import UIKit

protocol A where Self: UIView { }
extension A {
  func method () {
    print("protocol A")
  }
}

protocol B: A { }
extension B {
  func method () {
    print("protocol B")
    (self as A).method() // EXC_BAD_ACCESS
  }
}

class X: UIView, B {
  func method () {
    print("class X")
    (self as B).method()
  }
}

let x = X()
x.method()