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From: "aaron at aaronballman dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/107980] va_start does not warn about an arbitrary number of
 arguments in C2x mode
Date: Tue, 06 Dec 2022 13:37:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107980

--- 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 t=
he
> 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__ >=3D 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 tho=
se
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.
>=20
> But saying that I do think it is valid C2X code if you take the C2X stand=
ard
> seperately from the older standards.

I'm sad I noticed this after the NB comment period closed for the US NB bec=
ause
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 t=
his
valid in C2x, but given that the extra arguments are not expanded or evalua=
ted,
there is no reason to accept more than two arguments in any C standard vers=
ion.=