たまたま見つけたんですが

import UIKit protocol SomeChild where Self : UIViewController { func doSomething() } extension SomeChild { func doSomething() { print(self.title) } } class ChildViewController: UIViewController, SomeChild {} class ParentViewController: UIViewController { var children: [SomeChild] = [ChildViewController()] var child: SomeChild = ChildViewController() } let parent = ParentViewController() parent.child.doSomething() parent.children.forEach { $0.doSomething() }

childにアクセスすると問答無用でEXC_BAD_ACCESSになる

11:34 AM

extension側に where句付けると問題なし

protocol SomeChild { func doSomething() } extension SomeChild where Self : UIViewController { func doSomething() { print(self.title) } }