[Caml-list] typer strangeness (3.12.0)

[rixed@happyleptic.org]

Given these types:

(* A parser is given a list of items and returns either a Failure indication, a
   Wait for more inputs indication, or a result (composed of a new value and the list
   of tokens that were unused) *)
type ('a, 'b) parzer_result = Wait | Res of ('a * 'b list) | Fail
type ('a, 'b) parzer = 'b list -> ('a, 'b) parzer_result

And this function used to pipe two parsers together:

(* Use the results of the first parser as the input elements of the second.
   Stop as soon as p2 returns a result or fails.
   Notice that if p1 is a ('a, 'b) parzer and p2 a ('c, 'a) parzer
   then pipe p1 p2 is a ('c, 'b) parzer, which comes handy but p2 is then
   forced to consume everything ! *)
let (pipe : ('a, 'b) parzer -> ('c, 'a) parzer -> ('c, 'b) parzer) p1 p2 =
    let p1_rem = ref [] in
    fun bs -> match p1 (!p1_rem @ bs) with
        | Fail -> Fail
        | Wait -> Wait
        | Res (res, rem) ->
            p1_rem := rem ;
            (match p2 [res] with
                | Res (res', rem') ->
                    if rem' <> [] then Printf.printf "WRN: second end of a pipe did not consume eveything !\n" ;
                    Res (res', !p1_rem)
                | Fail -> Fail | Wait -> Wait)

This pipe function has the expected type :

# pipe;;
- : ('a, 'b) parzer -> ('c, 'a) parzer -> ('c, 'b) parzer = <fun>

Now, if I change it's last line for : "| x -> x)", ie not repeating Wait and Fail, the result is
very different :

# pipe;;
- : ('a, 'a) parzer -> ('b, 'a) parzer -> ('b, 'a) parzer = <fun>

How come?
And why didn't the compiler complain since I explicitely typed the function definition?