ocaml error Type char/2 is not compatible with type char/2312

obj.ml

module Types : sig
  type ('a, 'b) tag = Parsed of 'a * 'b option | Failed of 'b option
  type ('stream, 'parsed, 'error) result = ('parsed * 'stream, 'error) tag

  val bind : ('stream, 'parsed, 'error) result -> ('parsed -> [`Ok of 'parsed' | `Fail of 'error]) -> ('stream, 'parsed', 'error) result
end = struct
  type ('a, 'b) tag = Parsed of 'a * 'b option | Failed of 'b option
  type ('stream, 'parsed, 'error) result = ('parsed * 'stream, 'error) tag

  let bind _ _ = assert false
end

module Token : sig
    type t
    val repr : t -> string
end = struct
  type t
  let repr _ = assert false
end

module Reason = struct type t end

class virtual char s =
  let regexp = Re_str.regexp "'\\([^']\\|''\\|\\\\n\\|\\\\t\\)'" in
  object(self : 'a)
    method virtual get : String.t -> Re_str.regexp -> ('a, Token.t, Reason.t) Types.result
    method getCHAR     : ('a, char, Reason.t) Types.result =
      Types.bind
        (self#get "string constant" regexp)
        (fun t -> `Ok ((Token.repr t).[0]))

      (*
         (< get : String.t ->
                  Re_str.regexp -> ('a, Token.t, Reason.t) Types.result;
            getCHAR : ('a, char/2312, Reason.t) Types.result; .. >
          as 'a, char/2, Reason.t)
         Types.result = (char/2 * 'a, Reason.t) Types.tag
       but an expression was expected of type
         ('a, char/2312, Reason.t) Types.result =
           (char/2312 * 'a, Reason.t) Types.tag
       Type char/2 is not compatible with type char/2312 = < .. >
      *)
  end[@@warning "-3"]