WordPress at a glance
function is not described

comment_guid() WP 2.5.0

Display the feed GUID for the current comment.

No Hooks.

Return

Nothing (null).

Usage

comment_guid( $comment_id );
$comment_id(int/WP_Comment)
Optional comment object or id.
Default: global comment object

Code of comment guid: wp-includes/feed.php VER 5.1.1

<?php
function comment_guid( $comment_id = null ) {
	echo esc_url( get_comment_guid( $comment_id ) );
}