lib/formslib.php throws a JavaScript error when

• a form has a required autocomplete element,
• parameter multiple is true,
• client-side validation is used and
• 'strictformsrequired' parameter is enabled.

getValidationScript() is expecting a string but getting an array
https://github.com/moodle/moodle/blob/20302d4f8646fa367adbc48d6506d2aae07d7697/lib/formslib.php#L3434

if (!empty($CFG->strictformsrequired))

    if (!empty($format) && $format == FORMAT_HTML) {

        return array('', "{jsVar}.replace(/(<(?!img|hr|canvas)[^>]*>)|&nbsp;|\s+/ig, '') == ''");

    } else {

        return array('', "{jsVar}.replace(/^\s+$/g, '') == ''");

    }

}

Steps to reproduce (4.2).
0. Modify https://github.com/moodle/moodle/blob/20302d4f8646fa367adbc48d6506d2aae07d7697/admin/tool/cohortroles/classes/form/assign_role_cohort.php#L73 and add client-side validation rule

$mform->addRule('cohortids', null, 'required', null, 'client');

1. Open your JavaScript console.
2. Login as admin, go to 'Site Administration > Security > Site security settings' and enable 'Strict validation of required fields'.
3. Go to 'Site Administration > Users > Cohorts' and create a cohort.
4. Go to 'Site Administration > Users > Define roles', edit role 'Student' and set 'user context' as a context type that can be assigned.
5. Go to 'Site Administration > Permissions > Assign user roles to cohort' and select a cohort.
You get an error: Uncaught TypeError: value.replace is not a function