adt.scala

sealed trait Foo
case object X extends Foo
case class Y(i: Int) extends Foo

macro.scala

import scala.language.experimental.macros
import scala.reflect.macros.Context

object Example {
  def childPositions[A] = macro childPositions_impl[A]

  def childPositions_impl[A: c.WeakTypeTag](c: Context) = {
    import c.universe._

    weakTypeOf[A].typeSymbol.asClass.knownDirectSubclasses.toList.map(
      _.asInstanceOf[scala.reflect.internal.Symbols#Symbol].pos
    ).foreach(println)

    c.literalUnit
  }
}