Formatters for an ADT needs to be placed after all instances

[alexbergeron]

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The following code fails to compile with a could not find Lazy implicit value of type julienrf.json.derived.DerivedReads[A]:

import julienrf.json.derived
import play.api.libs.json._

sealed trait Minimize

object Minimize {
  implicit val formats: OFormat[Minimize] = derived.flat.oformat((__ \ "type").format[String])
}

case class MinimizeA(a: String) extends Minimize

case class MinimizeB(b: Int) extends Minimize

The following code compiles successfully:

import julienrf.json.derived
import play.api.libs.json._

sealed trait Minimize

case class MinimizeA(a: String) extends Minimize

case class MinimizeB(b: Int) extends Minimize

object Minimize {
  implicit val formats: OFormat[Minimize] = derived.flat.oformat((__ \ "type").format[String])
}

As there is a workaround, this is not a critical bug, but it felt like a "gotcha" moment.

[julienrf]

If I remember well it’s a limitation of knownDirectSubclasses. I think this PR https://github.com/scala/scala/pull/5284 fixed the issue, though. Are you using Scala 2.12?