Cannot specialize a Scala method with specializable trait as return type

[Tongfei Chen]

trait Eq[@specialized -X] {
  def eq(x: X, y: X): Boolean
}

trait Keyed[@specialized(Int) X] {
  def eqOnKey: Eq[X]
}

The method eqOnKey is not specialized in the generated class Keyed$mcI$sp.

How can I specialize this method, i.e. making the return type of method eqOnKey$mcI$sp in class Keyed$mcI$sp to be Eq$mcI$sp?