[libc++] Adds tests for LWG-3373.

[Mordante]

Summary

The code in libc++ already satisfy the requirements of LWG-3373. Since
the issue was written to specifically allow the types to be used in
structured bindings, tests have been added to validate the new
requirement.

Implements
LWG-3373 {to,from}_chars_result and format_to_n_result need the "we really mean what we say" wording

[Mordante]

Fix the CI.

libcxx/docs/Status/Cxx20Issues.csv

 "`3360 <https://wg21.link/LWG3360>`__","``three_way_comparable_with``\  is inconsistent with similar concepts","Prague","|Nothing To Do|","","|spaceship|"
 "`3362 <https://wg21.link/LWG3362>`__","Strike ``stop_source``\ 's ``operator!=``\ ","Prague","",""
 "`3363 <https://wg21.link/LWG3363>`__","``drop_while_view``\  should opt-out of ``sized_range``\ ","Prague","","","|ranges|"
 "`3364 <https://wg21.link/LWG3364>`__","Initialize data members of ranges and their iterators","Prague","","","|ranges|"
 "`3367 <https://wg21.link/LWG3367>`__","Integer-class conversions should not throw","Prague","",""
 "`3369 <https://wg21.link/LWG3369>`__","``span``\ 's deduction-guide for built-in arrays doesn't work","Prague","|Complete|","14.0"
 "`3371 <https://wg21.link/LWG3371>`__","``visit_format_arg``\  and ``make_format_args``\  are not hidden friends","Prague","|Complete|","14.0","|format|"
 "`3372 <https://wg21.link/LWG3372>`__","``vformat_to``\  should not try to deduce ``Out``\  twice","Prague","|Complete|","14.0","|format|"
-"`3373 <https://wg21.link/LWG3373>`__","``{to,from}_chars_result``\  and ``format_to_n_result``\  need the  ""we really mean what we say"" wording","Prague","","","|format|"
+"`3373 <https://wg21.link/LWG3373>`__","``{to,from}_chars_result``\  and ``format_to_n_result``\  need the  ""we really mean what we say"" wording","Prague","|Complete|","14.0","|format|"
 "`3374 <https://wg21.link/LWG3374>`__","P0653 + P1006 should have made the other ``std::to_address``\  overload ``constexpr``\ ","Prague","|Complete|","12.0"
 "`3375 <https://wg21.link/LWG3375>`__","``decay``\  in ``viewable_range``\  should be ``remove_cvref``\ ","Prague","","","|ranges|"
 "`3377 <https://wg21.link/LWG3377>`__","``elements_view::iterator``\  befriends a specialization of itself","Prague","","","|ranges|"
 "`3379 <https://wg21.link/LWG3379>`__","""``safe``\ "" in several library names is misleading","Prague","|In Progress|",""
 "`3380 <https://wg21.link/LWG3380>`__","``common_type``\  and comparison categories","Prague","","","|spaceship|"
 "`3381 <https://wg21.link/LWG3381>`__","``begin``\  and ``data``\  must agree for ``contiguous_range``\ ","Prague","","","|ranges|"
 "`3382 <https://wg21.link/LWG3382>`__","NTTP for ``pair``\  and ``array``\ ","Prague","",""
 "`3383 <https://wg21.link/LWG3383>`__","|sect|\ [time.zone.leap.nonmembers] ``sys_seconds``\  should be replaced with ``seconds``\ ","Prague","","","|chrono|"

libcxx/test/std/utilities/charconv/charconv.syn/from_chars_result.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: c++20 && libcpp-no-concepts
 
 // <charconv>
 
 // struct from_chars_result
 //   friend bool operator==(const from_chars_result&, const from_chars_result&) = default;
 
+// [charconv.syn]/2
+// The types to_chars_result and from_chars_result have the data members and
+// special members specified above. They have no base classes or members other
+// than those specified.
+
 #include <charconv>
 
 #include <cassert>
 #include <compare>
 #include <concepts>
 
 #include "test_macros.h"
 
 constexpr bool test() {
   std::from_chars_result lhs{nullptr, std::errc{}};
+#if TEST_STD_VER > 17
   std::from_chars_result rhs{nullptr, std::errc{}};
   assert(lhs == rhs);
   assert(!(lhs != rhs));
+#endif
+  auto [ptr, ec] = lhs;
+  static_assert(std::is_same_v<decltype(ptr), const char*>);
+  assert(ptr == lhs.ptr);
+  static_assert(std::is_same_v<decltype(ec), std::errc>);
+  assert(ec == lhs.ec);
 
   return true;
 }
 
 int main(int, char**) {
+#if TEST_STD_VER > 17
   static_assert(std::equality_comparable<std::from_chars_result>);
   static_assert(!std::totally_ordered<std::from_chars_result>);
   static_assert(!std::three_way_comparable<std::from_chars_result>);
+#endif
 
   assert(test());
   static_assert(test());
 
   return 0;
 }

libcxx/test/std/utilities/charconv/charconv.syn/to_chars_result.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: c++20 && libcpp-no-concepts
 
 // <charconv>
 
 // struct to_chars_result
 //   friend bool operator==(const to_chars_result&, const to_chars_result&) = default;
 
+// [charconv.syn]/2
+// The types to_chars_result and from_chars_result have the data members and
+// special members specified above. They have no base classes or members other
+// than those specified.
+
 #include <charconv>
 
 #include <cassert>
 #include <compare>
 #include <concepts>
 
 #include "test_macros.h"
 
 constexpr bool test() {
   std::to_chars_result lhs{nullptr, std::errc{}};
+#if TEST_STD_VER > 17
   std::to_chars_result rhs{nullptr, std::errc{}};
   assert(lhs == rhs);
   assert(!(lhs != rhs));
+#endif
+  auto [ptr, ec] = lhs;
+  static_assert(std::is_same_v<decltype(ptr), char*>);
+  assert(ptr == lhs.ptr);
+  static_assert(std::is_same_v<decltype(ec), std::errc>);
+  assert(ec == lhs.ec);
 
   return true;
 }
 
 int main(int, char**) {
+#if TEST_STD_VER > 17
   static_assert(std::equality_comparable<std::to_chars_result>);
   static_assert(!std::totally_ordered<std::to_chars_result>);
   static_assert(!std::three_way_comparable<std::to_chars_result>);
+#endif
 
   assert(test());
   static_assert(test());
 
   return 0;
 }

libcxx/test/std/utilities/format/format.syn/format_to_n_result.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// struct format_to_n_result
+
+// [format.syn]/1
+// The class template format_to_n_result has the template parameters, data
+// members, and special members specified above. It has no base classes or
+// members other than those specified.
+
+#include <format>
+
+#include <cassert>
+#include <concepts>
+
+#include "test_macros.h"
+
+template <class CharT>
+constexpr void test() {
+  std::format_to_n_result<CharT*> v{nullptr, std::iter_difference_t<CharT*>{42}};
+
+  auto [out, size] = v;
+  static_assert(std::same_as<decltype(out), CharT*>);
+  assert(out == v.out);
+  static_assert(std::same_as<decltype(size), std::iter_difference_t<CharT*>>);
+  assert(size == v.size);
+}
+
+constexpr bool test() {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return true;
+}
+
+int main(int, char**) {
+  assert(test());
+  static_assert(test());
+
+  return 0;
+}