knownDirectSubclasses.scala

scala> :paste
// Entering paste mode (ctrl-D to finish)

def impl[T: c.WeakTypeTag](c: Context): c.Tree = {
  import c.universe._
  def subs = weakTypeOf[T].typeSymbol.asClass.knownDirectSubclasses
  val subs1 = subs
  println(subs1)
  val global = c.universe.asInstanceOf[tools.nsc.Global]
  def checkSubsPostTyper = if (subs1 != subs)
    c.error(c.macroApplication.pos, "sealed descendents appears after macro exansion")
  val dummyTypTree =
    new global.TypeTreeWithDeferredRefCheck()(() => { checkSubsPostTyper ; global.TypeTree(global.NoType) }).asInstanceOf[TypTree]
  q"type T = $dummyTypTree; ${subs1.size} : _root_.scala.Int"
}
def numChildren[T]: Int = macro impl[T]

// Exiting paste mode, now interpreting.

impl: [T](c: scala.reflect.macros.blackbox.Context)(implicit evidence$1: c.WeakTypeTag[T])c.Tree
defined term macro numChildren: [T]=> Int

scala> numChildren[List[_]]
Set(class ::, object Nil)
res14: Int = 2

scala> object Test {sealed abstract class C; val x = numChildren[C]; class E extends C }
Set()
<console>:13: error: sealed descendents appears after macro exansion
       object Test {sealed abstract class C; val x = numChildren[C]; class E extends C }
                                                                ^

scala>