[Bug c/71870] New: wrong location of "%n$" directive in -Wformat

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71870

            Bug ID: 71870
           Summary: wrong location of "%n$" directive in -Wformat
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When encountering an invalid format specifier, the GCC -Wformat checker issues
a warning pointing to the specifier in the format string.  However, when it
encounters the dollar sign in an otherwise valid POSIX format directive, GCC
issues a warning that doesn't point to the directive.  Modulo the difficulties
discussed in bug 52952 and related there should be no problem pointing to the
'$' in the basic cases similarly to what's done for the 'r' in the test case
below.

$ cat xyz.c && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -S
-Wall -Wextra -Wpedantic xyz.c
char d [4];

void f (void)
{
  __builtin_sprintf (d, "%r");

  __builtin_sprintf (d, "%2$i%1$i", 1, 234);
}
xyz.c: In function ‘f’:
xyz.c:5:27: warning: unknown conversion type character ‘r’ in format
[-Wformat=]
   __builtin_sprintf (d, "%r");
                           ^
xyz.c:7:3: warning: ISO C does not support %n$ operand number formats
[-Wformat]
   __builtin_sprintf (d, "%2$i%1$i", 1, 234);
   ^~~~~~~~~~~~~~~~~