QPointer cannot be used with virtual inheritance

The following code fails to compile:

    struct Derived : virtual QObject {};

    QPointer<Derived> pd = new Derived;
    delete pd.get();                         // L76
    QPointer<const QObject> po = pd;
    QVERIFY(!po);

with

qpointer.h: In instantiation of ‘T* Qt6::QPointer<T>::data() const [with T = tst_QPointer::conversion()::Derived]’:
qpointer.h:55:14: required from ‘T* Qt6::QPointer<T>::get() const [with T = tst_QPointer::conversion()::Derived]’
tst_qpointer.cpp:76:22: required from here
qpointer.h:53:14: error: cannot convert from pointer to base class ‘Qt6::QObject’ to pointer to derived class ‘tst_QPointer::conversion()::Derived’ because the base is virtual
53 |

Unknown macro: { return static_cast<T*>(wp.internalData()); }

^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The reason is the SCARY optimization whereby a QPointer<T> is implemented in terms of QWeakPointer<const QObject>, which, in get(), has a static_cast<T*> from the QObject* returned by QWeakPointer<const QObject>::get(). If QObject is a virtual base, this cast is invalid.

The fix would be to undo the SCARY optimization, for which QTBUG-113366 might come in handy.

Acceptance criteria:

• The above code snippet compiles and is asan/valgrind-clean.