I have two structs like this

struct LastKey:Equatable, MyProtocol {
  var id: UUID  
  let lowercase:String?

  var isValid:Bool {
    // calculate
  }
} 

struct FirstKey:Equatable, MyProtocol {
  var id: UUID
  let name:String?
}

protocol MyProtocol {
  var id:UUID { get set }
}

Then I decided to use this

func setLastKey(_ lastKey:MyProtocol) {
  let lastKey = LastKey()
  if lastKey.isValid { // do something }
}

But because lastKey is of type MyProtocol, and isValid is not on the protocol I have to add it as optional, because firstKey doesn’t have this property.

If I change the protocol to

@objc protocol MyProtocol {
  var id:UUID { get set }
  @objc optional var isValid:Bool { get set }
}

then the structs whine with the following error:

Non-class type 'FirstKey' cannot conform to class protocol 'MyProtocol'
Non-class type 'LastKey' cannot conform to class protocol 'MyProtocol'