Wrapper for transient properties to use with synthesized protocol defaults

Transient.swift

/// Wraps a value but causes it to be treated as "value-less" for the purposes
/// of automatic Equatable, Hashable, and Codable synthesis. This allows one to
/// declare a "cache"-like property in a value type without giving up the rest
/// of the benefits of synthesis.
public enum Transient<Wrapped>: Equatable, Hashable, Codable {

  case none
  case some(Wrapped)

  public static func == (lhs: Transient<Wrapped>, rhs: Transient<Wrapped>) -> Bool {
    // By always returning true, transient values never produce false negatives
    // that cause two otherwise equal values to become unequal. In other words,
    // they are ignored for the purposes of equality.
    return true
  }

  public var hashValue: Int {
    // Transient values do not contribute to the hash value. (This could be any
    // constant.)
    return 0
  }

  public init(from decoder: Decoder) {
    // Since the value is transient, it's not encoded, so at decode-time we
    // merely clear it.
    self = .none
  }

  public func encode(to encoder: Encoder) throws {
    // Transient properties do not get encoded.
  }
}