enum Result<T> {
  case success(T)
  case failure(Error)
}

\Result<String>.success?.count
// KeyPath<Result<String>, Int?>

let kp = \Result<User>.success?.name
let result = Result.success(User(name: "an anonymous imp"))
result[keyPath: kp] = "Rumpelstiltskin"

if case let .success(user) = result {
  print(user.name)
}
// prints "Rumpelstiltskin"