[Regr: 5.15->6.4] Signals with containers using forward declared types don't compile

#include <QObject>

class TestObject;
using VectorOfTestObjects = std::vector<TestObject>;

class SignalTest : public QObject {    
    Q_OBJECT
public:
    SignalTest() = default;

signals:    
    void incompleteSignal(const VectorOfTestObjects& c) const;
};