Adding QPrivateSignal as an argument to the signal is meant to make it impossible to call from any other class than the owning class.
However, moc generates, in qt_static_metacall, the call for InvokeMetaMethod with the QPrivateSignal argument. For example:

        case 1: _t->pendingConnectionAvailable(QPrivateSignal()); break;

This means it is possible to still emit the signal by using the string-based lookup. Like so:

        QMetaObject::invokeMethod(this, "pendingConnectionAvailable");

The code snippet above was added to the patch below inside the DerivedServer::incomingConnection function, where 'pendingConnectionAvailable' is a private signal on the parent, QTcpServer.

See following patch for where the code is generated from/added to:
https://codereview.qt-project.org/c/qt/qtbase/+/413535