<lambda_1>,void>::ContextType *,
    Func2 &&,Qt::ConnectionType)' being compiled

This is nonsense, as SlotArgumentCount is used in the next line, to
construct the list of signal arguments, but the workaround to declare
the variable as [[maybe_unused]] is trivial.

Add a connect statement to the test case that creates such a connection.
This does not produce any warning with or without the attribute (and if
it did, the build would fail for CI configuratinos setting -Werror).

Fixes: QTBUG-114781
Change-Id: I4ee6f7d57c2836ef3dd9741d037d48181af2cdec
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 2b9ef2eb44c084d39ef8324cfe1ae42a98b3038f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qobject.h

@@ -207,6 +207,7 @@ public:
                             "Return type of the slot is not compatible with the return type of the signal.");
         } else {
             constexpr int FunctorArgumentCount = QtPrivate::ComputeFunctorArgumentCount<std::decay_t<Func2>, typename SignalType::Arguments>::Value;
+            [[maybe_unused]]
             constexpr int SlotArgumentCount = (FunctorArgumentCount >= 0) ? FunctorArgumentCount : 0;
             typedef typename QtPrivate::FunctorReturnType<std::decay_t<Func2>, typename QtPrivate::List_Left<typename SignalType::Arguments, SlotArgumentCount>::Value>::Value SlotReturnType;
 

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -6075,6 +6075,7 @@ void tst_QObject::connectFunctorArgDifference()
 
     connect(&timer, &QTimer::timeout, [=](){});
     connect(&timer, &QTimer::objectNameChanged, [=](const QString &){});
+    connect(&timer, &QTimer::objectNameChanged, this, [](){});
     connect(qApp, &QCoreApplication::aboutToQuit, [=](){});
 
     connect(&timer, &QTimer::objectNameChanged, [=](){});