Potential issue with deriving type classes that are already based on Mirrors #182
Description
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.