Classes should be disallowed from defining properties incompatible with their protocols...EVEN IF there is a default

This is particularly what I mean:

class Base {}
class Derived: Base {}

protocol Proto {
    var items: Array<Base> { get }
}

extension Proto {
    var items: Array<Base> {
        return []
    }

    func show() {
        print("Number of items: \(items.count)")
    }
}

class Example: Proto {
    let items = [Derived()]// as Array<Base>
}

let ex = Example()
ex.show()

This stumped me for a bit in one of my programs. Here, Example.items is actually Array<Derived>, so it’s not overriding the Proto's default value. If I cast it to Array<Base>, then it all works.

Should this be allowed? I can’t imagine how this would ever be useful…

I think this had been recently discussed at Can property shadowing warnings be improved? (re: SR-6689). @Marc_Palmer suggests that the compiler emits a warning in such case.