Restore proper error message when opening a functor.

typing/env.ml

@@ -1745,13 +1745,7 @@ let rec add_signature sg env =
 
 (* Open a signature path *)
 
-let open_signature slot root env0 =
-  let comps =
-    match get_components (find_module_descr root env0) with
-    | Structure_comps c -> c
-    | Functor_comps _ -> assert false
-  in
-
+let add_components slot root env0 comps =
   let add_l w comps env0 =
     Tbl.fold
       (fun name ->
@@ -1815,10 +1809,18 @@ let open_signature slot root env0 =
     modules;
   }
 
+let open_signature slot root env0 =
+  match get_components (find_module_descr root env0) with
+  | Functor_comps _ -> None
+  | Structure_comps comps -> Some (add_components slot root env0 comps)
+
+
 (* Open a signature from a file *)
 
 let open_pers_signature name env =
-  open_signature None (Pident(Ident.create_persistent name)) env
+  match open_signature None (Pident(Ident.create_persistent name)) env with
+  | Some env -> env
+  | None -> assert false (* a compilation unit cannot refer to a functor *)
 
 let open_signature ?(loc = Location.none) ?(toplevel = false) ovf root env =
   if not toplevel && ovf = Asttypes.Fresh && not loc.Location.loc_ghost

typing/env.mli

@@ -153,11 +153,12 @@ val add_item: signature_item -> t -> t
 val add_signature: signature -> t -> t
 
 (* Insertion of all fields of a signature, relative to the given path.
-   Used to implement open. *)
-
+   Used to implement open. Returns None if the path refers to a functor,
+   not a structure. *)
 val open_signature:
     ?loc:Location.t -> ?toplevel:bool -> Asttypes.override_flag -> Path.t ->
-      t -> t
+      t -> t option
+
 val open_pers_signature: string -> t -> t
 
 (* Insertion by name *)

typing/envaux.ml

@@ -63,7 +63,10 @@ let rec env_from_summary sum subst =
       | Env_open(s, path) ->
           let env = env_from_summary s subst in
           let path' = Subst.module_path subst path in
-          Env.open_signature Asttypes.Override path' env
+          begin match Env.open_signature Asttypes.Override path' env with
+          | Some env -> env
+          | None -> assert false
+          end
       | Env_functor_arg(Env_module(s, id, desc), id') when Ident.same id id' ->
           Env.add_module_declaration ~check:false
             id (Subst.module_declaration subst desc)

typing/typemod.ml

@@ -84,8 +84,12 @@ let extract_sig_open env loc mty =
 
 let type_open_ ?toplevel ovf env loc lid =
   let path = Typetexp.lookup_module ~load:true env lid.loc lid.txt in
-  (* TODO: handle same errors as in extract_sig_open *)
-  path, Env.open_signature ~loc ?toplevel ovf path env
+  match Env.open_signature ~loc ?toplevel ovf path env with
+  | Some env -> path, env
+  | None ->
+      let md = Env.find_module path env in
+      ignore (extract_sig_open env lid.loc md.md_type);
+      assert false
 
 let type_open ?toplevel env sod =
   let (path, newenv) =