SML code that leads to type inference fail.

fail.sml

fun all_answers func [] = SOME []
fun all_answers func xs = let 
  fun loop [] = []
  |   loop (h::tail) = case (func h) of 
                         SOME x => x :: loop tail
                       | NONE => loop tail
  in case loop xs of
         [] => NONE
       | lst => SOME lst
  end

(*
  val test = all_answers (fn x => x) []
  evaluates to dummy types: 
  val test = NONE : ?.X1 list option
*)