Declarations in extensions cannot override yet error in Swift 4

Question

I have an extension:

public extension UIWindow {
    override public func topMostController()->UIViewController? { ... }
}

but for my

Answer 1

It will work if you make the base implementation @objc. See Hamish's answer for a detailed explanation about the internals.

Overriding methods declared in extensions is a bit tricky to do correctly. Objective-C supports it, but it's not absolutely safe. Swift aims to do it better. The proposal is not completed yet.

Current version of the proposal available here.