duplicates:

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

Issue Description:

The following code has unexpected results.

protocol Component: class {
    func test() -> Int
}

extension Component {
    func test() -> Int {
        return -1
    }
}

class BaseComponent: Component {
    let delegate = ComponentDelegate<BaseComponent>()

    init() {
        delegate.component = self
    }
}

class ChildComponent: BaseComponent {
    func test() -> Int {
        return 10
    }
}

final class ComponentDelegate<COMPONENT: Component> {
    weak var component: COMPONENT?

    func runTest() -> Int? {
        return component?.test()
    }
}

let x = BaseComponent()
let y = ChildComponent()

x.test() // Returns -1
y.test() // Returns 10

x.delegate.runTest() // Returns -1
y.delegate.runTest() // Returns -1

Results from the delegated functions seem to be wrong and the default implementation is being used. A way to workaround this is to implement the `test` method in the `BaseComponent` class. Then all child classes work as well.

I tried changing the `delegate` property type to `ComponentDelegate<ChildComponent>` and then forcefully casting `self` in the init, but this produces the same results = calling the default implementation.

Is this a bug in Swift or intended behavior? If it's an intended behavior, it seems a bit confusing to me and might be unnecessary constraining.