Re: Varargs macros subtly broken

[Neil Booth <NeilB at earthling dot net>]

Hi Jamie,

Thanks for the bug reports.

>   Observe: gnu_count expansions differ.

Yes, this is a bug with identifying empty varargs (the VOID_REST
flag).  I'm bootstrapping a patch to fix this at the moment; I've got
rid of the VOID_REST stuff as it was too ugly to fix, and am trying
something else instead.

>            gnu_index makes preprocessing fail.

Those macros are, um, let's just say deep and complicated.  I'll try
and look into it when I've got time; it would take me ages just to
figure out what's supposed to be going on, never mind what actually
is.

>   Bonus observations:
>
>        - ## swallows the token in rest arg macros, even with
>          __VA_ARGS__.  According to the CPP manual, this is an
>          unreliable feature that is deprecated, and it's use is warned
>          about.  Actually there is no warning.  As you can see from
>          this code, that feature is essential, though only when the
>          discarded token is ",", to get the correct results here.  Can
>          we have a decision to not deprecate it at least in the comma
>          case, provided that is compatible with ISO C99?  (Presumably
>          ISO C99 doesn't define the behaviour of pasting "," before
>          another token anyway -- leaving a loophole which GCC can
>          exploit to discard it).

I do believe that what you describe is what Zack intended.  If he
concurs, I'll update the docs.  I think you only get a pedantic warning,
and then only if using std=c99.  Maybe that should be changed.

>        - The output has spurious newlines, followed by a corrective "#
>          LINE" directive.

As you've noticed in your other bug report, the output code is a mess.
I'm splitting it out to a separate file soon, to get all that code in
one place.  Hopefully it won't be too hard to clean up.  I think
there's an outstanding bug report about #pragma's causing similar
problems.  I suspect it's all the same problem.

Neil.