`WeakReference` content can't be compared with `null`.

[nicolasstucki]

def unapply[T <: AnyRef](wr: WeakReference[T]): Option[T] = {
    val x = wr.underlying.get
    if (x != null) Some(x) else None
  }

fails with

 33 |    if (x != null) Some(x) else None
    |       ^^^^^^^^^^^
    |       Values of types wr.underlying.java$lang$ref$WeakReference$$T and Null cannot be compared with == or !=

This happens when whitelisting scala/ref/WeakReference.scala.

[odersky]

Felix, I think this one is for you :-)