[Bug c++/89923] New: printf format check and char8_t

[drepper.fsp+rhbz at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89923

            Bug ID: 89923
           Summary: printf format check and char8_t
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drepper.fsp+rhbz at gmail dot com
  Target Milestone: ---

With the introduction of char8_t there is a new error case in the printf format
checks:

#include <stdio.h>

int main() {
  auto s = u8"hello world";
#pragma GCC diagnostic error "-Wformat"
  printf("%s\n", s);
}

Compiling this with C++2a results to the following output:

$ g++ -c u.cc -std=gnu++2a
u.cc: In function ‘int main()’:
u.cc:6:12: error: format ‘%s’ expects argument of type ‘char*’, but argument 2
has type ‘const char8_t*’ [-Werror=format=]
    6 |   printf("%s\n", s);
      |           ~^     ~
      |            |     |
      |            char* const char8_t*
      |           %hhn

I think char8_t* should be added to the allowed types for the %s parameter.  It
is arguably more likely to succeed then a char* argument since the latters
encoding is determined by the compiler.  At least with u8 strings the code can
make sure the locale used at runtime uses UTF-8.