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

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Warning for 3 or more arguments using preprocessor stuff only could look like:
#define __va_start_2()
#define __va_start_23(...) __builtin_warning ("too many va_arg arguments");
#define __va_start_1()
#define __va_start_12(v, ...) __va_start_2##__VA_OPT__(3) (__VA_ARGS__)
#define va_start(v, ...) __va_start_1##__VA_OPT__(2) (__VA_ARGS__)
__builtin_va_start (v, 0)
We are C2X here, so we can rely on __VA_OPT__.
As for warning when there are 2 arguments and second argument is not identifier
for the last argument, in the light of #c12 that would be fairly difficult.  In
the preprocessor it is going to be pretty hard to find out whether in the 2
arguments case the second one is a single identifier token, and on the compiler
side __builtin_va_start or __builtin_va_start_c23 would need to be a keyword
with its own parsing which would parse
normally as expression the first argument, but for the rest just warn if it is
more than 2 arguments or second argument is present and is not identifier of
the last argument, but otherwise ignore the tokens.  Still, isn't say:
#define A (
#define B )
va_start (ap, A B B );
va_start (ap, A A A );
valid in C2X?  That ruins both the argument counting and especially any
attempts to decide something after preprocessing.