transform fails using oneof sealed_value with empty case

[andyczerwonka]

I was trying to follow the docs here, but I'm either doing something terrible wrong, or....

The model looks like this.

syntax = "proto3";

import "scalapb/scalapb.proto";

package staging.model;

option (scalapb.options) = {
  scope: PACKAGE
  flat_package: true
  package_name: "staging.model.grpcgen"
  preserve_unknown_fields: false
};

message Measure {
  double value = 1;
  optional string units = 2;
}

message Label {
  string value = 1;
}

message PropertyValue {
  oneof sealed_value {      
    Measure measure = 1;
    Label label = 2;
  }
}

message Property {
  string name = 1;
  PropertyValue value = 2;
  optional string description = 3;
}

And the domain model I'd like to map it to is:

sealed trait PropertyValue
object PropertyValue {
  case class Measure(value: Double, units: Option[String] = None) extends PropertyValue
  case class Label(value: String) extends PropertyValue
}

case class Property(name: String, value: PropertyValue, description: Option[String] = None)

Given ⬆️ , the client code should look like:

import staging.model.grpcgen as pb
import io.scalaland.chimney.dsl.*
import io.scalaland.chimney.protobufs.* // includes support for empty scalapb.GeneratedSealedOneof

val domainPropertyValue: PropertyValue = PropertyValue.Label("wombat")
val pbPropertyValue: pb.PropertyValue = pb.Label("wombat")

pbPropertyValue.transformIntoPartial[PropertyValue].asOption == Some(domainPropertyValue)

This is failing to compile.

Chimney can't derive transformation from staging.model.grpcgen.PropertyValue to staging.model.PropertyValue

staging.model.PropertyValue
  can't transform coproduct instance staging.model.grpcgen.PropertyValue.Empty to staging.model.PropertyValue

staging.model.PropertyValue
  derivation from empty: staging.model.grpcgen.PropertyValue.Empty to staging.model.PropertyValue is not supported in Chimney!

Consult https://chimney.readthedocs.io for usage examples.

Am I doing something wrong here or do the docs need to be updated?

[andyczerwonka]

If I follow the docs and use sealed_value_optional, the documented transformation works.

[MateuszKubuszok]

If it's Scala 3, then the issue is probably related to this test, and a reason it's manually edited. This code:

  // among subtypes of GeneratedSealedOneof only Empty <:< Singleton (case object),
  // it allows us to precisely that value and no other
  implicit def partialTransformerFromEmptySealedOneOfInstance[From <: scalapb.GeneratedSealedOneof with Singleton, To]
      : PartialTransformer[From, To] =
    PartialTransformer(_ => partial.Result.fromEmpty)

Scala 2 can resolve this implicit inside a macro, while Scala 3 doesn't. Scala 3 CAN resolve it outside a macro. I suspect the issue is that outside a macro it can perform check that Empty.type <:< Singleton while inside a macro it do not have this information (I cannot reproduce it with simplified code but that's what happens with Chimney). I could have sworn, that I reported this issue but apparently not. CC @jchyb.

[MateuszKubuszok]

I actually did reproduce and reported it, it's scala/scala3#21122. I remember after finding it in #532