Using qmlRegisterType<QQuickWindow>(); triggers build failure

Building KWin (git://anongit.kde.org/kwin.git) fails against latest 5.4 branch:

[ 995s] ^
[ 996s] In file included from /usr/include/qt5/QtQml/qqml.h:45:0,
[ 996s] from /usr/include/qt5/QtQuick/qquickitem.h:46,
[ 996s] from /usr/include/qt5/QtQuick/qquickpainteditem.h:45,
[ 996s] from /usr/include/qt5/QtQuick/QQuickPaintedItem:1,
[ 996s] from /home/abuild/rpmbuild/BUILD/kwin-5.0.41git/scripting/../thumbnailitem.h:26,
[ 996s] from /home/abuild/rpmbuild/BUILD/kwin-5.0.41git/scripting/scripting.cpp:31:
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h: In instantiation of 'static QObject* QQmlPrivate::AttachedPropertySelector<T, 1>::attachedProperties(QObject*) [with T = QQuickWindow]':
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h:188:20: required from 'static QObject* (* QQmlPrivate::AttachedPropertySelector<T, 1>::func())(QObject*) [with T = QQuickWindow; QQmlAttachedPropertiesFunc = QObject* (*)(QObject*)]'
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h:198:130: required from 'QObject* (* QQmlPrivate::attachedPropertiesFunc())(QObject*) [with T = QQuickWindow; QQmlAttachedPropertiesFunc = QObject* (*)(QObject*)]'
[ 996s] /usr/include/qt5/QtQml/qqml.h:121:48: required from 'int qmlRegisterType() [with T = QQuickWindow]'
[ 996s] /home/abuild/rpmbuild/BUILD/kwin-5.0.41git/scripting/scripting.cpp:623:35: required from here
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h:180:48: error: cannot convert 'QQuickWindowAttached*' to 'QObject*' in return
[ 996s] return T::qmlAttachedProperties(obj);
[ 996s] ^
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h: In instantiation of 'static const QMetaObject* QQmlPrivate::AttachedPropertySelector<T, 1>::attachedPropertiesMetaObject(ReturnType* (QObject*)) [with ReturnType = QQuickWindowAttached; T = QQuickWindow]':
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h:191:74: required from 'static const QMetaObject* QQmlPrivate::AttachedPropertySelector<T, 1>::metaObject() [with T = QQuickWindow]'
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h:204:136: required from 'const QMetaObject* QQmlPrivate::attachedPropertiesMetaObject() [with T = QQuickWindow]'
[ 996s] /usr/include/qt5/QtQml/qqml.h:122:54: required from 'int qmlRegisterType() [with T = QQuickWindow]'
[ 996s] /home/abuild/rpmbuild/BUILD/kwin-5.0.41git/scripting/scripting.cpp:623:35: required from here
[ 996s] /usr/include/qt5/QtQml/qqmlprivate.h:184:20: error: incomplete type 'QQuickWindowAttached' used in nested name specifier
[ 996s] return &ReturnType::staticMetaObject;
[ 996s] ^

Downstream report - https://bugs.kde.org/show_bug.cgi?id=338277