[Bug c/102849] New: missing warning for unsigned bitfield tests for negativity

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102849

            Bug ID: 102849
           Summary: missing warning for unsigned bitfield tests for
                    negativity
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC warns for less than comparisons of unsigned integers to zero but it fails
to do the same for unsigned bit-fields (likely because they promote to int;
comparisons of unsigned bit-fields larger than int are diagnosed by
-Wtype-limits).  Diagnosing both kinds of expressions would be helpful.

$ cat x.c && gcc  -S -Wall -Wextra x.c
struct A
{
  unsigned i, j: 3;
};

void f (struct A *p)
{
  p->i = -1;              // missing warning
  p->j = -1;              // missing warning in C, -Woverflow in C++
}

void g (struct A *p)
{
  if (p->i < 0)           // -Wtype-limits in both C and C++, folded to false
    __builtin_abort ();
}

void h (struct A *p)
{
  if (p->j < 0)           // missing warning in both C and C++, folded to false
    __builtin_abort ();
}

x.c: In function ‘g’:
x.c:14:12: warning: comparison of unsigned expression in ‘< 0’ is always false
[-Wtype-limits]
   14 |   if (p->i < 0)           // -Wtype-limits in both C and C++, folded to
false
      |            ^