[Bug c/81453] New: relational expression involving null pointer not diagnosed with -Wall

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81453

            Bug ID: 81453
           Summary: relational expression involving null pointer not
                    diagnosed with -Wall
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

In C, relational expressions are defined only for pointers to the same object
or array.  Using a null pointer in such expressions is undefined.  Using a null
pointer constant in a relational expression is a common mistake, yet GCC only
diagnoses such uses with -Wextra or -Wpedantic but not with -Wall.  For
example, the following doesn't trigger a warning unless -Wextra is used, even
though the if statement is eliminated because the comparison is false
regardless of the value of i.

The history of the warning suggests that r9580 enabled it under -Wextra in
addition to -pedantic, but the commit comment indicates that -Wall may have
been intended.  Unfortunately, no test was added at the time to break the tie.

$ cat a.c && gcc -O2 -S -Wall a.c
int f (int *i)
{
  if (i < 0)
    return -1;
  return 1;
}