$app is of type object<Ratchet\MessageComponentInterface>, but the property $app was declared to be of type object<Ratchet\Http\HttpServerInterface>. Are you sure that you always receive this specific sub-class here, or does it make sense to add an instanceof check?
Our type inference engine has found a suspicous assignment of a value to a property.
This check raises an issue when a value that can be of a given class or a super-class
is assigned to a property that is type hinted more strictly.
Either this assignment is in error or an instanceof check should be added for that assignment.