relates to:

• SR-55 non-@objc protocol existentials do not conform to their own protocol type

Issue Description:

1. Let's declare three protocols Human, HasParent and CanCook:

protocol Human {
    func breathe()
}

protocol HasParent {
    associatedtype Parent

    var parent: Parent { get }
}

protocol CanCook {
    func cook()
}

2. Let's say we want to add a default implementation of the method cook() to every Human who also has a parent (conforming to HasParent protocol), but only if the parent can cook (Parent conforming to CanCook protocol).

extension Human where Self: CanCook & HasParent, Self.Parent: CanCook {
    func cook() {
        parent.cook()
    }
}

3. Lastly, let's have two classes that implement these protocols, one of them who implements the method cook() and another that has a parent, that we want to delegate the call to:

class Father: Human, CanCook {
    func breathe() { }

    func cook() { }
}


class Toddler: Human, HasParent, CanCook {
    let parent: Human & CanCook

    init(parent: Human & CanCook) {
        self.parent = parent
    }

    func breathe() { }
} 

Expected result would be that when calling cook() on the Toddler class, the parent.cook() would get called.

Actual result is compile error:

 error: SwiftBug.playground:27:7: error: type 'Toddler' does not conform to protocol 'CanCook'
class Toddler: Human, HasParent, CanCook {
      ^

SwiftBug.playground:16:10: note: candidate has non-matching type '<Self> () -> ()'
    func cook() {
         ^

SwiftBug.playground:12:10: note: protocol requires function 'cook()' with type '() -> ()'; do you want to add a stub?
    func cook()
         ^

Note that if we change the type of parent in Toddler to Father, it works as expected. As well as changing the type of parent in Toddler to CanCook and changing the extension definition so that the conformance states Self.Parent == CanCook instead of Self.Parent: CanCook.