QML "composite type" based on a C++ type does not pass through the Q_ENUMS declared

class QQuickAbstractMessageDialog : public QQuickAbstractDialog
{
    Q_OBJECT
    Q_ENUMS(Icon)
    Q_PROPERTY(Icon icon READ icon WRITE setIcon NOTIFY iconChanged)

public:
    enum Icon { NoIcon, Information, Warning, Question };
...
}

class QQuickQMessageBox : public QQuickAbstractMessageDialog
... implementation using QMessageBox ...

qmlRegisterType<QQuickQMessageBox>(uri, 1, 1, "QtMessageDialog");

WidgetMessageDialog.qml:

import QtQuick 2.1
import QtQuick.PrivateWidgets 1.1

QtMessageDialog { }

qmlRegisterType("file:///path/to/WidgetMessageDialog.qml", "QtQuick.Dialogs", 1, 1, "MessageDialog");

Now finally the test QML:

MessageDialog {
    icon: MessageDialog.Warning
}

MessageDialogs.qml:59:15: Unable to assign [undefined] to int

The reason is that MessageDialog is a "composite type". Even though it's based on a C++ class, the Q_ENUMS from the C++ class are not inherited inside MessageDialog. I can work around it by registering the abstract C++ type too:

qmlRegisterType<QQuickAbstractMessageDialog>(..., "AbstractMessageDialog")

then AbstractMessageDialog.Warning will exist and can be used in QML. But this breaks encapsulation so is not as nice as MessageDialog.Warning.

I can't use an alias either, but maybe we need a similar concept to be able to alias an enum in a QML type; or else inherit all the enums automatically so I don't have to do that.

As an alternative workaround I might have to register the enums in the QtQuick.Dialogs namespace or something like that (the way that Qt.xxx is a catchall for a lot of enums).