[Bug tree-optimization/80537] New: missing -Wformat-overflow on POSIX %C conversion specification

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80537

            Bug ID: 80537
           Summary: missing -Wformat-overflow on POSIX %C conversion
                    specification
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

POSIX specifies that the %C conversion specification is equivalent to %lc (used
to format wide characters).  The -Wformat-overflow doesn't recognize the %C
directive and doesn't issue warnings for it.  Worse, the directive has the
effect of disabling the checking for the rest of the format string.

$ cat c.c && gcc -O2 -S -Wall -Wextra -Wpedantic c.c
char d[1];

void f (void)
{
  __builtin_sprintf (d + 1, "%lc foo", 'a');
}

void g (void)
{
  __builtin_sprintf (d + 1, "%C bar", 'a');
}
c.c: In function ‘g’:
c.c:10:31: warning: ISO C does not support the ‘%C’ gnu_printf format
[-Wformat=]
   __builtin_sprintf (d + 1, "%C bar", 'a');
                               ^
c.c: In function ‘f’:
c.c:5:30: warning: ‘%lc’ directive writing up to 1 bytes into a region of size
0 [-Wformat-overflow=]
   __builtin_sprintf (d + 1, "%lc foo", 'a');
                              ^~~
c.c:5:3: note: ‘__builtin_sprintf’ output between 5 and 6 bytes into a
destination of size 0
   __builtin_sprintf (d + 1, "%lc foo", 'a');
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~