Issue Description:

NSFetchedResultsController is the class from Core Data

class Controller: NSFetchedResultsController<NSManagedObject> {}

assign Controller a delegate, and in the delegate method:

func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>)

I can't cast controller back to Controller.

Discussion on swift-users: https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20171002/006374.html