[libc++][format] Exposes basic-format-string

[Mordante]

Summary

This paper was accepted during the last plenary and is intended to be
backported to LLVM 15. When backporting the release notes in the branch
should be updated too.

Note the feature-test macro isn't updated since this will change; three
papers have updated the same macro in the same plenary.

Implements:

• P2508R1 Exposing std::basic-format-string

[Mordante]

Fixes CI issue.

[Mordante]

More CI fixes.

[Mordante]

Fixes CI.

[ldionne]

libcxx/docs/UsingLibcxx.rst

448

451–455

IMO it's not necessary to mention that we never shipped the previous names, since they were exposition-only. Also, it's mostly relevant for us libc++ developers to understand this change, but not for users who will consume this documentation.

libcxx/include/format

28–30

Here and elsewhere

libcxx/test/std/utilities/format/format.fmt.string/ctor.verify.cpp

10–12

Can you please set this line apart and add a comment like this? Applies elsewhere too.

libcxx/utils/generate_feature_test_macro_components.py

313–314

[Mordante]

Rebased and addresses review comments.
Make sure the CI passes after fixing the bootstrapping build.

libcxx/docs/Status/Cxx2bPapers.csv

 "`P2446R2 <https://wg21.link/P2446R2>`__","LWG","``views::as_rvalue``","July 2022","",""
 "`P2460R2 <https://wg21.link/P2460R2>`__","LWG","Relax requirements on ``wchar_t`` to match existing practices","July 2022","",""
 "`P2465R3 <https://wg21.link/P2465R3>`__","LWG","Standard Library Modules ``std`` and ``std.compat``","July 2022","",""
 "`P2467R1 <https://wg21.link/P2467R1>`__","LWG","Support exclusive mode for ``fstreams``","July 2022","",""
 "`P2474R2 <https://wg21.link/P2474R2>`__","LWG","``views::repeat``","July 2022","",""
 "`P2494R2 <https://wg21.link/P2494R2>`__","LWG","Relaxing range adaptors to allow for move only types","July 2022","",""
 "`P2499R0 <https://wg21.link/P2499R0>`__","LWG","``string_view`` range constructor should be ``explicit``","July 2022","",""
 "`P2502R2 <https://wg21.link/P2502R2>`__","LWG","``std::generator``: Synchronous Coroutine Generator for Ranges","July 2022","",""
-"`P2508R1 <https://wg21.link/P2508R1>`__","LWG","Exposing ``std::basic-format-string``","July 2022","",""
+"`P2508R1 <https://wg21.link/P2508R1>`__","LWG","Exposing ``std::basic-format-string``","July 2022","|Complete|","15.0"
 "`P2513R4 <https://wg21.link/P2513R4>`__","LWG","``char8_t`` Compatibility and Portability Fixes","July 2022","",""
 "`P2517R1 <https://wg21.link/P2517R1>`__","LWG","Add a conditional ``noexcept`` specification to ``std::apply``","July 2022","",""
 "`P2520R0 <https://wg21.link/P2520R0>`__","LWG","``move_iterator`` should be a random access iterator","July 2022","",""
 "`P2540R1 <https://wg21.link/P2540R1>`__","LWG","Empty Product for certain Views","July 2022","",""
 "`P2549R1 <https://wg21.link/P2549R1>`__","LWG","``std::unexpected`` should have ``error()`` as member accessor","July 2022","",""
 "`P2553R1 <https://wg21.link/P2553R1>`__","LWG","Make ``mdspan`` ``size_type`` controllable","July 2022","",""
 "`P2554R0 <https://wg21.link/P2554R0>`__","LWG","C-Array Interoperability of MDSpan","July 2022","",""
 "`P2585R0 <https://wg21.link/P2585R0>`__","LWG","Improving default container formatting","July 2022","",""
 "`P2590R2 <https://wg21.link/P2590R2>`__","LWG","Explicit lifetime management","July 2022","",""
 "`P2599R2 <https://wg21.link/P2599R2>`__","LWG","``mdspan::size_type`` should be ``index_type``","July 2022","",""
 "`P2604R0 <https://wg21.link/P2604R0>`__","LWG","mdspan: rename pointer and contiguous","July 2022","",""
 "`P2613R1 <https://wg21.link/P2613R1>`__","LWG","Add the missing ``empty`` to ``mdspan``","July 2022","",""
 "","","","","",""

