考虑：

protocol Meh {
    func doSomething();
}

//Extend protocol Meh,where `Self` is of type `UIViewController`
//func blah() will only exist for classes that inherit `UIViewController`. 
//In fact,this entire extension only exists for `UIViewController` subclasses.

extension Meh where Self: UIViewController {
    func blah() {
        print("Blah");
    }

    func foo() {
        print("Foo");
    }
}

class Foo : UIViewController,Meh { //This compiles and since Foo is a `UIViewController` subclass,it has access to all of `Meh` extension functions and `Meh` itself. IE: `doSomething,blah,foo`.
    func doSomething() {
        print("Do Something");
    }
}

class Obj : NSObject,Meh { //While this compiles,it won't have access to any of `Meh` extension functions. It only has access to `Meh.doSomething()`.
    func doSomething() {
        print("Do Something");
    }
}

以下将给出编译器错误,因为Obj无法访问Meh扩展函数.

let i = Obj();
i.blah();

但是下面的方法会奏效.

let j = Foo();
j.blah();

换句话说,Meh.blah()仅适用于UIViewController类型的类.