#16: page_link

page_link.php

<?php
/**
 * Return a "front page's" real permalink
 *
 * @param string $link The page permalink.
 * @param int $page_id The page id.
 * @param bool $sample Whether to return a sample permalink.
 *
 * @return string The filtered permalink.
 */
function wpdocs_real_home_permalink( $link, $page_id, $sample ) {
	if ( 'page' == get_option( 'show_on_front' ) && $post_id == get_option( 'page_on_front' ) )
		return _get_page_link( $page_id, false, $sample );
}
add_filter( 'page_link', 'wpdocs_real_home_permalink', 10, 3 );

There's a typo in there: $post_id instead of $page_id. I've fixed it in my fork: https://gist.github.com/JDGrimes/6065006