valac already does the type checking. gcc is giving warnings on the generated because the cast is not there.

Code below has this for example

/* GTK+ Vala Sample Code */
using GLib;
using Gtk;

public class Sample : Window {


        construct {
                destroy += Gtk.main_quit;
        }

	const ActionEntry[] action_entries = {
  		{"Action", null, null, null, null, on_action}
	};
	void on_action (Action action) {

	}

	void setup () {

		var ui_manager = new UIManager ();

		var actions = new ActionGroup ("Actions");
		actions.add_actions (action_entries, actions);
		ui_manager.insert_action_group (actions, 0);
	}

        static int main (string[] args) {
                Gtk.init (ref args);
	
                var sample = new Sample ();
		sample.setup ();
                sample.show_all ();

                Gtk.main ();
                return 0;
        }
}