Why can’t you use a generic Self reference in a Swift 3.2/4 KeyPath typealias?

Question

Why can't you use a generic Self reference in a Swift 3.2/4 KeyPath typealias?

Example: the below code compiles unless you uncomment the line marked // B . Why? From what I can reason, the line marked // B should mean exactly the same thing as the line marked // A , due to the statement typealias Path<T> = KeyPath<Self,T> .

Am I missing something here, or is this a Swift compiler bug?

protocol Fooer {
    associatedtype T
    typealias Path<T> = KeyPath<Self, T>
    var baz: T { get }
}

protocol FooPathContainable {
    associatedtype F: Fooer
    associatedtype T where F.T == T
    var fooerPathA: KeyPath<F,T> { get set } // A  
    var fooerPathB: F.Path<T> { get set } // B  <— causes compiler crash with Segmentation Fault 11
}

Answer 1

root, value of keypath, inside angular brackets, is required to declare a keypath. It is like declaring Array with type in Swift. This should work:

 var fooerPathB: F.Path<F, T> { get set }