qmlobject_cast: Make it useful again

The Qt 6 implementation of qmlobject_cast simply accessed the metaObject
of the object (canConvert -> QQmlMetaObject::QQmlMetaObject(QObject*)
-> QOject:metaObject). That would directly cause the creation of
dynamic meta-objects, if they weren't created so far.

Given that we still didn't get rid of the property-cache in QML, use it
to optimize this code path:
If the object has some associated QQmlData, retrieve it and check if we
have a  property cache. If we do, retrieve the first C++ meta object
from it. We can safely ignore any dynamic meta-objects as T must be a
static meta-object. Then do the normal inheritance check with the first
C++ meta-object and T::staticMetaObject.
If we don't have a property cache, fall back to the previous
implementation (but without the QQmlMetaObject detour).

Change-Id: Ie23d1a3dac6bf343ee2bc31e04c4592681ad9995
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/qml/qml/qqmlglobal.cpp

@@ -596,6 +596,16 @@ void QQmlApplication::setDomain(const QString &arg)
     QCoreApplication::instance()->setOrganizationDomain(arg);
 }
 
+bool qmlobject_can_cast(QObject *object, const QMetaObject *mo)
+{
+    Q_ASSERT(object);
+    Q_ASSERT(mo);
+    auto ddata = QQmlData::get(object, false);
+    if (!ddata || ! ddata->propertyCache)
+        return object->metaObject()->inherits(mo);
+   return ddata->propertyCache->firstCppMetaObject()->inherits(mo);
+}
+
 QT_END_NAMESPACE
 
 #include "moc_qqmlglobal_p.cpp"

src/qml/qml/qqmlglobal_p.h

@@ -122,6 +122,8 @@ do { \
     QMetaObject::disconnect(sender, signalIdx, receiver, methodIdx); \
 } while (0)
 
+Q_QML_PRIVATE_EXPORT bool qmlobject_can_cast(QObject *object, const QMetaObject *mo);
+
 /*!
     This method is identical to qobject_cast<T>() except that it does not require lazy
     QMetaObjects to be built, so should be preferred in all QML code that might interact
@@ -137,7 +139,9 @@ do { \
 template<class T>
 T qmlobject_cast(QObject *object)
 {
-    if (object && QQmlMetaObject::canConvert(object, &reinterpret_cast<T>(object)->staticMetaObject))
+    if (!object)
+        return nullptr;
+    if (qmlobject_can_cast(object, &(std::remove_pointer_t<T>::staticMetaObject)))
         return static_cast<T>(object);
     else
         return nullptr;