SubComponent.qml:

Item {
    property bool setMe: false
} 

Component.qml:

Item {    
    property alias firstComponent: firstComponent
    SubComponent {
        id: firstComponent
    }
} 

main.qml:

Item {
    MainComponent{
        firstComponent.setMe: true
    }
} 

when using the type compiler, the code generation fails with: "QML base type has unknown origin. Do you miss an import?".

as a workaround one can set `firstComponent.setMe` inside a `Component.onCompleted` handler, but one cannot initialize the setMe property via the property alias directly