Source

File: wp-includes/class-wp-query.php

public function is_feed( $feeds = '' ) {
		if ( empty( $feeds ) || ! $this->is_feed ) {
			return (bool) $this->is_feed;
		}

		$qv = $this->get( 'feed' );
		if ( 'feed' === $qv ) {
			$qv = get_default_feed();
		}

		return in_array( $qv, (array) $feeds, true );
	}