tailrec-2.scala:8: error: could not optimize @tailrec annotated method f: it contains a recursive call targeting a supertype
  @annotation.tailrec final def f[B >: A](mem: List[B]): List[B] = (null: Super[A]).f(mem)
                                                                                    ^
tailrec-2.scala:9: error: @tailrec annotated method contains no recursive calls
  @annotation.tailrec final def f1[B >: A](mem: List[B]): List[B] = this.g(mem)
                                ^
two errors found