public async int test_async ()
{
	Idle.add (() => {
		print ("In idle\n");
		test_async.callback ();
	});
	yield;
	return 0;
}


$ valac --pkg gio-2.0 callbacktest.vala 

ERROR:valamethod.c:3172:vala_method_get_callback_method: assertion failed: (self->priv->_coroutine)
Aborted



After some digging, I found out that get_callback_method is being called on the closure rather than the method containing it, which causes the problem.