Source

File: wp-includes/post-template.php

function walk_page_tree( $pages, $depth, $current_page, $r ) {
	if ( empty( $r['walker'] ) ) {
		$walker = new Walker_Page;
	} else {
		/**
		 * @var Walker $walker
		 */
		$walker = $r['walker'];
	}

	foreach ( (array) $pages as $page ) {
		if ( $page->post_parent ) {
			$r['pages_with_children'][ $page->post_parent ] = true;
		}
	}

	return $walker->walk( $pages, $depth, $r, $current_page );
}