SG16 will hold a telecon on Wednesday, October 25th, at 19:30 UTC
(timezone conversion
<https://www.timeanddate.com/worldclock/converter.html?iso=20231025T193000&p1=1440&p2=tz_pt&p3=tz_mt&p4=tz_ct&p5=tz_et&p6=tz_cest>).

The agenda follows.

  * charN_t, char_traits, codecvt, and iostreams:
      o P2873R0: Remove Deprecated Locale Category Facets For Unicode
        from C++26 <https://wg21.link/p2873r0>
      o LWG 3767: codecvt<charN_t, char8_t, mbstate_t> incorrectly added
        to locale <https://wg21.link/lwg3767>
      o LWG 2959: char_traits<char16_t>::eof is a valid UTF-16 code unit
        <https://wg21.link/lwg2959>
          + SG16 #32: std::char_traits<char16_t>::eof() requires
            uint_least16_t to be larger than 16 bits
            <https://github.com/sg16-unicode/sg16/issues/32>
      o SG16 #33: A correct codecvt facet that works with basic_filebuf
        can't do UTF conversions
        <https://github.com/sg16-unicode/sg16/issues/33>

Hang on, this is going to be a bumpy ride.

When char16_t and char32_t were added for C++11, the standard library
was extended to support corresponding specializations of
std::char_traits ([char.traits.general]p1
<http://eel.is/c++draft/char.traits.general#1>) and std::basic_string
([string.classes.general]p1
<http://eel.is/c++draft/string.classes#general-1>). Curiously, type
aliases were added for specializations of the std::fpos ([iosfwd.syn]
<http://eel.is/c++draft/iosfwd.syn#lib:fpos>) class template (but only
in the synopsis) and support for these types was added for the
std::codecvt ([tab:locale.category.facets]
<http://eel.is/c++draft/locale.category#tab:locale.category.facets>) and
std::codecvt_byname ([tab:locale.spec]
<http://eel.is/c++draft/locale.category#tab:locale.spec>) locale facets,
but not for any of the other locale facets nor for iostreams in general.
Support for these types was added to std::basic_string_view
([string.view.synop] <http://eel.is/c++draft/string.view.synop>) and
std::filesystem::path ([fs.path.type.cvt]p2
<http://eel.is/c++draft/fs.path.type.cvt#2>) in C++17, but no additional
support was ever extended to iostreams. The status quo is thus that the
standard requires implementations to provide some fragments (std::fpos,
std::codecvt, and std::codecvt_byname) of iostream support for these
types despite there being no use of these type aliases and
specializations in the standard; implementations are not required to
support streams of char16_t or char32_t.

std::char_traits is used by both the string library (e.g.,
std::basic_string) and iostreams. However, the string library only
depends on some of the std::char_traits members; it does not make use of
the int_type member type alias nor any of the member functions that
depend on that type (eof(), ​not_eof(), ​to_char_type(), ​to_int_type(),
​eq_int_type()). Per LWG 2959 <https://wg21.link/lwg2959> and SG16 #32
<https://github.com/sg16-unicode/sg16/issues/32>, the specified
std::char_traits<char16_t> specialization has a defect; all char16_t
values are valid code unit values, but the int_type member type alias is
defined as uint_least16_t (the same underlying type as char16_t) and it
is thus unable to hold a distinct value for EOF. The obvious fix is to
use a larger type for int_type, but that would result in an ABI break. I
recently asked the ABI review group if there are any known tricks they
could deploy to mitigate an ABI break, but no direct solutions were
identified; a suggestion to provide an alternative type for
std::char_traits<char16_t> that programmers would have to explicitly use
instead of the broken specialization was offered. That is an option, but
since the problematic int_type member is not actually used by any
functionality the standard requires implementors to provide, an ABI
break in this case might have little practical consequence.

When char8_t was added for C++20 via P0482R6 (char8_t: A type for UTF-8
characters and strings) <https://wg21.link/p0482>, I failed to
understand the intended purpose for which std::codecvt was added to the
standard. My impression of it at the time was that it was a poorly
designed general transcoding facility; I failed to appreciate its
significance as a locale facet as used by iostreams. This resulted in
two mistakes:

 1. I deprecated the following specializations (and their use as locale
    category facets):
    std::codecvt<char16_t, char, std::mbstate_t>
    std::codecvt<char32_t, char, std::mbstate_t>
    std::codecvt_byname<char16_t, char, std::mbstate_t>
    std::codecvt_byname<char32_t, char, std::mbstate_t>
 2. I added the following specializations as required locale category
    facets (adding the specializations themselves is arguably not a
    mistake, but adding them as locale category facets is):
    std::codecvt<char16_t, char8_t, std::mbstate_t>
    std::codecvt<char32_t, char8_t, std::mbstate_t>
    std::codecvt_byname<char16_t, char8_t, std::mbstate_t>
    std::codecvt_byname<char32_t, char8_t, std::mbstate_t>

Note that std::codecvt facets are only used by std::basic_filebuf which
only ever converts to and from elements of type char; the facets that
convert to and from char8_t are not substitutable for that purpose.

