on-watch rule triggers on return

[CxRes]

return statement must be treated equivalent to an assignment for the purpose of the on-watch rule.

My use case involves returning a function that ends with return $rootScope.$on(...) from a service into a controller. I can use the return value with the '$destroy' event. I know the workaround is to place $rootscope.$on() in the controller but with my method I can avoid a $rootScope DI in the controller.