QQmlEngine::retranslate no longer retranslates strings coming from C++

Hi,

Following fix of QTBUG-96192 (https://codereview.qt-project.org/c/qt/qtdeclarative/+/375491), QQmlEngine::retranslate properly reevaluates QML bindings, leading to their re-translation, but cannot detect translated strings coming from C++, leading to a regression compared to previous versions.

An example of binding that was working dynamically before the change of behavior:

Text {
    function textForThing()
    {
        return handler.getMyString();
    }
    anchors.centerIn: parent
    text: textForThing()
}

Example of implementation of getMyString():

class Handler : public QObject
{
    ...
    QString getMyString() const
    {
        return QObject::tr("This is a test");
    }
};

For this case, we have no way to force the reevaluation of everything, so we would have to rely on the trick of "+ emptyString" on all texts to have them update themselves each time there is a re-translation.

Please note that in the example above, I'm using QObject::tr(), but I could for example have written myTexts[getCurrentLang()] instead (with myTexts loaded from JSON configuration files). I mean that C++ will not always rely on QObject::tr(): it just expects to be called from QML bindings each time lang changes.

I attached a small test project to reproduce the issue: clicking on the window with Qt 5.15.2 leads to a re-translation, while with Qt 6.3.0 RC, it no longer does.

Not sure what we can do? To be honest, I'm very impacted by this change of behavior because I have a lot of strings coming from C++, so I liked very much the previous behavior, even if that would reload more than required.

Thank you,
Louis