Instantiating a forwarder trait

[kubukoz]

Hi! I'm trying to build a macro that builds a proxy for arbitrary traits. I'm having trouble using the parameters the method is called with.

Here's what the macro looks like:

//> using scala "3.2.1"
//> using option "-Ycheck:all"
//> using option "-Xcheck-macros"
////> using option "-Xprint:inlining"
//> using option "-Xprint-types"
import scala.quoted._
import scala.annotation.experimental

object macros {

  trait Proxxy[Alg] {
    def proxify(fa: Alg): Alg
  }

  inline def mkProxy[Alg]: Proxxy[Alg] = ${ mkProxyImpl[Alg] }

  @experimental
  def mkProxyImpl[Alg: Type](using Quotes): Expr[Proxxy[Alg]] = {
    val Typ = Type.of[Alg]

    import quotes.reflect._
    import quotes.reflect.given

    def instance(
      fExpr: Expr[Alg]
    )(
      using Quotes
    ): Expr[Alg] = {
      val par = TypeRepr.of[Alg]
      def methodSymbols(
        owner: Symbol
      ): List[Symbol] = par.typeSymbol.declaredMethods.map { baseMethod =>

        val deff = baseMethod.tree.asInstanceOf[DefDef]

        val paramVals = deff.paramss.flatMap(_.params.asInstanceOf[List[ValDef]])

        Symbol.newMethod(
          parent = owner,
          baseMethod.name,
          TypeRepr.of[Alg].memberType(baseMethod),
        )
      }

      val cl = Symbol.newClass(
        Symbol.spliceOwner,
        "$anon",
        parents = List(
          TypeRepr.of[Object],
          par,
        ),
        decls = methodSymbols(_),
        selfType = None,
      )

      val defs = methodSymbols(cl).map { m =>
        DefDef(
          m,
          args =>
            Some {
              fExpr
                .asTerm
                .select(cl.declaredMethod(m.name).head)
                .appliedToArgss(
                  args.map(
                    _.map { a =>
                      // the problematic part, apparently
                      a.asExpr.asTerm
                    }
                  )
                )
              // should I do this? doesn't seem to have an effect
              // .changeOwner(m)
            },
        )

      }

      Block(
        List(
          ClassDef(cl, Nil, defs)
        ),
        New(TypeTree.ref(cl))
          .select(cl.primaryConstructor)
          .appliedToArgs(Nil),
      ).asExprOf[Alg]
    }

    '{
      new Proxxy[Alg] {
        def proxify(fa: Alg): Alg = ${ instance('fa) }
      }
    }
  }

}

I'm trying it with this trait:

trait MyAlg {
  def hello(s: String): Int
}

and the error I'm getting is:

Error: Unexpected error when compiling project_466232d218: 'Could not find proxy for s: String in [parameter s, method hello, anonymous class Object with demo.MyAlg {...}, value macro, getter derivedProxy, object demo, package <empty>, package <root>], encl = package <empty>, owners = package <empty>, package <root>; enclosures = package <empty>, package <root>'

I've tried dozens of combinations of moving things around, changing owners, different ways to refer to the parameter, and I keep running into the same issue. I would appreciate any and all help and resources on this.

As you can see, I'm using -Xcheck-macros and -Xcheck:all, but they don't offer any help.

Related: #13571

[bishabosha]

I don't think you should be calling methodSymbols twice, that will create two sets of independent symbols

[kubukoz]

hmm, I guess there's a way I didn't consider before... lemme try that.

[kubukoz]

yeah, that does the trick! Is there something we can do about the error message to make the problem easier to understand?

[kubukoz]

also, are the Symbol.new*** the only impure parts of the reflect API or are there more?