(**************************************************************************)
(*                                                                        *)
(*                                 OCaml                                  *)
(*                                                                        *)
(*             Xavier Leroy, projet Cristal, INRIA Rocquencourt           *)
(*                                                                        *)
(*   Copyright 1996 Institut National de Recherche en Informatique et     *)
(*     en Automatique.                                                    *)
(*                                                                        *)
(*   All rights reserved.  This file is distributed under the terms of    *)
(*   the GNU Lesser General Public License version 2.1, with the          *)
(*   special exception on linking described in the file LICENSE.          *)
(*                                                                        *)
(**************************************************************************)

(* typetexp.ml,v 1.34.4.9 2002/01/07 08:39:16 garrigue Exp *)

(* Typechecking of type expressions for the core language *)

open Asttypes
open Misc
open Parsetree
open Typedtree
open Types
open Ctype

exception Already_bound

type error =
    Unbound_type_variable of string
  | Undefined_type_constructor of Path.t
  | Type_arity_mismatch of Longident.t * int * int
  | Bound_type_variable of string
  | Recursive_type
  | Unbound_row_variable of Longident.t
  | Type_mismatch of Ctype.Unification_trace.t
  | Alias_type_mismatch of Ctype.Unification_trace.t
  | Present_has_conjunction of string
  | Present_has_no_type of string
  | Constructor_mismatch of type_expr * type_expr
  | Not_a_variant of type_expr
  | Variant_tags of string * string
  | Invalid_variable_name of string
  | Cannot_quantify of string * type_expr
  | Multiple_constraints_on_type of Longident.t
  | Method_mismatch of string * type_expr * type_expr
  | Opened_object of Path.t option
  | Not_an_object of type_expr

exception Error of Location.t * Env.t * error
exception Error_forward of Location.error

(** Map indexed by type variable names. *)
module TyVarMap = Misc.Stdlib.String.Map

type variable_context = int * type_expr TyVarMap.t

(* Support for first-class modules. *)

let transl_modtype_longident = ref (fun _ -> assert false)
let transl_modtype = ref (fun _ -> assert false)

let create_package_mty fake loc env (p, l) =
  let l =
    List.sort
      (fun (s1, _t1) (s2, _t2) ->
         if s1.txt = s2.txt then
           raise (Error (loc, env, Multiple_constraints_on_type s1.txt));
         compare s1.txt s2.txt)
      l
  in
  l,
  List.fold_left
    (fun mty (s, t) ->
      let d = {ptype_name = mkloc (Longident.last s.txt) s.loc;
               ptype_params = [];
               ptype_cstrs = [];
               ptype_kind = Ptype_abstract;
               ptype_private = Asttypes.Public;
               ptype_manifest = if fake then None else Some t;
               ptype_attributes = [];
               ptype_loc = loc} in
      Ast_helper.Mty.mk ~loc
        (Pmty_with (mty, [ Pwith_type ({ txt = s.txt; loc }, d) ]))
    )
    (Ast_helper.Mty.mk ~loc (Pmty_ident p))
    l

(* Translation of type expressions *)

let type_variables = ref (TyVarMap.empty : type_expr TyVarMap.t)
let univars        = ref ([] : (string * type_expr) list)
let pre_univars    = ref ([] : type_expr list)
let used_variables = ref (TyVarMap.empty : (type_expr * Location.t) TyVarMap.t)

let reset_type_variables () =
  reset_global_level ();
  Ctype.reset_reified_var_counter ();
  type_variables := TyVarMap.empty

let narrow () =
  (increase_global_level (), !type_variables)

let widen (gl, tv) =
  restore_global_level gl;
  type_variables := tv

let strict_ident c = (c = '_' || c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z')

let validate_name = function
    None -> None
  | Some name as s ->
      if name <> "" && strict_ident name.[0] then s else None

let new_global_var ?name () =
  new_global_var ?name:(validate_name name) ()
let newvar ?name () =
  newvar ?name:(validate_name name) ()

