Proposed resolution:

This wording is relative to N4928.

1. Modify [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — and Table 106 ([tab:locale.spec]) "Required specializations" as indicated:

   Table 105: Locale category facets [tab:locale.category.facets]

   Category	Includes facets
   …
   ctype	ctype<char>, ctype<wchar_t> codecvt<char, char, mbstate_t> codecvt<char16_t, char8_t, mbstate_t> codecvt<char32_t, char8_t, mbstate_t> codecvt<wchar_t, char, mbstate_t>
   …

   […]

   Table 106: Required specializations [tab:locale.spec]

   Category	Includes facets
   …
   ctype	ctype_byname<char>, ctype_byname<wchar_t> codecvt_byname<char, char, mbstate_t> codecvt_byname<char16_t, char8_t, mbstate_t> codecvt_byname<char32_t, char8_t, mbstate_t> codecvt_byname<wchar_t, char, mbstate_t>
   …

2. Modify [locale.codecvt.general] as indicated:

   […]

   -3- The specializations required in Table 105 ([locale.category]) convert the implementation-defined native character set. codecvt<char, char, mbstate_t> implements a degenerate conversion; it does not convert at all. The specialization codecvt<char16_t, char8_t, mbstate_t> converts between the UTF-16 and UTF-8 encoding forms, and the specialization codecvt<char32_t, char8_t, mbstate_t> converts between the UTF-32 and UTF-8 encoding forms. codecvt<wchar_t, char, mbstate_t> converts between the native character sets for ordinary and wide characters. Specializations on mbstate_t perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-defined stateT type. Objects of type stateT can contain any state that is useful to communicate to or from the specialized do_in or do_out members.

3. Modify [depr.locale.category] (Deprecated locale category facets) in Annex D as indicated:

   -1- The ctype locale category includes the following facets as if they were specified in table Table 105 [tab:locale.category.facets] of [locale.codecvt.general].

   codecvt<char16_t, char, mbstate_t>
   codecvt<char32_t, char, mbstate_t>
   codecvt<char16_t, char8_t, mbstate_t>
   codecvt<char32_t, char8_t, mbstate_t>
   

   -1- The ctype locale category includes the following facets as if they were specified in table Table 106 [tab:locale.spec] of [locale.codecvt.general].

   codecvt_byname<char16_t, char, mbstate_t>
   codecvt_byname<char32_t, char, mbstate_t>
   codecvt_byname<char16_t, char8_t, mbstate_t>
   codecvt_byname<char32_t, char8_t, mbstate_t>
   

   -3- The following class template specializations are required in addition to those specified in [locale.codecvt]. The specializations codecvt<char16_t, char, mbstate_t> and codecvt<char16_t, char8_t, mbstate_t> converts between the UTF-16 and UTF-8 encoding forms, and the specializations codecvt<char32_t, char, mbstate_t> and codecvt<char32_t, char8_t, mbstate_t> converts between the UTF-32 and UTF-8 encoding forms.

[ Tokyo 2024-03-23; Status changed: Voting → WP. ]

[ Kona 2023-11-07; move to Ready ]

[ 2023-02-10; Victor Zverovich comments and provides improved wording ]

Per today's LWG discussion the following changes have been implemented in revised wording:

• Deprecated the facets instead of removing them (also _byname variants which were previously missed).

• Removed the changes to facet dtor since with deprecation it's no longer critical to provide other ways to access them.

[ Issaquah 2023-02-10; LWG issue processing ]

Removing these breaks most code using them today, because the most obvious way to use them is via use_facet on a locale, which would throw if they're removed (and because they were guaranteed to be present, code using them might have not bothered to check for them using has_facet). Instead of removing them, deprecate the guarantee that they're always present (so move them to [depr.locale.category]). Don't bother changing the destructor. Victor to update wording.

[ 2022-09-28; SG16 responds ]

SG16 agrees that the codecvt facets mentioned in LWG3767 "codecvt<charN_t, char8_t, mbstate_t> incorrectly added to locale" are intended to be invariant with respect to locale. Unanimously in favor.

[ 2022-09-23; Reflector poll ]

Set priority to 3 after reflector poll. Send to SG16 (then maybe LEWG).

Table [tab:locale.category.facets] includes the following two facets:

• codecvt<char16_t, char8_t, mbstate_t>

• codecvt<char32_t, char8_t, mbstate_t>

However, neither of those actually has anything to do with a locale and therefore it doesn't make sense to dynamically register them with std::locale. Instead they provide conversions between fixed encodings (UTF-8, UTF-16, UTF-32) that are unrelated to locale encodings other than they may happen to coincide with encodings of some locales by accident.

The issue was introduced when adding codecvt<char[16|32]_t, char, mbstate_t> in N2035 which gave no design rationale for using codecvt in the first place. Likely it was trying to do a minimal amount of changes and copied the wording for codecvt<wchar_t, char, mbstate_t> but unfortunately didn't consider encoding implications.

P0482 changed char to char8_t in these facets which made the issue more glaring but unfortunately, despite the breaking change, it failed to address it.

Apart from an obvious design mistake this also adds a small overhead for every locale construction because the implementation has to copy these pseudo-facets for no good reason violating "don't pay for what you don't use" principle.

A simple fix is to remove the two facets from table [tab:locale.category.facets] and make them directly constructible.