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

[redi at gcc dot gnu.org]

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

--- Comment #18 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I assume that testcase is meant to be C++ (because it isn't valid C) but it's
not valid C++ either:

17.6.4.3.1 [macro.names]
"A translation unit that includes a header shall not contain any macros that
define names declared or defined in that header. Nor shall such a translation
unit define macros for names lexically identical to keywords."