let type_variable loc name =
  try
    TyVarMap.find name !type_variables
  with Not_found ->
    raise(Error(loc, Env.empty, Unbound_type_variable ("'" ^ name)))

let valid_tyvar_name name =
  name <> "" && name.[0] <> '_'

let transl_type_param env styp =
  let loc = styp.ptyp_loc in
  match styp.ptyp_desc with
    Ptyp_any ->
      let ty = new_global_var ~name:"_" () in
        { ctyp_desc = Ttyp_any; ctyp_type = ty; ctyp_env = env;
          ctyp_loc = loc; ctyp_attributes = styp.ptyp_attributes; }
  | Ptyp_var name ->
      let ty =
        try
          if not (valid_tyvar_name name) then
            raise (Error (loc, Env.empty, Invalid_variable_name ("'" ^ name)));
          ignore (TyVarMap.find name !type_variables);
          raise Already_bound
        with Not_found ->
          let v = new_global_var ~name () in
            type_variables := TyVarMap.add name v !type_variables;
            v
      in
        { ctyp_desc = Ttyp_var name; ctyp_type = ty; ctyp_env = env;
          ctyp_loc = loc; ctyp_attributes = styp.ptyp_attributes; }
  | _ -> assert false

let transl_type_param env styp =
  (* Currently useless, since type parameters cannot hold attributes
     (but this could easily be lifted in the future). *)
  Builtin_attributes.warning_scope styp.ptyp_attributes
    (fun () -> transl_type_param env styp)


let new_pre_univar ?name () =
  let v = newvar ?name () in pre_univars := v :: !pre_univars; v

type policy = Fixed | Extensible | Univars

let rec transl_type env policy styp =
  Builtin_attributes.warning_scope styp.ptyp_attributes
    (fun () -> transl_type_aux env policy styp)

