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()

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.