P2873R0 <https://wg21.link/p2873r0>, which SG16 already approved (or,
rather, did not object to) during the 2023-05-26 SG16 meeting
<https://github.com/sg16-unicode/sg16-meetings#may-24th-2023>, now seeks
to remove the deprecated specializations. LWG 3767
<https://wg21.link/lwg3767> tracks addressing the incorrect addition of
the char8_t specializations as locale facets.

Arguably, P0482R6 <https://wg21.link/p0482> should have added the
following specializations as locale facets:

  * std::codecvt<char8_t, char, std::mbstate_t>
  * std::codecvt_byname<char8_t, char, std::mbstate_t>

The only specification for std::codecvt_byname in the standard is the
synopsis in [locale.codecvt.byname]
<http://eel.is/c++draft/locale.codecvt.byname>; there is no other
wording present.

As mentioned, the standard does not require implementations to provide
iostream support for the charN_t types. However, implementations may do
so as an extension. If they do, then, per [filebuf.general]p7
<http://eel.is/c++draft/input.output#filebuf.general-7>, specializations
of std::codecvt<charN_t, char, std::mbstate_t> are required to be
available via a call to std::use_facet() for the imbued locale. In which
case, per the standard, the status of the necessary specializations are:

  * std::codecvt<char8_t, char, std::mbstate_t> # Not specified.
  * std::codecvt<char16_t, char, std::mbstate_t> # Deprecated.
  * std::codecvt<char32_t, char, std::mbstate_t> # Deprecated.

If it is desirable to provide a better foundation for iostream support
of the charN_t types, either for a future version of the standard, or
for implementations that want to provide such support as an extension,
we could undeprecate the previously deprecated specializations and add
the missing one for char8_t. Since iostreams does not support charN_t in
the standard today and since the char16_t and char32_t specializations
have already been deprecated for two release cycles, perhaps it is even
reasonable to change their behavior so that they convert to and from the
locale encoding rather than UTF-8. This would remove the existing
inconsistency with the corresponding char and wchar_t specializations
that was part of the motivation for their deprecation in the first place
(see the discussion of codecvt in the Motivation section of P0482R6
<https://wg21.link/p0482r6#motivation>).

However, an endeavor to improve the situation for iostreams and charN_t
next runs into SG16 #33
<https://github.com/sg16-unicode/sg16/issues/33>; std::basic_fstream
does not support the UTF-8 and UTF-16 encodings for the "internal" side
of a std::codecvt conversion because std::basic_filebuf requires that,
per [locale.codecvt.virtuals]p4
<http://eel.is/c++draft/locale.codecvt#virtuals-4> and its related
footnote <http://eel.is/c++draft/locale.codecvt#footnote-246>,
"internal" characters are mapped 1-N to "external" characters. This is
an existing issue for std::basic_fstream<wchar_t> with UTF-16 data.

The Microsoft and libstdc++ standard library implementations appear to
support iostreams with charN_t types; at least on the surface. Libc++
intentionally does not provide definitions for charN_t specializations
of locale facets that are not required by the standard and this suffices
for basic usage to provoke compilation errors. I have not yet
investigated to what extent the Microsoft and libstdc++ implementations
work as might be expected. My impression is that, where they do produce
expected results, it is serendipity at work. See
https://godbolt.org/z/6T7hebY33 for a bit of fun (testing on Windows
requires changes to use an actual zero valued file since Windows doesn't
provide a builtin analog for /dev/zero, but in that case, MSVC produces
an executable that behaves as might be expected).

I haven't looked hard, but I have not yet identified any code in the
wild that uses iostreams with charN_t types. One would think that, if
any project did, it would be ICU. I confirmed that ICU, despite its use
of char16_t, makes no attempt to use it with iostreams.

So where is this all going? I see three general options that can be
pursued to resolve these various issues.

 1. We can fix these issues, despite the acknowledged ABI impact, so
    that the standard no longer actively hiders support for iostreams
    with the charN_t types. Optionally, we could further explore
    requiring such support in the standard (doing so would require
    adding charN_t support to more locale facets).
 2. We can declare that iostreams will never support the charN_t types
    in the standard and deprecate and remove the fragments of such
    support that are present. Implementations could of course provide
    support as an extension if they so desire.
 3. We can admit things are broken, choose to do nothing about it, and
    close the related LWG issues while chanting sorry-not-sorry.

The above issues are sufficiently complicated that I believe a paper is
warranted regardless of the direction that we favor. I'm signing up to
write that paper since I'm responsible for some of the mess. I do not
intend to poll any directions in this meeting; rather, the focus is to
ensure that the issues are well understood, to discuss decisions we
could make and their potential consequences, and to generally collect
information that will lead to a better paper.

Responses provided before the meeting to identify other existing related
issues or considerations would be appreciated. Ideal responses do not
include the phrase "burn it all to the ground".

Tom.