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