On Mon, Aug 2, 2021 at 11:25 PM Charlie Barto <Charles.Barto@microsoft.com> wrote:
➢ or if the locale is not in the implementation-defined set of known locales, the value of ... is locale-specific 

This is highly problematic, this would disallow using the "utf8" version of time_put in a user locale that was formed by copying some "known" locale and adding facets, requiring the slow implementation. It's better if we talk about known facets rather than known locales so that the behavior for copied / combined locales stays consistent.

But didn't you just say it was tricky such locales?
If you can identify them easily, you can put them in the implementation defined set of known locales (known locales here is a bit of a misnomer... "known not to produce mojibake for the purpose of chrono formatter" 

From: Corentin Jabot <corentinjabot@gmail.com>
Sent: Monday, August 2, 2021 2:21 PM
To: SG16 <sg16@lists.isocpp.org>
Cc: Charlie Barto <Charles.Barto@microsoft.com>
Subject: Re: [SG16] Additional concerns for LWG3565: Handling of encodings in localized formatting of chrono types is underspecified

There isn't mention in the wording that time_put is used.

> %a The locale's abbreviated weekday name. If the value does not contain a valid weekday, an exception of type format_error is thrown.

Can we say something roughly like: 

If the format string encoding is UTF-8, for any locale in the implementation-defined set of known locales, the value of %a %A %b, %B %c %C %p %x %X
is UTF-8 encoded. [Note: Whether time_put is called is unspecified].
If the format string encoding is not UTF-8, or if the locale is not in the implementation-defined set of known locales, the value of ... is locale-specific 
[Note: locale-specific value may not be in the same text encoding that the format string]

With some bonus wording if we want to support other utf encodings?

On Mon, Aug 2, 2021 at 9:51 PM Charlie Barto via SG16 <mailto:sg16@lists.isocpp.org> wrote:
I was discussing this with a coworker (Billy) and he brought up the point that even if we have an allow-list of locales in which to do the transcoding the proposed resolution is _still_ quite difficult to implement, because users can call “combine” or use “locale::locale(locale&,facet*)” (or various other constructors) to shove arbitrary new facets into a locale. The returned locale will have a different name, or be unnamed, but we still need to handle the facets from the original locale the same as before (i.e. do the transcoding). Otherwise users will be chugging along happily transcoding locale specific text, decide they’d like to add a new facet, and suddenly get mojibake.

To support this in a way that doesn’t have this surprise we’d have to take the facet we’d like to use and compare it against every single “supported” version of that facet. This means if we allow-list locale names we would have to allocate _every single locale’s_ version of a given facet the first time a locale-sensitive chrono specifier was used. The comparison may involve a dynamic_cast (although I’m not 100% sure that’s really necessary), making it potentially quite expensive. For me this is a bit of a dealbreaker if LWG3565 is applied without P2372.

It may be better to just allow implementations to use some custom facet type (maybe time_get<uchar_t>, or maybe _Utf8_time_get<char>, etc) when doing chrono formatting. This may already be allowed by the wording in [time.format]. It does not appear to actually say we must use a particular locale, although that may be specified in ISO8601:2004, a copy of which I am trying to get a hold of. We could also say that conversion goes through a hypothetical std::codecvt<char, char8_t, std::mbstate_t> facet.

In general, I’m OK with the “{:L}” specifiers being a little broken, there’s only so much we can do since facets don’t have a way of communicating the encoding of their output (besides the codecvt facets). If/when we add some better locale handling support, we can always add an “{:LEx}” specifier (maybe we could use “{:ℒ}” or “{:𝔏}” 😊).

+1 on that last point

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