error while extending existential class

Question

Let test simple construction:

case class ListRef (list : List[_]) extends ref.WeakReference(list)

It gives error:

<console>:8: error: illegal inheritance;
self-type ListRef does not conform to scala.ref.WeakReference[List[_$1]]'s selftype  scala.ref.WeakReference[List[_$1]]
   case class ListRef (list : List[_]) extends WeakReference(list)

What does it mean and how can it be escaped?

Answer 1

I don't know what that error message means but escaping it seems pretty straightforward.

case class ListRef[A](list: List[A]) extends ref.WeakReference(list)