windows. For obvious reasons, such warnings ought to be fixed.
Making compilation abort is the sledgehammer method to force that.
It's a stupid method for us, as we develop on various platforms, and
don't test every single commit under Windows. It gratuitously breaks
programs that work fine. Leave them as warnings, and fix them on
sight.
number. Check is redundant, because invalid values can come only from
natarg(), and those are checked above. Check is broken because
natarg() returns negative value on error, not 255.