S_3.scala:1: error: class Sub needs to be abstract, since:
it has 2 unimplemented members.
/** As seen from class Sub, the missing signatures are as follows.
 *  For convenience, these are usable as stub implementations.
 */
  def raw(x$1: M_1[_ <: String]): Unit = ???
  def raw(x$1: Any): Unit = ???

class Sub extends Raw_2 { }
      ^
one error found