So far we could assign QObjects of any type into any QmlListWrapper of another type. This is UB, since you may be holding the underlying QQmlListProperty in C++, and it's also not very consistent. Assign null instead if the type doesn't match and emit a warning. Likewise, also warn when initializing a list with mismatched elements. [ChangeLog][QtQml][Important Behavior Changes] Assignments to list properties in QML are now type-checked. Before you could, for example, insert a plain QObject into list<Item>, producing undefined behavior. Now it instead inserts null and warns. In particular, if you assign a JavaScript array of random objects to a list property, QML will check each individual element, and insert null as well as warn when encountering type incompatibilities. Pick-to: 6.8 6.7 6.5 Fixes: QTBUG-127343 Change-Id: I9b78afcd3ed40c80175a99861373588f56683954 Reviewed-by: Mitch Curtis <mitch.curtis@qt.io> Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>