Diverging implicits when building serializer for generic types.

[sherzodv]

Hi, thank you for a cool project!

I've encountered the issue with resolving implicits, here is somewhat minimal case reproducing error in header:

trait Res[+A] extends Product {
  def code: Res.Code
}

object Res {
  sealed trait Code
  final case object Succ extends Code
  final case object Fail extends Code

  final case class Ok[A](data: A, code: Code = Succ) extends Res[A]
  final case class Error[A](message: String, code: Code = Fail) extends Res[A]
}

final case class Item(name: String)

package object implicits {
  import tethys._
  import tethys.jackson._
  import tethys.derivation.semiauto._

  implicit val itemReader: JsonReader[Item] = jsonReader[Item]
  implicit val itemWriter: JsonObjectWriter[Item] = jsonWriter[Item]

  implicit def resReader[A: JsonReader]: JsonReader[Res[A]] = 
    JsonReader.builder
      .addField[String]("code")
      .selectReader[Res[A]]{
        case "SUCCESS" => JsonReader.builder.addField[A]("data").buildReader(Res.Ok[A](_))
        case "FAILURE" => JsonReader.builder.addField[String]("message").buildReader(Res.Error[A](_))
      }

  implicit def okWriter[A: JsonObjectWriter]: JsonObjectWriter[Res.Ok[A]] =
    JsonWriter.obj[Res.Ok[A]]
      .addField("code")(_ => "SUCCESS")
      .addField("data")(_.asJson)
  implicit val errorWriter: JsonObjectWriter[Res.Error[_]] =
    JsonWriter.obj[Res.Error[_]]
      .addField("code")(_ => "FAILURE")
      .addField("message")(_.message)
}

...

>>>          Ok(List(Item("a"), Item("b"))).asJson
...
> diverging implicit expansion for type tethys.JsonWriter[List[io.expload.api.Item]]

[eld0727]

@sherzodv that example is not correct because okWriter requires JsonObjectWriter for A and List does not have it

Do you have better example to reproduce this issue?