struct User {
  let firstName: String
  let lastName: String?
}

extension User: Codable {

    private enum CodingKeys: CodingKey {
      case firstName
      case lastName
    }

    init(from decoder: any Decoder) throws {
      let container: KeyedDecodingContainer<User.CodingKeys> = try decoder.container(keyedBy: User.CodingKeys.self)

      self.firstName = try container.decode(String.self, forKey: User.CodingKeys.firstName)
      self.lastName = try container.decodeIfPresent(String.self, forKey: User.CodingKeys.lastName)

    }

    func encode(to encoder: any Encoder) throws {
      var container = encoder.container(keyedBy: User.CodingKeys.self)

      try container.encode(self.firstName, forKey: User.CodingKeys.firstName)
      try container.encodeIfPresent(self.lastName, forKey: User.CodingKeys.lastName)
    }
}


struct Generic<Value> {
  var value: Value
}

extension Generic {
}

extension Generic: Codable where Value: Codable {
}


struct Outer {
  struct Inner {
    let value: Int
  }
}

extension Outer.Inner: Codable {
}