Swift 4 crash

File.swift

public protocol A {
    associatedtype V
    associatedtype E
    var a: V { get }
}

public protocol B {
    associatedtype V
    var b: V { get }
}

class C<V, E> {
    var v: V? = nil
    var e: E? = nil
}

extension C: A {
    typealias V = C.V
    typealias E = C.E
    var a: C.V {
        return v!
    }
}

extension C: B {
    typealias V = C.E
    var b: C.E {
        return e!
    }
}