Give this a try…
function save_url_before_login() {
if(!is_user_logged_in()) {
$_SESSION['referer_url'] = get_the_permalink();
}
add_action('wp', 'save_url_before_login');
and:
function redirect_after_login() {
$redirect_url = home_url("https://wordpress.stackexchange.com/");
if (isset($_SESSION['referer_url'])) {
$redirect_url = $_SESSION['referer_url'];
unset($_SESSION['referer_url']);
}
return $redirect_url;
}
add_filter('login_redirect', 'redirect_after_login');