Extending Swift type with method that requires a generic

Question

My question is about extending a type with a method that requires a generic.

I want to extend IndexPath and add a method to check if it's valid in a given NSFetchedResultsController . Here's my extension:

 public extension IndexPath {
    func isValid(in fetchedResultsController: NSFetchedResultsController<NSManagedObject>) -> Bool {
    guard let sections = fetchedResultsController.sections,
        section < sections.count,
        item < sections[section].numberOfObjects else {
            return false
    }
    return true
 }

In my UICollectionViewController class, I declare myFetchedResultsController :

private var myFetchedResultsController: NSFetchedResultsController<MyModel>!

MyModel is a NSManagedObject .

However, when I try to call my new method:

indexPath.isValid(in: myFetchedResultsController)

I get an error:

Cannot convert value of type 'NSFetchedResultsController!' to expected argument type 'NSFetchedResultsController'. Insert ' as! NSFetchedResultsController

What am I missing? MyModel is NSManagedObject so why is it asking me to cast it?

Answer 1

This was quicker than I thought. It turns out that changing the signature to include <T: NSManagedObject> fixes it:

func isValid<T: NSManagedObject>(in fetchedResultsController: NSFetchedResultsController<T>) -> Bool