This introduces support for nullptr and nullptr_t in C2x mode. The proposal accepted by WG14 is: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3042.htm
Note, there are quite a few incompatibilities with the C++ feature in some of the edge cases of this feature. Therefore, there are some FIXME comments in tests for testing behavior that might change after WG14 has resolved national body comments (a process we've not yet started). So this implementation might change slightly depending on the resolution of comments. This is called out explicitly in the release notes as well.