How to provide a localized description with an Error type in Swift?

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I am defining a custom error type with Swift 3 syntax and I want to provide a user-friendly description of the error which is returned by the localizedDescription property of the Error object. How can I do it? 

public enum MyError: Error {
  case customError

  var localizedDescription: String {
    switch self {
    case .customError:
      return NSLocalizedString("A user-friendly description of the error.", comment: "My error")
    }
  }
}

let error: Error = MyError.customError
error.localizedDescription
// "The operation couldn’t be completed. (MyError error 0.)"

Is there a way for the localizedDescription to return my custom error description (“A user-friendly description of the error.”)? Note that the error object here is of type Error and not MyError. I can, of course, cast the object to MyError

(error as? MyError)?.localizedDescription

but is there a way to make it work without casting to my error type?

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