Source

File: wp-includes/post-template.php

function walk_page_dropdown_tree( ...$args ) {
	if ( empty( $args[2]['walker'] ) ) { // The user's options are the third parameter.
		$walker = new Walker_PageDropdown;
	} else {
		/**
		 * @var Walker $walker
		 */
		$walker = $args[2]['walker'];
	}

	return $walker->walk( ...$args );
}