tailrec.scala:43: error: could not optimize @tailrec annotated method: it contains a recursive call not in tail position
  def facfail(n: Int): Int =
      ^
tailrec.scala:50: error: could not optimize @tailrec annotated method: it is neither private nor final so can be overridden
  @tailrec def fail1(x: Int): Int = fail1(x)
               ^
tailrec.scala:53: error: could not optimize @tailrec annotated method: it contains a recursive call not in tail position
  @tailrec final def fail2[T](xs: List[T]): List[T] = xs match {
                     ^
tailrec.scala:59: error: could not optimize @tailrec annotated method: it is called recursively with different type arguments
  @tailrec final def fail3[T](x: Int): Int = fail3(x - 1)
                     ^
tailrec.scala:63: error: could not optimize @tailrec annotated method: it changes type of 'this' on a polymorphic recursive call
    @tailrec final def fail4[U](other: Tom[U], x: Int): Int = other.fail4[U](other, x - 1)
                       ^
5 errors found