[libc++][format] LWG-3654 basic_format_context::arg(size_t) should be noexcept

[Mordante]

Summary

Note the update of the status table will be done in a separate commit.

[ldionne]

We have the LWG issues list now, so you could rebase onto that. LGTM but please rebase onto the LWG issue and mark it as fixed in this commit. Doing it in the same commit is useful because you can git blame the status page, and it shows you what commit fixed it exactly.

libcxx/docs/Status/Cxx2bIssues.csv

 "`3618 <https://wg21.link/LWG3618>`__","Unnecessary ``iter_move`` for ``transform_view::iterator`` ","February 2022","","","|ranges|"
 "`3619 <https://wg21.link/LWG3619>`__","Specification of ``vformat_to`` contains ill-formed ``formatted_size`` calls","February 2022","|Nothing to do|","","|format|"
 "`3621 <https://wg21.link/LWG3621>`__","Remove feature-test macro ``__cpp_lib_monadic_optional`` ","February 2022","",""
 "`3632 <https://wg21.link/LWG3632>`__","``unique_ptr`` ""Mandates: This constructor is not selected by class tmeplate argument deduction""","February 2022","|Nothing to do|",""
 "`3643 <https://wg21.link/LWG3643>`__","Missing ``constexpr`` in ``std::counted_iterator`` ","February 2022","",""
 "`3648 <https://wg21.link/LWG3648>`__","``format`` should not print ``bool`` with ``'c'`` ","February 2022","","","|format|"
 "`3649 <https://wg21.link/LWG3649>`__","[fund.ts.v2] Reinstate and bump ``__cpp_lib_experimental_memory_resource`` feature test macro","February 2022","",""
 "`3650 <https://wg21.link/LWG3650>`__","Are ``std::basic_string`` 's ``iterator`` and ``const_iterator`` constexpr iterators?","February 2022","|Nothing to do|",""
-"`3654 <https://wg21.link/LWG3654>`__","``basic_format_context::arg(size_t)`` should be ``noexcept`` ","February 2022","","","|format|"
+"`3654 <https://wg21.link/LWG3654>`__","``basic_format_context::arg(size_t)`` should be ``noexcept`` ","February 2022","|Complete|","15.0","|format|"
 "`3657 <https://wg21.link/LWG3657>`__","``std::hash<std::filesystem::path>`` is not enabled","February 2022","",""
 "`3660 <https://wg21.link/LWG3660>`__","``iterator_traits<common_iterator>::pointer`` should conform to §[iterator.traits]","February 2022","|Complete|","14.0"
 "`3661 <https://wg21.link/LWG3661>`__","``constinit atomic<shared_ptr<T>> a (nullptr);`` should work","February 2022","",""
 "","","","",""
 `3645 <https://wg21.link/LWG3645>`__,"``resize_and_overwrite`` is overspecified to call its callback with lvalues", "Not voted in","|Complete|","14.0",""
 "","","","",""

libcxx/include/__format/format_context.h

public:
   using char_type = _CharT;
   template <class _Tp>
   using formatter_type = formatter<_Tp, _CharT>;
 
   basic_format_context(const basic_format_context&) = delete;
   basic_format_context& operator=(const basic_format_context&) = delete;
 
   _LIBCPP_HIDE_FROM_ABI basic_format_arg<basic_format_context>
-  arg(size_t __id) const {
+  arg(size_t __id) const noexcept {
     return __args_.get(__id);
   }
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
   _LIBCPP_HIDE_FROM_ABI _VSTD::locale locale() {
     if (!__loc_)
       __loc_ = _VSTD::locale{};
     return *__loc_;
   }

libcxx/test/std/utilities/format/format.formatter/format.context/format.context/arg.pass.cpp

void test() {
   auto store = std::make_format_args<std::basic_format_context<OutIt, CharT>>(
       true, CharT('a'), 42, string);
   std::basic_format_args args = store;
 
   std::basic_string<CharT> output;
   const std::basic_format_context context =
       test_format_context_create(OutIt{output}, args);
   LIBCPP_ASSERT(args.__size() == 4);
+  ASSERT_NOEXCEPT(context.arg(0));
   for (size_t i = 0, e = args.__size(); i != e; ++i) {
     assert(context.arg(i));
   }
   assert(!context.arg(args.__size()));
 
   assert(test_basic_format_arg(context.arg(0), true));
   assert(test_basic_format_arg(context.arg(1), CharT('a')));
   assert(test_basic_format_arg(context.arg(2), 42));