Determines whether the query is for an existing single post of any post type
(post, attachment, page, custom post types).

If the $post_types parameter is specified, this function will additionally
check if the query is for one of the Posts Types specified.

For more information on this and similar theme functions, check out
the {@link Conditional Tags} article in the Theme Developer Handbook.

function is_singular( $post_types = '' ) {
	global $wp_query;

	if ( ! isset( $wp_query ) ) {
		_doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1.0' );
		return false;
	}

	return $wp_query->is_singular( $post_types );
}