[Bug c++/108646] nonnull attribute does not detect variables that are NULL being passed

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108646

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It already does detect it:

n.c: In function ‘test’:
n.c:6:2: warning: argument 1 null where non-null expected [-Wnonnull]
    6 |  mem2(dest);
      |  ^~~~~~~~~~
n.c:2:8: note: in a call to function ‘mem2’ declared ‘nonnull’
    2 | void * mem2(void *dest) __attribute__((nonnull));
      |        ^~~~

You need to enable optimization, otherwise there is no data flow analysis.

Without optimization, it detects it with -fanalyzer

n.c: In function ‘void test()’:
n.c:6:6: warning: use of NULL ‘dest’ where non-null expected [CWE-476]
[-Wanalyzer-null-argument]
    6 |  mem2(dest);
      |  ~~~~^~~~~~
  ‘void test()’: events 1-2
    |
    |    5 |  char *dest = NULL;
    |      |        ^~~~
    |      |        |
    |      |        (1) ‘dest’ is NULL
    |    6 |  mem2(dest);
    |      |  ~~~~~~~~~~
    |      |      |
    |      |      (2) argument 1 (‘dest’) NULL where non-null expected
    |
n.c:2:8: note: argument 1 of ‘void* mem2(void*)’ must be non-null
    2 | void * mem2(void *dest) __attribute__((nonnull));
      |        ^~~~


And it's also detected at runtime using -fsanitize=undefined:

n.c:6:6: runtime error: null pointer passed as argument 1, which is declared to
never be null