libcxx/docs/Status/FormatIssues.csv

 Number,Name,Standard,Assignee,Status,First released version
 `P0645 <https://wg21.link/P0645>`_,"Text Formatting","C++20",Mark de Wever,|Complete|,Clang 14
 `P1652 <https://wg21.link/P1652>`_,"Printf corner cases in std::format","C++20",Mark de Wever,|Complete|,Clang 14
 `P1892 <https://wg21.link/P1892>`_,"Extended locale-specific presentation specifiers for std::format","C++20",Mark de Wever,|Complete|,Clang 14
 `P1868 <https://wg21.link/P1868>`_,"width: clarifying units of width and precision in std::format (Implements the unicode support.)","C++20",Mark de Wever,|Complete|,Clang 14
 `P2216 <https://wg21.link/P2216>`_,"std::format improvements","C++20",Mark de Wever,|Complete|,Clang 15
 `P2418 <https://wg21.link/P2418>`__,"Add support for ``std::generator``-like types to ``std::format``","C++20",Mark de Wever,|Complete|, Clang 15
 "`P2093R14 <https://wg21.link/P2093R14>`__","Formatted output","C++23"
 "`P2286R8 <https://wg21.link/P2286R8>`__","Formatting Ranges","C++23","Mark de Wever","|In Progress|"
-"`P2508R1 <https://wg21.link/P2508R1>`__","Exposing ``std::basic-format-string``","C++23","Mark de Wever","|In Progress|"
+"`P2508R1 <https://wg21.link/P2508R1>`__","Exposing ``std::basic-format-string``","C++23","Mark de Wever","|Complete|", Clang 15
 "`P2585R0 <https://wg21.link/P2585R0>`__","Improving default container formatting","C++23",
 
 `P1361 <https://wg21.link/P1361>`_,"Integration of chrono with text formatting","C++20",Mark de Wever,|In Progress|,
 `P2372 <https://wg21.link/P2372>`__,"Fixing locale handling in chrono formatters","C++20",Mark de Wever,|In Progress|,
 "`P2419R2 <https://wg21.link/P2419R2>`__","Clarify handling of encodings in localized formatting of chrono types","C++23",
 

libcxx/docs/UsingLibcxx.rst

 Additional types supported in random distributions
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The `C++ Standard <http://eel.is/c++draft/rand#req.genl-1.5>`_ mentions that instantiating several random number
 distributions with types other than ``short``, ``int``, ``long``, ``long long``, and their unsigned versions is
 undefined. As an extension, libc++ supports instantiating ``binomial_distribution``, ``discrete_distribution``,
 ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``, and ``uniform_int_distribution``
 with ``int8_t``, ``__int128_t`` and their unsigned versions.
+
+Extensions ``<format>``
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The exposition only type ``basic-format-string`` and its typedefs
+``format-string`` and ``wformat-string`` became ``basic_format_string``,
+``format_string``, and ``wformat_string`` in C++23. As an extension libc++
+makes these types available in C++20. (The exposition only types were never
+shipped.)

libcxx/include/format

namespace std {
   using format_context = basic_format_context<unspecified, char>;
   using wformat_context = basic_format_context<unspecified, wchar_t>;
 
   // [format.args], class template basic_format_args
   template<class Context> class basic_format_args;
   using format_args = basic_format_args<format_context>;
   using wformat_args = basic_format_args<wformat_context>;
 
-  // [format.fmt.string], class template basic-format-string
+  // [format.fmt.string], class template basic_format_string
   template<class charT, class... Args>
-    struct basic-format-string;                       // exposition only
+    struct basic_format_string {                                // since C++23, before an exposition only type
+  private:
+    basic_string_view<charT> str;                             // exposition only
+
+    public:
+    template<class T> consteval basic_format_string(const T& s);
 