and transl_type_aux env policy styp =
  let loc = styp.ptyp_loc in
  let ctyp ctyp_desc ctyp_type =
    { ctyp_desc; ctyp_type; ctyp_env = env;
      ctyp_loc = loc; ctyp_attributes = styp.ptyp_attributes }
  in
  match styp.ptyp_desc with
    Ptyp_any ->
      let ty =
        if policy = Univars then new_pre_univar () else
          if policy = Fixed then
            raise (Error (styp.ptyp_loc, env, Unbound_type_variable "_"))
          else newvar ()
      in
      ctyp Ttyp_any ty
  | Ptyp_var name ->
    let ty =
      if not (valid_tyvar_name name) then
        raise (Error (styp.ptyp_loc, env, Invalid_variable_name ("'" ^ name)));
      begin try
        instance (List.assoc name !univars)
      with Not_found -> try
        instance (fst (TyVarMap.find name !used_variables))
      with Not_found ->
        let v =
          if policy = Univars then new_pre_univar ~name () else newvar ~name ()
        in
        used_variables := TyVarMap.add name (v, styp.ptyp_loc) !used_variables;
        v
      end
    in
    ctyp (Ttyp_var name) ty
  | Ptyp_arrow(l, st1, st2) ->
    let cty1 = transl_type env policy st1 in
    let cty2 = transl_type env policy st2 in
    let ty1 = cty1.ctyp_type in
    let ty1 =
      if Btype.is_optional l
      then newty (Tconstr(Predef.path_option,[ty1], ref Mnil))
      else ty1 in
    let ty = newty (Tarrow(l, ty1, cty2.ctyp_type, Cok)) in
    ctyp (Ttyp_arrow (l, cty1, cty2)) ty
  | Ptyp_tuple stl ->
    assert (List.length stl >= 2);
    let ctys = List.map (transl_type env policy) stl in
    let ty = newty (Ttuple (List.map (fun ctyp -> ctyp.ctyp_type) ctys)) in
    ctyp (Ttyp_tuple ctys) ty
  | Ptyp_constr(lid, stl) ->
      let (path, decl) = Env.lookup_type ~loc:lid.loc lid.txt env in
      let stl =
        match stl with
        | [ {ptyp_desc=Ptyp_any} as t ] when decl.type_arity > 1 ->
            List.map (fun _ -> t) decl.type_params
        | _ -> stl
      in
      if List.length stl <> decl.type_arity then
        raise(Error(styp.ptyp_loc, env,
                    Type_arity_mismatch(lid.txt, decl.type_arity,
                                        List.length stl)));
      let args = List.map (transl_type env policy) stl in
      let params = instance_list decl.type_params in
      let unify_param =
        match decl.type_manifest with
          None -> unify_var
        | Some ty ->
            if (repr ty).level = Btype.generic_level then unify_var else unify
      in
      List.iter2
        (fun (sty, cty) ty' ->
           try unify_param env ty' cty.ctyp_type with Unify trace ->
             let trace = Unification_trace.swap trace in
             raise (Error(sty.ptyp_loc, env, Type_mismatch trace))
        )
        (List.combine stl args) params;
      let constr =
        newconstr path (List.map (fun ctyp -> ctyp.ctyp_type) args) in
      begin try
        Ctype.enforce_constraints env constr
      with Unify trace ->
        raise (Error(styp.ptyp_loc, env, Type_mismatch trace))
      end;
      ctyp (Ttyp_constr (path, lid, args)) constr
  | Ptyp_object (fields, o) ->
      let ty, fields = transl_fields env policy o fields in
      ctyp (Ttyp_object (fields, o)) (newobj ty)
  | Ptyp_class(lid, stl) ->
      let (path, decl, _is_variant) =
        try
          let path, decl = Env.find_type_by_name lid.txt env in
          let rec check decl =
            match decl.type_manifest with
              None -> raise Not_found
            | Some ty ->
                match (repr ty).desc with
                  Tvariant row when Btype.static_row row -> ()
                | Tconstr (path, _, _) ->
                    check (Env.find_type path env)
                | _ -> raise Not_found
          in check decl;
          Location.deprecated styp.ptyp_loc
            "old syntax for polymorphic variant type";
          ignore(Env.lookup_type ~loc:lid.loc lid.txt env);
          (path, decl,true)
        with Not_found -> try
          let lid2 =
            match lid.txt with
              Longident.Lident s     -> Longident.Lident ("#" ^ s)
            | Longident.Ldot(r, s)   -> Longident.Ldot (r, "#" ^ s)
            | Longident.Lapply(_, _) -> fatal_error "Typetexp.transl_type"
          in
          let path, decl = Env.find_type_by_name lid2 env in
          ignore(Env.lookup_cltype ~loc:lid.loc lid.txt env);
          (path, decl, false)
        with Not_found ->
          ignore (Env.lookup_cltype ~loc:lid.loc lid.txt env); assert false
      in
      if List.length stl <> decl.type_arity then
        raise(Error(styp.ptyp_loc, env,
                    Type_arity_mismatch(lid.txt, decl.type_arity,
                                        List.length stl)));
