QVariant::convert() cannot deal with conversion across subclasses when using Q_GADGET

Assume the following setup:

class A {
    Q_GADGET
};

class B : public A {
    Q_GADGET
};

class Foo: public QObject {
    Q_PROPERTY(A propA READ propA WRITE setPropA)
};

and in QML we try to execute the following snippet:

Foo {
    propA: b // whereby b is an instance of gadget B
}

This fails to work as QQmlProperty cannot convert an instance of B to and instance of A. More precise QVariant::convert() relies on Q_OBJECT to work but it never got this treatment for Q_GADGET.

QVariant::canConvert and QVariant::convert should implement the conversion between two Q_GADGET if one is derived by the other.