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

[aaron at aaronballman dot com]

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

--- Comment #8 from Aaron Ballman <aaron at aaronballman dot com> ---
(Sorry, I meant __VA_ARGS__ and not __VA_OPT__; C23 on the brain!)

(In reply to Andrew Pinski from comment #6)
> Maybe add a _Static_warn builtin instead which is like _Static_assert but
> instead of an error, it is a warning.
> 
> That is:
> #define va_start(ap, ...) (_Static_warn(__builtin_va_opt_count(__VA_OPT__)
> <= 1, "More than one argument supplied to the macro va_start"),
> __builtin_va_start(ap, 0))
> 
> 
> THe only issue is that message does not get translated.

Huh, that's a really neat idea to consider! But _Static_assert is a
declaration, not an expression, so we'd have to use a statement expression
there instead of rely on the comma operator. But I could use that for both
diagnosing passing > 2 args as well as passing < 2 args (we sometimes like to
warn "this feature is incompatible with standards before <blah>" in Clang).


(In reply to Jakub Jelinek from comment #7)
> Why do you need a builtin for counting number of arguments in __VA_OPT__?
> That can be done in the preprocessor.  All one needs is 1, 2 or more
> arguments.

Yes, you can beat the preprocessor into doing this for you, but a builtin that
counts the number of arguments in __VA_ARGS__ would solve a problem users have
(https://stackoverflow.com/questions/2124339/c-preprocessor-va-args-number-of-arguments),
so I figured it was better than preprocessor games.