How to get a type of a case class from a reference type of its companion object

[soronpo]

Since it's not possible to get a type from a symbol, is there a way to get the case class type from the type of its companion object reference?

import scala.quoted.*
trait CaseClass[Companion <: AnyRef]:
  type CC <: Product
object CaseClass:
  transparent inline given [Comp <: AnyRef]: CaseClass[Comp] = ${
    macroImpl[Comp]
  }
  def macroImpl[Comp <: AnyRef](using
      Quotes,
      Type[Comp]
  ): Expr[CaseClass[Comp]] =
    import quotes.reflect.*
    val compObjTpe = TypeRepr.of[Comp]
    val clsSym = compObjTpe.typeSymbol.companionClass
    val clsType = ???
    '{
      new CaseClass[Comp]:
        type CC = clsType.Underlying
    }
end CaseClass

[bishabosha]

to do this, you can get the prefix of the type of the companion object, then use select on the prefix with the case class symbol, you need to explicitly refine the type of the anonymous class also to extract a usable CC type from the returned value (I use aux pattern also, but unnecessary):

import scala.quoted.*

trait CaseClass[Companion <: AnyRef]:
  type CC <: Product

object CaseClass:

  type Aux[Companion <: AnyRef, Case <: Product] = CaseClass[Companion] { type CC = Case }

  transparent inline given [Comp <: AnyRef]: CaseClass[Comp] = ${
    macroImpl[Comp]
  }
  def macroImpl[Comp <: AnyRef](using
      Quotes,
      Type[Comp]
  ): Expr[CaseClass[Comp]] =
    import quotes.reflect.*
    val compObjTpe = TypeRepr.of[Comp]
    val compPrefix = compObjTpe match
      case TermRef(pre, _) => pre
      case _ => report.throwError("Case class companion must be a term ref")
    val clsSym = compObjTpe.typeSymbol.companionClass
    if !clsSym.paramSymss.forall(_.headOption.forall(_.isTerm)) then
      report.throwError("Case class with type parameters are not supported")
    val clsTpe = compPrefix.select(clsSym)
    clsTpe.asType match
      case '[t & Product] =>
        type Case = t & Product
        '{ (new CaseClass[Comp] { type CC = Case }): CaseClass.Aux[Comp, Case] }
end CaseClass

and here is test code:

case class Foo(i: Int) derives CanEqual

@main def run =
  val cc = summon[CaseClass[Foo.type]]
  val mirror = summon[deriving.Mirror.ProductOf[cc.CC]]
  assert(mirror.fromProduct(Tuple1(23)) == Foo(23))

This code will need more work probably for inner/local classes, and for classes with type parameters, but there is the bones here of something that could work

[soronpo]

Thanks!