Fix usage of applied functors behind signatures

lib/module.ml

@@ -390,6 +390,12 @@ let rec add_to_env env foreign (mname, m) =
                 if not (Hashtbl.mem tbl name) then
                   (* Add decl to table *)
                   Hashtbl.add tbl name decl
+            | Mapplied_functor (loc, key, p, m) -> (
+                (* [register_applied_functor] adds the types to the decl table
+                   so they are readable later on *)
+                match register_applied_functor env loc key p m with
+                | Ok _ -> ()
+                | Error () -> raise (Error (loc, "Cannot apply functor")))
             | _ -> ())
           m.i;
         env

lib/monomorph_tree.ml

@@ -280,7 +280,9 @@ let rec cln ss p = function
             | Dabstract (Some dkind) -> cln_dkind dkind
           in
           cln_dkind decl.kind
-      | None -> failwith "Internal Error: Tconstr not available")
+      | None ->
+          failwith
+            ("Internal Error: Tconstr " ^ Path.show name ^ " not available"))
 
 and cln_kind ss p = function
   | Simple -> Simple

test/modules.t/hidden_functor_app.smu

@@ -0,0 +1,10 @@
+signature {
+  val do : () -> unit
+}
+
+module stbl = hashtbl/make(string)
+
+fun do() {
+  let _ : stbl/t[int] = stbl/create(16)
+  ()
+}

test/modules.t/use_hidden_functor_app.smu

@@ -0,0 +1,3 @@
+import hidden_functor_app
+
+hidden_functor_app/do()