It seems that its possible to declare a type with attached properties (MessageBoard), derive it in QML (MyMessageBoard), and then use the attached properties on the derived type (MyMessageBoard.expired instead of MessageBoard.expired, for example).

See the attached reproducer that does not trigger warnings.

There should be a warning if an attached property is found by looking at the type derived from the non-attached QML type, even if that works at runtime. And at runtime, it should warn.