Attached properties declared in C++ base type

// qquickabstractscrollindicator_p.h
class QQuickScrollIndicatorAttached : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QQuickAbstractScrollIndicator *horizontal ...)
    Q_PROPERTY(QQuickAbstractScrollIndicator *vertical ...)
};

class QQuickAbstractScrollIndicator : public QQuickItem
{
    Q_OBJECT

public:
    explicit QQuickAbstractScrollIndicator(QQuickItem *parent = Q_NULLPTR);

    static QQuickScrollIndicatorAttached *qmlAttachedProperties(QObject *object);
};

QML_DECLARE_TYPEINFO(QQuickAbstractScrollIndicator, QML_HAS_ATTACHED_PROPERTIES)

// qtquickcontrolsplugin.cpp
qmlRegisterType<QQuickAbstractScrollIndicator>(uri, 2, 0, "AbstractScrollIndicator");

// qmldir
ScrollIndicator 2.0 ScrollIndicator.qml

// ScrollIndicator.qml
AbstractScrollIndicator {
    id: control
    implicitWidth: ...
    implicitHeight: ...
    indicator: Rectangle { ... }
    background: Rectangle { ... }
}

Flickable {
    // ScrollIndicator.vertical: ScrollIndicator { ... } // "Non-existent attached object"
    // vs.
    AbstractScrollIndicator.vertical: ScrollIndicator { ... } // :(
}