Source

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

public function is_tag( $tag = '' ) {
		if ( ! $this->is_tag ) {
			return false;
		}

		if ( empty( $tag ) ) {
			return true;
		}

		$tag_obj = $this->get_queried_object();

		$tag = array_map( 'strval', (array) $tag );

		if ( in_array( (string) $tag_obj->term_id, $tag, true ) ) {
			return true;
		} elseif ( in_array( $tag_obj->name, $tag, true ) ) {
			return true;
		} elseif ( in_array( $tag_obj->slug, $tag, true ) ) {
			return true;
		}

		return false;
	}