QtScript: using methods from prototype of derived class

[svlasov]

I have two prototypes:
@
class QWidgetPrototype : public QObject, public QScriptable
{
...
Q_INVOKABLE void move(int x, int y);
};

class QLabelPrototype : public QObject, public QScriptable
{
...
Q_INVOKABLE void setText(QString text);
};

...

QWidgetPrototype widgetProto;
QLabelPrototype labelProto;
Q_DECLARE_METATYPE(QWidget *)
Q_DECLARE_METATYPE(QLabel *)
...
engine->setDefaultPrototype(qMetaTypeId<QWidget *>(), engine->newQObject(&widgetProto;));
engine->setDefaultPrototype(qMetaTypeId<QLabel *>(), engine->newQObject(&labelProto));
@

So in QtScript I can do something like that:
@
var widget = myWindowt.findChild("someWidget");
widget.move(100, 50);

var label = myWindowt.findChild("helloLabel");
label.setText("Hello there!");
...
@

But this doesn't work:
@
label.move(20, 30);
^TypeError: Result of expression 'label.move' [undefined] is not a function.
@

Because QLabel inherits QWidget I thought I could use the same methods from QWidgetPrototype on QLabelPrototype. Is it possible to typecast label to QPushButton inside QtScript?

EDIT: I found that the same problem described here: http://developer.qt.nokia.com/forums/viewthread/5082