Why won't this compile?

// Example of UIBarButtonItem instance
// let someButton = UIBarButtonItem(title: "A Title", style: .plain, target: nil, action: nil)

enum BarButton {
    case done(title: String)
    case edit(title: String)

    func button()->UIBarButtonItem {
        switch self {
      case .done(let title):
        return UIBarButtonItem(title: title, style: UIBarButtonItemStyle.done, target: nil, action: nil)
      case .edit(let title):
        return UIBarButtonItem(title: title, style: UIBarButtonItemStyle.plain, target: nil, action: nil)
      }
    }
}

let done = BarButton.done(title: "Save")
let button = done.button