Found when testing pure-qml example in Qt Wayland. The example has a local type called "Screen" and instantiates this in main.qml. In Qt 5, this worked fine, but in Qt 6, it now gives the error

qrc:/qml/main.qml:57:5: Screen can only be used via the attached property.

Changing the name to "CompositorScreen" fixes the issue, so it is probably resolving to the built-in type instead of the local type.