Source

File: wp-includes/comment-template.php

function get_comment_id_fields( $post_id = 0 ) {
	if ( empty( $post_id ) ) {
		$post_id = get_the_ID();
	}

	$reply_to_id = isset( $_GET['replytocom'] ) ? (int) $_GET['replytocom'] : 0;
	$result      = "<input type='hidden' name='comment_post_ID' value='$post_id' id='comment_post_ID' />\n";
	$result     .= "<input type='hidden' name='comment_parent' id='comment_parent' value='$reply_to_id' />\n";

	/**
	 * Filters the returned comment ID fields.
	 *
	 * @since 3.0.0
	 *
	 * @param string $result      The HTML-formatted hidden ID field comment elements.
	 * @param int    $post_id     The post ID.
	 * @param int    $reply_to_id The ID of the comment being replied to.
	 */
	return apply_filters( 'comment_id_fields', $result, $post_id, $reply_to_id );
}