Scala reflection: knownDirectSubclasses only works for sealed traits?

Given this question and SI-7046, this isn't at all what I expected.

scalac test.scala && scala Test in Scala 2.11.6 on the following prints an empty Set():

trait Foo
case class Bar() extends Foo
case class Baz() extends Foo

object Test {
  def main(args: Array[String]) {
    println( typeOf[Foo].typeSymbol.asClass.knownDirectSubclasses )
  }
}

However, if I change trait Foo to sealed trait Foo, it prints Set(class Bar, class Baz) as expected.

What's going on here?