Decode flat ADT with circe

error.json

{
  "error": "Some error message",
  "status": 409
}

response.json

{
  "type": "record",
  "id": 42,
  "version": 1
}

Response.scala

import io.circe._
import io.circe.generic.semiauto._

sealed trait Response

final case class Success(`type`: String, id: Int, version: Int)
  extends Response

object Success {
  implicit val successDecoder: Decoder[Success] = deriveDecoder[Success]
}

final case class Error(error: String, status: Int)
  extends Response

object Error {
  implicit val errorDecoder: Decoder[Error] = deriveDecoder[Error]
}

object Response {
  implicit def responseDecoder(
    implicit decodeError: Decoder[Error],
    decodeSuccess: Decoder[Success]
  ): Decoder[Response] = new Decoder[Response] {
    override def apply(cursor: HCursor): Result[Response] =
      if (cursor.downField("error").succeeded) {
        decodeError(cursor)
      } else {
        decodeSuccess(cursor)
      }
  }
}