https://isocpp.org/files/papers/P2728R3.html

This revision still does not have the change to the utfN_* concepts.
That change will make those concepts use same_as<charN_t> instead of
is_integral<T> && <<the right number of bits>>. It breaks my entire
implementation, and I want to implement it before changing the paper,
so I need a bit more time. This change will be in the next revision.

Zach