When Filter is defined, but returns void (does not return
anything), Latte says it is not defined.
Is that a bug or just an inaporpriate error message?
No it's not.
Here, checking if filter is defined (https://github.com/…e/Engine.php#L298)
Otherwise, return filter value (https://github.com/…e/Engine.php#L312)
Well than it should not throw an Exception.
@PavelJanda If you are sure that the filter is really registered, open an issue please.