Typedtree and GADT

a.ml

module _ = struct
  (* OCaml 3.12.1 *)
  open Typedtree
  type ttt = { self : 'a . 'a pattern_desc pattern_data -> bool}

  let _hack =
    let helper (type a)
        (self : ttt)
        (p1: a Typedtree.pattern_desc pattern_data)  =
      match p1.pat_desc with
      | Tpat_any
      | Tpat_var _ -> true
      | Tpat_constant c1 -> true
      | Tpat_tuple t1 -> List.for_all self.self t1
      | Tpat_construct (_, cd1, a1) -> List.for_all self.self a1
      | Tpat_value v1 ->
          self.self
            (v1 :> value Typedtree.pattern_desc Typedtree.pattern_data)
      | Tpat_exception _
      | _ -> false
    in
    let rec ans  = { self = (fun eta -> helper ans eta) } in
    ans

  let _hack_bad =
    let helper (type a)
        (self : a pattern_desc pattern_data -> bool)
        (p1: a Typedtree.pattern_desc pattern_data)  =
      match p1.pat_desc with
      | Tpat_any
      | Tpat_var _ -> true
      | Tpat_constant c1 -> true
      | Tpat_tuple t1 -> List.for_all self t1
      | Tpat_construct (_, cd1, a1) -> List.for_all self a1
      | Tpat_value v1 ->
          self
            (v1 :> value Typedtree.pattern_desc Typedtree.pattern_data)
          (*
          Error: This expression has type value Typedtree.pattern_desc pattern_data
                but an expression was expected of type
                  a Typedtree.pattern_desc pattern_data
                Type value is not compatible with type a
          *)
      | Tpat_exception _
      | _ -> false
    in
    let rec ans eta = helper ans eta  in
    ans
end