[Bug middle-end/78606] New: -Wformat-length/-fprintf-return-value incorrect for %+.0i and %.0o with zero value

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78606

            Bug ID: 78606
           Summary: -Wformat-length/-fprintf-return-value incorrect for
                    %+.0i and %.0o with zero value
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

There's an obscure rule in C that says that an integer conversion specification
with a zero precision and zero value (such as a "%.0i") results in bytes. 
There's an even more obscure rule that implies that the + flag (but no other
flag) results in a sole plus sign being output when used in such a directive
(i.e., "%+.0i").  And yet another obscure rule says that when used with the o
conversion (but none other), the # flag has the effect of increasing zero
precision from zero to 1 when used with a zero argument so that "%#.0o" results
in a 0 on output (while "%#.0x" results in nothing).  The test case below shows
that the gimple-ssa-sprintf pass doesn't get these subtleties right.

$ (set -x; cat c.c && for f in '"%+.0i"' '"%#.0o"'; do /build/gcc-git/gcc/xgcc
-B /build/gcc-git/gcc -DFMT="$f" -O2 -Wextra -Wformat-length c.c && ./a.out;
done)
+ cat c.c
const char* fmt = FMT;

int main (void)
{
   int n = __builtin_snprintf (0, 0, FMT, 0);

  if (n != __builtin_snprintf (0, 0, fmt, 0))
    __builtin_abort ();
}
+ for f in ''\''"%+.0i"'\''' ''\''"%#.0o"'\'''
+ /build/gcc-git/gcc/xgcc -B /build/gcc-git/gcc '-DFMT="%+.0i"' -O2 -Wextra
-Wformat-length c.c
+ ./a.out
+ for f in ''\''"%+.0i"'\''' ''\''"%#.0o"'\'''
+ /build/gcc-git/gcc/xgcc -B /build/gcc-git/gcc '-DFMT="%#.0o"' -O2 -Wextra
-Wformat-length c.c
+ ./a.out