Source

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

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

		$post_obj = $this->get_queried_object();

		return in_array( $post_obj->post_type, (array) $post_types, true );
	}