Protocol extensions unexpected implementation

protocol-extension-unexpected-implementation.swift

protocol MyProtocol {
    func doSomething()
}


extension MyProtocol {
    func doSomething() {
        print("default impl")
    }
}

class A: MyProtocol {}

class B: A {
    func doSomething() {
        print("B impl")
    }
}

let b: MyProtocol = B()
b.doSomething() // Prints "default impl"

////

protocol MyProtocol {
    func doSomething()
}


extension MyProtocol {
    func doSomething() {
        print("default impl")
    }
}

class A: MyProtocol {
    func doSomething() {
        print("A impl")
    }
}

class B: A {
    override func doSomething() {
        print("B impl")
    }
}

let b: MyProtocol = B()
b.doSomething() // Prints "B impl"

Xcode 7.2.1