I think there's another useful guarantee that could be provided, but I'm not sure whether it should be done as part of this proposal: In any encoding, it looks like alphabetic characters always appear in their lexicographical order, even if they aren't always contiguous.

Consequently, (a <=> b) is meaningful and useful between two characters if you know that a and b are letters.

On Sat, 28 Feb 2026 at 10:04, Jan Schultke <janschultke@googlemail.com> wrote:

Hi,

https://isocpp.org/files/papers/D4039R0.html

In the recent discussion of D4040R0, it was brought up that C2y now provides the guarantee that hexadecimal digit characters are contiguous. This was done by https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3192%2epdf

I've drafted up a proposal that adopts that guarantee into C++. Feedback is welcome.

I expect we can make this a DR against C++98 and pretend it's always been guaranteed.

Jan