Potential issue with deriving type classes that are already based on Mirrors

[matwojcik]

Hi,

I have a typeclass which is generated using Mirrors. There is a compilation failure returned when I use UnionDerivation on top of it.

Take a look at this minimal example:

//> using scala "3.5.2"
//> using lib "io.github.irevive::union-derivation-core:0.2.1"
//> using options "-Yretain-trees"

import scala.annotation.nowarn
import scala.deriving.Mirror

import io.github.irevive.union.derivation.IsUnion
import io.github.irevive.union.derivation.UnionDerivation

trait Show[A] {
  def show(value: A): String
}

object Show {
  inline given derivedUnion[A](using IsUnion[A]): Show[A] = UnionDerivation.derive[Show, A]

  @nowarn("msg=New anonymous class definition will be duplicated at each inline site")
  inline given instance[A](using m: Mirror.ProductOf[A]): Show[A] = {
    println(s"$m") // if you remove it, then it compiles
    new Show[A] {
      def show(value: A): String = value.toString()
    }
  }
}

case class A(x: String)
case class B(y: Int)

// it also compiles when you uncomment those:
// given Show[A] = Show.instance[A]
// given Show[B] = Show.instance[B]

@main def hello() = println(Show.derivedUnion[A | B].show(A("1")))

It returns

Error: Error while emitting example.scala
val m$proxy1

The error goes away if you either don't use the mirror (even if you declare it in using clause) or if you generate the instances outside of UnionDerivation macro.

[iRevive]

This change made the snippet compile:

- inline given instance[A](using m: Mirror.ProductOf[A]): Show[A] = {
+ inline given instance[A](using inline m: Mirror.ProductOf[A]): Show[A] = {

It may be a non-trivial compiler issue, but I didn't dive into it, to be honest.

[matwojcik]

Since 3.3.0 it is going to fail https://github.com/scala/scala3/issues/16804 when you use anything from this inlined param (like m.MirroredElemLabels) - see this for context.

(m : scala.deriving.Mirror.ProductOf[A]) is not a valid type prefix, since it is not an immutable path
[error]    |Inline parameters are not considered immutable paths and cannot be used as
[error]    |singleton types.
[error]    |
[error]    |Hint: Removing the `inline` qualifier from the `m` parameter
[error]    |may help resolve this issue.

I can see that they witnessed issues with it in circe - https://github.com/circe/circe/pull/2287 / https://github.com/circe/circe/pull/2278