+    constexpr basic_string_view<charT> get() const noexcept { return str; }
+  };
   template<class... Args>
-    using format-string =                             // exposition only
-      basic-format-string<char, type_identity_t<Args>...>;
+    using format_string =                                       // since C++23, before an exposition only type
+      basic_format_string<char, type_identity_t<Args>...>;
   template<class... Args>
-    using wformat-string =                            // exposition only
-      basic-format-string<wchar_t, type_identity_t<Args>...>;
+    using wformat_string =                                      // since C++23, before an exposition only type
+      basic_format_string<wchar_t, type_identity_t<Args>...>;
 
   // [format.functions], formatting functions
   template<class... Args>
     string format(format-string<Args...> fmt, Args&&... args);
   template<class... Args>
     wstring format(wformat-string<Args...> fmt, Args&&... args);
   template<class... Args>
     string format(const locale& loc, format-string<Args...> fmt, Args&&... args);
public:
   _LIBCPP_HIDE_FROM_ABI constexpr __compile_time_handle()
       : __parse_([](basic_format_parse_context<_CharT>&) { __throw_format_error("Not a handle"); }) {}
 
 private:
   void (*__parse_)(basic_format_parse_context<_CharT>&);
 };
 
 // Dummy format_context only providing the parts used during constant
-// validation of the basic-format-string.
+// validation of the basic_format_string.
 template <class _CharT>
 struct _LIBCPP_TEMPLATE_VIS __compile_time_basic_format_context {
 public:
   using char_type = _CharT;
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __compile_time_basic_format_context(
       const __arg_t* __args, const __compile_time_handle<_CharT>* __handles, size_t __size)
       : __args_(__args), __handles_(__handles), __size_(__size) {}
while (__begin != __end) {
     *__out_it++ = *__begin++;
   }
   return __out_it;
 }
 
 } // namespace __format
 
 template <class _CharT, class... _Args>
-struct _LIBCPP_TEMPLATE_VIS __basic_format_string {
-  basic_string_view<_CharT> __str_;
-
+struct _LIBCPP_TEMPLATE_VIS basic_format_string {
   template <class _Tp>
     requires convertible_to<const _Tp&, basic_string_view<_CharT>>
-  consteval __basic_format_string(const _Tp& __str) : __str_{__str} {
+  consteval basic_format_string(const _Tp& __str) : __str_{__str} {
     __format::__vformat_to(basic_format_parse_context<_CharT>{__str_, sizeof...(_Args)},
                            _Context{__types_.data(), __handles_.data(), sizeof...(_Args)});
   }
 
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT constexpr basic_string_view<_CharT> get() const noexcept {
+    return __str_;
+  }
+
 private:
+  basic_string_view<_CharT> __str_;
+
   using _Context = __format::__compile_time_basic_format_context<_CharT>;
 
   static constexpr array<__format::__arg_t, sizeof...(_Args)> __types_{
       __format::__determine_arg_t<_Context, remove_cvref_t<_Args>>()...};
 
   // TODO FMT remove this work-around when the AIX ICE has been resolved.
 #    if defined(_AIX) && defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER < 1400
   template <class _Tp>
if (__format::__determine_arg_t<_Context, _Tp>() == __format::__arg_t::__handle)
       __handle.template __enable<_Tp>();
 
     return __handle;
   }()...};
 #    endif
 };
 
 template <class... _Args>
-using __format_string_t = __basic_format_string<char, type_identity_t<_Args>...>;
+using format_string = basic_format_string<char, type_identity_t<_Args>...>;
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
-using __wformat_string_t = __basic_format_string<wchar_t, type_identity_t<_Args>...>;
+using wformat_string = basic_format_string<wchar_t, type_identity_t<_Args>...>;
 #endif
 
 template <class _OutIt, class _CharT, class _FormatOutIt>
 requires(output_iterator<_OutIt, const _CharT&>) _LIBCPP_HIDE_FROM_ABI _OutIt
     __vformat_to(
         _OutIt __out_it, basic_string_view<_CharT> __fmt,
         basic_format_args<basic_format_context<_FormatOutIt, _CharT>> __args) {
   if constexpr (same_as<_OutIt, _FormatOutIt>)