      let args = List.map (transl_type env policy) stl in
      let params = instance_list decl.type_params in
      List.iter2
        (fun (sty, cty) ty' ->
           try unify_var env ty' cty.ctyp_type with Unify trace ->
             let trace = Unification_trace.swap trace in
             raise (Error(sty.ptyp_loc, env, Type_mismatch trace))
        )
        (List.combine stl args) params;
        let ty_args = List.map (fun ctyp -> ctyp.ctyp_type) args in
      let ty =
        try Ctype.expand_head env (newconstr path ty_args)
        with Unify trace ->
          raise (Error(styp.ptyp_loc, env, Type_mismatch trace))
      in
      let ty = match ty.desc with
        Tvariant row ->
          let row = Btype.row_repr row in
          let fields =
            List.map
              (fun (l,f) -> l,
                match Btype.row_field_repr f with
                | Rpresent (Some ty) ->
                    Reither(false, [ty], false, ref None)
                | Rpresent None ->
                    Reither (true, [], false, ref None)
                | _ -> f)
              row.row_fields
          in
          let row = { row_closed = true; row_fields = fields;
                      row_bound = (); row_name = Some (path, ty_args);
                      row_fixed = None; row_more = newvar () } in
          let static = Btype.static_row row in
          let row =
            if static then { row with row_more = newty Tnil }
            else if policy <> Univars then row
            else { row with row_more = new_pre_univar () }
          in
          newty (Tvariant row)
      | Tobject (fi, _) ->
          let _, tv = flatten_fields fi in
          if policy = Univars then pre_univars := tv :: !pre_univars;
          ty
      | _ ->
          assert false
      in
      ctyp (Ttyp_class (path, lid, args)) ty
  | Ptyp_alias(st, alias) ->
      let cty =
        try
          let t =
            try List.assoc alias !univars
            with Not_found ->
              instance (fst(TyVarMap.find alias !used_variables))
          in
          let ty = transl_type env policy st in
          begin try unify_var env t ty.ctyp_type with Unify trace ->
            let trace = Unification_trace.swap trace in
            raise(Error(styp.ptyp_loc, env, Alias_type_mismatch trace))
          end;
          ty
        with Not_found ->
          if !Clflags.principal then begin_def ();
          let t = newvar () in
          used_variables :=
            TyVarMap.add alias (t, styp.ptyp_loc) !used_variables;
          let ty = transl_type env policy st in
          begin try unify_var env t ty.ctyp_type with Unify trace ->
            let trace = Unification_trace.swap trace in
            raise(Error(styp.ptyp_loc, env, Alias_type_mismatch trace))
          end;
          if !Clflags.principal then begin
            end_def ();
            generalize_structure t;
          end;
          let t = instance t in
          let px = Btype.proxy t in
          begin match px.desc with
          | Tvar None -> Btype.set_type_desc px (Tvar (Some alias))
          | Tunivar None -> Btype.set_type_desc px (Tunivar (Some alias))
          | _ -> ()
          end;
          { ty with ctyp_type = t }
      in
      ctyp (Ttyp_alias (cty, alias)) cty.ctyp_type
  | Ptyp_variant(fields, closed, present) ->
      let name = ref None in
      let mkfield l f =
        newty (Tvariant {row_fields=[l,f]; row_more=newvar();
                         row_bound=(); row_closed=true;
                         row_fixed=None; row_name=None}) in
      let hfields = Hashtbl.create 17 in
      let add_typed_field loc l f =
        let h = Btype.hash_variant l in
        try
          let (l',f') = Hashtbl.find hfields h in
          (* Check for tag conflicts *)
          if l <> l' then raise(Error(styp.ptyp_loc, env, Variant_tags(l, l')));
          let ty = mkfield l f and ty' = mkfield l f' in
          if equal env false [ty] [ty'] then () else
          try unify env ty ty'
          with Unify _trace ->
            raise(Error(loc, env, Constructor_mismatch (ty,ty')))
        with Not_found ->
          Hashtbl.add hfields h (l,f)
      in
      let add_field field =
        let rf_loc = field.prf_loc in
        let rf_attributes = field.prf_attributes in
        let rf_desc = match field.prf_desc with
        | Rtag (l, c, stl) ->
            name := None;
            let tl =
              Builtin_attributes.warning_scope rf_attributes
                (fun () -> List.map (transl_type env policy) stl)
            in
            let f = match present with
              Some present when not (List.mem l.txt present) ->
                let ty_tl = List.map (fun cty -> cty.ctyp_type) tl in
                Reither(c, ty_tl, false, ref None)
            | _ ->
                if List.length stl > 1 || c && stl <> [] then
                  raise(Error(styp.ptyp_loc, env,
                              Present_has_conjunction l.txt));
                match tl with [] -> Rpresent None
                | st :: _ ->
                      Rpresent (Some st.ctyp_type)
            in
            add_typed_field styp.ptyp_loc l.txt f;
              Ttag (l,c,tl)
        | Rinherit sty ->
            let cty = transl_type env policy sty in
            let ty = cty.ctyp_type in
            let nm =
              match repr cty.ctyp_type with
                {desc=Tconstr(p, tl, _)} -> Some(p, tl)
              | _                        -> None
            in
            name := if Hashtbl.length hfields <> 0 then None else nm;
            let fl = match expand_head env cty.ctyp_type, nm with
              {desc=Tvariant row}, _ when Btype.static_row row ->
                let row = Btype.row_repr row in
                row.row_fields
            | {desc=Tvar _}, Some(p, _) ->
                raise(Error(sty.ptyp_loc, env, Undefined_type_constructor p))
            | _ ->
                raise(Error(sty.ptyp_loc, env, Not_a_variant ty))
            in
            List.iter
              (fun (l, f) ->
                let f = match present with
                  Some present when not (List.mem l present) ->
                    begin match f with
                      Rpresent(Some ty) ->
                        Reither(false, [ty], false, ref None)
                    | Rpresent None ->
                        Reither(true, [], false, ref None)
                    | _ ->
                        assert false
                    end
                | _ -> f
                in
                add_typed_field sty.ptyp_loc l f)
              fl;
              Tinherit cty
        in
        { rf_desc; rf_loc; rf_attributes; }
      in
      let tfields = List.map add_field fields in
      let fields = Hashtbl.fold (fun _ p l -> p :: l) hfields [] in
      begin match present with None -> ()
      | Some present ->
          List.iter
            (fun l -> if not (List.mem_assoc l fields) then
              raise(Error(styp.ptyp_loc, env, Present_has_no_type l)))
            present
      end;
      let row =
        { row_fields = List.rev fields; row_more = newvar ();
          row_bound = (); row_closed = (closed = Closed);
          row_fixed = None; row_name = !name } in
      let static = Btype.static_row row in
      let row =
        if static then { row with row_more = newty Tnil }
        else if policy <> Univars then row
        else { row with row_more = new_pre_univar () }
      in
      let ty = newty (Tvariant row) in
      ctyp (Ttyp_variant (tfields, closed, present)) ty
  | Ptyp_poly(vars, st) ->
      let vars = List.map (fun v -> v.txt) vars in
      begin_def();
      let new_univars = List.map (fun name -> name, newvar ~name ()) vars in
      let old_univars = !univars in
      univars := new_univars @ !univars;
      let cty = transl_type env policy st in
      let ty = cty.ctyp_type in
      univars := old_univars;
      end_def();
      generalize ty;
      let ty_list =
        List.fold_left
          (fun tyl (name, ty1) ->
            let v = Btype.proxy ty1 in
            if deep_occur v ty then begin
              match v.desc with
                Tvar name when v.level = Btype.generic_level ->
                  v.desc <- Tunivar name;
                  v :: tyl
              | _ ->
                raise (Error (styp.ptyp_loc, env, Cannot_quantify (name, v)))
            end else tyl)
          [] new_univars
      in
      let ty' = Btype.newgenty (Tpoly(ty, List.rev ty_list)) in
      unify_var env (newvar()) ty';
      ctyp (Ttyp_poly (vars, cty)) ty'
  | Ptyp_package (p, l) ->
      let l, mty = create_package_mty true styp.ptyp_loc env (p, l) in
      let z = narrow () in
      let mty = !transl_modtype env mty in
      widen z;
      let ptys = List.map (fun (s, pty) ->
                             s, transl_type env policy pty
                          ) l in
      let path = !transl_modtype_longident styp.ptyp_loc env p.txt in
      let ty = newty (Tpackage (path,
                       List.map (fun (s, _pty) -> s.txt) l,
                       List.map (fun (_,cty) -> cty.ctyp_type) ptys))
      in
      ctyp (Ttyp_package {
            pack_path = path;
            pack_type = mty.mty_type;
            pack_fields = ptys;
            pack_txt = p;
           }) ty
  | Ptyp_extension ext ->
      raise (Error_forward (Builtin_attributes.error_of_extension ext))

and transl_poly_type env policy t =
  transl_type env policy (Ast_helper.Typ.force_poly t)

and transl_fields env policy o fields =
  let hfields = Hashtbl.create 17 in
  let add_typed_field loc l ty =
    try
      let ty' = Hashtbl.find hfields l in
      if equal env false [ty] [ty'] then () else
        try unify env ty ty'
        with Unify _trace ->
          raise(Error(loc, env, Method_mismatch (l, ty, ty')))
    with Not_found ->
      Hashtbl.add hfields l ty in
  let add_field {pof_desc; pof_loc; pof_attributes;} =
    let of_loc = pof_loc in
    let of_attributes = pof_attributes in
    let of_desc = match pof_desc with
    | Otag (s, ty1) -> begin
        let ty1 =
          Builtin_attributes.warning_scope of_attributes
            (fun () -> transl_poly_type env policy ty1)
        in
        let field = OTtag (s, ty1) in
        add_typed_field ty1.ctyp_loc s.txt ty1.ctyp_type;
        field
      end
    | Oinherit sty -> begin
        let cty = transl_type env policy sty in
        let nm =
          match repr cty.ctyp_type with
            {desc=Tconstr(p, _, _)} -> Some p
          | _                        -> None in
        let t = expand_head env cty.ctyp_type in
        match t, nm with
          {desc=Tobject ({desc=(Tfield _ | Tnil) as tf}, _)}, _ -> begin
            if opened_object t then
              raise (Error (sty.ptyp_loc, env, Opened_object nm));
            let rec iter_add = function
              | Tfield (s, _k, ty1, ty2) -> begin
                  add_typed_field sty.ptyp_loc s ty1;
                  iter_add ty2.desc
                end
              | Tnil -> ()
              | _ -> assert false in
            iter_add tf;
            OTinherit cty
            end
        | {desc=Tvar _}, Some p ->
            raise (Error (sty.ptyp_loc, env, Undefined_type_constructor p))
        | _ -> raise (Error (sty.ptyp_loc, env, Not_an_object t))
      end in
    { of_desc; of_loc; of_attributes; }
  in
  let object_fields = List.map add_field fields in
  let fields = Hashtbl.fold (fun s ty l -> (s, ty) :: l) hfields [] in
  let ty_init =
     match o, policy with
     | Closed, _ -> newty Tnil
     | Open, Univars -> new_pre_univar ()
     | Open, _ -> newvar () in
  let ty = List.fold_left (fun ty (s, ty') ->
      newty (Tfield (s, Fpresent, ty', ty))) ty_init fields in
  ty, object_fields


(* Make the rows "fixed" in this type, to make universal check easier *)
let rec make_fixed_univars ty =
  let ty = repr ty in
  if ty.level >= Btype.lowest_level then begin
    Btype.mark_type_node ty;
    match ty.desc with
    | Tvariant row ->
        let row = Btype.row_repr row in
        let more = Btype.row_more row in
        if Btype.is_Tunivar more then
          ty.desc <- Tvariant
              {row with row_fixed=Some(Univar more);
               row_fields = List.map
                 (fun (s,f as p) -> match Btype.row_field_repr f with
                   Reither (c, tl, _m, r) -> s, Reither (c, tl, true, r)
                 | _ -> p)
                 row.row_fields};
        Btype.iter_row make_fixed_univars row
    | _ ->
        Btype.iter_type_expr make_fixed_univars ty
  end

let make_fixed_univars ty =
  make_fixed_univars ty;
  Btype.unmark_type ty

let create_package_mty = create_package_mty false

let globalize_used_variables env fixed =
  let r = ref [] in
  TyVarMap.iter
    (fun name (ty, loc) ->
      let v = new_global_var () in
      let snap = Btype.snapshot () in
      if try unify env v ty; true with _ -> Btype.backtrack snap; false
      then try
        r := (loc, v,  TyVarMap.find name !type_variables) :: !r
      with Not_found ->
        if fixed && Btype.is_Tvar (repr ty) then
          raise(Error(loc, env, Unbound_type_variable ("'"^name)));
        let v2 = new_global_var () in
        r := (loc, v, v2) :: !r;
        type_variables := TyVarMap.add name v2 !type_variables)
    !used_variables;
  used_variables := TyVarMap.empty;
  fun () ->
    List.iter
      (function (loc, t1, t2) ->
        try unify env t1 t2 with Unify trace ->
          raise (Error(loc, env, Type_mismatch trace)))
      !r

let transl_simple_type env fixed styp =
  univars := []; used_variables := TyVarMap.empty;
  let typ = transl_type env (if fixed then Fixed else Extensible) styp in
  globalize_used_variables env fixed ();
  make_fixed_univars typ.ctyp_type;
  typ

let transl_simple_type_univars env styp =
  univars := []; used_variables := TyVarMap.empty; pre_univars := [];
  begin_def ();
  let typ = transl_type env Univars styp in
  (* Only keep already global variables in used_variables *)
  let new_variables = !used_variables in
  used_variables := TyVarMap.empty;
  TyVarMap.iter
    (fun name p ->
      if TyVarMap.mem name !type_variables then
        used_variables := TyVarMap.add name p !used_variables)
    new_variables;
  globalize_used_variables env false ();
  end_def ();
  generalize typ.ctyp_type;
  let univs =
    List.fold_left
      (fun acc v ->
        let v = repr v in
        match v.desc with
          Tvar name when v.level = Btype.generic_level ->
            v.desc <- Tunivar name; v :: acc
        | _ -> acc)
      [] !pre_univars
  in
  make_fixed_univars typ.ctyp_type;
    { typ with ctyp_type =
        instance (Btype.newgenty (Tpoly (typ.ctyp_type, univs))) }

let transl_simple_type_delayed env styp =
  univars := []; used_variables := TyVarMap.empty;
  begin_def ();
  let typ = transl_type env Extensible styp in
  end_def ();
  make_fixed_univars typ.ctyp_type;
  (* This brings the used variables to the global level, but doesn't link them
     to their other occurrences just yet. This will be done when [force] is
     called. *)
  let force = globalize_used_variables env false in
  (* Generalizes everything except the variables that were just globalized. *)
  generalize typ.ctyp_type;
  (typ, instance typ.ctyp_type, force)

let transl_type_scheme env styp =
  reset_type_variables();
  begin_def();
  let typ = transl_simple_type env false styp in
  end_def();
  generalize typ.ctyp_type;
  typ


(* Error report *)

