Don't understand how this wouldn't be exhaustive. What would be the default clause here then?

import Foundation

enum UIBarButtonStyle {
    case Done
    case Plain
    case Bordered
}

class UIBarButtonItem {

    var title: String?
    let style: UIBarButtonStyle
    var target: AnyObject?
    var action: Selector

    init(title: String?, style: UIBarButtonStyle, target: AnyObject?, action: Selector) {
        self.title = title
        self.style = style
        self.target = target
        self.action = action
    }
}

enum Button {
    case Done(String)
    case Edit(String)

    func ToUIBarButtonItem() -> UIBarButtonItem {
      switch self {
        case .Done("Done"): return UIBarButtonItem(title: "Done", style: UIBarButtonStyle.Done, target: nil, action: nil)
        case .Edit("Edit"): return UIBarButtonItem(title: "Edit", style: UIBarButtonStyle.Plain, target: nil, action: nil)
      }
    }  
}
let done = Button.Done("Done")