In Swift, why subclass method cannot override the one, provided by protocol extension in superclass

Question

I know the title of this question is confusing but the weird behaviour is explained in the example below:

protocol Protocol {
    func method() -> String
}

extension Protocol {
    func method() -> String {
        return "From Base"
    }
}

class SuperClass: Protocol {
}

class SubClass: SuperClass {
    func method() -> String {
        return "From Class2"
    }
}

let c1: Protocol = SuperClass()
c1.method() // "From Base"
let c2: Protocol = SubClass()
c2.method() // "From Base"

How come c1.method() and c2.method() return the same? How come the method() in SubClass doesn't work?

Interestingly, without declaring the type of c2, this is going to work:

let c2  = SubClass()
c2.method() // "From Class2"

Answer 1

The problem is that c1 and c2 are of type Protocol, as you've defined their type explicitly this way (remember: protocols are fully fledged types). This means, when calling method(), Swift calls Protocol.method.

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If you define something like:

let c3 = SuperClass()

...c3 is of type SuperClass. As SuperClass has no more specific method() declaration, Protocol.method() is still used, when calling c3.method().

---

If you define something like:

let c4 = SubClass()

...c4 is of type SubClass. As SubClass does have a more specific method() declaration, SubClass.method() is used, when calling c4.method().

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You could also get c2 to call SubClass.method(), by down-casting it to `SubClass:

(c2 as! SubClass).method() // returns "From Class2"

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Here's a demonstration on SwiftStub.