open Format
open Printtyp

let report_error env ppf = function
  | Unbound_type_variable name ->
      let add_name name _ l = if name = "_" then l else ("'" ^ name) :: l in
      let names = TyVarMap.fold add_name !type_variables [] in
    fprintf ppf "The type variable %s is unbound in this type declaration.@ %a"
      name
      did_you_mean (fun () -> Misc.spellcheck names name )
  | Undefined_type_constructor p ->
    fprintf ppf "The type constructor@ %a@ is not yet completely defined"
      path p
  | Type_arity_mismatch(lid, expected, provided) ->
    fprintf ppf
      "@[The type constructor %a@ expects %i argument(s),@ \
        but is here applied to %i argument(s)@]"
      longident lid expected provided
  | Bound_type_variable name ->
    fprintf ppf "Already bound type parameter %a" Pprintast.tyvar name
  | Recursive_type ->
    fprintf ppf "This type is recursive"
  | Unbound_row_variable lid ->
      (* we don't use "spellcheck" here: this error is not raised
         anywhere so it's unclear how it should be handled *)
      fprintf ppf "Unbound row variable in #%a" longident lid
  | Type_mismatch trace ->
      Printtyp.report_unification_error ppf Env.empty trace
        (function ppf ->
           fprintf ppf "This type")
        (function ppf ->
           fprintf ppf "should be an instance of type")
  | Alias_type_mismatch trace ->
      Printtyp.report_unification_error ppf Env.empty trace
        (function ppf ->
           fprintf ppf "This alias is bound to type")
        (function ppf ->
           fprintf ppf "but is used as an instance of type")
  | Present_has_conjunction l ->
      fprintf ppf "The present constructor %s has a conjunctive type" l
  | Present_has_no_type l ->
      fprintf ppf
        "@[<v>@[The constructor %s is missing from the upper bound@ \
         (between '<'@ and '>')@ of this polymorphic variant@ \
         but is present in@ its lower bound (after '>').@]@,\
         @[Hint: Either add `%s in the upper bound,@ \
         or remove it@ from the lower bound.@]@]"
         l l
  | Constructor_mismatch (ty, ty') ->
      wrap_printing_env ~error:true env (fun ()  ->
        Printtyp.reset_and_mark_loops_list [ty; ty'];
        fprintf ppf "@[<hov>%s %a@ %s@ %a@]"
          "This variant type contains a constructor"
          !Oprint.out_type (tree_of_typexp false ty)
          "which should be"
           !Oprint.out_type (tree_of_typexp false ty'))
  | Not_a_variant ty ->
      fprintf ppf
        "@[The type %a@ does not expand to a polymorphic variant type@]"
        Printtyp.type_expr ty;
      begin match ty.desc with
        | Tvar (Some s) ->
           (* PR#7012: help the user that wrote 'Foo instead of `Foo *)
           Misc.did_you_mean ppf (fun () -> ["`" ^ s])
        | _ -> ()
      end
  | Variant_tags (lab1, lab2) ->
      fprintf ppf
        "@[Variant tags `%s@ and `%s have the same hash value.@ %s@]"
        lab1 lab2 "Change one of them."
  | Invalid_variable_name name ->
      fprintf ppf "The type variable name %s is not allowed in programs" name
  | Cannot_quantify (name, v) ->
      fprintf ppf
        "@[<hov>The universal type variable %a cannot be generalized:@ "
        Pprintast.tyvar name;
      if Btype.is_Tvar v then
        fprintf ppf "it escapes its scope"
      else if Btype.is_Tunivar v then
        fprintf ppf "it is already bound to another variable"
      else
        fprintf ppf "it is bound to@ %a" Printtyp.type_expr v;
      fprintf ppf ".@]";
  | Multiple_constraints_on_type s ->
      fprintf ppf "Multiple constraints for type %a" longident s
  | Method_mismatch (l, ty, ty') ->
      wrap_printing_env ~error:true env (fun ()  ->
        fprintf ppf "@[<hov>Method '%s' has type %a,@ which should be %a@]"
          l Printtyp.type_expr ty Printtyp.type_expr ty')
  | Opened_object nm ->
      fprintf ppf
        "Illegal open object type%a"
        (fun ppf -> function
             Some p -> fprintf ppf "@ %a" path p
           | None -> fprintf ppf "") nm
  | Not_an_object ty ->
      fprintf ppf "@[The type %a@ is not an object type@]"
        Printtyp.type_expr ty

let () =
  Location.register_error_of_exn
    (function
      | Error (loc, env, err) ->
        Some (Location.error_of_printer ~loc (report_error env) err)
      | Error_forward err ->
        Some err
      | _ ->
        None
    )