Invalid _Static_assert expanded from HASH_CALLBACKS_CHECK

[Roberto Bagnara <roberto.bagnara@xxxxxxxxxxx>]

Hi there.

I stumbled upon parsing errors due to invalid uses of
_Static_assert expanded from HASH_CALLBACKS_CHECK where
the tested expression is not constant, as mandated by
the C standard.

Judging from the following comment, there is partial awareness
of the fact this is an issue:

#ifndef __clang__ /* At least some versions dislike some of the uses. */
#define HASH_CALLBACKS_CHECK(mask) \
    BUILD_BUG_ON((mask) > (1U << ARRAY_SIZE(callbacks)) - 1)

Indeed, this is not a fault of Clang: the point is that some
of the expansions of this macro are not C.  Moreover,
the fact that GCC sometimes accepts them is not
something we can rely upon:

$ cat p.c
void f() {
static const int x = 3;
_Static_assert(x < 4, "");
}
$ gcc -c -O p.c
$ gcc -c p.c
p.c: In function ‘f’:
p.c:3:20: error: expression in static assertion is not constant
3 | _Static_assert(x < 4, "");
| ~^~
$

Finally, I think this can be easily avoided: instead
of initializing a static const with a constant expression
and then static-asserting the static const, just static-assert
the constant initializer.

Kind regards,

   Roberto Bagnara