If a cpp type is exposed to qml via qmlRegisterType,
and this cpp type contains a signal `x` and a slot `onX`,
when a `SignalSpy` qml element is connected to signal `x`, an error is thrown:
`Error: Function.prototype.connect: this object is not a signal`
This makes it impossible to have interchangeable cpp and qml types with the same API based on the x/onX idiom.
example to reproduce: