(**************************************************************************)
(*                                                                        *)
(*                                 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.          *)
(*                                                                        *)
(**************************************************************************)

(* Typechecking for the core language *)

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

type type_forcing_context =
  | If_conditional
  | If_no_else_branch
  | While_loop_conditional
  | While_loop_body
  | For_loop_start_index
  | For_loop_stop_index
  | For_loop_body
  | Assert_condition
  | Sequence_left_hand_side
  | When_guard

type type_expected = {
  ty: type_expr;
  explanation: type_forcing_context option;
}

type to_unpack = {
  tu_name: string Location.loc;
  tu_loc: Location.t;
  tu_uid: Uid.t
}

module Datatype_kind = struct
  type t = Record | Variant

  let type_name = function
    | Record -> "record"
    | Variant -> "variant"

  let label_name = function
    | Record -> "field"
    | Variant -> "constructor"
end

type wrong_name = {
  type_path: Path.t;
  kind: Datatype_kind.t;
  name: string loc;
  valid_names: string list;
}

type wrong_kind_context =
  | Pattern
  | Expression of type_forcing_context option

type wrong_kind_sort =
  | Constructor
  | Record
  | Boolean
  | List
  | Unit

let wrong_kind_sort_of_constructor (lid : Longident.t) =
  match lid with
  | Lident "true" | Lident "false" | Ldot(_, "true") | Ldot(_, "false") ->
      Boolean
  | Lident "[]" | Lident "::" | Ldot(_, "[]") | Ldot(_, "::") -> List
  | Lident "()" | Ldot(_, "()") -> Unit
  | _ -> Constructor

type existential_restriction =
  | At_toplevel (** no existential types at the toplevel *)
  | In_group (** nor with let ... and ... *)
  | In_rec (** or recursive definition *)
  | With_attributes (** or let[@any_attribute] = ... *)
  | In_class_args (** or in class arguments *)
  | In_class_def  (** or in [class c = let ... in ...] *)
  | In_self_pattern (** or in self pattern *)

type error =
  | Constructor_arity_mismatch of Longident.t * int * int
  | Label_mismatch of Longident.t * Errortrace.unification_error
  | Pattern_type_clash :
      Errortrace.unification_error * _ pattern_desc option -> error
  | Or_pattern_type_clash of Ident.t * Errortrace.unification_error
  | Multiply_bound_variable of string
  | Orpat_vars of Ident.t * Ident.t list
  | Expr_type_clash of
      Errortrace.unification_error * type_forcing_context option
      * expression_desc option
  | Apply_non_function of type_expr
  | Apply_wrong_label of arg_label * type_expr * bool
  | Label_multiply_defined of string
  | Label_missing of Ident.t list
  | Label_not_mutable of Longident.t
  | Wrong_name of string * type_expected * wrong_name
  | Name_type_mismatch of
      Datatype_kind.t * Longident.t * (Path.t * Path.t) * (Path.t * Path.t) list
  | Invalid_format of string
  | Not_an_object of type_expr * type_forcing_context option
  | Undefined_method of type_expr * string * string list option
  | Undefined_self_method of string * string list
  | Virtual_class of Longident.t
  | Private_type of type_expr
  | Private_label of Longident.t * type_expr
  | Private_constructor of constructor_description * type_expr
  | Unbound_instance_variable of string * string list
  | Instance_variable_not_mutable of string
  | Not_subtype of Errortrace.Subtype.error
  | Outside_class
  | Value_multiply_overridden of string
  | Coercion_failure of
      Errortrace.expanded_type * Errortrace.unification_error * bool
  | Not_a_function of type_expr * type_forcing_context option
  | Too_many_arguments of type_expr * type_forcing_context option
  | Abstract_wrong_label of
      { got           : arg_label
      ; expected      : arg_label
      ; expected_type : type_expr
      ; explanation   : type_forcing_context option
      }
  | Scoping_let_module of string * type_expr
  | Not_a_polymorphic_variant_type of Longident.t
  | Incoherent_label_order
  | Less_general of string * Errortrace.unification_error
  | Modules_not_allowed
  | Cannot_infer_signature
  | Not_a_packed_module of type_expr
  | Unexpected_existential of existential_restriction * string * string list
  | Invalid_interval
  | Invalid_for_loop_index
  | No_value_clauses
  | Exception_pattern_disallowed
  | Mixed_value_and_exception_patterns_under_guard
  | Inlined_record_escape
  | Inlined_record_expected
  | Unrefuted_pattern of pattern
  | Invalid_extension_constructor_payload
  | Not_an_extension_constructor
  | Literal_overflow of string
  | Unknown_literal of string * char
  | Illegal_letrec_pat
  | Illegal_letrec_expr
  | Illegal_class_expr
  | Letop_type_clash of string * Errortrace.unification_error
  | Andop_type_clash of string * Errortrace.unification_error
  | Bindings_type_clash of Errortrace.unification_error
  | Unbound_existential of Ident.t list * type_expr
  | Missing_type_constraint
  | Wrong_expected_kind of wrong_kind_sort * wrong_kind_context * type_expr
  | Expr_not_a_record_type of type_expr

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

(* Forward declaration, to be filled in by Typemod.type_module *)

let type_module =
  ref ((fun _env _md -> assert false) :
       Env.t -> Parsetree.module_expr -> Typedtree.module_expr * Shape.t)

(* Forward declaration, to be filled in by Typemod.type_open *)

let type_open :
  (?used_slot:bool ref -> override_flag -> Env.t -> Location.t ->
   Longident.t loc -> Path.t * Env.t)
    ref =
  ref (fun ?used_slot:_ _ -> assert false)

let type_open_decl :
  (?used_slot:bool ref -> Env.t -> Parsetree.open_declaration
   -> open_declaration * Types.signature * Env.t)
    ref =
  ref (fun ?used_slot:_ _ -> assert false)

(* Forward declaration, to be filled in by Typemod.type_package *)

let type_package =
  ref (fun _ -> assert false)

(* Forward declaration, to be filled in by Typeclass.class_structure *)
let type_object =
  ref (fun _env _s -> assert false :
       Env.t -> Location.t -> Parsetree.class_structure ->
         Typedtree.class_structure * string list)

(*
  Saving and outputting type information.
  We keep these function names short, because they have to be
  called each time we create a record of type [Typedtree.expression]
  or [Typedtree.pattern] that will end up in the typed AST.
*)
let re node =
  Cmt_format.add_saved_type (Cmt_format.Partial_expression node);
  node

let rp node =
  Cmt_format.add_saved_type (Cmt_format.Partial_pattern (Value, node));
  node

let rcp node =
  Cmt_format.add_saved_type (Cmt_format.Partial_pattern (Computation, node));
  node


(* Context for inline record arguments; see [type_ident] *)

type recarg =
  | Allowed
  | Required
  | Rejected


let mk_expected ?explanation ty = { ty; explanation; }

let case lhs rhs =
  {c_lhs = lhs; c_guard = None; c_rhs = rhs}

(* Typing of constants *)

let type_constant = function
    Const_int _ -> instance Predef.type_int
  | Const_char _ -> instance Predef.type_char
  | Const_string _ -> instance Predef.type_string
  | Const_float _ -> instance Predef.type_float
  | Const_int32 _ -> instance Predef.type_int32
  | Const_int64 _ -> instance Predef.type_int64
  | Const_nativeint _ -> instance Predef.type_nativeint

let constant : Parsetree.constant -> (Asttypes.constant, error) result =
  function
  | Pconst_integer (i,None) ->
     begin
       try Ok (Const_int (Misc.Int_literal_converter.int i))
       with Failure _ -> Error (Literal_overflow "int")
     end
  | Pconst_integer (i,Some 'l') ->
     begin
       try Ok (Const_int32 (Misc.Int_literal_converter.int32 i))
       with Failure _ -> Error (Literal_overflow "int32")
     end
  | Pconst_integer (i,Some 'L') ->
     begin
       try Ok (Const_int64 (Misc.Int_literal_converter.int64 i))
       with Failure _ -> Error (Literal_overflow "int64")
     end
  | Pconst_integer (i,Some 'n') ->
     begin
       try Ok (Const_nativeint (Misc.Int_literal_converter.nativeint i))
       with Failure _ -> Error (Literal_overflow "nativeint")
     end
  | Pconst_integer (i,Some c) -> Error (Unknown_literal (i, c))
  | Pconst_char c -> Ok (Const_char c)
  | Pconst_string (s,loc,d) -> Ok (Const_string (s,loc,d))
  | Pconst_float (f,None)-> Ok (Const_float f)
  | Pconst_float (f,Some c) -> Error (Unknown_literal (f, c))

let constant_or_raise env loc cst =
  match constant cst with
  | Ok c -> c
  | Error err -> raise (Error (loc, env, err))

(* Specific version of type_option, using newty rather than newgenty *)

let type_option ty =
  newty (Tconstr(Predef.path_option,[ty], ref Mnil))

let mkexp exp_desc exp_type exp_loc exp_env =
  { exp_desc; exp_type; exp_loc; exp_env; exp_extra = []; exp_attributes = [] }

let option_none env ty loc =
  let lid = Longident.Lident "None" in
  let cnone = Env.find_ident_constructor Predef.ident_none env in
  mkexp (Texp_construct(mknoloc lid, cnone, [])) ty loc env

let option_some env texp =
  let lid = Longident.Lident "Some" in
  let csome = Env.find_ident_constructor Predef.ident_some env in
  mkexp ( Texp_construct(mknoloc lid , csome, [texp]) )
    (type_option texp.exp_type) texp.exp_loc texp.exp_env

let extract_option_type env ty =
  match get_desc (expand_head env ty) with
    Tconstr(path, [ty], _) when Path.same path Predef.path_option -> ty
  | _ -> assert false

let protect_expansion env ty =
  if Env.has_local_constraints env then generic_instance ty else ty

type record_extraction_result =
  | Record_type of Path.t * Path.t * Types.label_declaration list
  | Not_a_record_type
  | Maybe_a_record_type

let extract_concrete_typedecl_protected env ty =
  extract_concrete_typedecl env (protect_expansion env ty)

let extract_concrete_record env ty =
  match extract_concrete_typedecl_protected env ty with
  | Typedecl(p0, p, {type_kind=Type_record (fields, _)}) ->
    Record_type (p0, p, fields)
  | Has_no_typedecl | Typedecl(_, _, _) -> Not_a_record_type
  | May_have_typedecl -> Maybe_a_record_type

type variant_extraction_result =
  | Variant_type of Path.t * Path.t * Types.constructor_declaration list
  | Not_a_variant_type
  | Maybe_a_variant_type

let extract_concrete_variant env ty =
  match extract_concrete_typedecl_protected env ty with
  | Typedecl(p0, p, {type_kind=Type_variant (cstrs, _)}) ->
    Variant_type (p0, p, cstrs)
  | Typedecl(p0, p, {type_kind=Type_open}) ->
    Variant_type (p0, p, [])
  | Has_no_typedecl | Typedecl(_, _, _) -> Not_a_variant_type
  | May_have_typedecl -> Maybe_a_variant_type

let extract_label_names env ty =
  match extract_concrete_record env ty with
  | Record_type (_, _,fields) -> List.map (fun l -> l.Types.ld_id) fields
  | Not_a_record_type | Maybe_a_record_type -> assert false

let is_principal ty =
  not !Clflags.principal || get_level ty = generic_level

(* Typing of patterns *)

(* unification inside type_exp and type_expect *)
let unify_exp_types loc env ty expected_ty =
  (* Format.eprintf "@[%a@ %a@]@." Printtyp.raw_type_expr exp.exp_type
    Printtyp.raw_type_expr expected_ty; *)
  try
    unify env ty expected_ty
  with
    Unify err ->
      raise(Error(loc, env, Expr_type_clash(err, None, None)))
  | Tags(l1,l2) ->
      raise(Typetexp.Error(loc, env, Typetexp.Variant_tags (l1, l2)))

(* level at which to create the local type declarations *)
let gadt_equations_level = ref None
let get_gadt_equations_level () =
  match !gadt_equations_level with
    Some y -> y
  | None -> assert false

let nothing_equated = TypePairs.create 0

(* unification inside type_pat*)
let unify_pat_types_return_equated_pairs ?(refine = None) loc env ty ty' =
  try
    match refine with
    | Some allow_recursive ->
        unify_gadt ~equations_level:(get_gadt_equations_level ())
          ~allow_recursive env ty ty'
    | None ->
        unify !env ty ty';
        nothing_equated
  with
  | Unify err ->
      raise(Error(loc, !env, Pattern_type_clash(err, None)))
  | Tags(l1,l2) ->
      raise(Typetexp.Error(loc, !env, Typetexp.Variant_tags (l1, l2)))

let unify_pat_types ?refine loc env ty ty' =
  ignore (unify_pat_types_return_equated_pairs ?refine loc env ty ty')

let unify_pat ?refine env pat expected_ty =
  try unify_pat_types ?refine pat.pat_loc env pat.pat_type expected_ty
  with Error (loc, env, Pattern_type_clash(err, None)) ->
    raise(Error(loc, env, Pattern_type_clash(err, Some pat.pat_desc)))

(* unification of a type with a Tconstr with freshly created arguments *)
let unify_head_only ~refine loc env ty constr =
  let path = cstr_type_path constr in
  let decl = Env.find_type path !env in
  let ty' = Ctype.newconstr path (Ctype.instance_list decl.type_params) in
  unify_pat_types ~refine loc env ty' ty

(* Creating new conjunctive types is not allowed when typing patterns *)
(* make all Reither present in open variants *)
let finalize_variant pat tag opat r =
  let row =
    match get_desc (expand_head pat.pat_env pat.pat_type) with
      Tvariant row -> r := row; row
    | _ -> assert false
  in
  let f = get_row_field tag row in
  begin match row_field_repr f with
  | Rabsent -> () (* assert false *)
  | Reither (true, [], _) when not (row_closed row) ->
      link_row_field_ext ~inside:f (rf_present None)
  | Reither (false, ty::tl, _) when not (row_closed row) ->
      link_row_field_ext ~inside:f (rf_present (Some ty));
      begin match opat with None -> assert false
      | Some pat ->
          let env = ref pat.pat_env in List.iter (unify_pat env pat) (ty::tl)
      end
  | Reither (c, _l, true) when not (has_fixed_explanation row) ->
      link_row_field_ext ~inside:f (rf_either [] ~no_arg:c ~matched:false)
  | _ -> ()
  end
  (* Force check of well-formedness   WHY? *)
  (* unify_pat pat.pat_env pat
    (newty(Tvariant{row_fields=[]; row_more=newvar(); row_closed=false;
                    row_bound=(); row_fixed=false; row_name=None})); *)

let has_variants p =
  exists_general_pattern
    { f = fun (type k) (p : k general_pattern) -> match p.pat_desc with
     | (Tpat_variant _) -> true
     | _ -> false } p

let finalize_variants p =
  iter_general_pattern
    { f = fun (type k) (p : k general_pattern) -> match p.pat_desc with
     | Tpat_variant(tag, opat, r) ->
        finalize_variant p tag opat r
     | _ -> () } p

(* pattern environment *)
type pattern_variable =
  {
    pv_id: Ident.t;
    pv_type: type_expr;
    pv_loc: Location.t;
    pv_as_var: bool;
    pv_attributes: attributes;
  }

type module_variable =
  string loc * Location.t

let pattern_variables = ref ([] : pattern_variable list)
let pattern_force = ref ([] : (unit -> unit) list)
let allow_modules = ref false
let module_variables = ref ([] : module_variable list)
let reset_pattern allow =
  pattern_variables := [];
  pattern_force := [];
  allow_modules := allow;
  module_variables := []

let maybe_add_pattern_variables_ghost loc_let env pv =
  List.fold_right
    (fun {pv_id; _} env ->
       let name = Ident.name pv_id in
       if Env.bound_value name env then env
       else begin
         Env.enter_unbound_value name
           (Val_unbound_ghost_recursive loc_let) env
       end
    ) pv env

let enter_variable ?(is_module=false) ?(is_as_variable=false) loc name ty
    attrs =
  if List.exists (fun {pv_id; _} -> Ident.name pv_id = name.txt)
      !pattern_variables
  then raise(Error(loc, Env.empty, Multiply_bound_variable name.txt));
  let id = Ident.create_local name.txt in
  pattern_variables :=
    {pv_id = id;
     pv_type = ty;
     pv_loc = loc;
     pv_as_var = is_as_variable;
     pv_attributes = attrs} :: !pattern_variables;
  if is_module then begin
    (* Note: unpack patterns enter a variable of the same name *)
    if not !allow_modules then
      raise (Error (loc, Env.empty, Modules_not_allowed));
    module_variables := (name, loc) :: !module_variables
  end;
  id

let sort_pattern_variables vs =
  List.sort
    (fun {pv_id = x; _} {pv_id = y; _} ->
      Stdlib.compare (Ident.name x) (Ident.name y))
    vs

let enter_orpat_variables loc env  p1_vs p2_vs =
  (* unify_vars operate on sorted lists *)

  let p1_vs = sort_pattern_variables p1_vs
  and p2_vs = sort_pattern_variables p2_vs in

  let rec unify_vars p1_vs p2_vs =
    let vars vs = List.map (fun {pv_id; _} -> pv_id) vs in
    match p1_vs, p2_vs with
      | {pv_id = x1; pv_type = t1; _}::rem1, {pv_id = x2; pv_type = t2; _}::rem2
        when Ident.equal x1 x2 ->
          if x1==x2 then
            unify_vars rem1 rem2
          else begin
            begin try
              unify_var env (newvar ()) t1;
              unify env t1 t2
            with
            | Unify err ->
                raise(Error(loc, env, Or_pattern_type_clash(x1, err)))
            end;
          (x2,x1)::unify_vars rem1 rem2
          end
      | [],[] -> []
      | {pv_id; _}::_, [] | [],{pv_id; _}::_ ->
          raise (Error (loc, env, Orpat_vars (pv_id, [])))
      | {pv_id = x; _}::_, {pv_id = y; _}::_ ->
          let err =
            if Ident.name x < Ident.name y
            then Orpat_vars (x, vars p2_vs)
            else Orpat_vars (y, vars p1_vs) in
          raise (Error (loc, env, err)) in
  unify_vars p1_vs p2_vs

let rec build_as_type ~refine (env : Env.t ref) p =
  let as_ty = build_as_type_aux ~refine env p in
  (* Cf. #1655 *)
  List.fold_left (fun as_ty (extra, _loc, _attrs) ->
    match extra with
    | Tpat_type _ | Tpat_open _ | Tpat_unpack -> as_ty
    | Tpat_constraint cty ->
      (* [generic_instance] can only be used if the variables of the original
         type ([cty.ctyp_type] here) are not at [generic_level], which they are
         here.
         If we used [generic_instance] we would lose the sharing between
         [instance ty] and [ty].  *)
      begin_def ();
      let ty = instance cty.ctyp_type in
      end_def ();
      generalize_structure ty;
      (* This call to unify can't fail since the pattern is well typed. *)
      unify_pat_types ~refine p.pat_loc env (instance as_ty) (instance ty);
      ty
  ) as_ty p.pat_extra

and build_as_type_aux ~refine (env : Env.t ref) p =
  let build_as_type = build_as_type ~refine in
  match p.pat_desc with
    Tpat_alias(p1,_, _) -> build_as_type env p1
  | Tpat_tuple pl ->
      let tyl = List.map (build_as_type env) pl in
      newty (Ttuple tyl)
  | Tpat_construct(_, cstr, pl, vto) ->
      let keep =
        cstr.cstr_private = Private || cstr.cstr_existentials <> [] ||
        vto <> None (* be lazy and keep the type for node constraints *) in
      if keep then p.pat_type else
      let tyl = List.map (build_as_type env) pl in
      let ty_args, ty_res, _ =
        instance_constructor Keep_existentials_flexible cstr
      in
      List.iter2 (fun (p,ty) -> unify_pat ~refine env {p with pat_type = ty})
        (List.combine pl tyl) ty_args;
      ty_res
  | Tpat_variant(l, p', _) ->
      let ty = Option.map (build_as_type env) p' in
      let fields = [l, rf_present ty] in
      newty (Tvariant (create_row ~fields ~more:(newvar())
                         ~name:None ~fixed:None ~closed:false))
  | Tpat_record (lpl,_) ->
      let lbl = snd3 (List.hd lpl) in
      if lbl.lbl_private = Private then p.pat_type else
      let ty = newvar () in
      let ppl = List.map (fun (_, l, p) -> l.lbl_pos, p) lpl in
      let do_label lbl =
        let _, ty_arg, ty_res = instance_label false lbl in
        unify_pat ~refine env {p with pat_type = ty} ty_res;
        let refinable =
          lbl.lbl_mut = Immutable && List.mem_assoc lbl.lbl_pos ppl &&
          match get_desc lbl.lbl_arg with Tpoly _ -> false | _ -> true in
        if refinable then begin
          let arg = List.assoc lbl.lbl_pos ppl in
          unify_pat ~refine env
            {arg with pat_type = build_as_type env arg} ty_arg
        end else begin
          let _, ty_arg', ty_res' = instance_label false lbl in
          unify_pat_types ~refine p.pat_loc env ty_arg ty_arg';
          unify_pat ~refine env p ty_res'
        end in
      Array.iter do_label lbl.lbl_all;
      ty
  | Tpat_or(p1, p2, row) ->
      begin match row with
        None ->
          let ty1 = build_as_type env p1 and ty2 = build_as_type env p2 in
          unify_pat ~refine env {p2 with pat_type = ty2} ty1;
          ty1
      | Some row ->
          let Row {fields; fixed; name} = row_repr row in
          newty (Tvariant (create_row ~fields ~fixed ~name
                             ~closed:false ~more:(newvar())))
      end
  | Tpat_any | Tpat_var _ | Tpat_constant _
  | Tpat_array _ | Tpat_lazy _ -> p.pat_type

(* Constraint solving during typing of patterns *)

let solve_Ppat_poly_constraint ~refine env loc sty expected_ty =
  let cty, ty, force = Typetexp.transl_simple_type_delayed !env sty in
  unify_pat_types ~refine loc env ty (instance expected_ty);
  pattern_force := force :: !pattern_force;
  match get_desc ty with
  | Tpoly (body, tyl) ->
      begin_def ();
      init_def generic_level;
      let _, ty' = instance_poly ~keep_names:true false tyl body in
      end_def ();
      (cty, ty, ty')
  | _ -> assert false

let solve_Ppat_alias ~refine env pat =
  begin_def ();
  let ty_var = build_as_type ~refine env pat in
  end_def ();
  generalize ty_var;
  ty_var

let solve_Ppat_tuple (type a) ~refine loc env (args : a list) expected_ty =
  let vars = List.map (fun _ -> newgenvar ()) args in
  let ty = newgenty (Ttuple vars) in
  let expected_ty = generic_instance expected_ty in
  unify_pat_types ~refine loc env ty expected_ty;
  vars

let solve_constructor_annotation env name_list sty ty_args ty_ex =
  let expansion_scope = get_gadt_equations_level () in
  let ids =
    List.map
      (fun name ->
        let decl = new_local_type ~loc:name.loc () in
        let (id, new_env) =
          Env.enter_type ~scope:expansion_scope name.txt decl !env in
        env := new_env;
        {name with txt = id})
      name_list
  in
  begin_def ();
  let cty, ty, force = Typetexp.transl_simple_type_delayed !env sty in
  end_def ();
  generalize_structure ty;
  pattern_force := force :: !pattern_force;
  let ty_args =
    let ty1 = instance ty and ty2 = instance ty in
    match ty_args with
      [] -> assert false
    | [ty_arg] ->
        unify_pat_types cty.ctyp_loc env ty1 ty_arg;
        [ty2]
    | _ ->
        unify_pat_types cty.ctyp_loc env ty1 (newty (Ttuple ty_args));
        match get_desc (expand_head !env ty2) with
          Ttuple tyl -> tyl
        | _ -> assert false
  in
  if ids <> [] then ignore begin
    let ids = List.map (fun x -> x.txt) ids in
    let rem =
      List.fold_left
        (fun rem tv ->
          match get_desc tv with
            Tconstr(Path.Pident id, [], _) when List.mem id rem ->
              list_remove id rem
          | _ ->
              raise (Error (cty.ctyp_loc, !env,
                            Unbound_existential (ids, ty))))
        ids ty_ex
    in
    if rem <> [] then
      raise (Error (cty.ctyp_loc, !env,
                    Unbound_existential (ids, ty)))
  end;
  ty_args, Some (ids, cty)

let solve_Ppat_construct ~refine env loc constr no_existentials
        existential_styp expected_ty =
  (* if constructor is gadt, we must verify that the expected type has the
     correct head *)
  if constr.cstr_generalized then
    unify_head_only ~refine loc env (instance expected_ty) constr;
  begin_def ();
  let expected_ty = instance expected_ty in
  (* PR#7214: do not use gadt unification for toplevel lets *)
  let unify_res ty_res =
    let refine =
      match refine, no_existentials with
      | None, None when constr.cstr_generalized -> Some false
      | _ -> refine
    in
    unify_pat_types_return_equated_pairs ~refine loc env ty_res expected_ty
  in
  let expansion_scope = get_gadt_equations_level () in
  let ty_args, ty_res, equated_types, existential_ctyp =
    match existential_styp with
      None ->
        let ty_args, ty_res, _ =
          instance_constructor
            (Make_existentials_abstract { env; scope = expansion_scope }) constr
        in
        ty_args, ty_res, unify_res ty_res, None
    | Some (name_list, sty) ->
        let existential_treatment =
          if name_list = [] then
            Make_existentials_abstract { env; scope = expansion_scope }
          else
            (* we will unify them (in solve_constructor_annotation) with the
               local types provided by the user *)
            Keep_existentials_flexible
        in
        let ty_args, ty_res, ty_ex =
          instance_constructor existential_treatment constr
        in
        let equated_types = unify_res ty_res in
        let ty_args, existential_ctyp =
          solve_constructor_annotation env name_list sty ty_args ty_ex in
        ty_args, ty_res, equated_types, existential_ctyp
  in
  if constr.cstr_existentials <> [] then
    lower_variables_only !env expansion_scope ty_res;
  end_def ();
  generalize_structure expected_ty;
  generalize_structure ty_res;
  List.iter generalize_structure ty_args;
  if !Clflags.principal && refine = None then begin
    (* Do not warn for counter-examples *)
    let exception Warn_only_once in
    try
      TypePairs.iter
        (fun (t1, t2) ->
          generalize_structure t1;
          generalize_structure t2;
          if not (fully_generic t1 && fully_generic t2) then
            let msg =
              Format.asprintf
                "typing this pattern requires considering@ %a@ and@ %a@ as \
                equal.@,\
                But the knowledge of these types"
                    Printtyp.type_expr t1
                    Printtyp.type_expr t2
            in
            Location.prerr_warning loc (Warnings.Not_principal msg);
            raise Warn_only_once)
        equated_types
    with Warn_only_once -> ()
  end;
  (ty_args, existential_ctyp)

let solve_Ppat_record_field ~refine loc env label label_lid record_ty =
  begin_def ();
  let (_, ty_arg, ty_res) = instance_label false label in
  begin try
    unify_pat_types ~refine loc env ty_res (instance record_ty)
  with Error(_loc, _env, Pattern_type_clash(err, _)) ->
    raise(Error(label_lid.loc, !env,
                Label_mismatch(label_lid.txt, err)))
  end;
  end_def ();
  generalize_structure ty_res;
  generalize_structure ty_arg;
  ty_arg

let solve_Ppat_array ~refine loc env expected_ty =
  let ty_elt = newgenvar() in
  let expected_ty = generic_instance expected_ty in
  unify_pat_types ~refine
    loc env (Predef.type_array ty_elt) expected_ty;
  ty_elt

let solve_Ppat_lazy  ~refine loc env expected_ty =
  let nv = newgenvar () in
  unify_pat_types ~refine loc env (Predef.type_lazy_t nv)
    (generic_instance expected_ty);
  nv

let solve_Ppat_constraint ~refine loc env sty expected_ty =
  begin_def();
  let cty, ty, force = Typetexp.transl_simple_type_delayed !env sty in
  end_def();
  pattern_force := force :: !pattern_force;
  generalize_structure ty;
  let ty, expected_ty' = instance ty, ty in
  unify_pat_types ~refine loc env ty (instance expected_ty);
  (cty, ty, expected_ty')

let solve_Ppat_variant ~refine loc env tag no_arg expected_ty =
  let arg_type = if no_arg then [] else [newgenvar()] in
  let fields = [tag, rf_either ~no_arg arg_type ~matched:true] in
  let make_row more =
    create_row ~fields ~closed:false ~more ~fixed:None ~name:None
  in
  let row = make_row (newgenvar ()) in
  let expected_ty = generic_instance expected_ty in
  (* PR#7404: allow some_private_tag blindly, as it would not unify with
     the abstract row variable *)
  if tag <> Parmatch.some_private_tag then
    unify_pat_types ~refine loc env (newgenty(Tvariant row)) expected_ty;
  (arg_type, make_row (newvar ()), instance expected_ty)

(* Building the or-pattern corresponding to a polymorphic variant type *)
let build_or_pat env loc lid =
  let path, decl = Env.lookup_type ~loc:lid.loc lid.txt env in
  let tyl = List.map (fun _ -> newvar()) decl.type_params in
  let row0 =
    let ty = expand_head env (newty(Tconstr(path, tyl, ref Mnil))) in
    match get_desc ty with
      Tvariant row when static_row row -> row
    | _ -> raise(Error(lid.loc, env, Not_a_polymorphic_variant_type lid.txt))
  in
  let pats, fields =
    List.fold_left
      (fun (pats,fields) (l,f) ->
        match row_field_repr f with
          Rpresent None ->
            let f = rf_either [] ~no_arg:true ~matched:true in
            (l,None) :: pats,
            (l, f) :: fields
        | Rpresent (Some ty) ->
            let f = rf_either [ty] ~no_arg:false ~matched:true in
            (l, Some {pat_desc=Tpat_any; pat_loc=Location.none; pat_env=env;
                      pat_type=ty; pat_extra=[]; pat_attributes=[]})
            :: pats,
            (l, f) :: fields
        | _ -> pats, fields)
      ([],[]) (row_fields row0) in
  let fields = List.rev fields in
  let name = Some (path, tyl) in
  let make_row more =
    create_row ~fields ~more ~closed:false ~fixed:None ~name in
  let ty = newty (Tvariant (make_row (newvar()))) in
  let gloc = {loc with Location.loc_ghost=true} in
  let row' = ref (make_row (newvar())) in
  let pats =
    List.map
      (fun (l,p) ->
        {pat_desc=Tpat_variant(l,p,row'); pat_loc=gloc;
         pat_env=env; pat_type=ty; pat_extra=[]; pat_attributes=[]})
      pats
  in
  match pats with
    [] ->
      (* empty polymorphic variants: not possible with the concrete language
         but valid at the ast level *)
      raise(Error(lid.loc, env, Not_a_polymorphic_variant_type lid.txt))
  | pat :: pats ->
      let r =
        List.fold_left
          (fun pat pat0 ->
            {pat_desc=Tpat_or(pat0,pat,Some row0); pat_extra=[];
             pat_loc=gloc; pat_env=env; pat_type=ty; pat_attributes=[]})
          pat pats in
      (path, rp { r with pat_loc = loc })

let split_cases env cases =
  let add_case lst case = function
    | None -> lst
    | Some c_lhs -> { case with c_lhs } :: lst
  in
  List.fold_right (fun ({ c_lhs; c_guard } as case) (vals, exns) ->
    match split_pattern c_lhs with
    | Some _, Some _ when c_guard <> None ->
      raise (Error (c_lhs.pat_loc, env,
                    Mixed_value_and_exception_patterns_under_guard))
    | vp, ep -> add_case vals case vp, add_case exns case ep
  ) cases ([], [])

(* Type paths *)

let rec expand_path env p =
  let decl =
    try Some (Env.find_type p env) with Not_found -> None
  in
  match decl with
    Some {type_manifest = Some ty} ->
      begin match get_desc ty with
        Tconstr(p,_,_) -> expand_path env p
      | _ -> assert false
      end
  | _ ->
      let p' = Env.normalize_type_path None env p in
      if Path.same p p' then p else expand_path env p'

let compare_type_path env tpath1 tpath2 =
  Path.same (expand_path env tpath1) (expand_path env tpath2)

(* Records *)
exception Wrong_name_disambiguation of Env.t * wrong_name

let get_constr_type_path ty =
  match get_desc ty with
  | Tconstr(p, _, _) -> p
  | _ -> assert false

module NameChoice(Name : sig
  type t
  type usage
  val kind: Datatype_kind.t
  val get_name: t -> string
  val get_type: t -> type_expr
  val lookup_all_from_type:
    Location.t -> usage -> Path.t -> Env.t -> (t * (unit -> unit)) list

  (** Some names (for example the fields of inline records) are not
      in the typing environment -- they behave as structural labels
      rather than nominal labels.*)
  val in_env: t -> bool
end) = struct
  open Name

  let get_type_path d = get_constr_type_path (get_type d)

  let lookup_from_type env type_path usage lid =
    let descrs = lookup_all_from_type lid.loc usage type_path env in
    match lid.txt with
    | Longident.Lident name -> begin
        match
          List.find (fun (nd, _) -> get_name nd = name) descrs
        with
        | descr, use ->
            use ();
            descr
        | exception Not_found ->
            let valid_names = List.map (fun (nd, _) -> get_name nd) descrs in
            raise (Wrong_name_disambiguation (env, {
                    type_path;
                    name = { lid with txt = name };
                    kind;
                    valid_names;
              }))
      end
    | _ -> raise Not_found

  let rec unique eq acc = function
      [] -> List.rev acc
    | x :: rem ->
        if List.exists (eq x) acc then unique eq acc rem
        else unique eq (x :: acc) rem

  let ambiguous_types env lbl others =
    let tpath = get_type_path lbl in
    let others =
      List.map (fun (lbl, _) -> get_type_path lbl) others in
    let tpaths = unique (compare_type_path env) [tpath] others in
    match tpaths with
      [_] -> []
    | _ -> let open Printtyp in
        wrap_printing_env ~error:true env (fun () ->
            reset(); strings_of_paths Type tpaths)

  let disambiguate_by_type env tpath lbls =
    match lbls with
    | (Error _ : _ result) -> raise Not_found
    | Ok lbls ->
        let check_type (lbl, _) =
          let lbl_tpath = get_type_path lbl in
          compare_type_path env tpath lbl_tpath
        in
        List.find check_type lbls

  (* warn if there are several distinct candidates in scope *)
  let warn_if_ambiguous warn lid env lbl rest =
    if Warnings.is_active (Ambiguous_name ([],[],false,"")) then begin
      Printtyp.Conflicts.reset ();
      let paths = ambiguous_types env lbl rest in
      let expansion =
        Format.asprintf "%t" Printtyp.Conflicts.print_explanations in
      if paths <> [] then
        warn lid.loc
          (Warnings.Ambiguous_name ([Longident.last lid.txt],
                                    paths, false, expansion))
    end

  (* a non-principal type was used for disambiguation *)
  let warn_non_principal warn lid =
    let name = Datatype_kind.label_name kind in
    warn lid.loc
      (Warnings.Not_principal
         ("this type-based " ^ name ^ " disambiguation"))

  (* we selected a name out of the lexical scope *)
  let warn_out_of_scope warn lid env tpath =
    if Warnings.is_active (Name_out_of_scope ("",[],false)) then begin
      let path_s =
        Printtyp.wrap_printing_env ~error:true env
          (fun () -> Printtyp.string_of_path tpath) in
      warn lid.loc
        (Warnings.Name_out_of_scope (path_s, [Longident.last lid.txt], false))
    end

  (* warn if the selected name is not the last introduced in scope
     -- in these cases the resolution is different from pre-disambiguation OCaml
     (this warning is not enabled by default, it is specifically for people
      wishing to write backward-compatible code).
   *)
  let warn_if_disambiguated_name warn lid lbl scope =
    match scope with
    | Ok ((lab1,_) :: _) when lab1 == lbl -> ()
    | _ ->
        warn lid.loc
          (Warnings.Disambiguated_name (get_name lbl))

  let force_error : ('a, _) result -> 'a = function
    | Ok lbls -> lbls
    | Error (loc', env', err) ->
       Env.lookup_error loc' env' err

  type candidate = t * (unit -> unit)
  type nonempty_candidate_filter =
    candidate list -> (candidate list, candidate list) result
  (** This type is used for candidate filtering functions.
      Filtering typically proceeds in several passes, filtering
      candidates through increasingly precise conditions.

      We assume that the input list is non-empty, and the output is one of
      - [Ok result] for a non-empty list [result] of valid candidates
      - [Error candidates] with there are no valid candidates,
        and [candidates] is a non-empty subset of the input, typically
        the result of the last non-empty filtering step.
   *)

  (** [disambiguate] selects a concrete description for [lid] using
     some contextual information:
     - An optional [expected_type].
     - A list of candidates labels in the current lexical scope,
       [candidates_in_scope], that is actually at the type
       [(label_descr list, lookup_error) result] so that the
       lookup error is only raised when necessary.
     - A filtering criterion on candidates in scope [filter_candidates],
       representing extra contextual information that can help
       candidate selection (see [disambiguate_label_by_ids]).
   *)
  let disambiguate
        ?(warn=Location.prerr_warning)
        ?(filter : nonempty_candidate_filter = Result.ok)
        usage lid env
        expected_type
        candidates_in_scope =
    let lbl = match expected_type with
    | None ->
        (* no expected type => no disambiguation *)
        begin match filter (force_error candidates_in_scope) with
        | Ok [] | Error [] -> assert false
        | Error((lbl, _use) :: _rest) -> lbl (* will fail later *)
        | Ok((lbl, use) :: rest) ->
            use ();
            warn_if_ambiguous warn lid env lbl rest;
            lbl
        end
    | Some(tpath0, tpath, principal) ->
       (* If [expected_type] is available, the candidate selected
          will correspond to the type-based resolution.
          There are two reasons to still check the lexical scope:
          - for warning purposes
          - for extension types, the type environment does not contain
            a list of constructors, so using only type-based selection
            would fail.
        *)
        (* note that [disambiguate_by_type] does not
           force [candidates_in_scope]: we just skip this case if there
           are no candidates in scope *)
        begin match disambiguate_by_type env tpath candidates_in_scope with
        | lbl, use ->
          use ();
          if not principal then begin
            (* Check if non-principal type is affecting result *)
            match (candidates_in_scope : _ result) with
            | Error _ -> warn_non_principal warn lid
            | Ok lbls ->
            match filter lbls with
            | Error _ -> warn_non_principal warn lid
            | Ok [] -> assert false
            | Ok ((lbl', _use') :: rest) ->
            let lbl_tpath = get_type_path lbl' in
            (* no principality warning if the non-principal
               type-based selection corresponds to the last
               definition in scope *)
            if not (compare_type_path env tpath lbl_tpath)
            then warn_non_principal warn lid
            else warn_if_ambiguous warn lid env lbl rest;
          end;
          lbl
        | exception Not_found ->
        (* look outside the lexical scope *)
        match lookup_from_type env tpath usage lid with
        | lbl ->
          (* warn only on nominal labels;
             structural labels cannot be qualified anyway *)
          if in_env lbl then warn_out_of_scope warn lid env tpath;
          if not principal then warn_non_principal warn lid;
          lbl
        | exception Not_found ->
        match filter (force_error candidates_in_scope) with
        | Ok lbls | Error lbls ->
        let tp = (tpath0, expand_path env tpath) in
        let tpl =
          List.map
            (fun (lbl, _) ->
               let tp0 = get_type_path lbl in
               let tp = expand_path env tp0 in
               (tp0, tp))
            lbls
        in
        raise (Error (lid.loc, env,
                      Name_type_mismatch (kind, lid.txt, tp, tpl)));
        end
    in
    (* warn only on nominal labels *)
    if in_env lbl then
      warn_if_disambiguated_name warn lid lbl candidates_in_scope;
    lbl
end

let wrap_disambiguate msg ty f x =
  try f x with
  | Wrong_name_disambiguation (env, wrong_name) ->
    raise (Error (wrong_name.name.loc, env, Wrong_name (msg, ty, wrong_name)))

module Label = NameChoice (struct
  type t = label_description
  type usage = Env.label_usage
  let kind = Datatype_kind.Record
  let get_name lbl = lbl.lbl_name
  let get_type lbl = lbl.lbl_res
  let lookup_all_from_type loc usage path env =
    Env.lookup_all_labels_from_type ~loc usage path env
  let in_env lbl =
    match lbl.lbl_repres with
    | Record_regular | Record_float | Record_unboxed false -> true
    | Record_unboxed true | Record_inlined _ | Record_extension _ -> false
end)

(* In record-construction expressions and patterns, we have many labels
   at once; find a candidate type in the intersection of the candidates
   of each label. In the [closed] expression case, this candidate must
   contain exactly all the labels.

   If our successive refinements result in an empty list,
   return [Error] with the last non-empty list of candidates
   for use in error messages.
*)
let disambiguate_label_by_ids closed ids labels  : (_, _) result =
  let check_ids (lbl, _) =
    let lbls = Hashtbl.create 8 in
    Array.iter (fun lbl -> Hashtbl.add lbls lbl.lbl_name ()) lbl.lbl_all;
    List.for_all (Hashtbl.mem lbls) ids
  and check_closed (lbl, _) =
    (not closed || List.length ids = Array.length lbl.lbl_all)
  in
  match List.filter check_ids labels with
  | [] -> Error labels
  | labels ->
  match List.filter check_closed labels with
  | [] -> Error labels
  | labels ->
  Ok labels

(* Only issue warnings once per record constructor/pattern *)
let disambiguate_lid_a_list loc closed env usage expected_type lid_a_list =
  let ids = List.map (fun (lid, _) -> Longident.last lid.txt) lid_a_list in
  let w_pr = ref false and w_amb = ref []
  and w_scope = ref [] and w_scope_ty = ref "" in
  let warn loc msg =
    let open Warnings in
    match msg with
    | Not_principal _ -> w_pr := true
    | Ambiguous_name([s], l, _, ex) -> w_amb := (s, l, ex) :: !w_amb
    | Name_out_of_scope(ty, [s], _) ->
        w_scope := s :: !w_scope; w_scope_ty := ty
    | _ -> Location.prerr_warning loc msg
  in
  let process_label lid =
    let scope = Env.lookup_all_labels ~loc:lid.loc usage lid.txt env in
    let filter : Label.nonempty_candidate_filter =
      disambiguate_label_by_ids closed ids in
    Label.disambiguate ~warn ~filter usage lid env expected_type scope in
  let lbl_a_list =
    List.map (fun (lid,a) -> lid, process_label lid, a) lid_a_list in
  if !w_pr then
    Location.prerr_warning loc
      (Warnings.Not_principal "this type-based record disambiguation")
  else begin
    match List.rev !w_amb with
      (_,types,ex)::_ as amb ->
        let paths =
          List.map (fun (_,lbl,_) -> Label.get_type_path lbl) lbl_a_list in
        let path = List.hd paths in
        let fst3 (x,_,_) = x in
        if List.for_all (compare_type_path env path) (List.tl paths) then
          Location.prerr_warning loc
            (Warnings.Ambiguous_name (List.map fst3 amb, types, true, ex))
        else
          List.iter
            (fun (s,l,ex) -> Location.prerr_warning loc
                (Warnings.Ambiguous_name ([s],l,false, ex)))
            amb
    | _ -> ()
  end;
  if !w_scope <> [] then
    Location.prerr_warning loc
      (Warnings.Name_out_of_scope (!w_scope_ty, List.rev !w_scope, true));
  lbl_a_list

let rec find_record_qual = function
  | [] -> None
  | ({ txt = Longident.Ldot (modname, _) }, _) :: _ -> Some modname
  | _ :: rest -> find_record_qual rest

let map_fold_cont f xs k =
  List.fold_right (fun x k ys -> f x (fun y -> k (y :: ys)))
    xs (fun ys -> k (List.rev ys)) []

let type_label_a_list
      ?labels loc closed env usage type_lbl_a expected_type lid_a_list k =
  let lbl_a_list =
    match lid_a_list, labels with
      ({txt=Longident.Lident s}, _)::_, Some labels when Hashtbl.mem labels s ->
        (* Special case for rebuilt syntax trees *)
        List.map
          (function lid, a -> match lid.txt with
            Longident.Lident s -> lid, Hashtbl.find labels s, a
          | _ -> assert false)
          lid_a_list
    | _ ->
        let lid_a_list =
          match find_record_qual lid_a_list with
            None -> lid_a_list
          | Some modname ->
              List.map
                (fun (lid, a as lid_a) ->
                  match lid.txt with Longident.Lident s ->
                    {lid with txt=Longident.Ldot (modname, s)}, a
                  | _ -> lid_a)
                lid_a_list
        in
        disambiguate_lid_a_list loc closed env usage expected_type lid_a_list
  in
  (* Invariant: records are sorted in the typed tree *)
  let lbl_a_list =
    List.sort
      (fun (_,lbl1,_) (_,lbl2,_) -> compare lbl1.lbl_pos lbl2.lbl_pos)
      lbl_a_list
  in
  map_fold_cont type_lbl_a lbl_a_list k

(* Checks over the labels mentioned in a record pattern:
   no duplicate definitions (error); properly closed (warning) *)

let check_recordpat_labels loc lbl_pat_list closed =
  match lbl_pat_list with
  | [] -> ()                            (* should not happen *)
  | (_, label1, _) :: _ ->
      let all = label1.lbl_all in
      let defined = Array.make (Array.length all) false in
      let check_defined (_, label, _) =
        if defined.(label.lbl_pos)
        then raise(Error(loc, Env.empty, Label_multiply_defined label.lbl_name))
        else defined.(label.lbl_pos) <- true in
      List.iter check_defined lbl_pat_list;
      if closed = Closed
      && Warnings.is_active (Warnings.Missing_record_field_pattern "")
      then begin
        let undefined = ref [] in
        for i = 0 to Array.length all - 1 do
          if not defined.(i) then undefined := all.(i).lbl_name :: !undefined
        done;
        if !undefined <> [] then begin
          let u = String.concat ", " (List.rev !undefined) in
          Location.prerr_warning loc (Warnings.Missing_record_field_pattern u)
        end
      end

(* Constructors *)

module Constructor = NameChoice (struct
  type t = constructor_description
  type usage = Env.constructor_usage
  let kind = Datatype_kind.Variant
  let get_name cstr = cstr.cstr_name
  let get_type cstr = cstr.cstr_res
  let lookup_all_from_type loc usage path env =
    match Env.lookup_all_constructors_from_type ~loc usage path env with
    | _ :: _ as x -> x
    | [] ->
        match (Env.find_type path env).type_kind with
        | Type_open ->
            (* Extension constructors cannot be found by looking at the type
               declaration.
               We scan the whole environment to get an accurate spellchecking
               hint in the subsequent error message *)
            let filter lbl =
              compare_type_path env
                path (get_constr_type_path @@ get_type lbl) in
            let add_valid x acc = if filter x then (x,ignore)::acc else acc in
            Env.fold_constructors add_valid None env []
        | _ -> []
  let in_env _ = true
end)

(* Typing of patterns *)

(* "half typed" cases are produced in [type_cases] when we've just typechecked
   the pattern but haven't type-checked the body yet.
   At this point we might have added some type equalities to the environment,
   but haven't yet added identifiers bound by the pattern. *)
type 'case_pattern half_typed_case =
  { typed_pat: 'case_pattern;
    pat_type_for_unif: type_expr;
    untyped_case: Parsetree.case;
    branch_env: Env.t;
    pat_vars: pattern_variable list;
    unpacks: module_variable list;
    contains_gadt: bool; }

let rec has_literal_pattern p = match p.ppat_desc with
  | Ppat_constant _
  | Ppat_interval _ ->
     true
  | Ppat_any
  | Ppat_variant (_, None)
  | Ppat_construct (_, None)
  | Ppat_type _
  | Ppat_var _
  | Ppat_unpack _
  | Ppat_extension _ ->
     false
  | Ppat_exception p
  | Ppat_variant (_, Some p)
  | Ppat_construct (_, Some (_, p))
  | Ppat_constraint (p, _)
  | Ppat_alias (p, _)
  | Ppat_lazy p
  | Ppat_open (_, p) ->
     has_literal_pattern p
  | Ppat_tuple ps
  | Ppat_array ps ->
     List.exists has_literal_pattern ps
  | Ppat_record (ps, _) ->
     List.exists (fun (_,p) -> has_literal_pattern p) ps
  | Ppat_or (p, q) ->
     has_literal_pattern p || has_literal_pattern q

let check_scope_escape loc env level ty =
  try Ctype.check_scope_escape env level ty
  with Escape esc ->
    (* We don't expand the type here because if we do, we might expand to the
       type that escaped, leading to confusing error messages. *)
    let trace = Errortrace.[Escape (map_escape trivial_expansion esc)] in
    raise (Error(loc,
                 env,
                 Pattern_type_clash(Errortrace.unification_error ~trace, None)))

type pattern_checking_mode =
  | Normal
  (** We are checking user code. *)
  | Counter_example of counter_example_checking_info
  (** In [Counter_example] mode, we are checking a counter-example
      candidate produced by Parmatch. This is a syntactic pattern that
      represents a set of values by using or-patterns (p_1 | ... | p_n)
      to enumerate all alternatives in the counter-example
      search. These or-patterns occur at every choice point, possibly
      deep inside the pattern.

      Parmatch does not use type information, so this pattern may
      exhibit two issues:
      - some parts of the pattern may be ill-typed due to GADTs, and
      - some wildcard patterns may not match any values: their type is
        empty.

      The aim of [type_pat] in the [Counter_example] mode is to refine
      this syntactic pattern into a well-typed pattern, and ensure
      that it matches at least one concrete value.
      - It filters ill-typed branches of or-patterns.
        (see {!splitting_mode} below)
      - It tries to check that wildcard patterns are non-empty.
        (see {!explosion_fuel})
  *)

and counter_example_checking_info = {
    explosion_fuel: int;
    splitting_mode: splitting_mode;
    constrs: (string, Types.constructor_description) Hashtbl.t;
    labels: (string, Types.label_description) Hashtbl.t;
  }
(**
    [explosion_fuel] controls the checking of wildcard patterns.  We
    eliminate potentially-empty wildcard patterns by exploding them
    into concrete sub-patterns, for example (K1 _ | K2 _) or
    { l1: _; l2: _ }. [explosion_fuel] is the depth limit on wildcard
    explosion. Such depth limit is required to avoid non-termination
    and compilation-time blowups.

    [splitting_mode] controls the handling of or-patterns.  In
    [Counter_example] mode, we only need to select one branch that
    leads to a well-typed pattern. Checking all branches is expensive,
    we use different search strategies (see {!splitting_mode}) to
    reduce the number of explored alternatives.

    [constrs] and [labels] contain metadata produced by [Parmatch] to
    type-check the given syntactic pattern. [Parmatch] produces
    counter-examples by turning typed patterns into
    [Parsetree.pattern]. In this process, constructor and label paths
    are lost, and are replaced by generated strings. [constrs] and
    [labels] map those synthetic names back to the typed descriptions
    of the original names.
 *)

(** Due to GADT constraints, an or-pattern produced within
    a counter-example may have ill-typed branches. Consider for example

    {[
      type _ tag = Int : int tag | Bool : bool tag
    ]}

    then [Parmatch] will propose the or-pattern [Int | Bool] whenever
    a pattern of type [tag] is required to form a counter-example. For
    example, a function expects a (int tag option) and only [None] is
    handled by the user-written pattern. [Some (Int | Bool)] is not
    well-typed in this context, only the sub-pattern [Some Int] is.
    In this example, the expected type coming from the context
    suffices to know which or-pattern branch must be chosen.

    In the general case, choosing a branch can have non-local effects
    on the typability of the term. For example, consider a tuple type
    ['a tag * ...'a...], where the first component is a GADT.  All
    constructor choices for this GADT lead to a well-typed branch in
    isolation (['a] is unconstrained), but choosing one of them adds
    a constraint on ['a] that may make the other tuple elements
    ill-typed.

    In general, after choosing each possible branch of the or-pattern,
    [type_pat] has to check the rest of the pattern to tell if this
    choice leads to a well-typed term. This may lead to an explosion
    of typing/search work -- the rest of the term may in turn contain
    alternatives.

    We use careful strategies to try to limit counterexample-checking
    time; [splitting_mode] represents those strategies.
*)
and splitting_mode =
  | Backtrack_or
  (** Always backtrack in or-patterns.

      [Backtrack_or] selects a single alternative from an or-pattern
      by using backtracking, trying to choose each branch in turn, and
      to complete it into a valid sub-pattern. We call this
      "splitting" the or-pattern.

      We use this mode when looking for unused patterns or sub-patterns,
      in particular to check a refutation clause (p -> .).
    *)
  | Refine_or of { inside_nonsplit_or: bool; }
  (** Only backtrack when needed.

      [Refine_or] tries another approach for refining or-pattern.

      Instead of always splitting each or-pattern, It first attempts to
      find branches that do not introduce new constraints (because they
      do not contain GADT constructors). Those branches are such that,
      if they fail, all other branches will fail.

      If we find one such branch, we attempt to complete the subpattern
      (checking what's outside the or-pattern), ignoring other
      branches -- we never consider another branch choice again. If all
      branches are constrained, it falls back to splitting the
      or-pattern.

      We use this mode when checking exhaustivity of pattern matching.
  *)

(** This exception is only used internally within [type_pat_aux], in
    counter-example mode, to jump back to the parent or-pattern in the
    [Refine_or] strategy.

    Such a parent exists precisely when [inside_nonsplit_or = true];
    it's an invariant that we always setup an exception handler for
    [Need_backtrack] when we set this flag. *)
exception Need_backtrack

(** This exception is only used internally within [type_pat_aux], in
    counter-example mode. We use it to discard counter-example candidates
    that do not match any value. *)
exception Empty_branch

type abort_reason = Adds_constraints | Empty

(** Remember current typing state for backtracking.
   No variable information, as we only backtrack on
   patterns without variables (cf. assert statements). *)
type state =
 { snapshot: snapshot;
   levels: Ctype.levels;
   env: Env.t; }
let save_state env =
  { snapshot = Btype.snapshot ();
    levels = Ctype.save_levels ();
    env = !env; }
let set_state s env =
  Btype.backtrack s.snapshot;
  Ctype.set_levels s.levels;
  env := s.env

(** Find the first alternative in the tree of or-patterns for which
   [f] does not raise an error. If all fail, the last error is
   propagated *)
let rec find_valid_alternative f pat =
  match pat.ppat_desc with
  | Ppat_or(p1,p2) ->
      (try find_valid_alternative f p1 with
       | Empty_branch | Error _ -> find_valid_alternative f p2
      )
  | _ -> f pat

let no_explosion = function
  | Normal -> Normal
  | Counter_example info ->
     Counter_example { info with explosion_fuel = 0 }

let get_splitting_mode = function
  | Normal -> None
  | Counter_example {splitting_mode} -> Some splitting_mode

let enter_nonsplit_or mode = match mode with
  | Normal -> Normal
  | Counter_example info ->
     let splitting_mode = match info.splitting_mode with
       | Backtrack_or ->
          (* in Backtrack_or mode, or-patterns are always split *)
          assert false
       | Refine_or _ ->
          Refine_or {inside_nonsplit_or = true}
     in Counter_example { info with splitting_mode }

(** The typedtree has two distinct syntactic categories for patterns,
   "value" patterns, matching on values, and "computation" patterns
   that match on the effect of a computation -- typically, exception
   patterns (exception p).

   On the other hand, the parsetree has an unstructured representation
   where all categories of patterns are mixed together. The
   decomposition according to the value/computation structure has to
   happen during type-checking.

   We don't want to duplicate the type-checking logic in two different
   functions, depending on the kind of pattern to be produced. In
   particular, there are both value and computation or-patterns, and
   the type-checking logic for or-patterns is horribly complex; having
   it in two different places would be twice as horirble.

   The solution is to pass a GADT tag to [type_pat] to indicate whether
   a value or computation pattern is expected. This way, there is a single
   place where [Ppat_or] nodes are type-checked, the checking logic is shared,
   and only at the end do we inspect the tag to decide to produce a value
   or computation pattern.
*)
let pure
  : type k . k pattern_category -> value general_pattern -> k general_pattern
  = fun category pat ->
  match category with
  | Value -> pat
  | Computation -> as_computation_pattern pat

let only_impure
  : type k . k pattern_category ->
             computation general_pattern -> k general_pattern
  = fun category pat ->
  match category with
  | Value ->
     (* LATER: this exception could be renamed/generalized *)
     raise (Error (pat.pat_loc, pat.pat_env,
                   Exception_pattern_disallowed))
  | Computation -> pat

let as_comp_pattern
  : type k . k pattern_category ->
             k general_pattern -> computation general_pattern
  = fun category pat ->
  match category with
  | Value -> as_computation_pattern pat
  | Computation -> pat

(* type_pat propagates the expected type.
   Unification may update the typing environment.

   In counter-example mode, [Empty_branch] is raised when the counter-example
   does not match any value.  *)
let rec type_pat
  : type k r . k pattern_category ->
      no_existentials: existential_restriction option ->
      mode: pattern_checking_mode -> env: Env.t ref -> Parsetree.pattern ->
      type_expr -> (k general_pattern -> r) -> r
  = fun category ~no_existentials ~mode
        ~env sp expected_ty k ->
  Builtin_attributes.warning_scope sp.ppat_attributes
    (fun () ->
       type_pat_aux category ~no_existentials ~mode
         ~env sp expected_ty k
    )

and type_pat_aux
  : type k r . k pattern_category -> no_existentials:_ -> mode:_ ->
         env:_ -> _ -> _ -> (k general_pattern -> r) -> r
  = fun category ~no_existentials ~mode
      ~env sp expected_ty k ->
  let type_pat category ?(mode=mode) ?(env=env) =
    type_pat category ~no_existentials ~mode ~env
  in
  let loc = sp.ppat_loc in
  let refine =
    match mode with Normal -> None | Counter_example _ -> Some true in
  let solve_expected (x : pattern) : pattern =
    unify_pat ~refine env x (instance expected_ty);
    x
  in
  let rp x =
    let crp (x : k general_pattern) : k general_pattern =
      match category with
      | Value -> rp x
      | Computation -> rcp x in
    if mode = Normal then crp x else x in
  let rp k x = k (rp x)
  and rvp k x = k (rp (pure category x))
  and rcp k x = k (rp (only_impure category x)) in
  let construction_not_used_in_counterexamples = (mode = Normal) in
  let must_backtrack_on_gadt = match get_splitting_mode mode with
    | None -> false
    | Some Backtrack_or -> false
    | Some (Refine_or {inside_nonsplit_or}) -> inside_nonsplit_or
  in
  match sp.ppat_desc with
    Ppat_any ->
      let k' d = rvp k {
        pat_desc = d;
        pat_loc = loc; pat_extra=[];
        pat_type = instance expected_ty;
        pat_attributes = sp.ppat_attributes;
        pat_env = !env }
      in
      begin match mode with
      | Normal -> k' Tpat_any
      | Counter_example {explosion_fuel; _} when explosion_fuel <= 0 ->
          k' Tpat_any
      | Counter_example ({explosion_fuel; _} as info) ->
         let open Parmatch in
         begin match ppat_of_type !env expected_ty with
         | PT_empty -> raise Empty_branch
         | PT_any -> k' Tpat_any
         | PT_pattern (explosion, sp, constrs, labels) ->
            let explosion_fuel =
              match explosion with
              | PE_single -> explosion_fuel - 1
              | PE_gadt_cases ->
                  if must_backtrack_on_gadt then raise Need_backtrack;
                  explosion_fuel - 5
            in
            let mode =
              Counter_example { info with explosion_fuel; constrs; labels }
            in
            type_pat category ~mode sp expected_ty k
         end
      end
  | Ppat_var name ->
      let ty = instance expected_ty in
      let id = (* PR#7330 *)
        if name.txt = "*extension*" then
          Ident.create_local name.txt
        else
          enter_variable loc name ty sp.ppat_attributes
      in
      rvp k {
        pat_desc = Tpat_var (id, name);
        pat_loc = loc; pat_extra=[];
        pat_type = ty;
        pat_attributes = sp.ppat_attributes;
        pat_env = !env }
  | Ppat_unpack name ->
      assert construction_not_used_in_counterexamples;
      let t = instance expected_ty in
      begin match name.txt with
      | None ->
          rvp k {
            pat_desc = Tpat_any;
            pat_loc = sp.ppat_loc;
            pat_extra=[Tpat_unpack, name.loc, sp.ppat_attributes];
            pat_type = t;
            pat_attributes = [];
            pat_env = !env }
      | Some s ->
          let v = { name with txt = s } in
          let id = enter_variable loc v t ~is_module:true sp.ppat_attributes in
          rvp k {
            pat_desc = Tpat_var (id, v);
            pat_loc = sp.ppat_loc;
            pat_extra=[Tpat_unpack, loc, sp.ppat_attributes];
            pat_type = t;
            pat_attributes = [];
            pat_env = !env }
      end
  | Ppat_constraint(
      {ppat_desc=Ppat_var name; ppat_loc=lloc; ppat_attributes = attrs},
      ({ptyp_desc=Ptyp_poly _} as sty)) ->
      (* explicitly polymorphic type *)
      assert construction_not_used_in_counterexamples;
      let cty, ty, ty' =
        solve_Ppat_poly_constraint ~refine env lloc sty expected_ty in
      let id = enter_variable lloc name ty' attrs in
      rvp k { pat_desc = Tpat_var (id, name);
              pat_loc = lloc;
              pat_extra = [Tpat_constraint cty, loc, sp.ppat_attributes];
              pat_type = ty;
              pat_attributes = [];
              pat_env = !env }
  | Ppat_alias(sq, name) ->
      assert construction_not_used_in_counterexamples;
      type_pat Value sq expected_ty (fun q ->
        let ty_var = solve_Ppat_alias ~refine env q in
        let id =
          enter_variable ~is_as_variable:true loc name ty_var sp.ppat_attributes
        in
        rvp k {
          pat_desc = Tpat_alias(q, id, name);
          pat_loc = loc; pat_extra=[];
          pat_type = q.pat_type;
          pat_attributes = sp.ppat_attributes;
          pat_env = !env })
  | Ppat_constant cst ->
      let cst = constant_or_raise !env loc cst in
      rvp k @@ solve_expected {
        pat_desc = Tpat_constant cst;
        pat_loc = loc; pat_extra=[];
        pat_type = type_constant cst;
        pat_attributes = sp.ppat_attributes;
        pat_env = !env }
  | Ppat_interval (Pconst_char c1, Pconst_char c2) ->
      let open Ast_helper.Pat in
      let gloc = {loc with Location.loc_ghost=true} in
      let rec loop c1 c2 =
        if c1 = c2 then constant ~loc:gloc (Pconst_char c1)
        else
          or_ ~loc:gloc
            (constant ~loc:gloc (Pconst_char c1))
            (loop (Char.chr(Char.code c1 + 1)) c2)
      in
      let p = if c1 <= c2 then loop c1 c2 else loop c2 c1 in
      let p = {p with ppat_loc=loc} in
      type_pat category ~mode:(no_explosion mode) p expected_ty k
        (* TODO: record 'extra' to remember about interval *)
  | Ppat_interval _ ->
      raise (Error (loc, !env, Invalid_interval))
  | Ppat_tuple spl ->
      assert (List.length spl >= 2);
      let expected_tys = solve_Ppat_tuple ~refine loc env spl expected_ty in
      let spl_ann = List.combine spl expected_tys in
      map_fold_cont (fun (p,t) -> type_pat Value p t) spl_ann (fun pl ->
        rvp k {
        pat_desc = Tpat_tuple pl;
        pat_loc = loc; pat_extra=[];
        pat_type = newty (Ttuple(List.map (fun p -> p.pat_type) pl));
        pat_attributes = sp.ppat_attributes;
        pat_env = !env })
  | Ppat_construct(lid, sarg) ->
      let expected_type =
        match extract_concrete_variant !env expected_ty with
        | Variant_type(p0, p, _) ->
            Some (p0, p, is_principal expected_ty)
        | Maybe_a_variant_type -> None
        | Not_a_variant_type ->
            let srt = wrong_kind_sort_of_constructor lid.txt in
            let error = Wrong_expected_kind(srt, Pattern, expected_ty) in
            raise (Error (loc, !env, error))
      in
      let constr =
        match lid.txt, mode with
        | Longident.Lident s, Counter_example {constrs; _} ->
           (* assert: cf. {!counter_example_checking_info} documentation *)
            assert (Hashtbl.mem constrs s);
            Hashtbl.find constrs s
        | _ ->
        let candidates =
          Env.lookup_all_constructors Env.Pattern ~loc:lid.loc lid.txt !env in
        wrap_disambiguate "This variant pattern is expected to have"
          (mk_expected expected_ty)
          (Constructor.disambiguate Env.Pattern lid !env expected_type)
          candidates
      in
      if constr.cstr_generalized && must_backtrack_on_gadt then
        raise Need_backtrack;
      begin match no_existentials, constr.cstr_existentials with
      | None, _ | _, [] -> ()
      | Some r, (_ :: _ as exs)  ->
          let exs = List.map (Ctype.existential_name constr) exs in
          let name = constr.cstr_name in
          raise (Error (loc, !env, Unexpected_existential (r, name, exs)))
      end;
      let sarg', existential_styp =
        match sarg with
          None -> None, None
        | Some (vl, {ppat_desc = Ppat_constraint (sp, sty)})
          when vl <> [] || constr.cstr_arity > 1 ->
            Some sp, Some (vl, sty)
        | Some ([], sp) ->
            Some sp, None
        | Some (_, sp) ->
            raise (Error (sp.ppat_loc, !env, Missing_type_constraint))
      in
      let sargs =
        match sarg' with
          None -> []
        | Some {ppat_desc = Ppat_tuple spl} when
            constr.cstr_arity > 1 ||
            Builtin_attributes.explicit_arity sp.ppat_attributes
          -> spl
        | Some({ppat_desc = Ppat_any} as sp) when
            constr.cstr_arity = 0 && existential_styp = None
          ->
            Location.prerr_warning sp.ppat_loc
              Warnings.Wildcard_arg_to_constant_constr;
            []
        | Some({ppat_desc = Ppat_any} as sp) when constr.cstr_arity > 1 ->
            replicate_list sp constr.cstr_arity
        | Some sp -> [sp] in
      if Builtin_attributes.warn_on_literal_pattern constr.cstr_attributes then
        begin match List.filter has_literal_pattern sargs with
        | sp :: _ ->
           Location.prerr_warning sp.ppat_loc Warnings.Fragile_literal_pattern
        | _ -> ()
        end;
      if List.length sargs <> constr.cstr_arity then
        raise(Error(loc, !env, Constructor_arity_mismatch(lid.txt,
                                     constr.cstr_arity, List.length sargs)));

      let (ty_args, existential_ctyp) =
        solve_Ppat_construct ~refine env loc constr no_existentials
          existential_styp expected_ty
      in

      let rec check_non_escaping p =
        match p.ppat_desc with
        | Ppat_or (p1, p2) ->
            check_non_escaping p1;
            check_non_escaping p2
        | Ppat_alias (p, _) ->
            check_non_escaping p
        | Ppat_constraint _ ->
            raise (Error (p.ppat_loc, !env, Inlined_record_escape))
        | _ ->
            ()
      in
      if constr.cstr_inlined <> None then begin
        List.iter check_non_escaping sargs;
        Option.iter (fun (_, sarg) -> check_non_escaping sarg) sarg
      end;

      map_fold_cont
        (fun (p,t) -> type_pat Value p t)
        (List.combine sargs ty_args)
        (fun args ->
          rvp k {
            pat_desc=Tpat_construct(lid, constr, args, existential_ctyp);
            pat_loc = loc; pat_extra=[];
            pat_type = instance expected_ty;
            pat_attributes = sp.ppat_attributes;
            pat_env = !env })
  | Ppat_variant(tag, sarg) ->
      if tag = Parmatch.some_private_tag then
        assert (match mode with Normal -> false | Counter_example _ -> true);
      let constant = (sarg = None) in
      let arg_type, row, pat_type =
        solve_Ppat_variant ~refine loc env tag constant expected_ty in
      let k arg =
        rvp k {
        pat_desc = Tpat_variant(tag, arg, ref row);
        pat_loc = loc; pat_extra = [];
        pat_type = pat_type;
        pat_attributes = sp.ppat_attributes;
        pat_env = !env }
      in begin
        (* PR#6235: propagate type information *)
        match sarg, arg_type with
          Some p, [ty] -> type_pat Value p ty (fun p -> k (Some p))
        | _            -> k None
      end
  | Ppat_record(lid_sp_list, closed) ->
      assert (lid_sp_list <> []);
      let expected_type, record_ty =
        match extract_concrete_record !env expected_ty with
        | Record_type(p0, p, _) ->
            let ty = generic_instance expected_ty in
            Some (p0, p, is_principal expected_ty), ty
        | Maybe_a_record_type -> None, newvar ()
        | Not_a_record_type ->
          let error = Wrong_expected_kind(Record, Pattern, expected_ty) in
          raise (Error (loc, !env, error))
      in
      let type_label_pat (label_lid, label, sarg) k =
        let ty_arg =
          solve_Ppat_record_field ~refine loc env label label_lid record_ty in
        type_pat Value sarg ty_arg (fun arg ->
          k (label_lid, label, arg))
      in
      let make_record_pat lbl_pat_list =
        check_recordpat_labels loc lbl_pat_list closed;
        {
          pat_desc = Tpat_record (lbl_pat_list, closed);
          pat_loc = loc; pat_extra=[];
          pat_type = instance record_ty;
          pat_attributes = sp.ppat_attributes;
          pat_env = !env;
        }
      in
      let k' pat = rvp k @@ solve_expected pat in
      begin match mode with
      | Normal ->
          k' (wrap_disambiguate "This record pattern is expected to have"
               (mk_expected expected_ty)
               (type_label_a_list loc false !env Env.Projection
                  type_label_pat expected_type lid_sp_list)
               make_record_pat)
      | Counter_example {labels; _} ->
          type_label_a_list ~labels loc false !env Env.Projection
            type_label_pat expected_type lid_sp_list
            (fun lbl_pat_list -> k' (make_record_pat lbl_pat_list))
      end
  | Ppat_array spl ->
      let ty_elt = solve_Ppat_array ~refine loc env expected_ty in
      map_fold_cont (fun p -> type_pat Value p ty_elt) spl (fun pl ->
        rvp k {
        pat_desc = Tpat_array pl;
        pat_loc = loc; pat_extra=[];
        pat_type = instance expected_ty;
        pat_attributes = sp.ppat_attributes;
        pat_env = !env })
  | Ppat_or(sp1, sp2) ->
      begin match mode with
      | Normal ->
          let initial_pattern_variables = !pattern_variables in
          let initial_module_variables = !module_variables in
          let equation_level = !gadt_equations_level in
          let outter_lev = get_current_level () in
          (* introduce a new scope *)
          begin_def ();
          let lev = get_current_level () in
          gadt_equations_level := Some lev;
          let type_pat_rec env sp =
            type_pat category sp expected_ty ~env (fun x -> x) in
          let env1 = ref !env in
          let p1 = type_pat_rec env1 sp1 in
          let p1_variables = !pattern_variables in
          let p1_module_variables = !module_variables in
          pattern_variables := initial_pattern_variables;
          module_variables := initial_module_variables;
          let env2 = ref !env in
          let p2 = type_pat_rec env2 sp2 in
          end_def ();
          gadt_equations_level := equation_level;
          let p2_variables = !pattern_variables in
          (* Make sure no variable with an ambiguous type gets added to the
             environment. *)
          List.iter (fun { pv_type; pv_loc; _ } ->
            check_scope_escape pv_loc !env1 outter_lev pv_type
          ) p1_variables;
          List.iter (fun { pv_type; pv_loc; _ } ->
            check_scope_escape pv_loc !env2 outter_lev pv_type
          ) p2_variables;
          let alpha_env =
            enter_orpat_variables loc !env p1_variables p2_variables in
          let p2 = alpha_pat alpha_env p2 in
          pattern_variables := p1_variables;
          module_variables := p1_module_variables;
          rp k { pat_desc = Tpat_or (p1, p2, None);
                 pat_loc = loc; pat_extra = [];
                 pat_type = instance expected_ty;
                 pat_attributes = sp.ppat_attributes;
                 pat_env = !env }
      | Counter_example {splitting_mode; _} ->
          (* We are in counter-example mode, but try to avoid backtracking *)
          let must_split =
            match splitting_mode with
            | Backtrack_or -> true
            | Refine_or _ -> false in
          let state = save_state env in
          let split_or sp =
            let typ pat = type_pat category pat expected_ty k in
            find_valid_alternative (fun pat -> set_state state env; typ pat) sp
          in
          if must_split then split_or sp else
          let type_pat_result env sp : (_, abort_reason) result =
            let mode = enter_nonsplit_or mode in
            match type_pat category ~mode sp expected_ty ~env (fun x -> x) with
            | res -> Ok res
            | exception Need_backtrack -> Error Adds_constraints
            | exception Empty_branch -> Error Empty
          in
          let p1 = type_pat_result (ref !env) sp1 in
          let p2 = type_pat_result (ref !env) sp2 in
          match p1, p2 with
          | Error Empty, Error Empty ->
              raise Empty_branch
          | Error Adds_constraints, Error _
          | Error _, Error Adds_constraints ->
              let inside_nonsplit_or =
                match splitting_mode with
                | Backtrack_or -> false
                | Refine_or {inside_nonsplit_or} -> inside_nonsplit_or in
              if inside_nonsplit_or
              then raise Need_backtrack
              else split_or sp
          | Ok p, Error _
          | Error _, Ok p ->
              rp k p
          | Ok p1, Ok p2 ->
              rp k { pat_desc = Tpat_or (p1, p2, None);
                     pat_loc = loc; pat_extra = [];
                     pat_type = instance expected_ty;
                     pat_attributes = sp.ppat_attributes;
                     pat_env = !env }
      end
  | Ppat_lazy sp1 ->
      let nv = solve_Ppat_lazy ~refine loc env expected_ty in
      (* do not explode under lazy: PR#7421 *)
      type_pat Value ~mode:(no_explosion mode) sp1 nv (fun p1 ->
        rvp k {
        pat_desc = Tpat_lazy p1;
        pat_loc = loc; pat_extra=[];
        pat_type = instance expected_ty;
        pat_attributes = sp.ppat_attributes;
        pat_env = !env })
  | Ppat_constraint(sp, sty) ->
      assert construction_not_used_in_counterexamples;
      (* Pretend separate = true *)
      let cty, ty, expected_ty' =
        solve_Ppat_constraint ~refine loc env sty expected_ty in
      type_pat category sp expected_ty' (fun p ->
        (*Format.printf "%a@.%a@."
          Printtyp.raw_type_expr ty
          Printtyp.raw_type_expr p.pat_type;*)
        let extra = (Tpat_constraint cty, loc, sp.ppat_attributes) in
        let p : k general_pattern =
          match category, (p : k general_pattern) with
          | Value, {pat_desc = Tpat_var (id,s); _} ->
            {p with
              pat_type = ty;
              pat_desc =
                Tpat_alias
                  ({p with pat_desc = Tpat_any; pat_attributes = []}, id,s);
              pat_extra = [extra];
            }
          | _, p ->
             { p with pat_type = ty; pat_extra = extra::p.pat_extra }
        in k p)
  | Ppat_type lid ->
      assert construction_not_used_in_counterexamples;
      let (path, p) = build_or_pat !env loc lid in
      k @@ pure category @@ solve_expected
        { p with pat_extra = (Tpat_type (path, lid), loc, sp.ppat_attributes)
        :: p.pat_extra }
  | Ppat_open (lid,p) ->
      assert construction_not_used_in_counterexamples;
      let path, new_env =
        !type_open Asttypes.Fresh !env sp.ppat_loc lid in
      env := new_env;
      type_pat category ~env p expected_ty ( fun p ->
        let new_env = !env in
        begin match Env.remove_last_open path new_env with
        | None -> assert false
        | Some closed_env -> env := closed_env
        end;
        k { p with pat_extra = (Tpat_open (path,lid,new_env),
                                loc, sp.ppat_attributes) :: p.pat_extra }
      )
  | Ppat_exception p ->
      type_pat Value p Predef.type_exn (fun p_exn ->
      rcp k {
        pat_desc = Tpat_exception p_exn;
        pat_loc = sp.ppat_loc;
        pat_extra = [];
        pat_type = expected_ty;
        pat_env = !env;
        pat_attributes = sp.ppat_attributes;
      })
  | Ppat_extension ext ->
      raise (Error_forward (Builtin_attributes.error_of_extension ext))

let type_pat category ?no_existentials ?(mode=Normal)
    ?(lev=get_current_level()) env sp expected_ty =
  Misc.protect_refs [Misc.R (gadt_equations_level, Some lev)] (fun () ->
        type_pat category ~no_existentials ~mode
          ~env sp expected_ty (fun x -> x)
    )

(* this function is passed to Partial.parmatch
   to type check gadt nonexhaustiveness *)
let partial_pred ~lev ~splitting_mode ?(explode=0)
      env expected_ty constrs labels p =
  let env = ref env in
  let state = save_state env in
  let mode =
    Counter_example {
        splitting_mode;
        explosion_fuel = explode;
        constrs; labels;
      } in
  try
    reset_pattern true;
    let typed_p = type_pat Value ~lev ~mode env p expected_ty in
    set_state state env;
    (* types are invalidated but we don't need them here *)
    Some typed_p
  with Error _ | Empty_branch ->
    set_state state env;
    None

let check_partial ?(lev=get_current_level ()) env expected_ty loc cases =
  let explode = match cases with [_] -> 5 | _ -> 0 in
  let splitting_mode = Refine_or {inside_nonsplit_or = false} in
  Parmatch.check_partial
    (partial_pred ~lev ~splitting_mode ~explode env expected_ty) loc cases

let check_unused ?(lev=get_current_level ()) env expected_ty cases =
  Parmatch.check_unused
    (fun refute constrs labels spat ->
      match
        partial_pred ~lev ~splitting_mode:Backtrack_or ~explode:5
          env expected_ty constrs labels spat
      with
        Some pat when refute ->
          raise (Error (spat.ppat_loc, env, Unrefuted_pattern pat))
      | r -> r)
    cases

let iter_pattern_variables_type f : pattern_variable list -> unit =
  List.iter (fun {pv_type; _} -> f pv_type)

let add_pattern_variables ?check ?check_as env pv =
  List.fold_right
    (fun {pv_id; pv_type; pv_loc; pv_as_var; pv_attributes} env ->
       let check = if pv_as_var then check_as else check in
       Env.add_value ?check pv_id
         {val_type = pv_type; val_kind = Val_reg; Types.val_loc = pv_loc;
          val_attributes = pv_attributes;
          val_uid = Uid.mk ~current_unit:(Env.get_unit_name ());
         } env
    )
    pv env

let type_pattern category ~lev env spat expected_ty =
  reset_pattern true;
  let new_env = ref env in
  let pat = type_pat category ~lev new_env spat expected_ty in
  let pvs = get_ref pattern_variables in
  let unpacks = get_ref module_variables in
  (pat, !new_env, get_ref pattern_force, pvs, unpacks)

let type_pattern_list
    category no_existentials env spatl expected_tys allow
  =
  reset_pattern allow;
  let new_env = ref env in
  let type_pat (attrs, pat) ty =
    Builtin_attributes.warning_scope ~ppwarning:false attrs
      (fun () ->
         type_pat category ~no_existentials new_env pat ty
      )
  in
  let patl = List.map2 type_pat spatl expected_tys in
  let pvs = get_ref pattern_variables in
  let unpacks =
    List.map (fun (name, loc) ->
      {tu_name = name; tu_loc = loc;
       tu_uid = Uid.mk ~current_unit:(Env.get_unit_name ())}
    ) (get_ref module_variables)
  in
  let new_env = add_pattern_variables !new_env pvs in
  (patl, new_env, get_ref pattern_force, pvs, unpacks)

let type_class_arg_pattern cl_num val_env met_env l spat =
  reset_pattern false;
  let nv = newvar () in
  let pat =
    type_pat Value ~no_existentials:In_class_args (ref val_env) spat nv in
  if has_variants pat then begin
    Parmatch.pressure_variants val_env [pat];
    finalize_variants pat;
  end;
  List.iter (fun f -> f()) (get_ref pattern_force);
  if is_optional l then unify_pat (ref val_env) pat (type_option (newvar ()));
  let (pv, val_env, met_env) =
    List.fold_right
      (fun {pv_id; pv_type; pv_loc; pv_as_var; pv_attributes}
        (pv, val_env, met_env) ->
         let check s =
           if pv_as_var then Warnings.Unused_var s
           else Warnings.Unused_var_strict s in
         let id' = Ident.rename pv_id in
         let val_uid = Uid.mk ~current_unit:(Env.get_unit_name ()) in
         let val_env =
          Env.add_value pv_id
            { val_type = pv_type
            ; val_kind = Val_reg
            ; val_attributes = pv_attributes
            ; val_loc = pv_loc
            ; val_uid
            }
            val_env
         in
         let met_env =
          Env.add_value id' ~check
            { val_type = pv_type
            ; val_kind = Val_ivar (Immutable, cl_num)
            ; val_attributes = pv_attributes
            ; val_loc = pv_loc
            ; val_uid
            }
            met_env
         in
         ((id', pv_id, pv_type)::pv, val_env, met_env))
      !pattern_variables ([], val_env, met_env)
  in
  (pat, pv, val_env, met_env)

let type_self_pattern env spat =
  let open Ast_helper in
  let spat = Pat.mk(Ppat_alias (spat, mknoloc "selfpat-*")) in
  reset_pattern false;
  let nv = newvar() in
  let pat =
    type_pat Value ~no_existentials:In_self_pattern (ref env) spat nv in
  List.iter (fun f -> f()) (get_ref pattern_force);
  let pv = !pattern_variables in
  pattern_variables := [];
  pat, pv

let delayed_checks = ref []
let reset_delayed_checks () = delayed_checks := []
let add_delayed_check f =
  delayed_checks := (f, Warnings.backup ()) :: !delayed_checks

let force_delayed_checks () =
  (* checks may change type levels *)
  let snap = Btype.snapshot () in
  let w_old = Warnings.backup () in
  List.iter
    (fun (f, w) -> Warnings.restore w; f ())
    (List.rev !delayed_checks);
  Warnings.restore w_old;
  reset_delayed_checks ();
  Btype.backtrack snap

let rec final_subexpression exp =
  match exp.exp_desc with
    Texp_let (_, _, e)
  | Texp_sequence (_, e)
  | Texp_try (e, _)
  | Texp_ifthenelse (_, e, _)
  | Texp_match (_, {c_rhs=e} :: _, _)
  | Texp_letmodule (_, _, _, _, e)
  | Texp_letexception (_, e)
  | Texp_open (_, e)
    -> final_subexpression e
  | _ -> exp

(* Generalization criterion for expressions *)

let rec is_nonexpansive exp =
  match exp.exp_desc with
  | Texp_ident _
  | Texp_constant _
  | Texp_unreachable
  | Texp_function _
  | Texp_array [] -> true
  | Texp_let(_rec_flag, pat_exp_list, body) ->
      List.for_all (fun vb -> is_nonexpansive vb.vb_expr) pat_exp_list &&
      is_nonexpansive body
  | Texp_apply(e, (_,None)::el) ->
      is_nonexpansive e && List.for_all is_nonexpansive_opt (List.map snd el)
  | Texp_match(e, cases, _) ->
     (* Not sure this is necessary, if [e] is nonexpansive then we shouldn't
         care if there are exception patterns. But the previous version enforced
         that there be none, so... *)
      let contains_exception_pat pat =
        exists_general_pattern { f = fun (type k) (p : k general_pattern) ->
          match p.pat_desc with
          | Tpat_exception _ -> true
          | _ -> false } pat
      in
      is_nonexpansive e &&
      List.for_all
        (fun {c_lhs; c_guard; c_rhs} ->
           is_nonexpansive_opt c_guard && is_nonexpansive c_rhs
           && not (contains_exception_pat c_lhs)
        ) cases
  | Texp_tuple el ->
      List.for_all is_nonexpansive el
  | Texp_construct( _, _, el) ->
      List.for_all is_nonexpansive el
  | Texp_variant(_, arg) -> is_nonexpansive_opt arg
  | Texp_record { fields; extended_expression } ->
      Array.for_all
        (fun (lbl, definition) ->
           match definition with
           | Overridden (_, exp) ->
               lbl.lbl_mut = Immutable && is_nonexpansive exp
           | Kept _ -> true)
        fields
      && is_nonexpansive_opt extended_expression
  | Texp_field(exp, _, _) -> is_nonexpansive exp
  | Texp_ifthenelse(_cond, ifso, ifnot) ->
      is_nonexpansive ifso && is_nonexpansive_opt ifnot
  | Texp_sequence (_e1, e2) -> is_nonexpansive e2  (* PR#4354 *)
  | Texp_new (_, _, cl_decl) -> Btype.class_type_arity cl_decl.cty_type > 0
  (* Note: nonexpansive only means no _observable_ side effects *)
  | Texp_lazy e -> is_nonexpansive e
  | Texp_object ({cstr_fields=fields; cstr_type = { csig_vars=vars}}, _) ->
      let count = ref 0 in
      List.for_all
        (fun field -> match field.cf_desc with
            Tcf_method _ -> true
          | Tcf_val (_, _, _, Tcfk_concrete (_, e), _) ->
              incr count; is_nonexpansive e
          | Tcf_val (_, _, _, Tcfk_virtual _, _) ->
              incr count; true
          | Tcf_initializer e -> is_nonexpansive e
          | Tcf_constraint _ -> true
          | Tcf_inherit _ -> false
          | Tcf_attribute _ -> true)
        fields &&
      Vars.fold (fun _ (mut,_,_) b -> decr count; b && mut = Immutable)
        vars true &&
      !count = 0
  | Texp_letmodule (_, _, _, mexp, e)
  | Texp_open ({ open_expr = mexp; _}, e) ->
      is_nonexpansive_mod mexp && is_nonexpansive e
  | Texp_pack mexp ->
      is_nonexpansive_mod mexp
  (* Computations which raise exceptions are nonexpansive, since (raise e) is
     equivalent to (raise e; diverge), and a nonexpansive "diverge" can be
     produced using lazy values or the relaxed value restriction.
     See GPR#1142 *)
  | Texp_assert exp ->
      is_nonexpansive exp
  | Texp_apply (
      { exp_desc = Texp_ident (_, _, {val_kind =
             Val_prim {Primitive.prim_name =
                         ("%raise" | "%reraise" | "%raise_notrace")}}) },
      [Nolabel, Some e]) ->
     is_nonexpansive e
  | Texp_array (_ :: _)
  | Texp_apply _
  | Texp_try _
  | Texp_setfield _
  | Texp_while _
  | Texp_for _
  | Texp_send _
  | Texp_instvar _
  | Texp_setinstvar _
  | Texp_override _
  | Texp_letexception _
  | Texp_letop _
  | Texp_extension_constructor _ ->
    false

and is_nonexpansive_mod mexp =
  match mexp.mod_desc with
  | Tmod_ident _
  | Tmod_functor _ -> true
  | Tmod_unpack (e, _) -> is_nonexpansive e
  | Tmod_constraint (m, _, _, _) -> is_nonexpansive_mod m
  | Tmod_structure str ->
      List.for_all
        (fun item -> match item.str_desc with
          | Tstr_eval _ | Tstr_primitive _ | Tstr_type _
          | Tstr_modtype _ | Tstr_class_type _  -> true
          | Tstr_value (_, pat_exp_list) ->
              List.for_all (fun vb -> is_nonexpansive vb.vb_expr) pat_exp_list
          | Tstr_module {mb_expr=m;_}
          | Tstr_open {open_expr=m;_}
          | Tstr_include {incl_mod=m;_} -> is_nonexpansive_mod m
          | Tstr_recmodule id_mod_list ->
              List.for_all (fun {mb_expr=m;_} -> is_nonexpansive_mod m)
                id_mod_list
          | Tstr_exception {tyexn_constructor = {ext_kind = Text_decl _}} ->
              false (* true would be unsound *)
          | Tstr_exception {tyexn_constructor = {ext_kind = Text_rebind _}} ->
              true
          | Tstr_typext te ->
              List.for_all
                (function {ext_kind = Text_decl _} -> false
                        | {ext_kind = Text_rebind _} -> true)
                te.tyext_constructors
          | Tstr_class _ -> false (* could be more precise *)
          | Tstr_attribute _ -> true
        )
        str.str_items
  | Tmod_apply _ -> false

and is_nonexpansive_opt = function
  | None -> true
  | Some e -> is_nonexpansive e

let maybe_expansive e = not (is_nonexpansive e)

let check_recursive_bindings env valbinds =
  let ids = let_bound_idents valbinds in
  List.iter
    (fun {vb_expr} ->
       if not (Rec_check.is_valid_recursive_expression ids vb_expr) then
         raise(Error(vb_expr.exp_loc, env, Illegal_letrec_expr))
    )
    valbinds

let check_recursive_class_bindings env ids exprs =
  List.iter
    (fun expr ->
       if not (Rec_check.is_valid_class_expr ids expr) then
         raise(Error(expr.cl_loc, env, Illegal_class_expr)))
    exprs

let is_prim ~name funct =
  match funct.exp_desc with
  | Texp_ident (_, _, {val_kind=Val_prim{Primitive.prim_name; _}}) ->
      prim_name = name
  | _ -> false
(* Approximate the type of an expression, for better recursion *)

let rec approx_type env sty =
  match sty.ptyp_desc with
    Ptyp_arrow (p, _, sty) ->
      let ty1 = if is_optional p then type_option (newvar ()) else newvar () in
      newty (Tarrow (p, ty1, approx_type env sty, commu_ok))
  | Ptyp_tuple args ->
      newty (Ttuple (List.map (approx_type env) args))
  | Ptyp_constr (lid, ctl) ->
      let path, decl = Env.lookup_type ~use:false ~loc:lid.loc lid.txt env in
      if List.length ctl <> decl.type_arity then newvar ()
      else begin
        let tyl = List.map (approx_type env) ctl in
        newconstr path tyl
      end
  | Ptyp_poly (_, sty) ->
      approx_type env sty
  | _ -> newvar ()

let rec type_approx env sexp =
  match sexp.pexp_desc with
    Pexp_let (_, _, e) -> type_approx env e
  | Pexp_fun (p, _, _, e) ->
      let ty = if is_optional p then type_option (newvar ()) else newvar () in
      newty (Tarrow(p, ty, type_approx env e, commu_ok))
  | Pexp_function ({pc_rhs=e}::_) ->
      newty (Tarrow(Nolabel, newvar (), type_approx env e, commu_ok))
  | Pexp_match (_, {pc_rhs=e}::_) -> type_approx env e
  | Pexp_try (e, _) -> type_approx env e
  | Pexp_tuple l -> newty (Ttuple(List.map (type_approx env) l))
  | Pexp_ifthenelse (_,e,_) -> type_approx env e
  | Pexp_sequence (_,e) -> type_approx env e
  | Pexp_constraint (e, sty) ->
      let ty = type_approx env e in
      let ty1 = approx_type env sty in
      begin try unify env ty ty1 with Unify err ->
        raise(Error(sexp.pexp_loc, env, Expr_type_clash (err, None, None)))
      end;
      ty1
  | Pexp_coerce (e, sty1, sty2) ->
      let approx_ty_opt = function
        | None -> newvar ()
        | Some sty -> approx_type env sty
      in
      let ty = type_approx env e
      and ty1 = approx_ty_opt sty1
      and ty2 = approx_type env sty2 in
      begin try unify env ty ty1 with Unify err ->
        raise(Error(sexp.pexp_loc, env, Expr_type_clash (err, None, None)))
      end;
      ty2
  | _ -> newvar ()

(* List labels in a function type, and whether return type is a variable *)
let rec list_labels_aux env visited ls ty_fun =
  let ty = expand_head env ty_fun in
  if TypeSet.mem ty visited then
    List.rev ls, false
  else match get_desc ty with
    Tarrow (l, _, ty_res, _) ->
      list_labels_aux env (TypeSet.add ty visited) (l::ls) ty_res
  | _ ->
      List.rev ls, is_Tvar ty

let list_labels env ty =
  wrap_trace_gadt_instances env (list_labels_aux env TypeSet.empty []) ty

(* Check that all univars are safe in a type. Both exp.exp_type and
   ty_expected should already be generalized. *)
let check_univars env kind exp ty_expected vars =
  let pty = instance ty_expected in
  begin_def ();
  let exp_ty, vars =
    match get_desc pty with
      Tpoly (body, tl) ->
        (* Enforce scoping for type_let:
           since body is not generic,  instance_poly only makes
           copies of nodes that have a Tvar as descendant *)
        let _, ty' = instance_poly true tl body in
        let vars, exp_ty = instance_parameterized_type vars exp.exp_type in
        unify_exp_types exp.exp_loc env exp_ty ty';
        exp_ty, vars
    | _ -> assert false
  in
  end_def ();
  generalize exp_ty;
  List.iter generalize vars;
  let ty, complete = polyfy env exp_ty vars in
  if not complete then
    let ty_expected = instance ty_expected in
    raise (Error(exp.exp_loc,
                 env,
                 Less_general(kind,
                              Errortrace.unification_error
                                ~trace:[Ctype.expanded_diff env
                                          ~got:ty ~expected:ty_expected])))

let generalize_and_check_univars env kind exp ty_expected vars =
  generalize exp.exp_type;
  generalize ty_expected;
  List.iter generalize vars;
  check_univars env kind exp ty_expected vars

(* [check_statement] implements the [non-unit-statement] check.

   This check is called in contexts where the value of the expression is known
   to be discarded (eg. the lhs of a sequence). We check that [exp] has type
   unit, or has an explicit type annotation; otherwise we raise the
   [non-unit-statement] warning. *)

let check_statement exp =
  let ty = get_desc (expand_head exp.exp_env exp.exp_type) in
  match ty with
  | Tconstr (p, _, _)  when Path.same p Predef.path_unit -> ()
  | Tvar _ -> ()
  | _ ->
      let rec loop {exp_loc; exp_desc; exp_extra; _} =
        match exp_desc with
        | Texp_let (_, _, e)
        | Texp_sequence (_, e)
        | Texp_letexception (_, e)
        | Texp_letmodule (_, _, _, _, e) ->
            loop e
        | _ ->
            let loc =
              match List.find_opt (function
                  | (Texp_constraint _, _, _) -> true
                  | _ -> false) exp_extra
              with
              | Some (_, loc, _) -> loc
              | None -> exp_loc
            in
            Location.prerr_warning loc Warnings.Non_unit_statement
      in
      loop exp


(* [check_partial_application] implements the [ignored-partial-application]
   warning (and if [statement] is [true], also [non-unit-statement]).

   If [exp] has a function type, we check that it is not syntactically the
   result of a function application, as this is often a bug in certain contexts
   (eg the rhs of a let-binding or in the argument of [ignore]). For example,
   [ignore (List.map print_int)] written by mistake instead of [ignore (List.map
   print_int li)].

   The check can be disabled by explicitly annotating the expression with a type
   constraint, eg [(e : _ -> _)].

   If [statement] is [true] and the [ignored-partial-application] is {em not}
   triggered, then the [non-unit-statement] check is performed (see
   [check_statement]).

   If the type of [exp] is not known at the time this function is called, the
   check is retried again after typechecking. *)

let check_partial_application ~statement exp =
  let check_statement () = if statement then check_statement exp in
  let doit () =
    let ty = get_desc (expand_head exp.exp_env exp.exp_type) in
    match ty with
    | Tarrow _ ->
        let rec check {exp_desc; exp_loc; exp_extra; _} =
          if List.exists (function
              | (Texp_constraint _, _, _) -> true
              | _ -> false) exp_extra then check_statement ()
          else begin
            match exp_desc with
            | Texp_ident _ | Texp_constant _ | Texp_tuple _
            | Texp_construct _ | Texp_variant _ | Texp_record _
            | Texp_field _ | Texp_setfield _ | Texp_array _
            | Texp_while _ | Texp_for _ | Texp_instvar _
            | Texp_setinstvar _ | Texp_override _ | Texp_assert _
            | Texp_lazy _ | Texp_object _ | Texp_pack _ | Texp_unreachable
            | Texp_extension_constructor _ | Texp_ifthenelse (_, _, None)
            | Texp_function _ ->
                check_statement ()
            | Texp_match (_, cases, _) ->
                List.iter (fun {c_rhs; _} -> check c_rhs) cases
            | Texp_try (e, cases) ->
                check e; List.iter (fun {c_rhs; _} -> check c_rhs) cases
            | Texp_ifthenelse (_, e1, Some e2) ->
                check e1; check e2
            | Texp_let (_, _, e) | Texp_sequence (_, e) | Texp_open (_, e)
            | Texp_letexception (_, e) | Texp_letmodule (_, _, _, _, e) ->
                check e
            | Texp_apply _ | Texp_send _ | Texp_new _ | Texp_letop _ ->
                Location.prerr_warning exp_loc
                  Warnings.Ignored_partial_application
          end
        in
        check exp
    | _ ->
        check_statement ()
  in
  let ty = get_desc (expand_head exp.exp_env exp.exp_type) in
  match ty with
  | Tvar _ ->
      (* The type of [exp] is not known. Delay the check until after
         typechecking in order to give a chance for the type to become known
         through unification. *)
      add_delayed_check doit
  | _ ->
      doit ()

(* Check that a type is generalizable at some level *)
let generalizable level ty =
  let rec check ty =
    if not_marked_node ty then
      if get_level ty <= level then raise Exit else
      (flip_mark_node ty; iter_type_expr check ty)
  in
  try check ty; unmark_type ty; true
  with Exit -> unmark_type ty; false

(* Hack to allow coercion of self. Will clean-up later. *)
let self_coercion = ref ([] : (Path.t * Location.t list ref) list)

(* Helpers for type_cases *)

let contains_variant_either ty =
  let rec loop ty =
    if try_mark_node ty then
      begin match get_desc ty with
        Tvariant row ->
          if not (is_fixed row) then
            List.iter
              (fun (_,f) ->
                match row_field_repr f with Reither _ -> raise Exit | _ -> ())
              (row_fields row);
          iter_row loop row
      | _ ->
          iter_type_expr loop ty
      end
  in
  try loop ty; unmark_type ty; false
  with Exit -> unmark_type ty; true

let shallow_iter_ppat f p =
  match p.ppat_desc with
  | Ppat_any | Ppat_var _ | Ppat_constant _ | Ppat_interval _
  | Ppat_construct (_, None)
  | Ppat_extension _
  | Ppat_type _ | Ppat_unpack _ -> ()
  | Ppat_array pats -> List.iter f pats
  | Ppat_or (p1,p2) -> f p1; f p2
  | Ppat_variant (_, arg) -> Option.iter f arg
  | Ppat_tuple lst ->  List.iter f lst
  | Ppat_construct (_, Some (_, p))
  | Ppat_exception p | Ppat_alias (p,_)
  | Ppat_open (_,p)
  | Ppat_constraint (p,_) | Ppat_lazy p -> f p
  | Ppat_record (args, _flag) -> List.iter (fun (_,p) -> f p) args

let exists_ppat f p =
  let exception Found in
  let rec loop p =
    if f p then raise Found else ();
    shallow_iter_ppat loop p in
  match loop p with
  | exception Found -> true
  | () -> false

let contains_polymorphic_variant p =
  exists_ppat
    (function
     | {ppat_desc = (Ppat_variant _ | Ppat_type _)} -> true
     | _ -> false)
    p

let contains_gadt p =
  exists_general_pattern { f = fun (type k) (p : k general_pattern) ->
     match p.pat_desc with
     | Tpat_construct (_, cd, _, _) when cd.cstr_generalized -> true
     | _ -> false } p

(* There are various things that we need to do in presence of GADT constructors
   that aren't required if there are none.
   However, because of disambiguation, we can't know for sure whether the
   patterns contain some GADT constructors. So we conservatively assume that
   any constructor might be a GADT constructor. *)
let may_contain_gadts p =
  exists_ppat
  (function
   | {ppat_desc = Ppat_construct _} -> true
   | _ -> false)
  p

let check_absent_variant env =
  iter_general_pattern { f = fun (type k) (pat : k general_pattern) ->
    match pat.pat_desc with
    | Tpat_variant (s, arg, row) ->
      let row = !row in
      if List.exists (fun (s',fi) -> s = s' && row_field_repr fi <> Rabsent)
          (row_fields row)
      || not (is_fixed row) && not (static_row row)  (* same as Ctype.poly *)
      then () else
      let ty_arg =
        match arg with None -> [] | Some p -> [correct_levels p.pat_type] in
      let fields = [s, rf_either ty_arg ~no_arg:(arg=None) ~matched:true] in
      let row' =
        create_row ~fields
          ~more:(newvar ()) ~closed:false ~fixed:None ~name:None in
      (* Should fail *)
      unify_pat (ref env) {pat with pat_type = newty (Tvariant row')}
                          (correct_levels pat.pat_type)
    | _ -> () }

(* Getting proper location of already typed expressions.

   Used to avoid confusing locations on type error messages in presence of
   type constraints.
   For example:

       (* Before patch *)
       # let x : string = (5 : int);;
                           ^
       (* After patch *)
       # let x : string = (5 : int);;
                          ^^^^^^^^^
*)
let proper_exp_loc exp =
  let rec aux = function
    | [] -> exp.exp_loc
    | ((Texp_constraint _ | Texp_coerce _), loc, _) :: _ -> loc
    | _ :: rest -> aux rest
  in
  aux exp.exp_extra

(* To find reasonable names for let-bound and lambda-bound idents *)

let rec name_pattern default = function
    [] -> Ident.create_local default
  | p :: rem ->
    match p.pat_desc with
      Tpat_var (id, _) -> id
    | Tpat_alias(_, id, _) -> id
    | _ -> name_pattern default rem

let name_cases default lst =
  name_pattern default (List.map (fun c -> c.c_lhs) lst)

(* Typing of expressions *)

let unify_exp env exp expected_ty =
  let loc = proper_exp_loc exp in
  try
    unify_exp_types loc env exp.exp_type expected_ty
  with Error(loc, env, Expr_type_clash(err, tfc, None)) ->
    raise (Error(loc, env, Expr_type_clash(err, tfc, Some exp.exp_desc)))

(* If [is_inferred e] is true, [e] will be typechecked without using
   the "expected type" provided by the context. *)

let rec is_inferred sexp =
  match sexp.pexp_desc with
  | Pexp_ident _ | Pexp_apply _ | Pexp_field _ | Pexp_constraint _
  | Pexp_coerce _ | Pexp_send _ | Pexp_new _ -> true
  | Pexp_sequence (_, e) | Pexp_open (_, e) -> is_inferred e
  | Pexp_ifthenelse (_, e1, Some e2) -> is_inferred e1 && is_inferred e2
  | _ -> false

(* check if the type of %apply or %revapply matches the type expected by
   the specialized typing rule for those primitives.
*)
type apply_prim =
  | Apply
  | Revapply
let check_apply_prim_type prim typ =
  match get_desc typ with
  | Tarrow (Nolabel,a,b,_) ->
      begin match get_desc b with
      | Tarrow(Nolabel,c,d,_) ->
          let f, x, res =
            match prim with
            | Apply -> a, c, d
            | Revapply -> c, a, d
          in
          begin match get_desc f with
          | Tarrow(Nolabel,fl,fr,_) ->
              is_Tvar fl && is_Tvar fr && is_Tvar x && is_Tvar res
              && Types.eq_type fl x && Types.eq_type fr res
          | _ -> false
          end
      | _ -> false
      end
  | _ -> false

(* Merge explanation to type clash error *)

let with_explanation explanation f =
  match explanation with
  | None -> f ()
  | Some explanation ->
      try f ()
      with Error (loc', env', Expr_type_clash(err', None, exp'))
        when not loc'.Location.loc_ghost ->
        let err = Expr_type_clash(err', Some explanation, exp') in
        raise (Error (loc', env', err))

let rec type_exp ?recarg env sexp =
  (* We now delegate everything to type_expect *)
  type_expect ?recarg env sexp (mk_expected (newvar ()))

(* Typing of an expression with an expected type.
   This provide better error messages, and allows controlled
   propagation of return type information.
   In the principal case, [type_expected'] may be at generic_level.
 *)

and type_expect ?in_function ?recarg env sexp ty_expected_explained =
  let previous_saved_types = Cmt_format.get_saved_types () in
  let exp =
    Builtin_attributes.warning_scope sexp.pexp_attributes
      (fun () ->
         type_expect_ ?in_function ?recarg env sexp ty_expected_explained
      )
  in
  Cmt_format.set_saved_types
    (Cmt_format.Partial_expression exp :: previous_saved_types);
  exp

and type_expect_
    ?in_function ?(recarg=Rejected)
    env sexp ty_expected_explained =
  let { ty = ty_expected; explanation } = ty_expected_explained in
  let loc = sexp.pexp_loc in
  (* Record the expression type before unifying it with the expected type *)
  let with_explanation = with_explanation explanation in
  let rue exp =
    with_explanation (fun () ->
      unify_exp env (re exp) (instance ty_expected));
    exp
  in
  match sexp.pexp_desc with
  | Pexp_ident lid ->
      let path, desc = type_ident env ~recarg lid in
      let exp_desc =
        match desc.val_kind with
        | Val_ivar (_, cl_num) ->
            let (self_path, _) =
              Env.find_value_by_name
                (Longident.Lident ("self-" ^ cl_num)) env
            in
            Texp_instvar(self_path, path,
                         match lid.txt with
                             Longident.Lident txt -> { txt; loc = lid.loc }
                           | _ -> assert false)
        | Val_self (_, _, _, cl_num) ->
            let (path, _) =
              Env.find_value_by_name (Longident.Lident ("self-" ^ cl_num)) env
            in
            Texp_ident(path, lid, desc)
        | _ ->
            Texp_ident(path, lid, desc)
      in
      rue {
        exp_desc; exp_loc = loc; exp_extra = [];
        exp_type = instance desc.val_type;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_constant(Pconst_string (str, _, _) as cst) -> (
    let cst = constant_or_raise env loc cst in
    (* Terrible hack for format strings *)
    let ty_exp = expand_head env (protect_expansion env ty_expected) in
    let fmt6_path =
      Path.(Pdot (Pident (Ident.create_persistent "CamlinternalFormatBasics"),
                  "format6"))
    in
    let is_format = match get_desc ty_exp with
      | Tconstr(path, _, _) when Path.same path fmt6_path ->
        if !Clflags.principal && get_level ty_exp <> generic_level then
          Location.prerr_warning loc
            (Warnings.Not_principal "this coercion to format6");
        true
      | _ -> false
    in
    if is_format then
      let format_parsetree =
        { (type_format loc str env) with pexp_loc = sexp.pexp_loc }  in
      type_expect ?in_function env format_parsetree ty_expected_explained
    else
      rue {
        exp_desc = Texp_constant cst;
        exp_loc = loc; exp_extra = [];
        exp_type = instance Predef.type_string;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  )
  | Pexp_constant cst ->
      let cst = constant_or_raise env loc cst in
      rue {
        exp_desc = Texp_constant cst;
        exp_loc = loc; exp_extra = [];
        exp_type = type_constant cst;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_let(Nonrecursive,
             [{pvb_pat=spat; pvb_expr=sval; pvb_attributes=[]}], sbody)
    when may_contain_gadts spat ->
    (* TODO: allow non-empty attributes? *)
      type_expect ?in_function env
        {sexp with
         pexp_desc = Pexp_match (sval, [Ast_helper.Exp.case spat sbody])}
        ty_expected_explained
  | Pexp_let(rec_flag, spat_sexp_list, sbody) ->
      let existential_context =
        if rec_flag = Recursive then In_rec
        else if List.compare_length_with spat_sexp_list 1 > 0 then In_group
        else With_attributes in
      let (pat_exp_list, new_env, unpacks) =
        type_let existential_context env rec_flag spat_sexp_list true in
      let body = type_unpacks new_env unpacks sbody ty_expected_explained in
      let () =
        if rec_flag = Recursive then
          check_recursive_bindings env pat_exp_list
      in
      re {
        exp_desc = Texp_let(rec_flag, pat_exp_list, body);
        exp_loc = loc; exp_extra = [];
        exp_type = body.exp_type;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_fun (l, Some default, spat, sbody) ->
      assert(is_optional l); (* default allowed only with optional argument *)
      let open Ast_helper in
      let default_loc = default.pexp_loc in
      let scases = [
        Exp.case
          (Pat.construct ~loc:default_loc
             (mknoloc (Longident.(Ldot (Lident "*predef*", "Some"))))
             (Some ([], Pat.var ~loc:default_loc (mknoloc "*sth*"))))
          (Exp.ident ~loc:default_loc (mknoloc (Longident.Lident "*sth*")));

        Exp.case
          (Pat.construct ~loc:default_loc
             (mknoloc (Longident.(Ldot (Lident "*predef*", "None"))))
             None)
          default;
       ]
      in
      let sloc =
        { Location.loc_start = spat.ppat_loc.Location.loc_start;
          loc_end = default_loc.Location.loc_end;
          loc_ghost = true }
      in
      let smatch =
        Exp.match_ ~loc:sloc
          (Exp.ident ~loc (mknoloc (Longident.Lident "*opt*")))
          scases
      in
      let pat = Pat.var ~loc:sloc (mknoloc "*opt*") in
      let body =
        Exp.let_ ~loc Nonrecursive
          ~attrs:[Attr.mk (mknoloc "#default") (PStr [])]
          [Vb.mk spat smatch] sbody
      in
      type_function ?in_function loc sexp.pexp_attributes env
                    ty_expected_explained l [Exp.case pat body]
  | Pexp_fun (l, None, spat, sbody) ->
      type_function ?in_function loc sexp.pexp_attributes env
                    ty_expected_explained l [Ast_helper.Exp.case spat sbody]
  | Pexp_function caselist ->
      type_function ?in_function
        loc sexp.pexp_attributes env ty_expected_explained Nolabel caselist
  | Pexp_apply(sfunct, sargs) ->
      assert (sargs <> []);
      let rec lower_args seen ty_fun =
        let ty = expand_head env ty_fun in
        if TypeSet.mem ty seen then () else
          match get_desc ty with
            Tarrow (_l, ty_arg, ty_fun, _com) ->
              (try unify_var env (newvar()) ty_arg
               with Unify _ -> assert false);
              lower_args (TypeSet.add ty seen) ty_fun
          | _ -> ()
      in
      let type_sfunct sfunct =
        begin_def (); (* one more level for non-returning functions *)
        if !Clflags.principal then begin_def ();
        let funct = type_exp env sfunct in
        if !Clflags.principal then begin
          end_def ();
          generalize_structure funct.exp_type
        end;
        let ty = instance funct.exp_type in
        end_def ();
        wrap_trace_gadt_instances env (lower_args TypeSet.empty) ty;
        funct
      in
      let funct, sargs =
        let funct = type_sfunct sfunct in
        match funct.exp_desc, sargs with
        | Texp_ident (_, _,
                      {val_kind = Val_prim {prim_name="%revapply"}; val_type}),
          [Nolabel, sarg; Nolabel, actual_sfunct]
          when is_inferred actual_sfunct
            && check_apply_prim_type Revapply val_type ->
            type_sfunct actual_sfunct, [Nolabel, sarg]
        | Texp_ident (_, _,
                      {val_kind = Val_prim {prim_name="%apply"}; val_type}),
          [Nolabel, actual_sfunct; Nolabel, sarg]
          when check_apply_prim_type Apply val_type ->
            type_sfunct actual_sfunct, [Nolabel, sarg]
        | _ ->
            funct, sargs
      in
      begin_def ();
      let (args, ty_res) = type_application env funct sargs in
      end_def ();
      unify_var env (newvar()) funct.exp_type;
      let exp =
        { exp_desc = Texp_apply(funct, args);
          exp_loc = loc; exp_extra = [];
          exp_type = ty_res;
          exp_attributes = sexp.pexp_attributes;
          exp_env = env } in
      begin
        try rue exp
        with Error (_, _, Expr_type_clash _) as err ->
          Misc.reraise_preserving_backtrace err (fun () ->
            check_partial_application ~statement:false exp)
      end
  | Pexp_match(sarg, caselist) ->
      begin_def ();
      let arg = type_exp env sarg in
      end_def ();
      if maybe_expansive arg then lower_contravariant env arg.exp_type;
      generalize arg.exp_type;
      let cases, partial =
        type_cases Computation env
          arg.exp_type ty_expected_explained true loc caselist in
      re {
        exp_desc = Texp_match(arg, cases, partial);
        exp_loc = loc; exp_extra = [];
        exp_type = instance ty_expected;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_try(sbody, caselist) ->
      let body = type_expect env sbody ty_expected_explained in
      let cases, _ =
        type_cases Value env
          Predef.type_exn ty_expected_explained false loc caselist in
      re {
        exp_desc = Texp_try(body, cases);
        exp_loc = loc; exp_extra = [];
        exp_type = body.exp_type;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_tuple sexpl ->
      assert (List.length sexpl >= 2);
      let subtypes = List.map (fun _ -> newgenvar ()) sexpl in
      let to_unify = newgenty (Ttuple subtypes) in
      with_explanation (fun () ->
        unify_exp_types loc env to_unify (generic_instance ty_expected));
      let expl =
        List.map2 (fun body ty -> type_expect env body (mk_expected ty))
          sexpl subtypes
      in
      re {
        exp_desc = Texp_tuple expl;
        exp_loc = loc; exp_extra = [];
        (* Keep sharing *)
        exp_type = newty (Ttuple (List.map (fun e -> e.exp_type) expl));
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_construct(lid, sarg) ->
      type_construct env loc lid sarg ty_expected_explained sexp.pexp_attributes
  | Pexp_variant(l, sarg) ->
      (* Keep sharing *)
      let ty_expected1 = protect_expansion env ty_expected in
      let ty_expected0 = instance ty_expected in
      begin try match
        sarg, get_desc (expand_head env ty_expected1),
        get_desc (expand_head env ty_expected0)
      with
      | Some sarg, Tvariant row, Tvariant row0 ->
          begin match
            row_field_repr (get_row_field l row),
            row_field_repr (get_row_field l row0)
          with
            Rpresent (Some ty), Rpresent (Some ty0) ->
              let arg = type_argument env sarg ty ty0 in
              re { exp_desc = Texp_variant(l, Some arg);
                   exp_loc = loc; exp_extra = [];
                   exp_type = ty_expected0;
                   exp_attributes = sexp.pexp_attributes;
                   exp_env = env }
          | _ -> raise Exit
          end
      | _ -> raise Exit
      with Exit ->
        let arg = Option.map (type_exp env) sarg in
        let arg_type = Option.map (fun arg -> arg.exp_type) arg in
        let row =
          create_row
            ~fields: [l, rf_present arg_type]
            ~more:   (newvar ())
            ~closed: false
            ~fixed:  None
            ~name:   None
        in
        rue {
          exp_desc = Texp_variant(l, arg);
          exp_loc = loc; exp_extra = [];
          exp_type = newty (Tvariant row);
          exp_attributes = sexp.pexp_attributes;
          exp_env = env }
      end
  | Pexp_record(lid_sexp_list, opt_sexp) ->
      assert (lid_sexp_list <> []);
      let opt_exp =
        match opt_sexp with
          None -> None
        | Some sexp ->
            if !Clflags.principal then begin_def ();
            let exp = type_exp ~recarg env sexp in
            if !Clflags.principal then begin
              end_def ();
              generalize_structure exp.exp_type
            end;
            Some exp
      in
      let ty_record, expected_type =
        let expected_opath =
          match extract_concrete_record env ty_expected with
          | Record_type (p0, p, _) -> Some (p0, p, is_principal ty_expected)
          | Maybe_a_record_type -> None
          | Not_a_record_type ->
            let error =
              Wrong_expected_kind(Record, Expression explanation, ty_expected)
            in
            raise (Error (loc, env, error))
        in
        let opt_exp_opath =
          match opt_exp with
          | None -> None
          | Some exp ->
            match extract_concrete_record env exp.exp_type with
            | Record_type (p0, p, _) -> Some (p0, p, is_principal exp.exp_type)
            | Maybe_a_record_type -> None
            | Not_a_record_type ->
              let error = Expr_not_a_record_type exp.exp_type in
              raise (Error (exp.exp_loc, env, error))
        in
        match expected_opath, opt_exp_opath with
        | None, None -> newvar (), None
        | Some _, None -> ty_expected, expected_opath
        | Some(_, _, true), Some _ -> ty_expected, expected_opath
        | (None | Some (_, _, false)), Some (_, p', _) ->
            let decl = Env.find_type p' env in
            begin_def ();
            let ty = newconstr p' (instance_list decl.type_params) in
            end_def ();
            generalize_structure ty;
            ty, opt_exp_opath
      in
      let closed = (opt_sexp = None) in
      let lbl_exp_list =
        wrap_disambiguate "This record expression is expected to have"
          (mk_expected ty_record)
          (type_label_a_list loc closed env Env.Construct
             (fun e k -> k (type_label_exp true env loc ty_record e))
             expected_type lid_sexp_list)
          (fun x -> x)
      in
      with_explanation (fun () ->
        unify_exp_types loc env (instance ty_record) (instance ty_expected));

      (* type_label_a_list returns a list of labels sorted by lbl_pos *)
      (* note: check_duplicates would better be implemented in
         type_label_a_list directly *)
      let rec check_duplicates = function
        | (_, lbl1, _) :: (_, lbl2, _) :: _ when lbl1.lbl_pos = lbl2.lbl_pos ->
          raise(Error(loc, env, Label_multiply_defined lbl1.lbl_name))
        | _ :: rem ->
            check_duplicates rem
        | [] -> ()
      in
      check_duplicates lbl_exp_list;
      let opt_exp, label_definitions =
        let (_lid, lbl, _lbl_exp) = List.hd lbl_exp_list in
        let matching_label lbl =
          List.find
            (fun (_, lbl',_) -> lbl'.lbl_pos = lbl.lbl_pos)
            lbl_exp_list
        in
        match opt_exp with
          None ->
            let label_definitions =
              Array.map (fun lbl ->
                  match matching_label lbl with
                  | (lid, _lbl, lbl_exp) ->
                      Overridden (lid, lbl_exp)
                  | exception Not_found ->
                      let present_indices =
                        List.map (fun (_, lbl, _) -> lbl.lbl_pos) lbl_exp_list
                      in
                      let label_names = extract_label_names env ty_expected in
                      let rec missing_labels n = function
                          [] -> []
                        | lbl :: rem ->
                            if List.mem n present_indices
                            then missing_labels (n + 1) rem
                            else lbl :: missing_labels (n + 1) rem
                      in
                      let missing = missing_labels 0 label_names in
                      raise(Error(loc, env, Label_missing missing)))
                lbl.lbl_all
            in
            None, label_definitions
        | Some exp ->
            let ty_exp = instance exp.exp_type in
            let unify_kept lbl =
              let _, ty_arg1, ty_res1 = instance_label false lbl in
              unify_exp_types exp.exp_loc env ty_exp ty_res1;
              match matching_label lbl with
              | lid, _lbl, lbl_exp ->
                  (* do not connect result types for overridden labels *)
                  Overridden (lid, lbl_exp)
              | exception Not_found -> begin
                  let _, ty_arg2, ty_res2 = instance_label false lbl in
                  unify_exp_types loc env ty_arg1 ty_arg2;
                  with_explanation (fun () ->
                    unify_exp_types loc env (instance ty_expected) ty_res2);
                  Kept (ty_arg1, lbl.lbl_mut)
                end
            in
            let label_definitions = Array.map unify_kept lbl.lbl_all in
            Some {exp with exp_type = ty_exp}, label_definitions
      in
      let num_fields =
        match lbl_exp_list with [] -> assert false
        | (_, lbl,_)::_ -> Array.length lbl.lbl_all in
      if opt_sexp <> None && List.length lid_sexp_list = num_fields then
        Location.prerr_warning loc Warnings.Useless_record_with;
      let label_descriptions, representation =
        let (_, { lbl_all; lbl_repres }, _) = List.hd lbl_exp_list in
        lbl_all, lbl_repres
      in
      let fields =
        Array.map2 (fun descr def -> descr, def)
          label_descriptions label_definitions
      in
      re {
        exp_desc = Texp_record {
            fields; representation;
            extended_expression = opt_exp
          };
        exp_loc = loc; exp_extra = [];
        exp_type = instance ty_expected;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_field(srecord, lid) ->
      let (record, label, _) =
        type_label_access env srecord Env.Projection lid
      in
      let (_, ty_arg, ty_res) = instance_label false label in
      unify_exp env record ty_res;
      rue {
        exp_desc = Texp_field(record, lid, label);
        exp_loc = loc; exp_extra = [];
        exp_type = ty_arg;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_setfield(srecord, lid, snewval) ->
      let (record, label, expected_type) =
        type_label_access env srecord Env.Mutation lid in
      let ty_record =
        if expected_type = None then newvar () else record.exp_type in
      let (label_loc, label, newval) =
        type_label_exp false env loc ty_record (lid, label, snewval) in
      unify_exp env record ty_record;
      if label.lbl_mut = Immutable then
        raise(Error(loc, env, Label_not_mutable lid.txt));
      rue {
        exp_desc = Texp_setfield(record, label_loc, label, newval);
        exp_loc = loc; exp_extra = [];
        exp_type = instance Predef.type_unit;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_array(sargl) ->
      let ty = newgenvar() in
      let to_unify = Predef.type_array ty in
      with_explanation (fun () ->
        unify_exp_types loc env to_unify (generic_instance ty_expected));
      let argl =
        List.map (fun sarg -> type_expect env sarg (mk_expected ty)) sargl in
      re {
        exp_desc = Texp_array argl;
        exp_loc = loc; exp_extra = [];
        exp_type = instance ty_expected;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_ifthenelse(scond, sifso, sifnot) ->
      let cond = type_expect env scond
          (mk_expected ~explanation:If_conditional Predef.type_bool) in
      begin match sifnot with
        None ->
          let ifso = type_expect env sifso
              (mk_expected ~explanation:If_no_else_branch Predef.type_unit) in
          rue {
            exp_desc = Texp_ifthenelse(cond, ifso, None);
            exp_loc = loc; exp_extra = [];
            exp_type = ifso.exp_type;
            exp_attributes = sexp.pexp_attributes;
            exp_env = env }
      | Some sifnot ->
          let ifso = type_expect env sifso ty_expected_explained in
          let ifnot = type_expect env sifnot ty_expected_explained in
          (* Keep sharing *)
          unify_exp env ifnot ifso.exp_type;
          re {
            exp_desc = Texp_ifthenelse(cond, ifso, Some ifnot);
            exp_loc = loc; exp_extra = [];
            exp_type = ifso.exp_type;
            exp_attributes = sexp.pexp_attributes;
            exp_env = env }
      end
  | Pexp_sequence(sexp1, sexp2) ->
      let exp1 = type_statement ~explanation:Sequence_left_hand_side
          env sexp1 in
      let exp2 = type_expect env sexp2 ty_expected_explained in
      re {
        exp_desc = Texp_sequence(exp1, exp2);
        exp_loc = loc; exp_extra = [];
        exp_type = exp2.exp_type;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_while(scond, sbody) ->
      let cond = type_expect env scond
          (mk_expected ~explanation:While_loop_conditional Predef.type_bool) in
      let body = type_statement ~explanation:While_loop_body env sbody in
      rue {
        exp_desc = Texp_while(cond, body);
        exp_loc = loc; exp_extra = [];
        exp_type = instance Predef.type_unit;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_for(param, slow, shigh, dir, sbody) ->
      let low = type_expect env slow
          (mk_expected ~explanation:For_loop_start_index Predef.type_int) in
      let high = type_expect env shigh
          (mk_expected ~explanation:For_loop_stop_index Predef.type_int) in
      let id, new_env =
        match param.ppat_desc with
        | Ppat_any -> Ident.create_local "_for", env
        | Ppat_var {txt} ->
            Env.enter_value txt
              {val_type = instance Predef.type_int;
               val_attributes = [];
               val_kind = Val_reg;
               val_loc = loc;
               val_uid = Uid.mk ~current_unit:(Env.get_unit_name ());
              } env
              ~check:(fun s -> Warnings.Unused_for_index s)
        | _ ->
            raise (Error (param.ppat_loc, env, Invalid_for_loop_index))
      in
      let body = type_statement ~explanation:For_loop_body new_env sbody in
      rue {
        exp_desc = Texp_for(id, param, low, high, dir, body);
        exp_loc = loc; exp_extra = [];
        exp_type = instance Predef.type_unit;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_constraint (sarg, sty) ->
      (* Pretend separate = true, 1% slowdown for lablgtk *)
      begin_def ();
      let cty = Typetexp.transl_simple_type env false sty in
      let ty = cty.ctyp_type in
      end_def ();
      generalize_structure ty;
      let (arg, ty') = (type_argument env sarg ty (instance ty), instance ty) in
      rue {
        exp_desc = arg.exp_desc;
        exp_loc = arg.exp_loc;
        exp_type = ty';
        exp_attributes = arg.exp_attributes;
        exp_env = env;
        exp_extra =
          (Texp_constraint cty, loc, sexp.pexp_attributes) :: arg.exp_extra;
      }
  | Pexp_coerce(sarg, sty, sty') ->
      (* Pretend separate = true, 1% slowdown for lablgtk *)
      (* Also see PR#7199 for a problem with the following:
         let separate = !Clflags.principal || Env.has_local_constraints env in*)
      let (arg, ty',cty,cty') =
        match sty with
        | None ->
            let (cty', ty', force) =
              Typetexp.transl_simple_type_delayed env sty'
            in
            begin_def ();
            let arg = type_exp env sarg in
            end_def ();
            let tv = newvar () in
            let gen = generalizable (get_level tv) arg.exp_type in
            unify_var env tv arg.exp_type;
            begin match arg.exp_desc, !self_coercion, get_desc ty' with
              Texp_ident(_, _, {val_kind=Val_self _}), (path,r) :: _,
              Tconstr(path',_,_) when Path.same path path' ->
                (* prerr_endline "self coercion"; *)
                r := loc :: !r;
                force ()
            | _ when free_variables ~env arg.exp_type = []
                  && free_variables ~env ty' = [] ->
                if not gen && (* first try a single coercion *)
                  let snap = snapshot () in
                  let ty, _b = enlarge_type env ty' in
                  try
                    force (); Ctype.unify env arg.exp_type ty; true
                  with Unify _ ->
                    backtrack snap; false
                then ()
                else begin try
                  let force' = subtype env arg.exp_type ty' in
                  force (); force' ();
                  if not gen && !Clflags.principal then
                    Location.prerr_warning loc
                      (Warnings.Not_principal "this ground coercion");
                with Subtype err ->
                  (* prerr_endline "coercion failed"; *)
                  raise (Error(loc, env, Not_subtype err))
                end;
            | _ ->
                let ty, b = enlarge_type env ty' in
                force ();
                begin try Ctype.unify env arg.exp_type ty with Unify err ->
                  let expanded = full_expand ~may_forget_scope:true env ty' in
                  raise(Error(sarg.pexp_loc, env,
                              Coercion_failure({ty = ty'; expanded}, err, b)))
                end
            end;
            (arg, ty', None, cty')
        | Some sty ->
            begin_def ();
            let (cty, ty, force) =
              Typetexp.transl_simple_type_delayed env sty
            and (cty', ty', force') =
              Typetexp.transl_simple_type_delayed env sty'
            in
            end_def ();
            generalize_structure ty;
            generalize_structure ty';
            begin try
              let force'' = subtype env (instance ty) (instance ty') in
              force (); force' (); force'' ()
            with Subtype err ->
              raise (Error(loc, env, Not_subtype err))
            end;
            (type_argument env sarg ty (instance ty),
             instance ty', Some cty, cty')
      in
      rue {
        exp_desc = arg.exp_desc;
        exp_loc = arg.exp_loc;
        exp_type = ty';
        exp_attributes = arg.exp_attributes;
        exp_env = env;
        exp_extra = (Texp_coerce (cty, cty'), loc, sexp.pexp_attributes) ::
                       arg.exp_extra;
      }
  | Pexp_send (e, {txt=met}) ->
      if !Clflags.principal then begin_def ();
      let obj = type_exp env e in
      let (meth, typ) =
        match obj.exp_desc with
        | Texp_ident(_, _, {val_kind = Val_self(sign, meths, _, _)}) ->
            let id, typ =
              match meths with
              | Self_concrete meths ->
                  let id =
                    match Meths.find met meths with
                    | id -> id
                    | exception Not_found ->
                        let valid_methods =
                          Meths.fold (fun lab _ acc -> lab :: acc) meths []
                        in
                        raise (Error(e.pexp_loc, env,
                          Undefined_self_method (met, valid_methods)))
                  in
                  let typ = Btype.method_type met sign in
                  id, typ
              | Self_virtual meths_ref -> begin
                  match Meths.find met !meths_ref with
                  | id -> id, Btype.method_type met sign
                  | exception Not_found ->
                      let id = Ident.create_local met in
                      let ty = newvar () in
                      meths_ref := Meths.add met id !meths_ref;
                      add_method env met Private Virtual ty sign;
                      Location.prerr_warning loc
                        (Warnings.Undeclared_virtual_method met);
                      id, ty
                end
            in
            Tmeth_val id, typ
        | Texp_ident(_, _, {val_kind = Val_anc (sign, meths, cl_num)}) ->
            let id =
              match Meths.find met meths with
              | id -> id
              | exception Not_found ->
                  let valid_methods =
                    Meths.fold (fun lab _ acc -> lab :: acc) meths []
                  in
                  raise (Error(e.pexp_loc, env,
                    Undefined_self_method (met, valid_methods)))
            in
            let typ = Btype.method_type met sign in
            let (self_path, _) =
              Env.find_value_by_name
                (Longident.Lident ("self-" ^ cl_num)) env
            in
            Tmeth_ancestor(id, self_path), typ
        | _ ->
            let ty =
              match filter_method env met obj.exp_type with
              | ty -> ty
              | exception Filter_method_failed err ->
                let error =
                  match err with
                  | Unification_error err ->
                      Expr_type_clash(err, explanation, None)
                  | Not_an_object ty ->
                      Not_an_object(ty, explanation)
                  | Not_a_method ->
                      let valid_methods =
                        match get_desc (expand_head env obj.exp_type) with
                        | Tobject (fields, _) ->
                            let (fields, _) = Ctype.flatten_fields fields in
                            let collect_fields li (meth, meth_kind, _meth_ty) =
                              if field_kind_repr meth_kind = Fpublic
                              then meth::li else li
                            in
                            Some (List.fold_left collect_fields [] fields)
                        | _ -> None
                      in
                      Undefined_method(obj.exp_type, met, valid_methods)
                in
                raise (Error(e.pexp_loc, env, error))
            in
            Tmeth_name met, ty
      in
      if !Clflags.principal then begin
        end_def ();
        generalize_structure typ;
      end;
      let typ =
        match get_desc typ with
        | Tpoly (ty, []) ->
            instance ty
        | Tpoly (ty, tl) ->
            if !Clflags.principal && get_level typ <> generic_level then
              Location.prerr_warning loc
                (Warnings.Not_principal "this use of a polymorphic method");
            snd (instance_poly false tl ty)
        | Tvar _ ->
            let ty' = newvar () in
            unify env (instance typ) (newty(Tpoly(ty',[])));
            (* if not !Clflags.nolabels then
               Location.prerr_warning loc (Warnings.Unknown_method met); *)
            ty'
        | _ ->
            assert false
      in
      rue {
        exp_desc = Texp_send(obj, meth);
        exp_loc = loc; exp_extra = [];
        exp_type = typ;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_new cl ->
      let (cl_path, cl_decl) = Env.lookup_class ~loc:cl.loc cl.txt env in
      begin match cl_decl.cty_new with
          None ->
            raise(Error(loc, env, Virtual_class cl.txt))
        | Some ty ->
            rue {
              exp_desc = Texp_new (cl_path, cl, cl_decl);
              exp_loc = loc; exp_extra = [];
              exp_type = instance ty;
              exp_attributes = sexp.pexp_attributes;
              exp_env = env }
        end
  | Pexp_setinstvar (lab, snewval) -> begin
      let (path, mut, cl_num, ty) =
        Env.lookup_instance_variable ~loc lab.txt env
      in
      match mut with
      | Mutable ->
          let newval =
            type_expect env snewval (mk_expected (instance ty))
          in
          let (path_self, _) =
            Env.find_value_by_name (Longident.Lident ("self-" ^ cl_num)) env
          in
          rue {
            exp_desc = Texp_setinstvar(path_self, path, lab, newval);
            exp_loc = loc; exp_extra = [];
            exp_type = instance Predef.type_unit;
            exp_attributes = sexp.pexp_attributes;
            exp_env = env }
      | _ ->
          raise(Error(loc, env, Instance_variable_not_mutable lab.txt))
    end
  | Pexp_override lst ->
      let _ =
       List.fold_right
        (fun (lab, _) l ->
           if List.exists (fun l -> l.txt = lab.txt) l then
             raise(Error(loc, env,
                         Value_multiply_overridden lab.txt));
           lab::l)
        lst
        [] in
      begin match
        try
          Env.find_value_by_name (Longident.Lident "selfpat-*") env,
          Env.find_value_by_name (Longident.Lident "self-*") env
        with Not_found ->
          raise(Error(loc, env, Outside_class))
      with
        (_, {val_type = self_ty; val_kind = Val_self (sign, _, vars, _)}),
        (path_self, _) ->
          let type_override (lab, snewval) =
            begin try
              let id = Vars.find lab.txt vars in
              let ty = Btype.instance_variable_type lab.txt sign in
              (id, lab, type_expect env snewval (mk_expected (instance ty)))
            with
              Not_found ->
                let vars = Vars.fold (fun var _ li -> var::li) vars [] in
                raise(Error(loc, env,
                            Unbound_instance_variable (lab.txt, vars)))
            end
          in
          let modifs = List.map type_override lst in
          rue {
            exp_desc = Texp_override(path_self, modifs);
            exp_loc = loc; exp_extra = [];
            exp_type = self_ty;
            exp_attributes = sexp.pexp_attributes;
            exp_env = env }
      | _ ->
          assert false
      end
  | Pexp_letmodule(name, smodl, sbody) ->
      let ty = newvar() in
      (* remember original level *)
      begin_def ();
      let context = Typetexp.narrow () in
      let modl, md_shape = !type_module env smodl in
      Mtype.lower_nongen (get_level ty) modl.mod_type;
      let pres =
        match modl.mod_type with
        | Mty_alias _ -> Mp_absent
        | _ -> Mp_present
      in
      let scope = create_scope () in
      let md =
        { md_type = modl.mod_type; md_attributes = []; md_loc = name.loc;
          md_uid = Uid.mk ~current_unit:(Env.get_unit_name ()); }
      in
      let (id, new_env) =
        match name.txt with
        | None -> None, env
        | Some name ->
          let id, env =
            Env.enter_module_declaration ~scope ~shape:md_shape name pres md env
          in
          Some id, env
      in
      Typetexp.widen context;
      (* ideally, we should catch Expr_type_clash errors
         in type_expect triggered by escaping identifiers from the local module
         and refine them into Scoping_let_module errors
      *)
      let body = type_expect new_env sbody ty_expected_explained in
      (* go back to original level *)
      end_def ();
      Ctype.unify_var new_env ty body.exp_type;
      re {
        exp_desc = Texp_letmodule(id, name, pres, modl, body);
        exp_loc = loc; exp_extra = [];
        exp_type = ty;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_letexception(cd, sbody) ->
      let (cd, newenv) = Typedecl.transl_exception env cd in
      let body = type_expect newenv sbody ty_expected_explained in
      re {
        exp_desc = Texp_letexception(cd, body);
        exp_loc = loc; exp_extra = [];
        exp_type = body.exp_type;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }

  | Pexp_assert (e) ->
      let cond = type_expect env e
          (mk_expected ~explanation:Assert_condition Predef.type_bool) in
      let exp_type =
        match cond.exp_desc with
        | Texp_construct(_, {cstr_name="false"}, _) ->
            instance ty_expected
        | _ ->
            instance Predef.type_unit
      in
      rue {
        exp_desc = Texp_assert cond;
        exp_loc = loc; exp_extra = [];
        exp_type;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env;
      }
  | Pexp_lazy e ->
      let ty = newgenvar () in
      let to_unify = Predef.type_lazy_t ty in
      with_explanation (fun () ->
        unify_exp_types loc env to_unify (generic_instance ty_expected));
      let arg = type_expect env e (mk_expected ty) in
      re {
        exp_desc = Texp_lazy arg;
        exp_loc = loc; exp_extra = [];
        exp_type = instance ty_expected;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env;
      }
  | Pexp_object s ->
      let desc, meths = !type_object env loc s in
      rue {
        exp_desc = Texp_object (desc, meths);
        exp_loc = loc; exp_extra = [];
        exp_type = desc.cstr_type.csig_self;
        exp_attributes = sexp.pexp_attributes;
        exp_env = env;
      }
  | Pexp_poly(sbody, sty) ->
      if !Clflags.principal then begin_def ();
      let ty, cty =
        match sty with None -> protect_expansion env ty_expected, None
        | Some sty ->
            let sty = Ast_helper.Typ.force_poly sty in
            let cty = Typetexp.transl_simple_type env false sty in
            cty.ctyp_type, Some cty
      in
      if !Clflags.principal then begin
        end_def ();
        generalize_structure ty
      end;
      if sty <> None then
        with_explanation (fun () ->
          unify_exp_types loc env (instance ty) (instance ty_expected));
      let exp =
        match get_desc (expand_head env ty) with
          Tpoly (ty', []) ->
            let exp = type_expect env sbody (mk_expected ty') in
            { exp with exp_type = instance ty }
        | Tpoly (ty', tl) ->
            (* One more level to generalize locally *)
            begin_def ();
            if !Clflags.principal then begin_def ();
            let vars, ty'' = instance_poly true tl ty' in
            if !Clflags.principal then begin
              end_def ();
              generalize_structure ty''
            end;
            let exp = type_expect env sbody (mk_expected ty'') in
            end_def ();
            generalize_and_check_univars env "method" exp ty_expected vars;
            { exp with exp_type = instance ty }
        | Tvar _ ->
            let exp = type_exp env sbody in
            let exp = {exp with exp_type = newty (Tpoly (exp.exp_type, []))} in
            unify_exp env exp ty;
            exp
        | _ -> assert false
      in
      re { exp with exp_extra =
             (Texp_poly cty, loc, sexp.pexp_attributes) :: exp.exp_extra }
  | Pexp_newtype({txt=name}, sbody) ->
      let ty =
        if Typetexp.valid_tyvar_name name then
          newvar ~name ()
        else
          newvar ()
      in
      (* remember original level *)
      begin_def ();
      (* Create a fake abstract type declaration for name. *)
      let decl = new_local_type ~loc () in
      let scope = create_scope () in
      let (id, new_env) = Env.enter_type ~scope name decl env in

      let body = type_exp new_env sbody in
      (* Replace every instance of this type constructor in the resulting
         type. *)
      let seen = Hashtbl.create 8 in
      let rec replace t =
        if Hashtbl.mem seen (get_id t) then ()
        else begin
          Hashtbl.add seen (get_id t) ();
          match get_desc t with
          | Tconstr (Path.Pident id', _, _) when id == id' -> link_type t ty
          | _ -> Btype.iter_type_expr replace t
        end
      in
      let ety = Subst.type_expr Subst.identity body.exp_type in
      replace ety;
      (* back to original level *)
      end_def ();
      (* lower the levels of the result type *)
      (* unify_var env ty ety; *)

      (* non-expansive if the body is non-expansive, so we don't introduce
         any new extra node in the typed AST. *)
      rue { body with exp_loc = loc; exp_type = ety;
            exp_extra =
            (Texp_newtype name, loc, sexp.pexp_attributes) :: body.exp_extra }
  | Pexp_pack m ->
      let (p, fl) =
        match get_desc (Ctype.expand_head env (instance ty_expected)) with
          Tpackage (p, fl) ->
            if !Clflags.principal &&
              get_level (Ctype.expand_head env
                           (protect_expansion env ty_expected))
                < Btype.generic_level
            then
              Location.prerr_warning loc
                (Warnings.Not_principal "this module packing");
            (p, fl)
        | Tvar _ ->
            raise (Error (loc, env, Cannot_infer_signature))
        | _ ->
            raise (Error (loc, env, Not_a_packed_module ty_expected))
      in
      let (modl, fl') = !type_package env m p fl in
      rue {
        exp_desc = Texp_pack modl;
        exp_loc = loc; exp_extra = [];
        exp_type = newty (Tpackage (p, fl'));
        exp_attributes = sexp.pexp_attributes;
        exp_env = env }
  | Pexp_open (od, e) ->
      let tv = newvar () in
      let (od, _, newenv) = !type_open_decl env od in
      let exp = type_expect newenv e ty_expected_explained in
      (* Force the return type to be well-formed in the original
         environment. *)
      unify_var newenv tv exp.exp_type;
      re {
        exp_desc = Texp_open (od, exp);
        exp_type = exp.exp_type;
        exp_loc = loc;
        exp_extra = [];
        exp_attributes = sexp.pexp_attributes;
        exp_env = env;
      }
  | Pexp_letop{ let_ = slet; ands = sands; body = sbody } ->
      let rec loop spat_acc ty_acc sands =
        match sands with
        | [] -> spat_acc, ty_acc
        | { pbop_pat = spat; _} :: rest ->
            let ty = newvar () in
            let loc = { slet.pbop_op.loc with Location.loc_ghost = true } in
            let spat_acc = Ast_helper.Pat.tuple ~loc [spat_acc; spat] in
            let ty_acc = newty (Ttuple [ty_acc; ty]) in
            loop spat_acc ty_acc rest
      in
      if !Clflags.principal then begin_def ();
      let let_loc = slet.pbop_op.loc in
      let op_path, op_desc = type_binding_op_ident env slet.pbop_op in
      let op_type = instance op_desc.val_type in
      let spat_params, ty_params = loop slet.pbop_pat (newvar ()) sands in
      let ty_func_result = newvar () in
      let ty_func =
        newty (Tarrow(Nolabel, ty_params, ty_func_result, commu_ok)) in
      let ty_result = newvar () in
      let ty_andops = newvar () in
      let ty_op =
        newty (Tarrow(Nolabel, ty_andops,
          newty (Tarrow(Nolabel, ty_func, ty_result, commu_ok)), commu_ok))
      in
      begin try
        unify env op_type ty_op
      with Unify err ->
        raise(Error(let_loc, env, Letop_type_clash(slet.pbop_op.txt, err)))
      end;
      if !Clflags.principal then begin
        end_def ();
        generalize_structure ty_andops;
        generalize_structure ty_params;
        generalize_structure ty_func_result;
        generalize_structure ty_result
      end;
      let exp, ands = type_andops env slet.pbop_exp sands ty_andops in
      let scase = Ast_helper.Exp.case spat_params sbody in
      let cases, partial =
        type_cases Value env
          ty_params (mk_expected ty_func_result) true loc [scase]
      in
      let body =
        match cases with
        | [case] -> case
        | _ -> assert false
      in
      let param = name_cases "param" cases in
      let let_ =
        { bop_op_name = slet.pbop_op;
          bop_op_path = op_path;
          bop_op_val = op_desc;
          bop_op_type = op_type;
          bop_exp = exp;
          bop_loc = slet.pbop_loc; }
      in
      let desc =
        Texp_letop{let_; ands; param; body; partial}
      in
      rue { exp_desc = desc;
            exp_loc = sexp.pexp_loc;
            exp_extra = [];
            exp_type = instance ty_result;
            exp_env = env;
            exp_attributes = sexp.pexp_attributes; }

  | Pexp_extension ({ txt = ("ocaml.extension_constructor"
                             |"extension_constructor"); _ },
                    payload) ->
      begin match payload with
      | PStr [ { pstr_desc =
                   Pstr_eval ({ pexp_desc = Pexp_construct (lid, None); _ }, _)
               } ] ->
          let path =
            let cd =
              Env.lookup_constructor Env.Positive ~loc:lid.loc lid.txt env
            in
            match cd.cstr_tag with
            | Cstr_extension (path, _) -> path
            | _ -> raise (Error (lid.loc, env, Not_an_extension_constructor))
          in
          rue {
            exp_desc = Texp_extension_constructor (lid, path);
            exp_loc = loc; exp_extra = [];
            exp_type = instance Predef.type_extension_constructor;
            exp_attributes = sexp.pexp_attributes;
            exp_env = env }
      | _ ->
          raise (Error (loc, env, Invalid_extension_constructor_payload))
      end
  | Pexp_extension ext ->
      raise (Error_forward (Builtin_attributes.error_of_extension ext))

  | Pexp_unreachable ->
      re { exp_desc = Texp_unreachable;
           exp_loc = loc; exp_extra = [];
           exp_type = instance ty_expected;
           exp_attributes = sexp.pexp_attributes;
           exp_env = env }

and type_ident env ?(recarg=Rejected) lid =
  let (path, desc) = Env.lookup_value ~loc:lid.loc lid.txt env in
  let is_recarg =
    match get_desc desc.val_type with
    | Tconstr(p, _, _) -> Path.is_constructor_typath p
    | _ -> false
  in
  begin match is_recarg, recarg, get_desc desc.val_type with
  | _, Allowed, _
  | true, Required, _
  | false, Rejected, _ -> ()
  | true, Rejected, _
  | false, Required, (Tvar _ | Tconstr _) ->
      raise (Error (lid.loc, env, Inlined_record_escape))
  | false, Required, _  -> () (* will fail later *)
  end;
  path, desc

and type_binding_op_ident env s =
  let loc = s.loc in
  let lid = Location.mkloc (Longident.Lident s.txt) loc in
  let path, desc = type_ident env lid in
  let path =
    match desc.val_kind with
    | Val_ivar _ ->
        fatal_error "Illegal name for instance variable"
    | Val_self (_, _, _, cl_num) ->
        let path, _ =
          Env.find_value_by_name (Longident.Lident ("self-" ^ cl_num)) env
        in
        path
    | _ -> path
  in
  path, desc

and type_function ?(in_function : (Location.t * type_expr) option)
    loc attrs env ty_expected_explained arg_label caselist =
  let { ty = ty_expected; explanation } = ty_expected_explained in
  let (loc_fun, ty_fun) =
    match in_function with Some p -> p
    | None -> (loc, instance ty_expected)
  in
  let separate = !Clflags.principal || Env.has_local_constraints env in
  if separate then begin_def ();
  let (ty_arg, ty_res) =
    try filter_arrow env (instance ty_expected) arg_label
    with Filter_arrow_failed err ->
      let err = match err with
        | Unification_error unif_err ->
            Expr_type_clash(unif_err, explanation, None)
        | Label_mismatch { got; expected; expected_type} ->
            Abstract_wrong_label { got; expected; expected_type; explanation }
        | Not_a_function -> begin
            match in_function with
            | Some _ -> Too_many_arguments(ty_fun, explanation)
            | None   -> Not_a_function(ty_fun, explanation)
          end
      in
      raise (Error(loc_fun, env, err))
  in
  let ty_arg =
    if is_optional arg_label then
      let tv = newvar() in
      begin
        try unify env ty_arg (type_option tv)
        with Unify _ -> assert false
      end;
      type_option tv
    else ty_arg
  in
  if separate then begin
    end_def ();
    generalize_structure ty_arg;
    generalize_structure ty_res
  end;
  let cases, partial =
    type_cases Value ~in_function:(loc_fun,ty_fun) env
      ty_arg (mk_expected ty_res) true loc caselist in
  let not_nolabel_function ty =
    let ls, tvar = list_labels env ty in
    List.for_all ((<>) Nolabel) ls && not tvar
  in
  if is_optional arg_label && not_nolabel_function ty_res then
    Location.prerr_warning (List.hd cases).c_lhs.pat_loc
      Warnings.Unerasable_optional_argument;
  let param = name_cases "param" cases in
  re {
    exp_desc = Texp_function { arg_label; param; cases; partial; };
    exp_loc = loc; exp_extra = [];
    exp_type =
      instance (newgenty (Tarrow(arg_label, ty_arg, ty_res, commu_ok)));
    exp_attributes = attrs;
    exp_env = env }


and type_label_access env srecord usage lid =
  if !Clflags.principal then begin_def ();
  let record = type_exp ~recarg:Allowed env srecord in
  if !Clflags.principal then begin
    end_def ();
    generalize_structure record.exp_type
  end;
  let ty_exp = record.exp_type in
  let expected_type =
    match extract_concrete_record env ty_exp with
    | Record_type(p0, p, _) ->
        Some(p0, p, is_principal ty_exp)
    | Maybe_a_record_type -> None
    | Not_a_record_type ->
        let error = Expr_not_a_record_type ty_exp in
        raise (Error (record.exp_loc, env, error))
  in
  let labels = Env.lookup_all_labels ~loc:lid.loc usage lid.txt env in
  let label =
    wrap_disambiguate "This expression has" (mk_expected ty_exp)
      (Label.disambiguate usage lid env expected_type) labels in
  (record, label, expected_type)

(* Typing format strings for printing or reading.
   These formats are used by functions in modules Printf, Format, and Scanf.
   (Handling of * modifiers contributed by Thorsten Ohl.) *)

and type_format loc str env =
  let loc = {loc with Location.loc_ghost = true} in
  try
    CamlinternalFormatBasics.(CamlinternalFormat.(
      let mk_exp_loc pexp_desc = {
        pexp_desc = pexp_desc;
        pexp_loc = loc;
        pexp_loc_stack = [];
        pexp_attributes = [];
      } and mk_lid_loc lid = {
        txt = lid;
        loc = loc;
      } in
      let mk_constr name args =
        let lid = Longident.(Ldot(Lident "CamlinternalFormatBasics", name)) in
        let arg = match args with
          | []          -> None
          | [ e ]       -> Some e
          | _ :: _ :: _ -> Some (mk_exp_loc (Pexp_tuple args)) in
        mk_exp_loc (Pexp_construct (mk_lid_loc lid, arg)) in
      let mk_cst cst = mk_exp_loc (Pexp_constant cst) in
      let mk_int n = mk_cst (Pconst_integer (Int.to_string n, None))
      and mk_string str = mk_cst (Pconst_string (str, loc, None))
      and mk_char chr = mk_cst (Pconst_char chr) in
      let rec mk_formatting_lit fmting = match fmting with
        | Close_box ->
          mk_constr "Close_box" []
        | Close_tag ->
          mk_constr "Close_tag" []
        | Break (org, ns, ni) ->
          mk_constr "Break" [ mk_string org; mk_int ns; mk_int ni ]
        | FFlush ->
          mk_constr "FFlush" []
        | Force_newline ->
          mk_constr "Force_newline" []
        | Flush_newline ->
          mk_constr "Flush_newline" []
        | Magic_size (org, sz) ->
          mk_constr "Magic_size" [ mk_string org; mk_int sz ]
        | Escaped_at ->
          mk_constr "Escaped_at" []
        | Escaped_percent ->
          mk_constr "Escaped_percent" []
        | Scan_indic c ->
          mk_constr "Scan_indic" [ mk_char c ]
      and mk_formatting_gen : type a b c d e f .
          (a, b, c, d, e, f) formatting_gen -> Parsetree.expression =
        fun fmting -> match fmting with
        | Open_tag (Format (fmt', str')) ->
          mk_constr "Open_tag" [ mk_format fmt' str' ]
        | Open_box (Format (fmt', str')) ->
          mk_constr "Open_box" [ mk_format fmt' str' ]
      and mk_format : type a b c d e f .
          (a, b, c, d, e, f) CamlinternalFormatBasics.fmt -> string ->
          Parsetree.expression = fun fmt str ->
        mk_constr "Format" [ mk_fmt fmt; mk_string str ]
      and mk_side side = match side with
        | Left  -> mk_constr "Left"  []
        | Right -> mk_constr "Right" []
        | Zeros -> mk_constr "Zeros" []
      and mk_iconv iconv = match iconv with
        | Int_d  -> mk_constr "Int_d"  [] | Int_pd -> mk_constr "Int_pd" []
        | Int_sd -> mk_constr "Int_sd" [] | Int_i  -> mk_constr "Int_i"  []
        | Int_pi -> mk_constr "Int_pi" [] | Int_si -> mk_constr "Int_si" []
        | Int_x  -> mk_constr "Int_x"  [] | Int_Cx -> mk_constr "Int_Cx" []
        | Int_X  -> mk_constr "Int_X"  [] | Int_CX -> mk_constr "Int_CX" []
        | Int_o  -> mk_constr "Int_o"  [] | Int_Co -> mk_constr "Int_Co" []
        | Int_u  -> mk_constr "Int_u"  [] | Int_Cd -> mk_constr "Int_Cd" []
        | Int_Ci -> mk_constr "Int_Ci" [] | Int_Cu -> mk_constr "Int_Cu" []
      and mk_fconv fconv =
        let flag = match fst fconv with
        | Float_flag_ -> mk_constr "Float_flag_" []
        | Float_flag_p -> mk_constr "Float_flag_p" []
        | Float_flag_s -> mk_constr "Float_flag_s" [] in
        let kind = match snd fconv with
        | Float_f  -> mk_constr "Float_f"  []
        | Float_e  -> mk_constr "Float_e"  []
        | Float_E  -> mk_constr "Float_E"  []
        | Float_g  -> mk_constr "Float_g"  []
        | Float_G  -> mk_constr "Float_G"  []
        | Float_h  -> mk_constr "Float_h"  []
        | Float_H  -> mk_constr "Float_H"  []
        | Float_F  -> mk_constr "Float_F"  []
        | Float_CF -> mk_constr "Float_CF" [] in
        mk_exp_loc (Pexp_tuple [flag; kind])
      and mk_counter cnt = match cnt with
        | Line_counter  -> mk_constr "Line_counter"  []
        | Char_counter  -> mk_constr "Char_counter"  []
        | Token_counter -> mk_constr "Token_counter" []
      and mk_int_opt n_opt = match n_opt with
        | None ->
          let lid_loc = mk_lid_loc (Longident.Lident "None") in
          mk_exp_loc (Pexp_construct (lid_loc, None))
        | Some n ->
          let lid_loc = mk_lid_loc (Longident.Lident "Some") in
          mk_exp_loc (Pexp_construct (lid_loc, Some (mk_int n)))
      and mk_fmtty : type a b c d e f g h i j k l .
          (a, b, c, d, e, f, g, h, i, j, k, l) fmtty_rel -> Parsetree.expression
          =
      fun fmtty -> match fmtty with
        | Char_ty rest      -> mk_constr "Char_ty"      [ mk_fmtty rest ]
        | String_ty rest    -> mk_constr "String_ty"    [ mk_fmtty rest ]
        | Int_ty rest       -> mk_constr "Int_ty"       [ mk_fmtty rest ]
        | Int32_ty rest     -> mk_constr "Int32_ty"     [ mk_fmtty rest ]
        | Nativeint_ty rest -> mk_constr "Nativeint_ty" [ mk_fmtty rest ]
        | Int64_ty rest     -> mk_constr "Int64_ty"     [ mk_fmtty rest ]
        | Float_ty rest     -> mk_constr "Float_ty"     [ mk_fmtty rest ]
        | Bool_ty rest      -> mk_constr "Bool_ty"      [ mk_fmtty rest ]
        | Alpha_ty rest     -> mk_constr "Alpha_ty"     [ mk_fmtty rest ]
        | Theta_ty rest     -> mk_constr "Theta_ty"     [ mk_fmtty rest ]
        | Any_ty rest       -> mk_constr "Any_ty"       [ mk_fmtty rest ]
        | Reader_ty rest    -> mk_constr "Reader_ty"    [ mk_fmtty rest ]
        | Ignored_reader_ty rest ->
          mk_constr "Ignored_reader_ty" [ mk_fmtty rest ]
        | Format_arg_ty (sub_fmtty, rest) ->
          mk_constr "Format_arg_ty" [ mk_fmtty sub_fmtty; mk_fmtty rest ]
        | Format_subst_ty (sub_fmtty1, sub_fmtty2, rest) ->
          mk_constr "Format_subst_ty"
            [ mk_fmtty sub_fmtty1; mk_fmtty sub_fmtty2; mk_fmtty rest ]
        | End_of_fmtty -> mk_constr "End_of_fmtty" []
      and mk_ignored : type a b c d e f .
          (a, b, c, d, e, f) ignored -> Parsetree.expression =
      fun ign -> match ign with
        | Ignored_char ->
          mk_constr "Ignored_char" []
        | Ignored_caml_char ->
          mk_constr "Ignored_caml_char" []
        | Ignored_string pad_opt ->
          mk_constr "Ignored_string" [ mk_int_opt pad_opt ]
        | Ignored_caml_string pad_opt ->
          mk_constr "Ignored_caml_string" [ mk_int_opt pad_opt ]
        | Ignored_int (iconv, pad_opt) ->
          mk_constr "Ignored_int" [ mk_iconv iconv; mk_int_opt pad_opt ]
        | Ignored_int32 (iconv, pad_opt) ->
          mk_constr "Ignored_int32" [ mk_iconv iconv; mk_int_opt pad_opt ]
        | Ignored_nativeint (iconv, pad_opt) ->
          mk_constr "Ignored_nativeint" [ mk_iconv iconv; mk_int_opt pad_opt ]
        | Ignored_int64 (iconv, pad_opt) ->
          mk_constr "Ignored_int64" [ mk_iconv iconv; mk_int_opt pad_opt ]
        | Ignored_float (pad_opt, prec_opt) ->
          mk_constr "Ignored_float" [ mk_int_opt pad_opt; mk_int_opt prec_opt ]
        | Ignored_bool pad_opt ->
          mk_constr "Ignored_bool" [ mk_int_opt pad_opt ]
        | Ignored_format_arg (pad_opt, fmtty) ->
          mk_constr "Ignored_format_arg" [ mk_int_opt pad_opt; mk_fmtty fmtty ]
        | Ignored_format_subst (pad_opt, fmtty) ->
          mk_constr "Ignored_format_subst" [
            mk_int_opt pad_opt; mk_fmtty fmtty ]
        | Ignored_reader ->
          mk_constr "Ignored_reader" []
        | Ignored_scan_char_set (width_opt, char_set) ->
          mk_constr "Ignored_scan_char_set" [
            mk_int_opt width_opt; mk_string char_set ]
        | Ignored_scan_get_counter counter ->
          mk_constr "Ignored_scan_get_counter" [
            mk_counter counter
          ]
        | Ignored_scan_next_char ->
          mk_constr "Ignored_scan_next_char" []
      and mk_padding : type x y . (x, y) padding -> Parsetree.expression =
      fun pad -> match pad with
        | No_padding         -> mk_constr "No_padding" []
        | Lit_padding (s, w) -> mk_constr "Lit_padding" [ mk_side s; mk_int w ]
        | Arg_padding s      -> mk_constr "Arg_padding" [ mk_side s ]
      and mk_precision : type x y . (x, y) precision -> Parsetree.expression =
      fun prec -> match prec with
        | No_precision    -> mk_constr "No_precision" []
        | Lit_precision w -> mk_constr "Lit_precision" [ mk_int w ]
        | Arg_precision   -> mk_constr "Arg_precision" []
      and mk_fmt : type a b c d e f .
          (a, b, c, d, e, f) fmt -> Parsetree.expression =
      fun fmt -> match fmt with
        | Char rest ->
          mk_constr "Char" [ mk_fmt rest ]
        | Caml_char rest ->
          mk_constr "Caml_char" [ mk_fmt rest ]
        | String (pad, rest) ->
          mk_constr "String" [ mk_padding pad; mk_fmt rest ]
        | Caml_string (pad, rest) ->
          mk_constr "Caml_string" [ mk_padding pad; mk_fmt rest ]
        | Int (iconv, pad, prec, rest) ->
          mk_constr "Int" [
            mk_iconv iconv; mk_padding pad; mk_precision prec; mk_fmt rest ]
        | Int32 (iconv, pad, prec, rest) ->
          mk_constr "Int32" [
            mk_iconv iconv; mk_padding pad; mk_precision prec; mk_fmt rest ]
        | Nativeint (iconv, pad, prec, rest) ->
          mk_constr "Nativeint" [
            mk_iconv iconv; mk_padding pad; mk_precision prec; mk_fmt rest ]
        | Int64 (iconv, pad, prec, rest) ->
          mk_constr "Int64" [
            mk_iconv iconv; mk_padding pad; mk_precision prec; mk_fmt rest ]
        | Float (fconv, pad, prec, rest) ->
          mk_constr "Float" [
            mk_fconv fconv; mk_padding pad; mk_precision prec; mk_fmt rest ]
        | Bool (pad, rest) ->
          mk_constr "Bool" [ mk_padding pad; mk_fmt rest ]
        | Flush rest ->
          mk_constr "Flush" [ mk_fmt rest ]
        | String_literal (s, rest) ->
          mk_constr "String_literal" [ mk_string s; mk_fmt rest ]
        | Char_literal (c, rest) ->
          mk_constr "Char_literal" [ mk_char c; mk_fmt rest ]
        | Format_arg (pad_opt, fmtty, rest) ->
          mk_constr "Format_arg" [
            mk_int_opt pad_opt; mk_fmtty fmtty; mk_fmt rest ]
        | Format_subst (pad_opt, fmtty, rest) ->
          mk_constr "Format_subst" [
            mk_int_opt pad_opt; mk_fmtty fmtty; mk_fmt rest ]
        | Alpha rest ->
          mk_constr "Alpha" [ mk_fmt rest ]
        | Theta rest ->
          mk_constr "Theta" [ mk_fmt rest ]
        | Formatting_lit (fmting, rest) ->
          mk_constr "Formatting_lit" [ mk_formatting_lit fmting; mk_fmt rest ]
        | Formatting_gen (fmting, rest) ->
          mk_constr "Formatting_gen" [ mk_formatting_gen fmting; mk_fmt rest ]
        | Reader rest ->
          mk_constr "Reader" [ mk_fmt rest ]
        | Scan_char_set (width_opt, char_set, rest) ->
          mk_constr "Scan_char_set" [
            mk_int_opt width_opt; mk_string char_set; mk_fmt rest ]
        | Scan_get_counter (cnt, rest) ->
          mk_constr "Scan_get_counter" [ mk_counter cnt; mk_fmt rest ]
        | Scan_next_char rest ->
          mk_constr "Scan_next_char" [ mk_fmt rest ]
        | Ignored_param (ign, rest) ->
          mk_constr "Ignored_param" [ mk_ignored ign; mk_fmt rest ]
        | End_of_format ->
          mk_constr "End_of_format" []
        | Custom _ ->
          (* Custom formatters have no syntax so they will never appear
             in formats parsed from strings. *)
          assert false
      in
      let legacy_behavior = not !Clflags.strict_formats in
      let Fmt_EBB fmt = fmt_ebb_of_string ~legacy_behavior str in
      mk_constr "Format" [ mk_fmt fmt; mk_string str ]
    ))
  with Failure msg ->
    raise (Error (loc, env, Invalid_format msg))

and type_label_exp create env loc ty_expected
          (lid, label, sarg) =
  (* Here also ty_expected may be at generic_level *)
  begin_def ();
  let separate = !Clflags.principal || Env.has_local_constraints env in
  if separate then (begin_def (); begin_def ());
  let (vars, ty_arg, ty_res) = instance_label true label in
  if separate then begin
    end_def ();
    (* Generalize label information *)
    generalize_structure ty_arg;
    generalize_structure ty_res
  end;
  begin try
    unify env (instance ty_res) (instance ty_expected)
  with Unify err ->
    raise (Error(lid.loc, env, Label_mismatch(lid.txt, err)))
  end;
  (* Instantiate so that we can generalize internal nodes *)
  let ty_arg = instance ty_arg in
  if separate then begin
    end_def ();
    (* Generalize information merged from ty_expected *)
    generalize_structure ty_arg
  end;
  if label.lbl_private = Private then
    if create then
      raise (Error(loc, env, Private_type ty_expected))
    else
      raise (Error(lid.loc, env, Private_label(lid.txt, ty_expected)));
  let arg =
    let snap = if vars = [] then None else Some (Btype.snapshot ()) in
    let arg = type_argument env sarg ty_arg (instance ty_arg) in
    end_def ();
    try
      if (vars = []) then arg
      else begin
        if maybe_expansive arg then
          lower_contravariant env arg.exp_type;
        generalize_and_check_univars env "field value" arg label.lbl_arg vars;
        {arg with exp_type = instance arg.exp_type}
      end
    with exn when maybe_expansive arg -> try
      (* Try to retype without propagating ty_arg, cf PR#4862 *)
      Option.iter Btype.backtrack snap;
      begin_def ();
      let arg = type_exp env sarg in
      end_def ();
      lower_contravariant env arg.exp_type;
      begin_def ();
      let arg = {arg with exp_type = instance arg.exp_type} in
      unify_exp env arg (instance ty_arg);
      end_def ();
      generalize_and_check_univars env "field value" arg label.lbl_arg vars;
      {arg with exp_type = instance arg.exp_type}
    with Error (_, _, Less_general _) as e -> raise e
    | _ -> raise exn    (* In case of failure return the first error *)
  in
  (lid, label, arg)

and type_argument ?explanation ?recarg env sarg ty_expected' ty_expected =
  (* ty_expected' may be generic *)
  let no_labels ty =
    let ls, tvar = list_labels env ty in
    not tvar && List.for_all ((=) Nolabel) ls
  in
  let may_coerce =
    if not (is_inferred sarg) then None else
    let work () =
      let te = expand_head env ty_expected' in
      match get_desc te with
        Tarrow(Nolabel,_,ty_res0,_) ->
          Some (no_labels ty_res0, get_level te)
      | _ -> None
    in
    (* Need to be careful not to expand local constraints here *)
    if Env.has_local_constraints env then
      let snap = Btype.snapshot () in
      try_finally ~always:(fun () -> Btype.backtrack snap) work
    else work ()
  in
  match may_coerce with
    Some (safe_expect, lv) ->
      (* apply optional arguments when expected type is "" *)
      (* we must be very careful about not breaking the semantics *)
      if !Clflags.principal then begin_def ();
      let texp = type_exp env sarg in
      if !Clflags.principal then begin
        end_def ();
        generalize_structure texp.exp_type
      end;
      let rec make_args args ty_fun =
        match get_desc (expand_head env ty_fun) with
        | Tarrow (l,ty_arg,ty_fun,_) when is_optional l ->
            let ty = option_none env (instance ty_arg) sarg.pexp_loc in
            make_args ((l, Some ty) :: args) ty_fun
        | Tarrow (l,_,ty_res',_) when l = Nolabel || !Clflags.classic ->
            List.rev args, ty_fun, no_labels ty_res'
        | Tvar _ ->  List.rev args, ty_fun, false
        |  _ -> [], texp.exp_type, false
      in
      let args, ty_fun', simple_res = make_args [] texp.exp_type
      and texp = {texp with exp_type = instance texp.exp_type} in
      if not (simple_res || safe_expect) then begin
        unify_exp env texp ty_expected;
        texp
      end else begin
      let warn = !Clflags.principal &&
        (lv <> generic_level || get_level ty_fun' <> generic_level)
      and ty_fun = instance ty_fun' in
      let ty_arg, ty_res =
        match get_desc (expand_head env ty_expected) with
          Tarrow(Nolabel,ty_arg,ty_res,_) -> ty_arg, ty_res
        | _ -> assert false
      in
      unify_exp env {texp with exp_type = ty_fun} ty_expected;
      if args = [] then texp else
      (* eta-expand to avoid side effects *)
      let var_pair name ty =
        let id = Ident.create_local name in
        let desc =
          { val_type = ty; val_kind = Val_reg;
            val_attributes = [];
            val_loc = Location.none;
            val_uid = Uid.mk ~current_unit:(Env.get_unit_name ());
          }
        in
        let exp_env = Env.add_value id desc env in
        {pat_desc = Tpat_var (id, mknoloc name); pat_type = ty;pat_extra=[];
         pat_attributes = [];
         pat_loc = Location.none; pat_env = env},
        {exp_type = ty; exp_loc = Location.none; exp_env = exp_env;
         exp_extra = []; exp_attributes = [];
         exp_desc =
         Texp_ident(Path.Pident id, mknoloc (Longident.Lident name), desc)}
      in
      let eta_pat, eta_var = var_pair "eta" ty_arg in
      let func texp =
        let e =
          {texp with exp_type = ty_res; exp_desc =
           Texp_apply
             (texp,
              args @ [Nolabel, Some eta_var])}
        in
        let cases = [case eta_pat e] in
        let param = name_cases "param" cases in
        { texp with exp_type = ty_fun; exp_desc =
          Texp_function { arg_label = Nolabel; param; cases;
            partial = Total; } }
      in
      Location.prerr_warning texp.exp_loc
        (Warnings.Eliminated_optional_arguments
           (List.map (fun (l, _) -> Printtyp.string_of_label l) args));
      if warn then Location.prerr_warning texp.exp_loc
          (Warnings.Non_principal_labels "eliminated optional argument");
      (* let-expand to have side effects *)
      let let_pat, let_var = var_pair "arg" texp.exp_type in
      re { texp with exp_type = ty_fun; exp_desc =
           Texp_let (Nonrecursive,
                     [{vb_pat=let_pat; vb_expr=texp; vb_attributes=[];
                       vb_loc=Location.none;
                      }],
                     func let_var) }
      end
  | None ->
      let texp = type_expect ?recarg env sarg
        (mk_expected ?explanation ty_expected') in
      unify_exp env texp ty_expected;
      texp

and type_application env funct sargs =
  (* funct.exp_type may be generic *)
  let result_type omitted ty_fun =
    List.fold_left
      (fun ty_fun (l,ty,lv) -> newty2 ~level:lv (Tarrow(l,ty,ty_fun,commu_ok)))
      ty_fun omitted
  in
  let has_label l ty_fun =
    let ls, tvar = list_labels env ty_fun in
    tvar || List.mem l ls
  in
  let eliminated_optional_arguments = ref [] in
  let omitted_parameters = ref [] in
  let type_unknown_arg (ty_fun, typed_args) (lbl, sarg) =
    let (ty_arg, ty_res) =
      let ty_fun = expand_head env ty_fun in
      match get_desc ty_fun with
      | Tvar _ ->
          let t1 = newvar () and t2 = newvar () in
          if get_level ty_fun >= get_level t1 &&
             not (is_prim ~name:"%identity" funct)
          then
            Location.prerr_warning sarg.pexp_loc
              Warnings.Ignored_extra_argument;
          unify env ty_fun (newty (Tarrow(lbl,t1,t2,commu_var ())));
          (t1, t2)
      | Tarrow (l,t1,t2,_) when l = lbl
        || !Clflags.classic && lbl = Nolabel && not (is_optional l) ->
          (t1, t2)
      | td ->
          let ty_fun = match td with Tarrow _ -> newty td | _ -> ty_fun in
          let ty_res =
            result_type (!omitted_parameters @ !eliminated_optional_arguments)
              ty_fun
          in
          match get_desc ty_res with
          | Tarrow _ ->
              if !Clflags.classic || not (has_label lbl ty_fun) then
                raise (Error(sarg.pexp_loc, env,
                             Apply_wrong_label(lbl, ty_res, false)))
              else
                raise (Error(funct.exp_loc, env, Incoherent_label_order))
          | _ ->
              raise(Error(funct.exp_loc, env, Apply_non_function
                            (expand_head env funct.exp_type)))
    in
    let arg () =
      let arg = type_expect env sarg (mk_expected ty_arg) in
      if is_optional lbl then
        unify_exp env arg (type_option(newvar()));
      arg
    in
    (ty_res, (lbl, Some arg) :: typed_args)
  in
  let ignore_labels =
    !Clflags.classic ||
    begin
      let ls, tvar = list_labels env funct.exp_type in
      not tvar &&
      let labels = List.filter (fun l -> not (is_optional l)) ls in
      List.length labels = List.length sargs &&
      List.for_all (fun (l,_) -> l = Nolabel) sargs &&
      List.exists (fun l -> l <> Nolabel) labels &&
      (Location.prerr_warning
         funct.exp_loc
         (Warnings.Labels_omitted
            (List.map Printtyp.string_of_label
                      (List.filter ((<>) Nolabel) labels)));
       true)
    end
  in
  let warned = ref false in
  let rec type_args args ty_fun ty_fun0 sargs =
    let type_unknown_args () =
      (* We're not looking at a *known* function type anymore, or there are no
         arguments left. *)
      let ty_fun, typed_args =
        List.fold_left type_unknown_arg (ty_fun0, args) sargs
      in
      let args =
        (* Force typing of arguments.
           Careful: the order matters here. Using [List.rev_map] would be
           incorrect. *)
        List.map
          (function
            | l, None -> l, None
            | l, Some f -> l, Some (f ()))
          (List.rev typed_args)
      in
      let result_ty = instance (result_type !omitted_parameters ty_fun) in
      args, result_ty
    in
    if sargs = [] then type_unknown_args () else
    let ty_fun' = expand_head env ty_fun in
    match get_desc ty_fun', get_desc (expand_head env ty_fun0) with
    | Tarrow (l, ty, ty_fun, com), Tarrow (_, ty0, ty_fun0, _)
      when is_commu_ok com ->
        let lv = get_level ty_fun' in
        let may_warn loc w =
          if not !warned && !Clflags.principal && lv <> generic_level
          then begin
            warned := true;
            Location.prerr_warning loc w
          end
        in
        let name = label_name l
        and optional = is_optional l in
        let use_arg sarg l' =
          Some (
            if not optional || is_optional l' then
              (fun () -> type_argument env sarg ty ty0)
            else begin
              may_warn sarg.pexp_loc
                (Warnings.Not_principal "using an optional argument here");
              (fun () -> option_some env (type_argument env sarg
                                            (extract_option_type env ty)
                                            (extract_option_type env ty0)))
            end
          )
        in
        let eliminate_optional_arg () =
          may_warn funct.exp_loc
            (Warnings.Non_principal_labels "eliminated optional argument");
          eliminated_optional_arguments :=
            (l,ty,lv) :: !eliminated_optional_arguments;
          Some (fun () -> option_none env (instance ty) Location.none)
        in
        let remaining_sargs, arg =
          if ignore_labels then begin
            (* No reordering is allowed, process arguments in order *)
            match sargs with
            | [] -> assert false
            | (l', sarg) :: remaining_sargs ->
                if name = label_name l' || (not optional && l' = Nolabel) then
                  (remaining_sargs, use_arg sarg l')
                else if
                  optional &&
                  not (List.exists (fun (l, _) -> name = label_name l)
                         remaining_sargs) &&
                  List.exists (function (Nolabel, _) -> true | _ -> false)
                    sargs
                then
                  (sargs, eliminate_optional_arg ())
                else
                  raise(Error(sarg.pexp_loc, env,
                              Apply_wrong_label(l', ty_fun', optional)))
          end else
            (* Arguments can be commuted, try to fetch the argument
               corresponding to the first parameter. *)
            match extract_label name sargs with
            | Some (l', sarg, commuted, remaining_sargs) ->
                if commuted then begin
                  may_warn sarg.pexp_loc
                    (Warnings.Not_principal "commuting this argument")
                end;
                if not optional && is_optional l' then
                  Location.prerr_warning sarg.pexp_loc
                    (Warnings.Nonoptional_label (Printtyp.string_of_label l));
                remaining_sargs, use_arg sarg l'
            | None ->
                sargs,
                if optional && List.mem_assoc Nolabel sargs then
                  eliminate_optional_arg ()
                else begin
                  (* No argument was given for this parameter, we abstract over
                     it. *)
                  may_warn funct.exp_loc
                    (Warnings.Non_principal_labels "commuted an argument");
                  omitted_parameters := (l,ty,lv) :: !omitted_parameters;
                  None
                end
        in
        type_args ((l,arg)::args) ty_fun ty_fun0 remaining_sargs
    | _ ->
        type_unknown_args ()
  in
  let is_ignore funct =
    is_prim ~name:"%ignore" funct &&
    (try ignore (filter_arrow env (instance funct.exp_type) Nolabel); true
     with Filter_arrow_failed _ -> false)
  in
  match sargs with
  | (* Special case for ignore: avoid discarding warning *)
    [Nolabel, sarg] when is_ignore funct ->
      let ty_arg, ty_res = filter_arrow env (instance funct.exp_type) Nolabel in
      let exp = type_expect env sarg (mk_expected ty_arg) in
      check_partial_application ~statement:false exp;
      ([Nolabel, Some exp], ty_res)
  | _ ->
    let ty = funct.exp_type in
    type_args [] ty (instance ty) sargs

and type_construct env loc lid sarg ty_expected_explained attrs =
  let { ty = ty_expected; explanation } = ty_expected_explained in
  let expected_type =
    match extract_concrete_variant env ty_expected with
    | Variant_type(p0, p,_) ->
        Some(p0, p, is_principal ty_expected)
    | Maybe_a_variant_type -> None
    | Not_a_variant_type ->
        let srt = wrong_kind_sort_of_constructor lid.txt in
        let ctx = Expression explanation in
        let error = Wrong_expected_kind(srt, ctx, ty_expected) in
        raise (Error (loc, env, error))
  in
  let constrs =
    Env.lookup_all_constructors ~loc:lid.loc Env.Positive lid.txt env
  in
  let constr =
    wrap_disambiguate "This variant expression is expected to have"
      ty_expected_explained
      (Constructor.disambiguate Env.Positive lid env expected_type) constrs
  in
  let sargs =
    match sarg with
      None -> []
    | Some {pexp_desc = Pexp_tuple sel} when
        constr.cstr_arity > 1 || Builtin_attributes.explicit_arity attrs
      -> sel
    | Some se -> [se] in
  if List.length sargs <> constr.cstr_arity then
    raise(Error(loc, env, Constructor_arity_mismatch
                            (lid.txt, constr.cstr_arity, List.length sargs)));
  let separate = !Clflags.principal || Env.has_local_constraints env in
  if separate then (begin_def (); begin_def ());
  let (ty_args, ty_res, _) =
    instance_constructor Keep_existentials_flexible constr
  in
  let texp =
    re {
      exp_desc = Texp_construct(lid, constr, []);
      exp_loc = loc; exp_extra = [];
      exp_type = ty_res;
      exp_attributes = attrs;
      exp_env = env } in
  if separate then begin
    end_def ();
    generalize_structure ty_res;
    with_explanation explanation (fun () ->
      unify_exp env {texp with exp_type = instance ty_res}
        (instance ty_expected));
    end_def ();
    List.iter generalize_structure ty_args;
    generalize_structure ty_res;
  end;
  let ty_args0, ty_res =
    match instance_list (ty_res :: ty_args) with
      t :: tl -> tl, t
    | _ -> assert false
  in
  let texp = {texp with exp_type = ty_res} in
  if not separate then unify_exp env texp (instance ty_expected);
  let recarg =
    match constr.cstr_inlined with
    | None -> Rejected
    | Some _ ->
      begin match sargs with
      | [{pexp_desc =
            Pexp_ident _ |
            Pexp_record (_, (Some {pexp_desc = Pexp_ident _}| None))}] ->
        Required
      | _ ->
        raise (Error(loc, env, Inlined_record_expected))
      end
  in
  let args =
    List.map2 (fun e (t,t0) -> type_argument ~recarg env e t t0) sargs
      (List.combine ty_args ty_args0) in
  if constr.cstr_private = Private then
    begin match constr.cstr_tag with
    | Cstr_extension _ ->
        raise(Error(loc, env, Private_constructor (constr, ty_res)))
    | Cstr_constant _ | Cstr_block _ | Cstr_unboxed ->
        raise (Error(loc, env, Private_type ty_res));
    end;
  (* NOTE: shouldn't we call "re" on this final expression? -- AF *)
  { texp with
    exp_desc = Texp_construct(lid, constr, args) }

(* Typing of statements (expressions whose values are discarded) *)

and type_statement ?explanation env sexp =
  begin_def();
  let exp = type_exp env sexp in
  end_def();
  let ty = expand_head env exp.exp_type and tv = newvar() in
  if is_Tvar ty && get_level ty > get_level tv then
    Location.prerr_warning
      (final_subexpression exp).exp_loc
      Warnings.Nonreturning_statement;
  if !Clflags.strict_sequence then
    let expected_ty = instance Predef.type_unit in
    with_explanation explanation (fun () ->
      unify_exp env exp expected_ty);
    exp
  else begin
    check_partial_application ~statement:true exp;
    unify_var env tv ty;
    exp
  end

and type_unpacks ?(in_function : (Location.t * type_expr) option)
    env (unpacks : to_unpack list) sbody expected_ty =
  if unpacks = [] then type_expect ?in_function env sbody expected_ty else
  let ty = newvar() in
  (* remember original level *)
  let extended_env, tunpacks =
    List.fold_left (fun (env, tunpacks) unpack ->
      begin_def ();
      let context = Typetexp.narrow () in
      let modl, md_shape =
        !type_module env
          Ast_helper.(
            Mod.unpack ~loc:unpack.tu_loc
              (Exp.ident ~loc:unpack.tu_name.loc
                 (mkloc (Longident.Lident unpack.tu_name.txt)
                    unpack.tu_name.loc)))
      in
      Mtype.lower_nongen (get_level ty) modl.mod_type;
      let pres =
        match modl.mod_type with
        | Mty_alias _ -> Mp_absent
        | _ -> Mp_present
      in
      let scope = create_scope () in
      let md =
        { md_type = modl.mod_type; md_attributes = [];
          md_loc = unpack.tu_name.loc;
          md_uid = unpack.tu_uid; }
      in
      let (id, env) =
        Env.enter_module_declaration ~scope ~shape:md_shape
          unpack.tu_name.txt pres md env
      in
      Typetexp.widen context;
      env, (id, unpack.tu_name, pres, modl) :: tunpacks
    ) (env, []) unpacks
  in
  (* ideally, we should catch Expr_type_clash errors
     in type_expect triggered by escaping identifiers from the local module
     and refine them into Scoping_let_module errors
  *)
  let body = type_expect ?in_function extended_env sbody expected_ty in
  let exp_loc = { body.exp_loc with loc_ghost = true } in
  let exp_attributes = [Ast_helper.Attr.mk (mknoloc "#modulepat") (PStr [])] in
  List.fold_left (fun body (id, name, pres, modl) ->
    (* go back to parent level *)
    end_def ();
    Ctype.unify_var extended_env ty body.exp_type;
    re {
      exp_desc = Texp_letmodule(Some id, { name with txt = Some name.txt },
                                pres, modl, body);
      exp_loc;
      exp_attributes;
      exp_extra = [];
      exp_type = ty;
      exp_env = env }
  ) body tunpacks

(* Typing of match cases *)
and type_cases
    : type k . k pattern_category ->
           ?in_function:_ -> _ -> _ -> _ -> _ -> _ -> Parsetree.case list ->
           k case list * partial
  = fun category ?in_function env
        ty_arg ty_res_explained partial_flag loc caselist ->
  (* ty_arg is _fully_ generalized *)
  let { ty = ty_res; explanation } = ty_res_explained in
  let patterns = List.map (fun {pc_lhs=p} -> p) caselist in
  let contains_polyvars = List.exists contains_polymorphic_variant patterns in
  let erase_either = contains_polyvars && contains_variant_either ty_arg in
  let may_contain_gadts = List.exists may_contain_gadts patterns in
  let ty_arg =
    if (may_contain_gadts || erase_either) && not !Clflags.principal
    then correct_levels ty_arg else ty_arg
  in
  let rec is_var spat =
    match spat.ppat_desc with
      Ppat_any | Ppat_var _ -> true
    | Ppat_alias (spat, _) -> is_var spat
    | _ -> false in
  let needs_exhaust_check =
    match caselist with
      [{pc_rhs = {pexp_desc = Pexp_unreachable}}] -> true
    | [{pc_lhs}] when is_var pc_lhs -> false
    | _ -> true
  in
  let outer_level = get_current_level () in
  let lev =
    if may_contain_gadts then begin_def ();
    get_current_level ()
  in
  let take_partial_instance =
    if erase_either
    then Some false else None
  in
  begin_def (); (* propagation of the argument *)
  let pattern_force = ref [] in
(*  Format.printf "@[%i %i@ %a@]@." lev (get_current_level())
    Printtyp.raw_type_expr ty_arg; *)
  let half_typed_cases =
    List.map
      (fun ({pc_lhs; pc_guard = _; pc_rhs = _} as case) ->
        if !Clflags.principal then begin_def (); (* propagation of pattern *)
        begin_def ();
        let ty_arg = instance ?partial:take_partial_instance ty_arg in
        end_def ();
        generalize_structure ty_arg;
        let (pat, ext_env, force, pvs, unpacks) =
          type_pattern category ~lev env pc_lhs ty_arg
        in
        pattern_force := force @ !pattern_force;
        let pat =
          if !Clflags.principal then begin
            end_def ();
            iter_pattern_variables_type generalize_structure pvs;
            { pat with pat_type = instance pat.pat_type }
          end else pat
        in
        (* Ensure that no ambivalent pattern type escapes its branch *)
        check_scope_escape pat.pat_loc env outer_level ty_arg;
        { typed_pat = pat;
          pat_type_for_unif = ty_arg;
          untyped_case = case;
          branch_env = ext_env;
          pat_vars = pvs;
          unpacks;
          contains_gadt = contains_gadt (as_comp_pattern category pat); }
        )
      caselist in
  let patl = List.map (fun { typed_pat; _ } -> typed_pat) half_typed_cases in
  let does_contain_gadt =
    List.exists (fun { contains_gadt; _ } -> contains_gadt) half_typed_cases
  in
  let ty_res, do_copy_types =
    if does_contain_gadt && not !Clflags.principal then
      correct_levels ty_res, Env.make_copy_of_types env
    else ty_res, (fun env -> env)
  in
  (* Unify all cases (delayed to keep it order-free) *)
  let ty_arg' = newvar () in
  let unify_pats ty =
    List.iter (fun { typed_pat = pat; pat_type_for_unif = pat_ty; _ } ->
      unify_pat_types pat.pat_loc (ref env) pat_ty ty
    ) half_typed_cases
  in
  unify_pats ty_arg';
  (* Check for polymorphic variants to close *)
  if List.exists has_variants patl then begin
      Parmatch.pressure_variants_in_computation_pattern env
        (List.map (as_comp_pattern category) patl);
      List.iter finalize_variants patl
  end;
  (* `Contaminating' unifications start here *)
  List.iter (fun f -> f()) !pattern_force;
  (* Post-processing and generalization *)
  if take_partial_instance <> None then unify_pats (instance ty_arg);
  List.iter (fun { pat_vars; _ } ->
    iter_pattern_variables_type (fun t -> unify_var env (newvar()) t) pat_vars
  ) half_typed_cases;
  end_def ();
  generalize ty_arg';
  List.iter (fun { pat_vars; _ } ->
    iter_pattern_variables_type generalize pat_vars
  ) half_typed_cases;
  (* type bodies *)
  let in_function = if List.length caselist = 1 then in_function else None in
  let ty_res' = instance ty_res in
  if !Clflags.principal then begin_def ();
  let cases =
    List.map
      (fun { typed_pat = pat; branch_env = ext_env; pat_vars = pvs; unpacks;
             untyped_case = {pc_lhs = _; pc_guard; pc_rhs};
             contains_gadt; _ }  ->
        let ext_env =
          if contains_gadt then
            do_copy_types ext_env
          else
            ext_env
        in
        let ext_env =
          add_pattern_variables ext_env pvs
            ~check:(fun s -> Warnings.Unused_var_strict s)
            ~check_as:(fun s -> Warnings.Unused_var s)
        in
        let unpacks =
          List.map (fun (name, loc) ->
            {tu_name = name; tu_loc = loc;
             tu_uid = Uid.mk ~current_unit:(Env.get_unit_name ())}
          ) unpacks
        in
        let ty_expected =
          if contains_gadt && not !Clflags.principal then
            (* allow propagation from preceding branches *)
            correct_levels ty_res
          else ty_res in
        let guard =
          match pc_guard with
          | None -> None
          | Some scond ->
              Some
                (type_unpacks ext_env unpacks scond
                   (mk_expected ~explanation:When_guard Predef.type_bool))
        in
        let exp =
          type_unpacks ?in_function ext_env
            unpacks pc_rhs (mk_expected ?explanation ty_expected)
        in
        {
         c_lhs = pat;
         c_guard = guard;
         c_rhs = {exp with exp_type = ty_res'}
        }
      )
      half_typed_cases
  in
  if !Clflags.principal then end_def ();
  let do_init = may_contain_gadts || needs_exhaust_check in
  let ty_arg_check =
    if do_init then
      (* Hack: use for_saving to copy variables too *)
      Subst.type_expr (Subst.for_saving Subst.identity) ty_arg'
    else ty_arg'
  in
  let val_cases, exn_cases =
    match category with
      | Value -> (cases : value case list), []
      | Computation -> split_cases env cases in
  if val_cases = [] && exn_cases <> [] then
    raise (Error (loc, env, No_value_clauses));
  let partial =
    if partial_flag then
      check_partial ~lev env ty_arg_check loc val_cases
    else
      Partial
  in
  let unused_check delayed =
    List.iter (fun { typed_pat; branch_env; _ } ->
      check_absent_variant branch_env (as_comp_pattern category typed_pat)
    ) half_typed_cases;
    if delayed then (begin_def (); init_def lev);
    check_unused ~lev env ty_arg_check val_cases ;
    check_unused ~lev env Predef.type_exn exn_cases ;
    if delayed then end_def ();
    Parmatch.check_ambiguous_bindings val_cases ;
    Parmatch.check_ambiguous_bindings exn_cases
  in
  if contains_polyvars then
    add_delayed_check (fun () -> unused_check true)
  else
    (* Check for unused cases, do not delay because of gadts *)
    unused_check false;
  if may_contain_gadts then begin
    end_def ();
    (* Ensure that existential types do not escape *)
    unify_exp_types loc env ty_res' (newvar ()) ;
  end;
  cases, partial

(* Typing of let bindings *)

and type_let
    ?(check = fun s -> Warnings.Unused_var s)
    ?(check_strict = fun s -> Warnings.Unused_var_strict s)
    existential_context
    env rec_flag spat_sexp_list allow =
  let open Ast_helper in
  begin_def();
  if !Clflags.principal then begin_def ();

  let is_fake_let =
    match spat_sexp_list with
    | [{pvb_expr={pexp_desc=Pexp_match(
           {pexp_desc=Pexp_ident({ txt = Longident.Lident "*opt*"})},_)}}] ->
        true (* the fake let-declaration introduced by fun ?(x = e) -> ... *)
    | _ ->
        false
  in
  let check = if is_fake_let then check_strict else check in

  let spatl =
    List.map
      (fun {pvb_pat=spat; pvb_expr=sexp; pvb_attributes=attrs} ->
        attrs,
        match spat.ppat_desc, sexp.pexp_desc with
          (Ppat_any | Ppat_constraint _), _ -> spat
        | _, Pexp_coerce (_, _, sty)
        | _, Pexp_constraint (_, sty) when !Clflags.principal ->
            (* propagate type annotation to pattern,
               to allow it to be generalized in -principal mode *)
            Pat.constraint_
              ~loc:{spat.ppat_loc with Location.loc_ghost=true}
              spat
              sty
        | _ -> spat)
      spat_sexp_list in
  let nvs = List.map (fun _ -> newvar ()) spatl in
  let (pat_list, new_env, force, pvs, unpacks) =
    type_pattern_list Value existential_context env spatl nvs allow in
  let attrs_list = List.map fst spatl in
  let is_recursive = (rec_flag = Recursive) in
  (* If recursive, first unify with an approximation of the expression *)
  if is_recursive then
    List.iter2
      (fun pat binding ->
        let pat =
          match get_desc pat.pat_type with
          | Tpoly (ty, tl) ->
              {pat with pat_type =
               snd (instance_poly ~keep_names:true false tl ty)}
          | _ -> pat
        in unify_pat (ref env) pat (type_approx env binding.pvb_expr))
      pat_list spat_sexp_list;
  (* Polymorphic variant processing *)
  List.iter
    (fun pat ->
      if has_variants pat then begin
        Parmatch.pressure_variants env [pat];
        finalize_variants pat
      end)
    pat_list;
  (* Generalize the structure *)
  let pat_list =
    if !Clflags.principal then begin
      end_def ();
      iter_pattern_variables_type generalize_structure pvs;
      List.map (fun pat ->
        generalize_structure pat.pat_type;
        {pat with pat_type = instance pat.pat_type}
      ) pat_list
    end else
      pat_list
  in
  (* Only bind pattern variables after generalizing *)
  List.iter (fun f -> f()) force;
  let sexp_is_fun { pvb_expr = sexp; _ } =
    match sexp.pexp_desc with
    | Pexp_fun _ | Pexp_function _ -> true
    | _ -> false
  in
  let exp_env =
    if is_recursive then new_env
    else if List.for_all sexp_is_fun spat_sexp_list
    then begin
      (* Add ghost bindings to help detecting missing "rec" keywords.

         We only add those if the body of the definition is obviously a
         function. The rationale is that, in other cases, the hint is probably
         wrong (and the user is using "advanced features" anyway (lazy,
         recursive values...)).

