overloaded-unapply.scala:18: error: ambiguous reference to overloaded definition,
both method unapply in object List of type [a](xs: List[a])Option[Null]
and  method unapply in object List of type [a](xs: List[a])Option[(a, List[a])]
match argument types (List[a])
    case List(x, xs) => 7
         ^
overloaded-unapply.scala:22: error: cannot resolve overloaded unapply
    case List(x, xs) => 7
         ^
overloaded-unapply.scala:12: error: method unapply is defined twice
  conflicting symbols both originated in file 'overloaded-unapply.scala'
  def unapply[a](xs: List[a]): Option[Null] = xs match {
      ^
three errors found