Swift 4: generic conforming to AnyObject does not accept a protocol conforming to AnyObject

Question

I need to create an array of type-safe weak references

1. a structure that holds a 'type-safe' weak reference and can be an element of an array:

   public struct WeakRef<T: AnyObject>: Hashable {
       public weak var ref: T?
       public let hashValue: Int
   
       init(_ ref: T) {
           self.ref = ref
           self.hashValue = ObjectIdentifier(ref).hashValue
       }
   }
   
   extension WeakRef: Equatable {
       public static func ==(lhs: WeakRef<T>, rhs: WeakRef<T>) -> Bool {
           return lhs.hashValue == rhs.hashValue
       }
   }
   
   extension WeakRef: Comparable {
       static public func < (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue < rhs.hashValue
       }
       static public func <= (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue <= rhs.hashValue
       }
       static public func >= (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue >= rhs.hashValue
       }
       static public func > (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue > rhs.hashValue
       }
   }
   

2. I have a protocol which needs to be used with this weak ref:

   protocol LemmingTrackingProtocol: AnyObject {
       func onLemmingZPositionChanged()
       func onLemmingDrop()
   }
   

3. this is possible:

   var trackers = [WeakRef<LemmingTrackingProtocol>]()
   

4. but this is not:

   func addTracker(_ tracker: LemmingTrackingProtocol) {
       let tracker = WeakRef(tracker) // <-- Cannot invoke initializer...
   
       ...
   }
   

Please kindly give me a hint of what I am doing wrong. Thanks!

Answer 1

TL;DR:

just add @objc to required protocol (in my case LemmingTrackingProtocol)

Warning

though this works, you'll probably get stuck with not being able to create an extension with your @objc protocol

Detailed

according to @Hamish links provided:

1. this is a (sort of) bug: https://bugs.swift.org/browse/SR-55, and it is still present in Swift 4
2. protocols do not (should not) always conform to themselves