Source

File: wp-includes/category-template.php

function walk_category_dropdown_tree( ...$args ) {
	// The user's options are the third parameter.
	if ( empty( $args[2]['walker'] ) || ! ( $args[2]['walker'] instanceof Walker ) ) {
		$walker = new Walker_CategoryDropdown;
	} else {
		/**
		 * @var Walker $walker
		 */
		$walker = $args[2]['walker'];
	}
	return $walker->walk( ...$args );
}