Anticipated by https://bugs.swift.org/browse/SR-1576

A not-so-pretty workaround: use a wrapper type.

import Photos

struct ResultSequence<Element: AnyObject>: Sequence {
  var result: PHFetchResult<Element>
  init(_ result: PHFetchResult<Element>) {
    self.result = result
  }
  func makeIterator() -> NSFastEnumerationIterator {
    return NSFastEnumerationIterator(self.result)
  }
}


func test(_ request: PHFetchResult<PHCollection>) {
  for elem in ResultSequence(request) {
    print(elem)
  }
}

@belkadan thanks!

Yes, this is the same problem as SR-1576. Conforming with AnyObject as your element type should work too.

@jckarter Does it depend on https://github.com/apple/swift-evolution/blob/master/proposals/0143-conditional-conformances.md being implemented? If so, not usable currently.

No, just unconditionally use `typealias Element = AnyObject` and force-cast to PHFetchResult<AnyObject>.

@jckarter I'm lost. Where to put `typealias Element = AnyObject`? `Sequence` doesn't have the associated type `Element`.

I meant something like this:

import Photos

extension PHFetchResult: Sequence {
    public func makeIterator() -> FetchResultIterator {
        return FetchResultIterator(self as! PHFetchResult<AnyObject>)
    }
}

@objc public class FetchResultIterator: NSObject, IteratorProtocol {
  // iterator implementation here
}

Extensions of ObjC generic classes are limited to being @objc-compatible, so you need to use an iterator class that's @objc. NSFastEnumeratorIterator is not ObjC-representable.

@jckarter Thanks! It's a better workaround because only the extension implementation needs patches.

Comment by Marc Knaup (JIRA)

The following code works fine in release mode but crashes at runtime in debug mode in some cases (e.g. when calling .map() on the sequence) 🙁

extension NSFastEnumeration {

    public func makeIterator() -> NSFastEnumerationIterator {
        return NSFastEnumerationIterator(self)
    }
}

extension PHFetchResult: Sequence {}

Looks like that no matter how you make PHFetchResult conform to Sequence, it will randomly crash at runtime in debug mode - also with @jckarter's suggestion.

@swift-ci create