Open Radar

 
As of WWDC 2016 the Core Data team updated their framework to be friendlier to generics and the new beefed up `NSFetchRequest` now returns objects that conform to the `NSFetchRequestResult`. 

So in the code below I have my base protocol that conforms to the said `NSFetchRequestResult` protocol, and in an extension of its child protocol `ManagedObjectFetchable` I want to return fetch objects of conforming types:

    import CoreData

    @objc protocol ManagedObjectType: class, NSFetchRequestResult { }

    protocol ManagedObjectFetchable: ManagedObjectType { }

    extension ManagedObjectFetchable {
      static func preFetch(for predicate: Predicate, sortedBy sortDescriptors: [SortDescriptor]) throws -> [Self] {
        let fetchRequest = NSFetchRequest<Self>(entityName: Self.entityName)
        fetchRequest.predicate = predicate
        fetchRequest.sortDescriptors = sortDescriptors

        let dummyContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
        return try dummyContext.fetch(fetchRequest)
      }
    }

The problem here is this line:

    // Error: Binary operator '<' cannot be applied to operands of type 'NSFetchRequest<_>.Type' and 'Self.Type'
    let fetchRequest = NSFetchRequest<Self>(entityName: Self.entityName)

I don't think this has anything to do with binary operators but rather a bug in Swift

As pointed out by a Stackoverflow answer this code works:

    let fetchRequest: NSFetchRequest<Self> = NSFetchRequest(entityName: Self.entityName)

Stackoverflow link: http://stackoverflow.com/questions/38678318/extending-protocols-conforming-to-nsfetchrequestresult/38687263#38687263