 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT _OutIt
 vformat_to(_OutIt __out_it, wstring_view __fmt, wformat_args __args) {
   return _VSTD::__vformat_to(_VSTD::move(__out_it), __fmt, __args);
 }
 #endif
 
 template <output_iterator<const char&> _OutIt, class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT _OutIt
-format_to(_OutIt __out_it, __format_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::vformat_to(_VSTD::move(__out_it), __fmt.__str_,
+format_to(_OutIt __out_it, format_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::vformat_to(_VSTD::move(__out_it), __fmt.get(),
                            _VSTD::make_format_args(__args...));
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <output_iterator<const wchar_t&> _OutIt, class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT _OutIt
-format_to(_OutIt __out_it, __wformat_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::vformat_to(_VSTD::move(__out_it), __fmt.__str_,
+format_to(_OutIt __out_it, wformat_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::vformat_to(_VSTD::move(__out_it), __fmt.get(),
                            _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 _LIBCPP_ALWAYS_INLINE inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string
 vformat(string_view __fmt, format_args __args) {
   string __res;
   _VSTD::vformat_to(_VSTD::back_inserter(__res), __fmt, __args);
   return __res;
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 _LIBCPP_ALWAYS_INLINE inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT wstring
 vformat(wstring_view __fmt, wformat_args __args) {
   wstring __res;
   _VSTD::vformat_to(_VSTD::back_inserter(__res), __fmt, __args);
   return __res;
 }
 #endif
 
 template <class... _Args>
-_LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string format(__format_string_t<_Args...> __fmt,
+_LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string format(format_string<_Args...> __fmt,
                                                                                       _Args&&... __args) {
-  return _VSTD::vformat(__fmt.__str_, _VSTD::make_format_args(__args...));
+  return _VSTD::vformat(__fmt.get(), _VSTD::make_format_args(__args...));
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT wstring
-format(__wformat_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::vformat(__fmt.__str_, _VSTD::make_wformat_args(__args...));
+format(wformat_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::vformat(__fmt.get(), _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 template <class _Context, class _OutIt, class _CharT>
 _LIBCPP_HIDE_FROM_ABI format_to_n_result<_OutIt> __vformat_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n,
                                                                 basic_string_view<_CharT> __fmt,
                                                                 basic_format_args<_Context> __args) {
   __format::__format_to_n_buffer<_OutIt, _CharT> __buffer{_VSTD::move(__out_it), __n};
   _VSTD::__format::__vformat_to(basic_format_parse_context{__fmt, __args.__size()},
                                 _VSTD::__format_context_create(__buffer.make_output_iterator(), __args));
   return _VSTD::move(__buffer).result();
 }
 
 template <output_iterator<const char&> _OutIt, class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT format_to_n_result<_OutIt>
-format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, __format_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::__vformat_to_n<format_context>(_VSTD::move(__out_it), __n, __fmt.__str_, _VSTD::make_format_args(__args...));
+format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, format_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::__vformat_to_n<format_context>(_VSTD::move(__out_it), __n, __fmt.get(), _VSTD::make_format_args(__args...));
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <output_iterator<const wchar_t&> _OutIt, class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT format_to_n_result<_OutIt>
-format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, __wformat_string_t<_Args...> __fmt,
+format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, wformat_string<_Args...> __fmt,
             _Args&&... __args) {
-  return _VSTD::__vformat_to_n<wformat_context>(_VSTD::move(__out_it), __n, __fmt.__str_, _VSTD::make_wformat_args(__args...));
+  return _VSTD::__vformat_to_n<wformat_context>(_VSTD::move(__out_it), __n, __fmt.get(), _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 template <class _CharT>
 _LIBCPP_HIDE_FROM_ABI size_t __vformatted_size(basic_string_view<_CharT> __fmt, auto __args) {
   __format::__formatted_size_buffer<_CharT> __buffer;
   _VSTD::__format::__vformat_to(basic_format_parse_context{__fmt, __args.__size()},
                                 _VSTD::__format_context_create(__buffer.make_output_iterator(), __args));
   return _VSTD::move(__buffer).result();
 }
 
