我有以下代碼：import UIKitprotocol Fooable: class where Self: UIViewController {    func foo()}class SampleViewController: UIViewController, Fooable {    func foo() {        print("foo")    }}let vc1: Fooable = SampleViewController()let vc2: Fooable = SampleViewController()// vc1.show(vc2, sender: nil) - error: Value of type 'Fooable' has no member 'show'// (vc1 as! UIViewController).show(vc2, sender: nil) - error: Cannot convert value of type 'Fooable' to expected argument type 'UIViewController'(vc1 as! UIViewController).show((vc2 as! UIViewController), sender: nil)注釋行無法編譯。為什么UIViewController即使Fooable協(xié)議要求，我也必須強(qiáng)制將協(xié)議類型對(duì)象強(qiáng)制轉(zhuǎn)換為符合它的類型的繼承對(duì)象UIViewController？