enum Payload: Codable {
  case plain(String)
  case pair(key: String, value: String)
}

enum Payload_D: Decodable {
  case plain(String)
  case pair(key: String, value: String)

  private enum CodingKeys: CodingKey {
    case plain
    case pair
  }

  private enum PlainCodingKeys: CodingKey {
    case _0
  }

  private enum PairCodingKeys: CodingKey {
    case key
    case value
  }

  init(from decoder: any Decoder) throws {
    let container = try decoder.container(keyedBy: Payload_D.CodingKeys.self)

    var allKeys = ArraySlice(container.allKeys)

    guard let onlyKey = allKeys.popFirst(), allKeys.isEmpty else {
      throw DecodingError.typeMismatch(Payload_D.self, DecodingError.Context.init(codingPath: container.codingPath, debugDescription: "Invalid number of keys found, expected one.", underlyingError: nil))
    }
    switch onlyKey {
    case .plain:
      let nestedContainer = try container.nestedContainer(keyedBy: Payload_D.PlainCodingKeys.self, forKey: Payload_D.CodingKeys.plain)

      self = Payload_D.plain(try nestedContainer.decode(String.self, forKey: Payload_D.PlainCodingKeys._0))
    case .pair:
      let nestedContainer = try container.nestedContainer(keyedBy: Payload_D.PairCodingKeys.self, forKey: Payload_D.CodingKeys.pair)

      self = Payload_D.pair(key: try nestedContainer.decode(String.self, forKey: Payload_D.PairCodingKeys.key), value: try nestedContainer.decode(String.self, forKey: Payload_D.PairCodingKeys.value))
    }
  }
}

enum Payload_E: Encodable {
  case plain(String)
  case pair(key: String, value: String)
}