[Bug c/110402] New: Bogus -Waddress warning that pointer comparison is always true

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110402

            Bug ID: 110402
           Summary: Bogus -Waddress warning that pointer comparison is
                    always true
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lsof at mailbox dot org
  Target Milestone: ---

Created attachment 55398
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55398&action=edit
testcase

In the following testcase:


struct m { float *v; int t; };

_Bool chk(struct m *m, int key) {
    return (m->t = key) < 0 ? 0 : &m->v[key];
}

/* a comma expression can suppress it */
_Bool chk2(struct m *m, int key) {
    return (m->t = key, m->t < 0 ? 0 : &m->v[key]);
}

/* however, this does NOT warn */
_Bool chk3(struct m *m, int key) {
    return (m->t = key) < 0 ? &m->v[key] : &m->v[key];
}


gcc -Waddress gives this warning:

x.c: In function ‘chk’:
x.c:4:5: warning: the comparison will always evaluate as ‘true’ for the pointer
operand in ‘m->v + (sizetype)((long unsigned int)key * 4)’ must not be NULL
[-Waddress]
    4 |     return key < 0 ? 0 : &m->v[key];
      |     ^~~~~~

In particular the second function does not produce the warning (correctly), but
the third function for which the warning actually applies does not give a
warning.