[Bug c++/60304] Including <atomic> disables -Wconversion-null

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60304

--- Comment #22 from Manuel LÃpez-IbÃÃez <manu at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #21)
> (In reply to Manuel LÃpez-IbÃÃez from comment #19) 
> > This is exactly what G++'s stdbool.h is doing with -std=gnu++98 and
> > -std=c++98.
> 
> <stdbool.h> is a "header" which is C++ standardese for a standard library
> header. The rule only applies to user code, not std::lib headers. In your
> testcase sys.h is OK but nonsys.h is not.

We can drop the nonsys.h case. It is only testing that the no system_headers
case also works (according to your other comment, it will be valid if no
system_header is included).

My point is that the fix cannot be only limited to tweaking stdbool.h, as long
as any C++ mode still defines false as a macro.