Fix check of undeclared types in any operators

Bug introduced in be290ed. Notice that desugaring of
any operators occurs before type checking.
Previous code, `SI.add id ty_vars`, does not work because it leads to
incorrect positions being reported in error messages.

src/lustre/typeCheckerContext.ml

@@ -807,7 +807,7 @@ and ty_vars_of_type ctx node_name ty =
   | UserType (_, ty_args, id) -> (
     match lookup_ty_syn ctx id ty_args with
     | Some ty -> ty_vars_of_type ctx node_name ty
-    | None -> assert false
+    | None -> SI.empty
   )
   | RefinementType (_, (_, _, ty), e) 
   | ArrayType (_, (ty, e)) -> 

tests/ounit/lustre/lustreTypeChecker/undeclared_type_06.lus

@@ -0,0 +1,4 @@
+node N() returns (y: int);
+let
+  y = any Foo;
+tel

tests/ounit/lustre/testLustreFrontend.ml

@@ -599,6 +599,10 @@ let _ = run_test_tt_main ("frontend LustreTypeChecker error tests" >::: [
     match load_file "./lustreTypeChecker/undeclared_type_05.lus" with
     | Error (`LustreTypeCheckerError (_, UndeclaredType _)) -> true
     | _ -> false);
+  mk_test "test undeclared 6" (fun () ->
+    match load_file "./lustreTypeChecker/undeclared_type_06.lus" with
+    | Error (`LustreTypeCheckerError (_, UndeclaredType _)) -> true
+    | _ -> false);
   mk_test "test arity incorrect node call" (fun () ->
     match load_file "./lustreTypeChecker/arity_incorrect_node_call.lus" with
     | Error (`LustreTypeCheckerError (_, IlltypedCall _)) -> true