CAS SSO and multi-authentication - SSO doesn't work as expected

Steps to reproduce the error:

1. Enable both CAS SSO authentication and Email-based self-registration authentication plugins.
2. Login to other system with CAS SSO authentication, and then redirect to Moodle to access a course (https://www.examplemoodle.com/course/view.php?id=99) 
3. Then moodle direct back to https://www.examplemoodle.com/login/index.php page to select login method.
4. When user click  https://www.examplemoodle.com/login/index.php?authCAS=CAS it shows as user already logged in. 

Expected outcome:

 User will direct to the particular course page without landing into  https://www.examplemoodle.com/login/index.php 

 

Background:

This happens because of loginpage_hook method in auth/cas/auth.php. 

 

// If the multi-authentication setting is used, check for the param before connecting to CAS.
if ($this->config->multiauth) {

// If there is an authentication error, stay on the default authentication page.
if (!empty($SESSION->loginerrormsg))

{ return; }
$authCAS = optional_param('authCAS', '', PARAM_RAW);

if ($authCAS == 'NOCAS') { return; }

// Show authentication form for multi-authentication.
// Test pgtIou parameter for proxy mode (https connection in background from CAS server to the php server).
if ($authCAS != 'CAS' && !isset($_GET['pgtIou']))

{ $PAGE->set_url('/login/index.php'); $PAGE->navbar->add($CASform); $PAGE->set_title("$site->fullname: $CASform"); $PAGE->set_heading($site->fullname); echo$OUTPUT->header(); include($CFG->dirroot.'/auth/cas/cas_form.html'); echo$OUTPUT->footer(); exit(); }

}
When $authCAS is not equal to 'CAS' it redirects users into '/login/index.php'. On the other hand this is important to get manual login option.

However if user already logged in using CAS, it should go here,
if (phpCAS::checkAuthentication()) {
$frm = newstdClass();
$frm->username = phpCAS::getUser();
$frm->password = 'passwdCas';

// Redirect to a course if multi-auth is activated, authCAS is set to CAS and the courseid is specified.
if ($this->config->multiauth && !empty($courseid))

{ redirect(newmoodle_url('/course/view.php', array('id'=>$courseid))); }

return;
}

// Force CAS authentication (if needed).
if (!phpCAS::isAuthenticated())

{ phpCAS::setLang($this->config->language); phpCAS::forceAuthentication(); }

Challenge here is, it should only happen if user already logged in using CAS SSO, otherwise both options should available to login.