Comment by Ben A (JIRA)

This is pretty confusing to me. I'd be grateful to know if there's a workaround that doesn't involve modifying the protocol, since the protocol I'm dealing with in the real world lives in UIKit.

:-( I think the only workaround right now is to define an Objective-C function that does it (it can be static inline). We've put off dealing with this because it comes up fairly rarely.

Comment by Ben A (JIRA)

Darn okay. I've come up with another workaround that's slightly more verbose, but it helps when dealing with UIKit (or other one off cases where something like this is needed). In my real word case, I want to configure the UITextInputTraits keyboard properties the same way for both a UITextView and a UITextField. To accomplish that, I ended up with something like this:

protocol TextInputKeyboardConfiguration: class {
    var keyboardType: UIKeyboardType { get set }
    /* copy pattern here for other configuration you want to do */
}

extension UITextField: TextInputKeyboardConfiguration {}
extension UITextView: TextInputKeyboardConfiguration {}

// You can now use a subtype existential like this to manage their setup (add as subviews, configure layout, and configure UITextInputTraits properties that you add to this protocol):
typealias KeyboardConfigurableView = UIView & TextInputKeyboardConfiguration