Martinho Fernandes:
> But why does the user need to know about it? When presented this way,
> it's just an implementation detail. When would the user ever actually
> want to create and use objects of type code_unit_sequence directly?

Same thing with std::char_traits. I never needed to call
std::char_traits<...> member functions yet they are specified in the
standard.

> If you say that
> basic_string can do this job just fine, what is a strong motivating
> example for adding another specialized sequence container type?

The main motivation is actually replacing it. std::basic_string was
designed without any Unicode handling in mind so it doesn't belong in
the standard nowadays. I hope to see it deprecated by C++26 and
completely removed in C++32.