+ $error = self::validateTarget( $nu, $sender );
+
+ return $error ? $error : $nu;
+ }
+
+ /**
+ * Validate target User
+ *
+ * @param User $target Target user
+ * @param User|null $sender User sending the email
+ * @return string Error message or empty string if valid.
+ * @since 1.30
+ */
+ public static function validateTarget( $target, User $sender = null ) {
+ if ( $sender === null ) {
+ wfDeprecated( __METHOD__ . ' without specifying the sending user', '1.30' );
+ }
+
+ if ( !$target instanceof User || !$target->getId() ) {