[libc++][format] Addresses LWG3881.

[Mordante]

Summary

LWG3881 Incorrect formatting of container adapters backed by std::string

[Mordante]

CI fixes.

[ldionne]

libcxx/include/__format/container_adaptor.h

42

Any reason why you're using ref_view instead of all_t? Those are not always equivalent -- is the difference not relevant?

[Mordante]

libcxx/include/__format/container_adaptor.h

42

Mainly since that is what the wording in the LWG issue is. Victor's original proposed wording did use views::all_t.
This is the rationale in the issue to make the change:

[2023-02-10 Tim provides updated wording]

The container elements may not be const-formattable so we cannot use the const formatter unconditionally. Also the current wording is broken because an adaptor is not range and we cannot use fmt-maybe-const on the adaptor — only the underlying container.

[tcanens]

libcxx/include/__format/container_adaptor.h

42

This is for passing a slightly disguised container into a function, not for view composition. It does not need the sometimes-by-reference-sometimes-by-copy-sometimes-ill-formed behavior of all_t.

[Mordante]

libcxx/include/__format/container_adaptor.h

42

Thanks for the information.

[ldionne]

libcxx/include/__format/container_adaptor.h

42

Ah thanks, I think I had missed that the original resolution was superseded.

libcxx/test/std/containers/container.adaptors/container.adaptors.format/format.functions.tests.h

254

libcxx/docs/Status/Cxx2bIssues.csv

 "`3871 <https://wg21.link/LWG3871>`__","Adjust note about ``terminate``","February 2023","","",""
 "`3872 <https://wg21.link/LWG3872>`__","``basic_const_iterator`` should have custom ``iter_move``","February 2023","","",""
 "`3875 <https://wg21.link/LWG3875>`__","``std::ranges::repeat_view<T, IntegerClass>::iterator`` may be ill-formed","February 2023","","","|ranges|"
 "`3876 <https://wg21.link/LWG3876>`__","Default constructor of ``std::layout_XX::mapping`` misses precondition","February 2023","","",""
 "`3877 <https://wg21.link/LWG3877>`__","Incorrect constraints on ``const``-qualified monadic overloads for ``std::expected``","February 2023","","",""
 "`3878 <https://wg21.link/LWG3878>`__","import ``std;`` should guarantee initialization of standard iostreams objects","February 2023","","",""
 "`3879 <https://wg21.link/LWG3879>`__","``erase_if`` for ``flat_{,multi}set`` is incorrectly specified","February 2023","","",""
 "`3880 <https://wg21.link/LWG3880>`__","Clarify ``operator+=`` complexity for ``{chunk,stride}_view::iterator``","February 2023","","","|ranges|"
-"`3881 <https://wg21.link/LWG3881>`__","Incorrect formatting of container adapters backed by ``std::string``","February 2023","","","|format|"
+"`3881 <https://wg21.link/LWG3881>`__","Incorrect formatting of container adapters backed by ``std::string``","February 2023","|Complete|","17.0","|format|"
 "","","","","",""
 "`3343 <https://wg21.link/LWG3343>`__","Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Yet Adopted","|Complete|","16.0",""

libcxx/include/__format/container_adaptor.h

 #  pragma GCC system_header
 #endif
 
 #include <__availability>
 #include <__config>
 #include <__format/concepts.h>
 #include <__format/formatter.h>
 #include <__format/range_default_formatter.h>
+#include <__ranges/all.h>
+#include <__ranges/ref_view.h>
 #include <queue>
 #include <stack>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 23
 
 // [container.adaptors.format] only specifies the library should provide the
 // formatter specializations, not which header should provide them.
 // Since <format> includes a lot of headers, add these headers here instead of
 // adding more dependencies like, locale, optinal, string, tuple, etc. to the
 // adaptor headers. To use the format functions users already include <format>.
 
 template <class _Adaptor, class _CharT>
 struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __formatter_container_adaptor {
 private:
-  using __maybe_const_adaptor = __fmt_maybe_const<_Adaptor, _CharT>;
-  formatter<typename _Adaptor::container_type, _CharT> __underlying_;
+  using __maybe_const_container = __fmt_maybe_const<typename _Adaptor::container_type, _CharT>;
+  using __maybe_const_adaptor   = __maybe_const<is_const_v<__maybe_const_container>, _Adaptor>;
+  formatter<ranges::ref_view<__maybe_const_container>, _CharT> __underlying_;
 
 public:
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
     return __underlying_.parse(__ctx);
   }
 
   template <class _FormatContext>

libcxx/test/std/containers/container.adaptors/container.adaptors.format/format.functions.tests.h

void test_char(TestFunction check, ExceptionTest check_exception) {
   test_char_string<CharT>(check, check_exception, std::queue{input.begin(), input.end()});
   test_char_string<CharT>(check, check_exception, std::priority_queue{input.begin(), input.end(), std::greater{}});
   test_char_string<CharT>(check, check_exception, std::stack{input.begin(), input.end()});
 
   test_char_escaped_string<CharT>(check, check_exception, std::queue{input.begin(), input.end()});
   test_char_escaped_string<CharT>(
       check, check_exception, std::priority_queue{input.begin(), input.end(), std::greater{}});
   test_char_escaped_string<CharT>(check, check_exception, std::stack{input.begin(), input.end()});
+
+  // LWG3881 fixes formatting container adaptors backed by a std::string.
+  test_char_default<CharT>(check, check_exception, std::queue{std::basic_string<CharT>{input.begin(), input.end()}});
+  test_char_default<CharT>(
+      check,
+      check_exception,
+      std::priority_queue{std::greater{}, std::basic_string<CharT>{input.begin(), input.end()}});
+  test_char_default<CharT>(check, check_exception, std::stack{std::basic_string<CharT>{input.begin(), input.end()}});
 }
 
 //
 // char -> wchar_t
 //
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class TestFunction, class ExceptionTest>