//> 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) }
 }
 }
 }
}

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

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>'