t6574.scala:4: error: could not optimize @tailrec annotated method notTailPos$extension: it contains a recursive call not in tail position
    println("tail")
    ^
t6574.scala:8: error: could not optimize @tailrec annotated method differentTypeArgs$extension: it is called recursively with different type arguments
    {(); new Bad[String, Unit](0)}.differentTypeArgs
                                   ^
two errors found