[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 #14 from Aaron Ballman <aaron at aaronballman dot com> ---
(In reply to joseph@codesourcery.com from comment #12)
> The standard rule about not using extra arguments means that any warnings 
> would need to avoid even converting those arguments from pp-tokens to 
> tokens; it's OK for them to contain pp-tokens that cannot be converted to 
> tokens.

Agreed, that's why I filed an NB comment to remove the restriction that the
arguments can't be expanded. Hopefully WG14 will relax that so we can give good
diagnostic behavior.

> I think the accepted change to the standard was entirely clear about 
> ignoring the extra arguments; it wasn't some obscure non-obvious 
> consequence that such code would become valid.

It was not discussed in the paper, or the meeting minutes, or my personal
notes. I may have missed something, but I have no reason to believe WG14
explicitly considered the ramifications of this. Also, I cannot find another
instance where a function-like macro accepts arguments that it is not allowed
to *expand*; that's a novel WG14 invention.

Also, it was a potentially silently breaking change (if you don't mind horribly
contrived examples of breakage): https://godbolt.org/z/T4bG6179f