POC print variant runtime repr

As in `type t = @as(undefined) A`

Triggered in error message:

```res
Type declarations do not match:
    type t = @as(undefined) A
  is not included in
    type t = @as(null) A
```

jscomp/ml/oprint.ml

@@ -499,13 +499,13 @@ and print_out_signature ppf =
  match items with
  Osig_typext(ext, Oext_next) :: items ->
  gather_extensions
- ((ext.oext_name, ext.oext_args, ext.oext_ret_type) :: acc)
+ ((ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr) :: acc)
  items
  | _ -> (List.rev acc, items)
  in
  let exts, items =
  gather_extensions
- [(ext.oext_name, ext.oext_args, ext.oext_ret_type)]
+ [(ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)]
  items
  in
  let te =
@@ -531,7 +531,7 @@ and print_out_sig_item ppf =
  name !out_class_type clt
  | Osig_typext (ext, Oext_exception) ->
  fprintf ppf "@[<2>exception %a@]"
- print_out_constr (ext.oext_name, ext.oext_args, ext.oext_ret_type)
+ print_out_constr (ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)
  | Osig_typext (ext, _es) ->
  print_out_extension_constructor ppf ext
  | Osig_modtype (name, Omty_abstract) ->
@@ -639,7 +639,10 @@ and print_out_type_decl kwd ppf td =
  print_immediate
  print_unboxed
 
-and print_out_constr ppf (name, tyl,ret_type_opt) =
+and print_out_constr ppf (name, tyl, ret_type_opt, repr) =
+ let () = match repr with
+ | None -> ()
+ | Some s -> pp_print_string ppf s in
  let name =
  match name with
  | "::" -> "(::)" (* #7200 *)
@@ -686,7 +689,7 @@ and print_out_extension_constructor ppf ext =
  fprintf ppf "@[<hv 2>type %t +=%s@;<1 2>%a@]"
  print_extended_type
  (if ext.oext_private = Asttypes.Private then " private" else "")
- print_out_constr (ext.oext_name, ext.oext_args, ext.oext_ret_type)
+ print_out_constr (ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)
 
 and print_out_type_extension ppf te =
  let print_extended_type ppf =
@@ -736,13 +739,13 @@ let rec print_items ppf =
  match items with
  (Osig_typext(ext, Oext_next), None) :: items ->
  gather_extensions
- ((ext.oext_name, ext.oext_args, ext.oext_ret_type) :: acc)
+ ((ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr) :: acc)
  items
  | _ -> (List.rev acc, items)
  in
  let exts, items =
  gather_extensions
- [(ext.oext_name, ext.oext_args, ext.oext_ret_type)]
+ [(ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)]
  items
  in
  let te =

jscomp/ml/outcometree.ml

@@ -63,7 +63,7 @@ type out_type =
  | Otyp_object of (string * out_type) list * bool option
  | Otyp_record of (string * bool * bool * out_type) list
  | Otyp_stuff of string
- | Otyp_sum of (string * out_type list * out_type option) list
+ | Otyp_sum of (string * out_type list * out_type option * string option) list
  | Otyp_tuple of out_type list
  | Otyp_var of bool * string
  | Otyp_variant of
@@ -118,11 +118,12 @@ and out_extension_constructor =
  oext_type_params: string list;
  oext_args: out_type list;
  oext_ret_type: out_type option;
+ oext_repr: string option;
  oext_private: Asttypes.private_flag }
 and out_type_extension =
  { otyext_name: string;
  otyext_params: string list;
- otyext_constructors: (string * out_type list * out_type option) list;
+ otyext_constructors: (string * out_type list * out_type option * string option) list;
  otyext_private: Asttypes.private_flag }
 and out_val_decl =
  { oval_name: string;

