duplicates:

• SR-103 Protocol Extension: function's implementation cannot be overridden by a subclass

Issue Description:

The following code compiles:

protocol P {
    func foo()
}

extension P {
    func foo() {
        print("P.foo()")
    }
}

class B : P {}

class D : B {
    func foo() {
        super.foo()
        print("D.foo()")
    }
}

Note in particular that foo() isn't decorated with override, but appears to be overriding the function. (Aside: in fact D's foo() is not an override of B's foo() which can be observed by casting D to be and calling foo(): (D() as B).foo() prints P.foo().)

Ideally, the behavior of

protocol P {
    func foo()
}
extension P {
    func foo() {
        print("P.foo()")
    }   
}

class B : P {
}

and

protocol P {
    func foo()
}

class B : P {
    func foo() {
        print("B.foo()")
    }   
}

would behave "the same" in the context of subclassing: In either case, the first code snippet would not compile (since the override keyword is missing), and instead the following would (which currently does not):

protocol P {
    func foo()
}

extension P {
    func foo() {
        print("P.foo()")
    }
}

class B : P {
}

class D : B {
    override func foo() {
        super.foo()
        print("D.foo()")
    }
}

and writing (D() as P).foo() would print

P.foo()
D.foo()