Enum cases as protocol witnesses

Evolution Pitches
134

I’ve been aching for this feature for what feels like ages! My main use case is an event-driven UI (and not only UI) architecture:


public protocol InputEventHandler {
    associatedtype InputEvent
    func handle(_ event: InputEvent)
}

public protocol CanCloseInputEvent {
    static var close: Self { get }
}

public protocol CanMinimizeInputEvent {
    static var minimize: Self { get }
}

public func doSomethingWith<W>(window: W) where W: InputEventHandler, W.Event: CanCloseInputEvent & CanMinimizeInputEvent {
    // ...
}

struct MyWindow: InputEventHandler {
    enum InputEvent: CanCloseInputEvent, CanMinimizeInputEvent {
        case close
        case minimize
    }
    func handle(_ event: InputEvent) {
        // ...
    }
}
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