[Caml-list] Strange type inference error

[Maxence Guesdon <Maxence.Guesdon@inria.fr>]

Hello,

I encountered a strange typing problem and reduced it to the following
code example:

====[t.ml]====
let mk_env () = None
let list_of_string (s: string) = [s]

let apply : 'a -> 'a option -> string list -> 'a * string list =
  fun acc env l -> (acc, ["hello"])

let f =
   let tmpl = ["coucou"] in
    fun ?(env=mk_env()) ->
      fun ~x ->
        fun () -> apply () env tmpl

let g =
   let tmpl = list_of_string "coucou" in
    fun ?(env=mk_env()) ->
      fun ~x ->
        fun () -> apply () env tmpl
====
Note that f and g only differ on the way tmpl is defined.

When I compile this file with ocaml 4.02.1, I get the following error:
File "t.ml", line 14, characters 3-119:
Error: The type of this expression,
       ?env:unit option -> x:'_a -> unit -> unit * string list,
       contains type variables that cannot be generalized

Am I missing something or should I file a bug report ?

Regards,

Maxence