Opaque types with bounds

[kamilkloch]

In the code below deriving JsonValueCodec[A] ends in a compilation error.
When definition of ReqId is changed to opaque type ReqId = Option[String], derivation succeeds.

object Wrappers {
    opaque type ReqId <: Option[String] = Option[String]

    object ReqId {
      inline def apply(x: Option[String]): ReqId = x

      given JsonValueCodec[ReqId] = JsonCodecMaker.make[Option[String]]
    }
}

object JsoniterOpaque extends App {
  import Wrappers.*

  println(writeToString(ReqId(None))) // null
  println(readFromString[ReqId]("null")) // None
  
  case class A(r: ReqId)

/** Exception occurred while executing macro expansion.
*    java.util.NoSuchElementException: head of empty list
*/
  // given JsonValueCodec[A] = JsonCodecMaker.make 
}

[plokhotnyuk]

Hi, Kamil!

Thanks for reporting an issue!

Opaque types are not supported yet and I suppose that a custom codec for ReqId cannot be summoned due to a known bug in the Scala 3 compiler: scala/scala3#21199

There are several examples in tests where custom codec for opaque type works fine:
https://github.com/plokhotnyuk/jsoniter-scala/tree/master/jsoniter-scala-macros/shared/src/test/scala-3/com/github/plokhotnyuk/jsoniter_scala/macros

Probably opaque type ReqId = Option[String] works only because TypeRepr.dealias converts it into Option[String].

In v2.35.2 release I've added a proper message for the compilation failure instead of java.util.NoSuchElementException: head of empty list.