case objects desugaring needs to implement Product0

[DarkDimius]

./scala-scala/src/library/scala/collection/immutable/List.scala:417: error: object creation impossible, since:
it has 2 unimplemented members.
/** As seen from module class Nil$, the missing signatures are as follows.
 *  For convenience, these are usable as stub implementations.
 */
  def productArity: => Int = ???
  def productElement: (n: Int)Any = ???

case object Nil extends List[Nothing] {
                  ^

[smarter]

Duplicate of #723.

[odersky]

Fixed by now.