jscomp/ml/printtyp.ml

@@ -917,16 +917,29 @@ and tree_of_constructor_arguments = function
 
 and tree_of_constructor cd =
  let name = Ident.name cd.cd_id in
+ let nullary = Ast_untagged_variants.is_nullary_variant cd.cd_args in
+ let repr =
+ if not nullary then None
+ else match Ast_untagged_variants.process_tag_type cd.cd_attributes with
+ | Some Null -> Some "@as(null)"
+ | Some Undefined -> Some "@as(undefined)"
+ | Some (String s) -> Some (Printf.sprintf "@as(%S)" s)
+ | Some (Int i) -> Some (Printf.sprintf "@as(%d)" i)
+ | Some (Float f) -> Some (Printf.sprintf "@as(%s)" f)
+ | Some (Bool b) -> Some (Printf.sprintf "@as(%b)" b)
+ | Some (BigInt s) -> Some (Printf.sprintf "@as(%sn)" s)
+ | Some (Untagged _) (* should never happen *)
+ | None -> None in
  let arg () = tree_of_constructor_arguments cd.cd_args in
  match cd.cd_res with
- | None -> (name, arg (), None)
+ | None -> (name, arg (), None, repr)
  | Some res ->
  let nm = !names in
  names := [];
  let ret = tree_of_typexp false res in
  let args = arg () in
  names := nm;
- (name, args, Some ret)
+ (name, args, Some ret, repr)
 
 and tree_of_label l =
  let opt = l.ld_attributes |> List.exists (fun ({txt}, _) -> txt = "ns.optional" || txt = "res.optional") in
@@ -982,6 +995,7 @@ let tree_of_extension_constructor id ext es =
  oext_type_params = ty_params;
  oext_args = args;
  oext_ret_type = ret;
+ oext_repr = None;
  oext_private = ext.ext_private }
  in
  let es =

jscomp/syntax/src/res_outcome_printer.ml

@@ -429,8 +429,13 @@ and print_out_constructors_doc constructors =
  constructors);
  ]))
 
-and print_out_constructor_doc (name, args, gadt) =
- let gadt_doc =
+and print_out_constructor_doc (name, args, gadt, repr) =
+ let reprDoc =
+ match repr with
+ | None -> Doc.nil
+ | Some s -> Doc.text (s ^ " ")
+ in
+ let gadtDoc =
  match gadt with
  | Some out_type -> Doc.concat [Doc.text ": "; print_out_type_doc out_type]
  | None -> Doc.nil
@@ -469,7 +474,7 @@ and print_out_constructor_doc (name, args, gadt) =
  Doc.rparen;
  ])
  in
- Doc.group (Doc.concat [Doc.text name; args_doc; gadt_doc])
+ Doc.group (Doc.concat [reprDoc; Doc.text name; args_doc; gadtDoc])
 
 and print_record_decl_row_doc (name, mut, opt, arg) =
  Doc.group
@@ -758,13 +763,14 @@ and print_out_signature_doc (signature : Outcometree.out_sig_item list) =
  match items with
  | Outcometree.Osig_typext (ext, Oext_next) :: items ->
  gather_extensions
- ((ext.oext_name, ext.oext_args, ext.oext_ret_type) :: acc)
+ ((ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)
+ :: acc)
  items
  | _ -> (List.rev acc, items)
  in
  let exts, items =
  gather_extensions
- [(ext.oext_name, ext.oext_args, ext.oext_ret_type)]
+ [(ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)]
  items
  in
  let te =
@@ -822,7 +828,10 @@ and print_out_extension_constructor_doc
  (if out_ext.oext_private = Asttypes.Private then Doc.text "private "
  else Doc.nil);
  print_out_constructor_doc
- (out_ext.oext_name, out_ext.oext_args, out_ext.oext_ret_type);
+ ( out_ext.oext_name,
+ out_ext.oext_args,
+ out_ext.oext_ret_type,
+ out_ext.oext_repr );
  ])
 
 and print_out_type_extension_doc
@@ -1035,13 +1044,14 @@ let print_out_phrase_signature signature =
  match items with
  | (Outcometree.Osig_typext (ext, Oext_next), None) :: items ->
  gather_extensions
- ((ext.oext_name, ext.oext_args, ext.oext_ret_type) :: acc)
+ ((ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)
+ :: acc)
  items
  | _ -> (List.rev acc, items)
  in
  let exts, signature =
  gather_extensions
- [(ext.oext_name, ext.oext_args, ext.oext_ret_type)]
+ [(ext.oext_name, ext.oext_args, ext.oext_ret_type, ext.oext_repr)]
  signature
  in
  let te =