[Bug libc/31470] New: inttypes.h provides incorrect macro definitions

["luigighiron at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31470

            Bug ID: 31470
           Summary: inttypes.h provides incorrect macro definitions
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: luigighiron at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

The inttypes.h header provides macros for printing out the types defined in
inttypes.h and stdint.h. In C23, the following text was added to the C standard
in section 7.8.1 "Macros for format specifiers":

> The functions in the fprintf and fwprintf families shall behave as if they use
> va_arg with a type argument naming the type resulting from applying the
> default argument promotions to the type corresponding to the macro and then
> convert the result of the va_arg expansion to the type corresponding to the
> macro.

For example:

#include<stdio.h>
#include<inttypes.h>
int main(){
    uint8_t u=-1;
    printf("%"PRIu8"\n",u+1);
}

This program should output 0, at least in C23. u contains the value 255, then
one is added which results in the value 256 (after integer promotions). 256 is
provided to printf which will do va_arg with int (the promoted type of uint8_t)
and that value is then converted back to uint8_t which will result in the value
0.

However, libc (I tested it on godbolt) seems to print 256 here because it does
not do the conversion back to uint8_t. This seems to be because libc defines
PRIu8 here as "u". I tested this on MSVC and Cygwin and it printed 0 as
expected, with those compilers it is defined as "hhu". The text which requires
that the conversion back after va_arg was added by GB-108 (N3148). See N3167
for discussion about accepting GB-108.

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