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(__format_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::__vformatted_size(__fmt.__str_, basic_format_args{_VSTD::make_format_args(__args...)});
+formatted_size(format_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::__vformatted_size(__fmt.get(), basic_format_args{_VSTD::make_format_args(__args...)});
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(__wformat_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::__vformatted_size(__fmt.__str_, basic_format_args{_VSTD::make_wformat_args(__args...)});
+formatted_size(wformat_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::__vformatted_size(__fmt.get(), basic_format_args{_VSTD::make_wformat_args(__args...)});
 }
 #endif
 
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
 
 template <class _OutIt, class _CharT, class _FormatOutIt>
 requires(output_iterator<_OutIt, const _CharT&>) _LIBCPP_HIDE_FROM_ABI _OutIt
     __vformat_to(
_LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT _OutIt vformat_to(
     _OutIt __out_it, locale __loc, wstring_view __fmt, wformat_args __args) {
   return _VSTD::__vformat_to(_VSTD::move(__out_it), _VSTD::move(__loc), __fmt,
                              __args);
 }
 #endif
 
 template <output_iterator<const char&> _OutIt, class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT _OutIt
-format_to(_OutIt __out_it, locale __loc, __format_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::vformat_to(_VSTD::move(__out_it), _VSTD::move(__loc), __fmt.__str_,
+format_to(_OutIt __out_it, locale __loc, format_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::vformat_to(_VSTD::move(__out_it), _VSTD::move(__loc), __fmt.get(),
                            _VSTD::make_format_args(__args...));
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <output_iterator<const wchar_t&> _OutIt, class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT _OutIt
-format_to(_OutIt __out_it, locale __loc, __wformat_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::vformat_to(_VSTD::move(__out_it), _VSTD::move(__loc), __fmt.__str_,
+format_to(_OutIt __out_it, locale __loc, wformat_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::vformat_to(_VSTD::move(__out_it), _VSTD::move(__loc), __fmt.get(),
                            _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 _LIBCPP_ALWAYS_INLINE inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string
 vformat(locale __loc, string_view __fmt, format_args __args) {
   string __res;
   _VSTD::vformat_to(_VSTD::back_inserter(__res), _VSTD::move(__loc), __fmt,
                     __args);
   return __res;
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 _LIBCPP_ALWAYS_INLINE inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT wstring
 vformat(locale __loc, wstring_view __fmt, wformat_args __args) {
   wstring __res;
   _VSTD::vformat_to(_VSTD::back_inserter(__res), _VSTD::move(__loc), __fmt,
                     __args);
   return __res;
 }
 #endif
 
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string format(locale __loc,
-                                                                                      __format_string_t<_Args...> __fmt,
+                                                                                      format_string<_Args...> __fmt,
                                                                                       _Args&&... __args) {
-  return _VSTD::vformat(_VSTD::move(__loc), __fmt.__str_,
+  return _VSTD::vformat(_VSTD::move(__loc), __fmt.get(),
                         _VSTD::make_format_args(__args...));
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT wstring
-format(locale __loc, __wformat_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::vformat(_VSTD::move(__loc), __fmt.__str_,
+format(locale __loc, wformat_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::vformat(_VSTD::move(__loc), __fmt.get(),
                         _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 template <class _Context, class _OutIt, class _CharT>
 _LIBCPP_HIDE_FROM_ABI format_to_n_result<_OutIt> __vformat_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n,
                                                                 locale __loc, basic_string_view<_CharT> __fmt,
                                                                 basic_format_args<_Context> __args) {
   __format::__format_to_n_buffer<_OutIt, _CharT> __buffer{_VSTD::move(__out_it), __n};
   _VSTD::__format::__vformat_to(
       basic_format_parse_context{__fmt, __args.__size()},
       _VSTD::__format_context_create(__buffer.make_output_iterator(), __args, _VSTD::move(__loc)));
   return _VSTD::move(__buffer).result();
 }
 
