Scala3: Deriving a JsonDecoder for a type having a phantom type doesn't seem to work

[jivanic-demystdata]

I have this code:

final case class Example[State <: ValidationState](a: String, b: Int) derives JsonDecoder

object Example {
  type UnvalidatedExample = Example[ValidationState.Unvalidated]
  type ValidatedExample   = Example[ValidationState.Validated]
  
  enum ValidationState {
    case Unvalidated, Validated 
  }
  object ValidationState {
    type Unvalidated = Unvalidated.type
    type Validated   = Validated.type
  }
  
  // Doesn't compile
  given JsonDecoder[UnvalidatedExample] = JsonDecoder[Example[?]].asInstanceOf[JsonDecoder[UnvalidatedExample]]
}

which fails to compile with error:

No given instance of type zio.json.JsonDecoder[com.example.Example[?]] was found for parameter a of method apply in object JsonDecoder.

[johnhungerford]

Why are you trying to summon the decoder instead of deriving it? This compiles:

import zio.json.*
import Example.ValidationState


final case class Example[State <: ValidationState](a: String, b: Int) derives JsonDecoder

object Example {
  type UnvalidatedExample = Example[ValidationState.Unvalidated]
  type ValidatedExample   = Example[ValidationState.Validated]
  
  enum ValidationState {
    case Unvalidated, Validated 
  }
  object ValidationState {
    type Unvalidated = Unvalidated.type
    type Validated   = Validated.type
  }
  
  given JsonDecoder[UnvalidatedExample] = DeriveJsonDecoder.gen[Example[?]].asInstanceOf[JsonDecoder[UnvalidatedExample]]
}

[johnhungerford]

Never mind -- I missed that it's the deriving bit you were asking about. Not sure why that doesn't work, as JsonDecoder.derived just calls DeriveJsonDecoder.gen.