Chaining decoders

EntityDecoder.scala

import cats.effect.{IO, Sync}
import fs2.Stream
import io.circe.{Decoder, Encoder, Json}
import io.circe.generic.semiauto._
import io.circe.syntax._
import org.http4s.{EntityDecoder, EntityEncoder, Response, Status}
import org.http4s.circe._

sealed trait A
final case class B(error: String)   extends A
final case class C(message: String) extends A

implicit val bd: Decoder[B] = deriveDecoder[B]
implicit val be: Encoder[B] = deriveEncoder[B]
implicit val cd: Decoder[C] = deriveDecoder[C]
implicit val ce: Encoder[C] = deriveEncoder[C]

def decoder[F[_]: Sync]: EntityDecoder[F, A] =
  jsonOf[F, B].widen.orElse(jsonOf[F, C].widen)

def createBody[F[_]](js: Json): Stream[F, Byte] =
  implicitly[EntityEncoder[F, Json]].toEntity(js).body

val response1 = Response[IO](status = Status(200), body = createBody(B("error").asJson))
println(response1.attemptAs[A](decoder).value.unsafeRunSync())
// Left(org.http4s.InvalidMessageBodyFailure: Invalid message body: Could not decode JSON: {
//  "error" : "error"
//})


val response2 = Response[IO](status = Status(200), body = createBody(C("msg").asJson))
println(response2.attemptAs[A](decoder).value.unsafeRunSync())
// Right(C(msg))

Ouch!

Are you sure this isn't an issue with Circe?

Afaict no, because the solution was to rely on circe decoders directly: https://github.com/47degrees/github4s/pull/440/files#diff-b93ab619ec70a2dfcfee92460b1d5ac0R164-R167