 template <output_iterator<const char&> _OutIt, class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT format_to_n_result<_OutIt>
-format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, locale __loc, __format_string_t<_Args...> __fmt,
+format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, locale __loc, format_string<_Args...> __fmt,
             _Args&&... __args) {
-  return _VSTD::__vformat_to_n<format_context>(_VSTD::move(__out_it), __n, _VSTD::move(__loc), __fmt.__str_,
+  return _VSTD::__vformat_to_n<format_context>(_VSTD::move(__out_it), __n, _VSTD::move(__loc), __fmt.get(),
                                                _VSTD::make_format_args(__args...));
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <output_iterator<const wchar_t&> _OutIt, class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT format_to_n_result<_OutIt>
-format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, locale __loc, __wformat_string_t<_Args...> __fmt,
+format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, locale __loc, wformat_string<_Args...> __fmt,
             _Args&&... __args) {
-  return _VSTD::__vformat_to_n<wformat_context>(_VSTD::move(__out_it), __n, _VSTD::move(__loc), __fmt.__str_,
+  return _VSTD::__vformat_to_n<wformat_context>(_VSTD::move(__out_it), __n, _VSTD::move(__loc), __fmt.get(),
                                                 _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 template <class _CharT>
 _LIBCPP_HIDE_FROM_ABI size_t __vformatted_size(locale __loc, basic_string_view<_CharT> __fmt, auto __args) {
   __format::__formatted_size_buffer<_CharT> __buffer;
   _VSTD::__format::__vformat_to(
       basic_format_parse_context{__fmt, __args.__size()},
       _VSTD::__format_context_create(__buffer.make_output_iterator(), __args, _VSTD::move(__loc)));
   return _VSTD::move(__buffer).result();
 }
 
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(locale __loc, __format_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::__vformatted_size(_VSTD::move(__loc), __fmt.__str_, basic_format_args{_VSTD::make_format_args(__args...)});
+formatted_size(locale __loc, format_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::__vformatted_size(_VSTD::move(__loc), __fmt.get(), basic_format_args{_VSTD::make_format_args(__args...)});
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(locale __loc, __wformat_string_t<_Args...> __fmt, _Args&&... __args) {
-  return _VSTD::__vformatted_size(_VSTD::move(__loc), __fmt.__str_, basic_format_args{_VSTD::make_wformat_args(__args...)});
+formatted_size(locale __loc, wformat_string<_Args...> __fmt, _Args&&... __args) {
+  return _VSTD::__vformatted_size(_VSTD::move(__loc), __fmt.get(), basic_format_args{_VSTD::make_wformat_args(__args...)});
 }
 #endif
 
 #endif // _LIBCPP_HAS_NO_LOCALIZATION
 
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 
 #endif // _LIBCPP_FORMAT

libcxx/test/std/utilities/format/format.fmt.string/ctor.verify.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: !stdlib=libc++ && c++20
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// <format>
+
+// template<class charT, class... Args>
+// class basic_format_string<charT, type_identity_t<Args>...>
+//
+// template<class T> consteval basic_format_string(const T& s);
+//
+// This constructor does the compile-time format string validation for the
+// std::format* functions.
+
+#include <format>
+
+#include <string_view>
+
+#include "test_macros.h"
+#include "make_string.h"
+
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+void test() {
+  (void)std::basic_format_string<CharT>{SV("foo")};
+  (void)std::basic_format_string<CharT>{SV("{}")}; // expected-error-re 2 {{call to consteval function{{.*}}is not a constant expression}}
+  (void)std::basic_format_string<CharT, int>{SV("{0:{0}P}")}; // expected-error-re 2 {{call to consteval function{{.*}}is not a constant expression}}
+  (void)std::basic_format_string<CharT, int>{SV("{0:{0}}")};
+  (void)std::basic_format_string<CharT, float>{SV("{0:{0}}")}; // expected-error-re 2 {{call to consteval function{{.*}}is not a constant expression}}
+  (void)std::basic_format_string<CharT, int>{SV("{.3}")}; // expected-error-re 2 {{call to consteval function{{.*}}is not a constant expression}}
+}
+
+void run() {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#else
+  test<char>(); // Run again the get the expected number of errors.
+#endif
+}

