[Bug c/95554] New: spurious -Wnonnull on a conditional call

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95554

            Bug ID: 95554
           Summary: spurious -Wnonnull on a conditional call
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The C test case below was reduced from gcc/var-tracking.c which triggers the
-Wnonnull (in C++) when the this pointer in member functions is considered
implicitly declared nonnull (or, alternatively, when vec::is_empty() is
declared with attribute nonnull):

In file included from /ssd/test/src/gcc/86568/gcc/var-tracking.c:89:
/ssd/test/src/gcc/86568/gcc/var-tracking.c: In function ‘void
loc_exp_dep_alloc(variable*, int)’:
/ssd/test/src/gcc/86568/gcc/var-tracking.c:8109:46: error: ‘this’ pointer null
[-Werror=nonnull]
 8109 |          || VAR_LOC_DEP_VEC (var)->is_empty ());
      |                                              ^
/ssd/test/src/gcc/86568/gcc/system.h:748:14: note: in definition of macro
‘gcc_assert’
  748 |    ((void)(!(EXPR) ? fancy_abort (__FILE__, __LINE__, __FUNCTION__), 0
: 0))
      |              ^~~~
/ssd/test/src/gcc/86568/gcc/var-tracking.c:8107:3: note: in expansion of macro
‘gcc_checking_assert’
 8107 |   gcc_checking_assert (!count
      |   ^~~~~~~~~~~~~~~~~~~


In the test case the P macro corresponds to the VAR_LOC_DEP_VEC() macro defined
in the file, function g() to loc_exp_dep_alloc (), and the return statement to
the gcc_checking_assert.

$ cat q.c && gcc -O2 -S -Wall -fdump-tree-original=/dev/stdout q.c
__attribute__ ((nonnull)) int f (void*);

struct A
{
  int i;
};

struct B
{
  struct A *p;
};

#define P(p) (p->p ? &p->p : 0)

int g (struct B *p)
{
  return P (p) == 0 || f (P (p));
}
q.c: In function ‘g’:
q.c:17:24: warning: null argument where non-null required (argument 1)
[-Wnonnull]
   17 |   return P (p) == 0 || f (P (p));
      |                        ^

;; Function g (null)
;; enabled by -tree-original


{
  return (p->p == 0B || &p->p == 0B) || f (p->p != 0B ? (void *) &p->p : 0B) !=
0;
}