Source Reference:

• Component: bbPress :: bbp-forum

  File: /bbp-includes/bbp-forum-template.php :: Trac Source Line: 1425

  		$ancestors = bbp_get_forum_ancestors( $forum_id );
  
  		foreach ( (array) $ancestors as $ancestor ) {
  			if ( bbp_is_forum( $ancestor ) && bbp_is_forum_public( $ancestor, false ) ) {
  				$retval = true;
  			}
  		}
  	}
  
  	return apply_filters( 'bbp_is_forum_public', (bool) $retval, $forum_id, $check_ancestors );
  }
  
  /**
   * Is the forum private?
   *
   * @since bbPress (r2746)
   *
   * @param int $forum_id Optional. Forum id
   * @param bool $check_ancestors Check if the ancestors are private (only if
   *                               they're a category)