Polymorphic Method Macro

poly.scala

object PolymorphicMethod {
  inline def make[T]: Unit = ${ impl[T] }

  def impl[T: Type](using Quotes): Expr[Unit] = {
    import quotes.reflect.*

    def decls(owner: Symbol): List[Symbol] = {
      val targetSymbol = TypeRepr.of[T].typeSymbol
      val abstractDef: DefDef = targetSymbol.declaredMethod("poly").head.tree.asInstanceOf[DefDef]

      val methodType = PolyType(List("A"))(
        _ => List(TypeBounds(TypeRepr.of[Nothing], TypeRepr.of[Any])),
        pt => MethodType(List("x"))(
          _ => abstractDef.termParamss.head.params.map(_.tpt.tpe),
          _ => abstractDef.returnTpt.tpe
        )
      )

      List(
        Symbol.newMethod(owner, "poly", methodType)
      )
    }

    val parents = List(TypeTree.of[Object], TypeTree.of[T])
    val classSymbol = Symbol.newClass(
      Symbol.spliceOwner,
      "ImplT",
      parents.map(_.tpe),
      decls,
      None
    )

    val impls = {
      val polySymbol = classSymbol.declaredMethod("poly").head

      List(
        DefDef(polySymbol, argss => {
          val paramTerm = argss.drop(1).head.head match {
            case i: Ident => i.underlying
          }
          
          Some(Select.unique(paramTerm, "head"))
        })
      )
    }

    val classDef = ClassDef(classSymbol, parents, impls)
    val unit = '{ () }

    val result = Block(List(classDef), unit.asTerm).asExprOf[Unit]

    println(result.show)

    result
  }

}

I am calling this code from a test:

trait TestTrait {
  def poly[A](x: List[A]): A
}

object Thing {
  PolymorphicMethod.make[TestTrait]
}

The println produces this:

{
  class ImplT extends java.lang.Object with com.gregghz.macros.TestTrait {
    override def poly[A](x: scala.collection.immutable.List[A]): A = x.head
  }
  ()
}

But compilation fails with this:

[error] 8 |  PolymorphicMethod.make[TestTrait]
[error]   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]   |class ImplT needs to be abstract, since def poly[A](x: List[A]): A in trait TestTrait in package com.gregghz.macros is not defined 
[error]   |(Note that
[error]   | parameter List[A] in def poly[A](x: List[A]): A in trait TestTrait in package com.gregghz.macros does not match
[error]   | parameter List[A] in def poly[A](x: List[A]): A in class ImplT
[error]   | their type parameters differ)