If you do the following in MSVC, the application will crash:

//header:
signals:
    void signal1(QPrivateSignal);
    void signal2(QPrivateSignal);

//source
connect(this, &MyClass::signal1, this, &MyClass::signal2);
emit signal1(QPrivateSignal());

It works fine on linux, and it works for all platforms if the connect is done via SIGNAL(signal1()).

See the attached project for a full example.