- The only exception is that the flag -std=c++2a is still used not to break compatibility with older compilers (clang <= 9, gcc <= 9).
- Bump _LIBCPP_STD_VER for C++20 to 20 and use 21 for the future standard (C++2b).
That's a preparation step to add c++2b support to libc++.
My understanding is that you're planning to make other changes in a separate PR to finish up adding a c++2b mode for real. And I like this patch and don't want to slow it down. But I echo what @miscco said: shouldn't you just go all the way to c++2b in this patch? Is there really a good reason to land this intermediate state rather than go all the way to c++2b in one hop?