https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88736
--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> --- The rationale for removing nullptr_t from <stddef.h> given in LWG 3484 seems to have gone away, now that C *does* define nullptr_t. So I think stddef.h should indeed define it, at least for C++26. And so it's unspecified whether or not <cstddef> defines it in the global namespace as well as in namespace std.
