STATICALLY-WIDEN is not a hack, it's just a convenient pseudo-function that
simplifies and formalizes a bunch of wording originated from p0355. In case
of chrono it will work identically for Unicode and the only reason we
didn't mention charX_t is that neither std::format nor iostreams support
those. As Corentin wrote you could provide formatter specializations
including for Unicode types and once we have charX_t overloads of
std::format (I plan to propose these for C++23) it will just pick up those
specialization. In your case STATICALLY-WIDEN won't help because you have
different representation of units for different character types.

Cheers,
Victor

On Thu, Oct 17, 2019 at 12:21 PM Mateusz Pusz <mateusz.pusz_at_[hidden]>
wrote:

> Hi everyone,
>
> Right now I am in the process of designing and implementing a Physical
> Units library that hopefully will be a start for having such a feature in
> the C++ Standard Library. You can find more info on the library here:
> https://github.com/mpusz/units.
>
> Recently, I started to work on the text output of quantities. Quantities
> consist of value and a unit symbol. The latter is a perfect use case for
> Unicode. Consider:
>
> 10 us vs 10 μs
> 2 kg*m/s^2 vs 2 kg⋅m/s²
>
> Before C++20 we could get away with a hack by providing Unicode characters
> to `char`-based types and streams, but with the introduction of `char8_t`
> in C++20 it seems it will be a bigger issue from now on. The library
> implementors will have to provide 2 separate implementations:
> 1. For `char`-based types (string_view, ostream) without Unicode signs
> 2. For Unicode char based types
>
> However, there are a few issues here:
> 1. As of now, we do not have std::u8cout or even std::u8ostream. So there
> is really no easy way to create and use a stream for Unicode characters. So
> even if I implement
>
> template<class CharT, class Traits>
> friend std::basic_ostream<CharT, Traits>&
> operator<<(std::basic_ostream<CharT, Traits>& os, const quantity& q)
>
> correctly, we do not have an easy way to use it.
>
> 2. In order to implement the above, I could imagine such an interface for
> a symbol prefix:
>
> template<typename CharT, typename Traits, typename Prefix, typename Ratio>
> inline constexpr std::basic_string_view<CharT, Traits> prefix_symbol;
>
> and its partial specializations for different prefixes/ratios:
>
> template<typename CharT, typename Traits>
> inline constexpr std::basic_string_view<char, Traits> prefix_symbol<char,
> Traits, si_prefix, std::micro> = "u";
> template<typename CharT, typename Traits>
> inline constexpr std::basic_string_view<CharT,
> Traits> prefix_symbol<CharT, Traits, si_prefix, std::micro> = u8"\u00b5";
> // µ
> template<typename CharT, typename Traits>
> inline constexpr std::basic_string_view<CharT,
> Traits> prefix_symbol<CharT, Traits, si_prefix, std::milli> = "m";
>
> The problem is that the above code will not compile. Specialization for
> all `CharT` will not be possible to be initialized with a literal like "m".
> Also, there is no generic mechanism to initialize all Unicode-based
> versions of the type with the same literal as each of them requires a
> different prefix (u8, u, U). Providing a specialization for every character
> type here is going to be a nightmare for library authors.
>
> To solve the second problem fmt and chrono defined something called
> STATICALLY-WIDEN (http://wg21.link/time.general) but it seems that it is
> more a specification hack rather than the implementation technique. I call
> it a hack as it currently addresses only `char` and `wchar_t` and does not
> mention Unicode characters at all as of now.
>
> Dear SG16 members, do you have any BKMs or suggestions on how to write a
> library that is Unicode aware and safe in an easy and approachable way?
> Should we strive to provide a nice-looking representation of units for
> outputs that support Unicode (console, files, etc) or should we, as ever
> before, just support only `char` and `wchar_t` and ignore the existence of
> Unicode in C++?
>
> Please keep in mind that the library is hoped to target C++23.
>
> Best
>
> Mat
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