[Bug c/107980] va_start does not warn about an arbitrary number of arguments in C2x mode

["aaron at aaronballman dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107980

--- Comment #5 from Aaron Ballman <aaron at aaronballman dot com> ---
(In reply to Andrew Pinski from comment #3)
> I think we should warn but how to warn is going to have to special case the
> macro I think.

I was contemplating a perhaps terrible idea of adding some new builtins:
```
unsigned __builtin_va_opt_count(...);
void __builtin_c23_va_start(va_list list, unsigned arg_count);
```
and then doing something along these lines in stdarg.h:
```
#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
#define va_start(ap, ...) __builtin_c23_va_start(ap,
__builtin_va_opt_count(__VA_OPT__))
#else
...
#endif
```
but I've not put enough thought into it yet. WDYT about something along those
lines?

(In reply to Andrew Pinski from comment #4)
> (In reply to Andrew Pinski from comment #3)
> > I think we should warn but how to warn is going to have to special case the
> > macro I think.
> 
> But saying that I do think it is valid C2X code if you take the C2X standard
> seperately from the older standards.

I'm sad I noticed this after the NB comment period closed for the US NB because
otherwise I'd leave a comment to make it a constraint violation to pass more
than two arguments to the macro (or at least a recommended practice to
diagnose). Because I agree with you that the letter of the standard makes this
valid in C2x, but given that the extra arguments are not expanded or evaluated,
there is no reason to accept more than two arguments in any C standard version.