libcxx/test/std/utilities/format/format.fmt.string/get.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: !stdlib=libc++ && c++20
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// <format>
+
+// template<class charT, class... Args>
+// class basic_format_string<charT, type_identity_t<Args>...>
+//
+// constexpr basic_string_view<charT> get() const noexcept { return str; }
+
+#include <format>
+
+#include <cassert>
+#include <concepts>
+#include <string_view>
+
+#include "test_macros.h"
+#include "make_string.h"
+
+#define CSTR(S) MAKE_CSTRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+constexpr bool test() {
+  assert((std::basic_format_string<CharT>{CSTR("foo")}.get() == SV("foo")));
+  assert((std::basic_format_string<CharT, int>{CSTR("{}")}.get() == SV("{}")));
+  assert((std::basic_format_string<CharT, int, float>{CSTR("{} {:01.23L}")}.get() == SV("{} {:01.23L}")));
+
+  // Embedded NUL character
+  assert((std::basic_format_string<CharT, void*, double>{SV("{}\0{}")}.get() == SV("{}\0{}")));
+  return true;
+}
+
+int main(int, char**) {
+  test<char>();
+  static_assert(test<char>());
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+  static_assert(test<wchar_t>());
+#endif
+  return 0;
+}

libcxx/test/std/utilities/format/format.fmt.string/types.compile.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: !stdlib=libc++ && c++20
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// <format>
+
+//  template<class... Args>
+//    using format_string =
+//      basic_format_string<char, type_identity_t<Args>...>;
+//  template<class... Args>
+//    using wformat_string =
+//      basic_format_string<wchar_t, type_identity_t<Args>...>;
+
+#include <format>
+
+#include <concepts>
+
+#include "test_macros.h"
+
+static_assert(std::same_as<std::format_string<>, std::basic_format_string<char>>);
+static_assert(std::same_as<std::format_string<int>, std::basic_format_string<char, int>>);
+static_assert(std::same_as<std::format_string<int, float>, std::basic_format_string<char, int, float>>);
+static_assert(std::same_as<std::format_string<int, float, void*>, std::basic_format_string<char, int, float, void*>>);
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+static_assert(std::same_as<std::wformat_string<>, std::basic_format_string<wchar_t>>);
+static_assert(std::same_as<std::wformat_string<int>, std::basic_format_string<wchar_t, int>>);
+static_assert(std::same_as<std::wformat_string<int, float>, std::basic_format_string<wchar_t, int, float>>);
+static_assert(
+    std::same_as<std::wformat_string<int, float, void*>, std::basic_format_string<wchar_t, int, float, void*>>);
+#endif

libcxx/utils/generate_feature_test_macro_components.py

feature_test_macros = [ add_version_header(x) for x in [
   }, {
     "name": "__cpp_lib_filesystem",
     "values": { "c++17": 201703 },
     "headers": ["filesystem"],
     "test_suite_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_filesystem)",
     "libcxx_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_filesystem)"
   }, {
     "name": "__cpp_lib_format",
+    # P2508, P2286, and P2419 were accepted in the same plenary and modify this
+    # feature-test macro. I expect to see an LWG issue soon. For now keep the
+    # value as is.
+    # TODO FMT Set P2508's feature-test macro.
+    #"values": { "c++20": 202106, "c++23": 202207" },
     "values": { "c++20": 202106 },
     "headers": ["format"],
     "test_suite_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format)",
     "libcxx_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format)",
     "unimplemented": True,
   }, {
     "name": "__cpp_lib_gcd_lcm",
     "values": { "c++17": 201606 },