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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109162] C++23 improvements to std::format
Date: Wed, 10 Jan 2024 22:52:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109162

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> (In reply to Jonathan Wakely from comment #0)
> > https://wg21.link/P2419R2 localized chrono formatting (also p2372r3)
>=20
> I think this this requires using nl_langinfo_l(CODESET, loc) to find out =
if
> the locale uses UTF-8, and then use iconv to convert to UTF-8 if it doesn=
't.
> But I'm not sure how we do that for an arbitrary std::locale which doesn't
> have an associated C locale, and we can't get its locale_t identifier any=
way.

I remain confused about how to implement this. We could use
newlocale(loc.name()) to try to open a C locale from the C++ locale's name,=
 and
if that works use nl_langinfo_l to get the locale's encoding, then use icon=
v to
convert to UTF-8.

Libc++ doesn't implement this yet. {fmt} does ... but it seems broken. It u=
ses
std::codecvt<char32_t, char, mbstate_t> to convert the locale's encoding to
UTF-32 and then converts that to UTF-8. But that's wrong. That codecvt
specialization converts between UTF-8 and UTF-32, it's locale-independent. =
So
the example in P2419 fails with a format_error exception when I run it:

#include <locale>
#include <print>
#include <chrono>
#include <iostream>
#include <fmt/format.h>
#include <fmt/chrono.h>

int main()
{
  std::locale::global(std::locale("ru_UA.koi8u"));
  std::string s =3D fmt::format("=D0=94=D0=B5=D0=BD=D1=8C =D0=BD=D0=B5=D0=
=B4=D0=B5=D0=BB=D0=B8: {:L}", std::chrono::Monday);
  std::cout << s << '\n';
}

terminate called after throwing an instance of 'fmt::v9::format_error'
  what():  failed to format time
Aborted (core dumped)=