tailrec-3.scala:4: error: could not optimize @tailrec annotated method quux: it contains a recursive call not in tail position
  @tailrec private def quux(xs: List[String]): List[String] = quux(quux(xs))
                                                                       ^
tailrec-3.scala:6: error: could not optimize @tailrec annotated method quux2: it contains a recursive call not in tail position
    case x1 :: x2 :: rest => quux2(x1 :: quux2(rest))
                                              ^
tailrec-3.scala:10: error: could not optimize @tailrec annotated method quux3: it contains a recursive call not in tail position
    case x :: xs if quux3(List("abc"))  => quux3(xs)
                         ^
three errors found