Attachment: Download

Issue Description:

Hi all,

Really unsure whether or not this is a bug, limitation, or I'm doing something stupid, but I've tried hard to resolve this and this seems like something that should be supported, but is not. Code:

import UIKit

protocol ContentViewController: class {
  var view: UIView! { get set }
}

protocol MapViewController: ContentViewController {}

protocol ContainerTabbleViewController: class {
  associatedtype HeaderViewController: ContentViewController
  var headerViewController: ContentViewController? { get set }
}

final class ViewController: UIViewController, ContainerTabbleViewController {
    typealias HeaderViewController = MapViewController
    var headerViewController: MapViewController?
}

I have a protocol that will be used to abstract some layout behaviour. It should work for any child view controller, but in the concrete implementation (ViewController) I would still like to have headerViewController expressed using a protocol (which as far as I can see, correctly inherits from the base protocol). This does not work, however:

error: type 'ViewController' does not conform to protocol 'ContainerTabbleViewController'
final class ViewController: UIViewController, ContainerTabbleViewController {
            ^
note: possibly intended match 'ViewController.HeaderViewController' (aka 'MapViewController') does not conform to 'ContentViewController'
    typealias HeaderViewController = MapViewController
              ^
note: protocol requires nested type 'HeaderViewController'; do you want to add it?
    associatedtype HeaderViewController: ContentViewController
                   ^

The odd part is the middle error: possibly intended match 'ViewController.HeaderViewController' (aka 'MapViewController') does not conform to 'ContentViewController'. However, in my implementation the MapViewController protocol certainly does inherit from ContentViewController. So it would seem that conforming to the protocol with a subtype of the protocol is not supported. This seems like it should work. I would like to use an extension to ContainerTabbleViewController to do the layout, which would only rely on the availability of the view property, which is expressed in ContentViewController. Then, in the context of ViewController I ought to be able to refer to the characteristics of my protocol subtype, providing it inherits from the correct protocol to satisfy ContainerTabbleViewController

Apologies in advance if I'm making an error here, but this certainly seems wrong and ought to be supported. Many thanks!