Tagged unions with custom serializers not working

[edwardcwang]

Scala 3.1.1, upickle 2.0.0

import upickle.default._

sealed trait BooleanOrInt
object BooleanOrInt {
  implicit val rw: upickle.default.ReadWriter[BooleanOrInt] = upickle.default.macroRW
}
case class IsBoolean(val value: Boolean) extends BooleanOrInt
object IsBoolean {
  implicit val rw: upickle.default.ReadWriter[IsBoolean] = upickle.default.readwriter[Boolean].bimap[IsBoolean](_.value, IsBoolean(_))
}
case class IsInt(val value: Int) extends BooleanOrInt
object IsInt {
  implicit val rw: upickle.default.ReadWriter[IsInt] = upickle.default.readwriter[Int].bimap[IsInt](_.value, IsInt(_))
}

val x: BooleanOrInt = IsBoolean(false); upickle.default.write(x)

Stacktrace:

java.lang.ClassCastException: class upickle.core.Types$$anon$1 cannot be cast to class upickle.core.Types$TaggedWriter (upickle.core.Types$$anon$1 and upickle.core.Types$TaggedWriter are in unnamed module of loader 'app')
  upickle.core.Types.upickle$core$Types$$scanChildren(Types.scala:323)
  upickle.core.Types$TaggedWriter$Node.findWriter(Types.scala:361)
  upickle.core.Types$$anon$2.findWriter(Types.scala:50)
  upickle.core.Types$TaggedWriter.write0(Types.scala:348)
  upickle.core.Types$TaggedWriter.write0$(Types.scala:345)
  upickle.core.Types$$anon$2.write0(Types.scala:47)
  upickle.core.Types$Writer.write(Types.scala:130)
  upickle.core.Types$Writer.write$(Types.scala:118)
  upickle.core.Types$$anon$2.write(Types.scala:47)
  upickle.core.Types$Writer.transform(Types.scala:126)
  upickle.core.Types$Writer.transform$(Types.scala:118)
  upickle.core.Types$$anon$2.transform(Types.scala:47)
  upickle.Api$transform.transform(Api.scala:124)
  upickle.Api$transform.to(Api.scala:125)
  upickle.Api.write(Api.scala:48)
  upickle.Api.write$(Api.scala:17)
  upickle.default$.write(Api.scala:157)

[lihaoyi]

This does not work in Scala 2 as well. The issue seems to be that macroRW expects all subclasses to be macroRW as well, and does not play nicely with the custom bimaped readwriters. A workaround could be to make BooleanOrInt.rw a custom readwriter as well