         [pvb_loc] (below) is the location of the first let-binding (in case of
         a let .. and ..), and is where the missing "rec" hint suggests to add a
         "rec" keyword. *)
      match spat_sexp_list with
      | {pvb_loc; _} :: _ -> maybe_add_pattern_variables_ghost pvb_loc env pvs
      | _ -> assert false
    end
    else env in

  let current_slot = ref None in
  let rec_needed = ref false in
  let warn_about_unused_bindings =
    List.exists
      (fun attrs ->
         Builtin_attributes.warning_scope ~ppwarning:false attrs (fun () ->
           Warnings.is_active (check "") || Warnings.is_active (check_strict "")
           || (is_recursive && (Warnings.is_active Warnings.Unused_rec_flag))))
      attrs_list
  in
  let pat_slot_list =
    (* Algorithm to detect unused declarations in recursive bindings:
       - During type checking of the definitions, we capture the 'value_used'
         events on the bound identifiers and record them in a slot corresponding
         to the current definition (!current_slot).
         In effect, this creates a dependency graph between definitions.

       - After type checking the definition (!current_slot = None),
         when one of the bound identifier is effectively used, we trigger
         again all the events recorded in the corresponding slot.
         The effect is to traverse the transitive closure of the graph created
         in the first step.

       We also keep track of whether *all* variables in a given pattern
       are unused. If this is the case, for local declarations, the issued
       warning is 26, not 27.
     *)
    List.map2
      (fun attrs pat ->
         Builtin_attributes.warning_scope ~ppwarning:false attrs (fun () ->
           if not warn_about_unused_bindings then pat, None
           else
             let some_used = ref false in
             (* has one of the identifier of this pattern been used? *)
             let slot = ref [] in
             List.iter
               (fun id ->
                  let vd = Env.find_value (Path.Pident id) new_env in
                  (* note: Env.find_value does not trigger the value_used
                           event *)
                  let name = Ident.name id in
                  let used = ref false in
                  if not (name = "" || name.[0] = '_' || name.[0] = '#') then
                    add_delayed_check
                      (fun () ->
                         if not !used then
                           Location.prerr_warning vd.Types.val_loc
                             ((if !some_used then check_strict else check) name)
                      );
                  Env.set_value_used_callback
                    vd
                    (fun () ->
                       match !current_slot with
                       | Some slot ->
                         slot := vd.val_uid :: !slot; rec_needed := true
                       | None ->
                         List.iter Env.mark_value_used (get_ref slot);
                         used := true;
                         some_used := true
                    )
               )
               (Typedtree.pat_bound_idents pat);
             pat, Some slot
         ))
      attrs_list
      pat_list
  in
  let exp_list =
    List.map2
      (fun {pvb_expr=sexp; pvb_attributes; _} (pat, slot) ->
        if is_recursive then current_slot := slot;
        match get_desc pat.pat_type with
        | Tpoly (ty, tl) ->
            if !Clflags.principal then begin_def ();
            let vars, ty' = instance_poly ~keep_names:true true tl ty in
            if !Clflags.principal then begin
              end_def ();
              generalize_structure ty'
            end;
            let exp =
              Builtin_attributes.warning_scope pvb_attributes (fun () ->
                if rec_flag = Recursive then
                  type_unpacks exp_env unpacks sexp (mk_expected ty')
                else
                  type_expect exp_env sexp (mk_expected ty')
              )
            in
            exp, Some vars
        | _ ->
            let exp =
              Builtin_attributes.warning_scope pvb_attributes (fun () ->
                  if rec_flag = Recursive then
                    type_unpacks exp_env unpacks sexp (mk_expected pat.pat_type)
                  else
                    type_expect exp_env sexp (mk_expected pat.pat_type))
            in
            exp, None)
      spat_sexp_list pat_slot_list in
  current_slot := None;
  if is_recursive && not !rec_needed then begin
    let {pvb_pat; pvb_attributes} = List.hd spat_sexp_list in
    (* See PR#6677 *)
    Builtin_attributes.warning_scope ~ppwarning:false pvb_attributes
      (fun () ->
         Location.prerr_warning pvb_pat.ppat_loc Warnings.Unused_rec_flag
      )
  end;
  List.iter2
    (fun pat (attrs, exp) ->
       Builtin_attributes.warning_scope ~ppwarning:false attrs
         (fun () ->
            ignore(check_partial env pat.pat_type pat.pat_loc
                     [case pat exp])
         )
    )
    pat_list
    (List.map2 (fun (attrs, _) (e, _) -> attrs, e) spatl exp_list);
  let pvs = List.map (fun pv -> { pv with pv_type = instance pv.pv_type}) pvs in
  end_def();
  List.iter2
    (fun pat (exp, _) ->
       if maybe_expansive exp then
         lower_contravariant env pat.pat_type)
    pat_list exp_list;
  iter_pattern_variables_type generalize pvs;
  List.iter2
    (fun pat (exp, vars) ->
       match vars with
       | None ->
         (* We generalize expressions even if they are not bound to a variable
            and do not have an expliclit polymorphic type annotation.  This is
            not needed in general, however those types may be shown by the
            interactive toplevel, for example:
            {[
              let _ = Array.get;;
              - : 'a array -> int -> 'a = <fun>
            ]}
            so we do it anyway. *)
         generalize exp.exp_type
       | Some vars ->
           if maybe_expansive exp then
             lower_contravariant env exp.exp_type;
           generalize_and_check_univars env "definition" exp pat.pat_type vars)
    pat_list exp_list;
  let l = List.combine pat_list exp_list in
  let l =
    List.map2
      (fun (p, (e, _)) pvb ->
        {vb_pat=p; vb_expr=e; vb_attributes=pvb.pvb_attributes;
         vb_loc=pvb.pvb_loc;
        })
      l spat_sexp_list
  in
  if is_recursive then
    List.iter
      (fun {vb_pat=pat} -> match pat.pat_desc with
           Tpat_var _ -> ()
         | Tpat_alias ({pat_desc=Tpat_any}, _, _) -> ()
         | _ -> raise(Error(pat.pat_loc, env, Illegal_letrec_pat)))
      l;
  List.iter (function
      | {vb_pat = {pat_desc = Tpat_any; pat_extra; _}; vb_expr; _} ->
          if not (List.exists (function (Tpat_constraint _, _, _) -> true
                                      | _ -> false) pat_extra) then
            check_partial_application ~statement:false vb_expr
      | _ -> ()) l;
  (l, new_env, unpacks)

and type_andops env sarg sands expected_ty =
  let rec loop env let_sarg rev_sands expected_ty =
    match rev_sands with
    | [] -> type_expect env let_sarg (mk_expected expected_ty), []
    | { pbop_op = sop; pbop_exp = sexp; pbop_loc = loc; _ } :: rest ->
        if !Clflags.principal then begin_def ();
        let op_path, op_desc = type_binding_op_ident env sop in
        let op_type = instance op_desc.val_type in
        let ty_arg = newvar () in
        let ty_rest = newvar () in
        let ty_result = newvar() in
        let ty_rest_fun =
          newty (Tarrow(Nolabel, ty_arg, ty_result, commu_ok)) in
        let ty_op = newty (Tarrow(Nolabel, ty_rest, ty_rest_fun, commu_ok)) in
        begin try
          unify env op_type ty_op
        with Unify err ->
          raise(Error(sop.loc, env, Andop_type_clash(sop.txt, err)))
        end;
        if !Clflags.principal then begin
          end_def ();
          generalize_structure ty_rest;
          generalize_structure ty_arg;
          generalize_structure ty_result
        end;
        let let_arg, rest = loop env let_sarg rest ty_rest in
        let exp = type_expect env sexp (mk_expected ty_arg) in
        begin try
          unify env (instance ty_result) (instance expected_ty)
        with Unify err ->
          raise(Error(loc, env, Bindings_type_clash(err)))
        end;
        let andop =
          { bop_op_name = sop;
            bop_op_path = op_path;
            bop_op_val = op_desc;
            bop_op_type = op_type;
            bop_exp = exp;
            bop_loc = loc }
        in
        let_arg, andop :: rest
  in
  let let_arg, rev_ands = loop env sarg (List.rev sands) expected_ty in
  let_arg, List.rev rev_ands

(* Typing of toplevel bindings *)

let type_binding env rec_flag spat_sexp_list =
  Typetexp.reset_type_variables();
  let (pat_exp_list, new_env, _unpacks) =
    type_let
      ~check:(fun s -> Warnings.Unused_value_declaration s)
      ~check_strict:(fun s -> Warnings.Unused_value_declaration s)
      At_toplevel
      env rec_flag spat_sexp_list false
  in
  (pat_exp_list, new_env)

let type_let existential_ctx env rec_flag spat_sexp_list =
  let (pat_exp_list, new_env, _unpacks) =
    type_let existential_ctx env rec_flag spat_sexp_list false in
  (pat_exp_list, new_env)

(* Typing of toplevel expressions *)

let type_expression env sexp =
  Typetexp.reset_type_variables();
  begin_def();
  let exp = type_exp env sexp in
  end_def();
  if maybe_expansive exp then lower_contravariant env exp.exp_type;
  generalize exp.exp_type;
  match sexp.pexp_desc with
    Pexp_ident lid ->
      let loc = sexp.pexp_loc in
      (* Special case for keeping type variables when looking-up a variable *)
      let (_path, desc) = Env.lookup_value ~use:false ~loc lid.txt env in
      {exp with exp_type = desc.val_type}
  | _ -> exp

(* Error report *)

let spellcheck ppf unbound_name valid_names =
  Misc.did_you_mean ppf (fun () ->
    Misc.spellcheck valid_names unbound_name
  )

let spellcheck_idents ppf unbound valid_idents =
  spellcheck ppf (Ident.name unbound) (List.map Ident.name valid_idents)

open Format

let longident = Printtyp.longident

(* Returns the first diff of the trace *)
let type_clash_of_trace trace =
  Errortrace.(explain trace (fun ~prev:_ -> function
    | Diff diff -> Some diff
    | _ -> None
  ))

(* Hint on type error on integer literals
   To avoid confusion, it is disabled on float literals
   and when the expected type is `int` *)
let report_literal_type_constraint expected_type const =
  let const_str = match const with
    | Const_int n -> Some (Int.to_string n)
    | Const_int32 n -> Some (Int32.to_string n)
    | Const_int64 n -> Some (Int64.to_string n)
    | Const_nativeint n -> Some (Nativeint.to_string n)
    | _ -> None
  in
  let suffix =
    if Path.same expected_type Predef.path_int32 then
      Some 'l'
    else if Path.same expected_type Predef.path_int64 then
      Some 'L'
    else if Path.same expected_type Predef.path_nativeint then
      Some 'n'
    else if Path.same expected_type Predef.path_float then
      Some '.'
    else None
  in
  match const_str, suffix with
  | Some c, Some s -> [ Location.msg "@[Hint: Did you mean `%s%c'?@]" c s ]
  | _, _ -> []

let report_literal_type_constraint const = function
  | Some tr ->
      begin match get_desc Errortrace.(tr.expected.ty) with
        Tconstr (typ, [], _) ->
          report_literal_type_constraint typ const
      | _ -> []
      end
  | None -> []

let report_expr_type_clash_hints exp diff =
  match exp with
  | Some (Texp_constant const) -> report_literal_type_constraint const diff
  | _ -> []

let report_pattern_type_clash_hints
      (type k) (pat : k pattern_desc option) diff =
  match pat with
  | Some (Tpat_constant const) -> report_literal_type_constraint const diff
  | _ -> []

let report_type_expected_explanation expl ppf =
  let because expl_str = fprintf ppf "@ because it is in %s" expl_str in
  match expl with
  | If_conditional ->
      because "the condition of an if-statement"
  | If_no_else_branch ->
      because "the result of a conditional with no else branch"
  | While_loop_conditional ->
      because "the condition of a while-loop"
  | While_loop_body ->
      because "the body of a while-loop"
  | For_loop_start_index ->
      because "a for-loop start index"
  | For_loop_stop_index ->
      because "a for-loop stop index"
  | For_loop_body ->
      because "the body of a for-loop"
  | Assert_condition ->
      because "the condition of an assertion"
  | Sequence_left_hand_side ->
      because "the left-hand side of a sequence"
  | When_guard ->
      because "a when-guard"

let report_type_expected_explanation_opt expl ppf =
  match expl with
  | None -> ()
  | Some expl -> report_type_expected_explanation expl ppf

let report_unification_error ~loc ?sub env err
    ?type_expected_explanation txt1 txt2 =
  Location.error_of_printer ~loc ?sub (fun ppf () ->
    Printtyp.report_unification_error ppf env err
      ?type_expected_explanation txt1 txt2
  ) ()

let report_error ~loc env = function
  | Constructor_arity_mismatch(lid, expected, provided) ->
      Location.errorf ~loc
       "@[The constructor %a@ expects %i argument(s),@ \
        but is applied here to %i argument(s)@]"
       longident lid expected provided
  | Label_mismatch(lid, err) ->
      report_unification_error ~loc env err
        (function ppf ->
           fprintf ppf "The record field %a@ belongs to the type"
                   longident lid)
        (function ppf ->
           fprintf ppf "but is mixed here with fields of type")
  | Pattern_type_clash (err, pat) ->
      let diff = type_clash_of_trace err.trace in
      let sub = report_pattern_type_clash_hints pat diff in
      report_unification_error ~loc ~sub env err
        (function ppf ->
          fprintf ppf "This pattern matches values of type")
        (function ppf ->
          fprintf ppf "but a pattern was expected which matches values of \
                       type");
  | Or_pattern_type_clash (id, err) ->
      report_unification_error ~loc env err
        (function ppf ->
          fprintf ppf "The variable %s on the left-hand side of this \
                       or-pattern has type" (Ident.name id))
        (function ppf ->
          fprintf ppf "but on the right-hand side it has type")
  | Multiply_bound_variable name ->
      Location.errorf ~loc
        "Variable %s is bound several times in this matching"
        name
  | Orpat_vars (id, valid_idents) ->
      Location.error_of_printer ~loc (fun ppf () ->
        fprintf ppf
          "Variable %s must occur on both sides of this | pattern"
          (Ident.name id);
        spellcheck_idents ppf id valid_idents
      ) ()
  | Expr_type_clash (err, explanation, exp) ->
      let diff = type_clash_of_trace err.trace in
      let sub = report_expr_type_clash_hints exp diff in
      report_unification_error ~loc ~sub env err
        ~type_expected_explanation:
          (report_type_expected_explanation_opt explanation)
        (function ppf ->
           fprintf ppf "This expression has type")
        (function ppf ->
           fprintf ppf "but an expression was expected of type");
  | Apply_non_function typ ->
      begin match get_desc typ with
        Tarrow _ ->
          Location.errorf ~loc
            "@[<v>@[<2>This function has type@ %a@]\
             @ @[It is applied to too many arguments;@ %s@]@]"
            Printtyp.type_expr typ "maybe you forgot a `;'.";
      | _ ->
          Location.errorf ~loc "@[<v>@[<2>This expression has type@ %a@]@ %s@]"
            Printtyp.type_expr typ
            "This is not a function; it cannot be applied."
      end
  | Apply_wrong_label (l, ty, extra_info) ->
      let print_label ppf = function
        | Nolabel -> fprintf ppf "without label"
        | l -> fprintf ppf "with label %s" (prefixed_label_name l)
      in
      let extra_info =
        if not extra_info then
          []
        else
          [ Location.msg
              "Since OCaml 4.11, optional arguments do not commute when \
               -nolabels is given" ]
      in
      Location.errorf ~loc ~sub:extra_info
        "@[<v>@[<2>The function applied to this argument has type@ %a@]@.\
         This argument cannot be applied %a@]"
        Printtyp.type_expr ty print_label l
  | Label_multiply_defined s ->
      Location.errorf ~loc "The record field label %s is defined several times"
        s
  | Label_missing labels ->
      let print_labels ppf =
        List.iter (fun lbl -> fprintf ppf "@ %s" (Ident.name lbl)) in
      Location.errorf ~loc "@[<hov>Some record fields are undefined:%a@]"
        print_labels labels
  | Label_not_mutable lid ->
      Location.errorf ~loc "The record field %a is not mutable" longident lid
  | Wrong_name (eorp, ty_expected, { type_path; kind; name; valid_names; }) ->
      Location.error_of_printer ~loc (fun ppf () ->
        Printtyp.wrap_printing_env ~error:true env (fun () ->
          let { ty; explanation } = ty_expected in
          if Path.is_constructor_typath type_path then begin
            fprintf ppf
              "@[The field %s is not part of the record \
               argument for the %a constructor@]"
              name.txt
              Printtyp.type_path type_path;
          end else begin
            fprintf ppf
              "@[@[<2>%s type@ %a%t@]@ \
               There is no %s %s within type %a@]"
              eorp Printtyp.type_expr ty
              (report_type_expected_explanation_opt explanation)
              (Datatype_kind.label_name kind)
              name.txt (*kind*) Printtyp.type_path type_path;
          end;
          spellcheck ppf name.txt valid_names
      )) ()
  | Name_type_mismatch (kind, lid, tp, tpl) ->
      let type_name = Datatype_kind.type_name kind in
      let name = Datatype_kind.label_name kind in
      Location.error_of_printer ~loc (fun ppf () ->
        Printtyp.report_ambiguous_type_error ppf env tp tpl
          (function ppf ->
             fprintf ppf "The %s %a@ belongs to the %s type"
               name longident lid type_name)
          (function ppf ->
             fprintf ppf "The %s %a@ belongs to one of the following %s types:"
               name longident lid type_name)
          (function ppf ->
             fprintf ppf "but a %s was expected belonging to the %s type"
               name type_name)
      ) ()
  | Invalid_format msg ->
      Location.errorf ~loc "%s" msg
  | Not_an_object (ty, explanation) ->
    Location.error_of_printer ~loc (fun ppf () ->
      fprintf ppf "This expression is not an object;@ \
                   it has type %a"
        Printtyp.type_expr ty;
      report_type_expected_explanation_opt explanation ppf
    ) ()
  | Undefined_method (ty, me, valid_methods) ->
      Location.error_of_printer ~loc (fun ppf () ->
        Printtyp.wrap_printing_env ~error:true env (fun () ->
          fprintf ppf
            "@[<v>@[This expression has type@;<1 2>%a@]@,\
             It has no method %s@]" Printtyp.type_expr ty me;
          begin match valid_methods with
            | None -> ()
            | Some valid_methods -> spellcheck ppf me valid_methods
          end
      )) ()
  | Undefined_self_method (me, valid_methods) ->
      Location.error_of_printer ~loc (fun ppf () ->
        fprintf ppf "This expression has no method %s" me;
        spellcheck ppf me valid_methods;
      ) ()
  | Virtual_class cl ->
      Location.errorf ~loc "Cannot instantiate the virtual class %a"
        longident cl
  | Unbound_instance_variable (var, valid_vars) ->
      Location.error_of_printer ~loc (fun ppf () ->
        fprintf ppf "Unbound instance variable %s" var;
        spellcheck ppf var valid_vars;
      ) ()
  | Instance_variable_not_mutable v ->
      Location.errorf ~loc "The instance variable %s is not mutable" v
  | Not_subtype err ->
      Location.error_of_printer ~loc (fun ppf () ->
        Printtyp.Subtype.report_error ppf env err "is not a subtype of"
      ) ()
  | Outside_class ->
      Location.errorf ~loc
        "This object duplication occurs outside a method definition"
  | Value_multiply_overridden v ->
      Location.errorf ~loc
        "The instance variable %s is overridden several times"
        v
  | Coercion_failure (ty_exp, err, b) ->
      Location.error_of_printer ~loc (fun ppf () ->
        Printtyp.report_unification_error ppf env err
          (function ppf ->
             let ty_exp = Printtyp.prepare_expansion ty_exp in
             fprintf ppf "This expression cannot be coerced to type@;<1 2>%a;@ \
                          it has type"
             (Printtyp.type_expansion Type) ty_exp)
          (function ppf ->
             fprintf ppf "but is here used with type");
        if b then
          fprintf ppf ".@.@[<hov>%s@ %s@ %s@]"
            "This simple coercion was not fully general."
            "Hint: Consider using a fully explicit coercion"
            "of the form: `(foo : ty1 :> ty2)'."
      ) ()
  | Not_a_function (ty, explanation) ->
      Location.errorf ~loc
        "This expression should not be a function,@ \
         the expected type is@ %a%t"
        Printtyp.type_expr ty
        (report_type_expected_explanation_opt explanation)
  | Too_many_arguments (ty, explanation) ->
      Location.errorf ~loc
        "This function expects too many arguments,@ \
         it should have type@ %a%t"
        Printtyp.type_expr ty
        (report_type_expected_explanation_opt explanation)
  | Abstract_wrong_label {got; expected; expected_type; explanation} ->
      let label ~long = function
        | Nolabel -> "unlabeled"
        | l       -> (if long then "labeled " else "") ^ prefixed_label_name l
      in
      let second_long = match got, expected with
        | Nolabel, _ | _, Nolabel -> true
        | _                       -> false
      in
      Location.errorf ~loc
        "@[<v>@[<2>This function should have type@ %a%t@]@,\
         @[but its first argument is %s@ instead of %s%s@]@]"
        Printtyp.type_expr expected_type
        (report_type_expected_explanation_opt explanation)
        (label ~long:true got)
        (if second_long then "being " else "")
        (label ~long:second_long expected)
  | Scoping_let_module(id, ty) ->
      Location.errorf ~loc
        "This `let module' expression has type@ %a@ \
         In this type, the locally bound module name %s escapes its scope"
        Printtyp.type_expr ty id
  | Private_type ty ->
      Location.errorf ~loc "Cannot create values of the private type %a"
        Printtyp.type_expr ty
  | Private_label (lid, ty) ->
      Location.errorf ~loc "Cannot assign field %a of the private type %a"
        longident lid Printtyp.type_expr ty
  | Private_constructor (constr, ty) ->
      Location.errorf ~loc
        "Cannot use private constructor %s to create values of type %a"
        constr.cstr_name Printtyp.type_expr ty
  | Not_a_polymorphic_variant_type lid ->
      Location.errorf ~loc "The type %a@ is not a variant type" longident lid
  | Incoherent_label_order ->
      Location.errorf ~loc
        "This function is applied to arguments@ \
        in an order different from other calls.@ \
        This is only allowed when the real type is known."
  | Less_general (kind, err) ->
      report_unification_error ~loc env err
        (fun ppf -> fprintf ppf "This %s has type" kind)
        (fun ppf -> fprintf ppf "which is less general than")
  | Modules_not_allowed ->
      Location.errorf ~loc "Modules are not allowed in this pattern."
  | Cannot_infer_signature ->
      Location.errorf ~loc
        "The signature for this packaged module couldn't be inferred."
  | Not_a_packed_module ty ->
      Location.errorf ~loc
        "This expression is packed module, but the expected type is@ %a"
        Printtyp.type_expr ty
  | Unexpected_existential (reason, name, types) ->
      let reason_str =
        match reason with
        | In_class_args ->
            "Existential types are not allowed in class arguments"
        | In_class_def ->
            "Existential types are not allowed in bindings inside \
             class definition"
        | In_self_pattern ->
            "Existential types are not allowed in self patterns"
        | At_toplevel ->
            "Existential types are not allowed in toplevel bindings"
        | In_group ->
            "Existential types are not allowed in \"let ... and ...\" bindings"
        | In_rec ->
            "Existential types are not allowed in recursive bindings"
        | With_attributes ->
            "Existential types are not allowed in presence of attributes"
      in
      begin match List.find (fun ty -> ty <> "$" ^ name) types with
      | example ->
          Location.errorf ~loc
            "%s,@ but this pattern introduces the existential type %s."
            reason_str example
      | exception Not_found ->
          Location.errorf ~loc
            "%s,@ but the constructor %s introduces existential types."
            reason_str name
      end
  | Invalid_interval ->
      Location.errorf ~loc
        "@[Only character intervals are supported in patterns.@]"
  | Invalid_for_loop_index ->
      Location.errorf ~loc
        "@[Invalid for-loop index: only variables and _ are allowed.@]"
  | No_value_clauses ->
      Location.errorf ~loc
        "None of the patterns in this 'match' expression match values."
  | Exception_pattern_disallowed ->
      Location.errorf ~loc
        "@[Exception patterns are not allowed in this position.@]"
  | Mixed_value_and_exception_patterns_under_guard ->
      Location.errorf ~loc
        "@[Mixing value and exception patterns under when-guards is not \
         supported.@]"
  | Inlined_record_escape ->
      Location.errorf ~loc
        "@[This form is not allowed as the type of the inlined record could \
         escape.@]"
  | Inlined_record_expected ->
      Location.errorf ~loc
        "@[This constructor expects an inlined record argument.@]"
  | Unrefuted_pattern pat ->
      Location.errorf ~loc
        "@[%s@ %s@ %a@]"
        "This match case could not be refuted."
        "Here is an example of a value that would reach it:"
        Printpat.top_pretty pat
  | Invalid_extension_constructor_payload ->
      Location.errorf ~loc
        "Invalid [%%extension_constructor] payload, a constructor is expected."
  | Not_an_extension_constructor ->
      Location.errorf ~loc
        "This constructor is not an extension constructor."
  | Literal_overflow ty ->
      Location.errorf ~loc
        "Integer literal exceeds the range of representable integers of type %s"
        ty
  | Unknown_literal (n, m) ->
      Location.errorf ~loc "Unknown modifier '%c' for literal %s%c" m n m
  | Illegal_letrec_pat ->
      Location.errorf ~loc
        "Only variables are allowed as left-hand side of `let rec'"
  | Illegal_letrec_expr ->
      Location.errorf ~loc
        "This kind of expression is not allowed as right-hand side of `let rec'"
  | Illegal_class_expr ->
      Location.errorf ~loc
        "This kind of recursive class expression is not allowed"
  | Letop_type_clash(name, err) ->
      report_unification_error ~loc env err
        (function ppf ->
          fprintf ppf "The operator %s has type" name)
        (function ppf ->
          fprintf ppf "but it was expected to have type")
  | Andop_type_clash(name, err) ->
      report_unification_error ~loc env err
        (function ppf ->
          fprintf ppf "The operator %s has type" name)
        (function ppf ->
          fprintf ppf "but it was expected to have type")
  | Bindings_type_clash(err) ->
      report_unification_error ~loc env err
        (function ppf ->
          fprintf ppf "These bindings have type")
        (function ppf ->
          fprintf ppf "but bindings were expected of type")
  | Unbound_existential (ids, ty) ->
      Location.errorf ~loc
        "@[<2>%s:@ @[type %s.@ %a@]@]"
        "This type does not bind all existentials in the constructor"
        (String.concat " " (List.map Ident.name ids))
        Printtyp.type_expr ty
  | Missing_type_constraint ->
      Location.errorf ~loc
        "@[%s@ %s@]"
        "Existential types introduced in a constructor pattern"
        "must be bound by a type constraint on the argument."
  | Wrong_expected_kind(sort, ctx, ty) ->
      let ctx, explanation =
        match ctx with
        | Expression explanation -> "expression", explanation
        | Pattern -> "pattern", None
      in
      let sort =
        match sort with
        | Constructor -> "constructor"
        | Boolean -> "boolean literal"
        | List -> "list literal"
        | Unit -> "unit literal"
        | Record -> "record"
      in
      Location.errorf ~loc
        "This %s should not be a %s,@ \
         the expected type is@ %a%t"
        ctx sort Printtyp.type_expr ty
        (report_type_expected_explanation_opt explanation)
  | Expr_not_a_record_type ty ->
      Location.errorf ~loc
        "This expression has type %a@ \
         which is not a record type."
        Printtyp.type_expr ty

let report_error ~loc env err =
  Printtyp.wrap_printing_env ~error:true env
    (fun () -> report_error ~loc env err)

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

let () =
  Persistent_env.add_delayed_check_forward := add_delayed_check;
  Env.add_delayed_check_forward := add_delayed_check;
  ()

(* drop ?recarg argument from the external API *)
let type_expect ?in_function env e ty = type_expect ?in_function env e ty
let type_exp env e = type_exp env e
let type_argument env e t1 t2 = type_argument env e t1 t2
