format

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// <format> Formatting -*- C++ -*-
 
// Copyright The GNU Toolchain Authors.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/format
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_FORMAT
#define _GLIBCXX_FORMAT 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <bits/requires_hosted.h> // for std::string
 
#define __glibcxx_want_format
#define __glibcxx_want_format_ranges
#define __glibcxx_want_format_uchar
#include <bits/version.h>
 
#ifdef __cpp_lib_format // C++ >= 20 && HOSTED
 
#include <array>
#include <charconv>
#include <concepts>
#include <limits>
#include <locale>
#include <optional>
#include <span>
#include <string_view>
#include <string>
#include <bits/monostate.h>
#include <bits/formatfwd.h>
#include <bits/ranges_base.h>  // input_range, range_reference_t
#include <bits/ranges_util.h>  // subrange
#include <bits/ranges_algobase.h> // ranges::copy
#include <bits/stl_iterator.h> // back_insert_iterator
#include <bits/stl_pair.h>     // __is_pair
#include <bits/unicode.h>      // __is_scalar_value, _Utf_view, etc.
#include <bits/utility.h>      // tuple_size_v
#include <ext/numeric_traits.h> // __int_traits
 
#if !__has_builtin(__builtin_toupper)
# include <cctype>
#endif
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpedantic" // __int128
#pragma GCC diagnostic ignored "-Wc++23-extensions" // bf16
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  // [format.fmt.string], class template basic_format_string
  template<typename _CharT, typename... _Args> struct basic_format_string;
 
/// @cond undocumented
namespace __format
{
  // STATICALLY-WIDEN, see C++20 [time.general]
  // It doesn't matter for format strings (which can only be char or wchar_t)
  // but this returns the narrow string for anything that isn't wchar_t. This
  // is done because const char* can be inserted into any ostream type, and
  // will be widened at runtime if necessary.
  template<typename _CharT>
    consteval auto
    _Widen(const char* __narrow, const wchar_t* __wide)
    {
      if constexpr (is_same_v<_CharT, wchar_t>)
        return __wide;
      else
        return __narrow;
    }
#define _GLIBCXX_WIDEN_(C, S) ::std::__format::_Widen<C>(S, L##S)
#define _GLIBCXX_WIDEN(S) _GLIBCXX_WIDEN_(_CharT, S)
 
  // Size for stack located buffer
  template<typename _CharT>
  constexpr size_t __stackbuf_size = 32 * sizeof(void*) / sizeof(_CharT);
 
  // Type-erased character sinks.
  template<typename _CharT> class _Sink;
  template<typename _CharT> class _Fixedbuf_sink;
  template<typename _Seq> class _Seq_sink;
 
  template<typename _CharT, typename _Alloc = allocator<_CharT>>
    using _Str_sink
      = _Seq_sink<basic_string<_CharT, char_traits<_CharT>, _Alloc>>;
 
  // template<typename _CharT, typename _Alloc = allocator<_CharT>>
  // using _Vec_sink = _Seq_sink<vector<_CharT, _Alloc>>;
 
  // Output iterator that writes to a type-erase character sink.
  template<typename _CharT>
    class _Sink_iter;
 
  // An unspecified output iterator type used in the `formattable` concept.
  template<typename _CharT>
    struct _Iter_for
    { using type = back_insert_iterator<basic_string<_CharT>>; };
 
  template<typename _CharT>
    using __format_context = basic_format_context<_Sink_iter<_CharT>, _CharT>;
 
  template<typename _CharT>
    struct _Runtime_format_string
    {
      [[__gnu__::__always_inline__]]
      _Runtime_format_string(basic_string_view<_CharT> __s) noexcept
      : _M_str(__s) { }
 
      _Runtime_format_string(const _Runtime_format_string&) = delete;
      void operator=(const _Runtime_format_string&) = delete;
 
    private:
      basic_string_view<_CharT> _M_str;
 
      template<typename, typename...> friend struct std::basic_format_string;
    };
 
} // namespace __format
/// @endcond
 
  using format_context  = __format::__format_context<char>;
#ifdef _GLIBCXX_USE_WCHAR_T
  using wformat_context = __format::__format_context<wchar_t>;
#endif
 
  // [format.args], class template basic_format_args
  template<typename _Context> class basic_format_args;
  using format_args = basic_format_args<format_context>;
#ifdef _GLIBCXX_USE_WCHAR_T
  using wformat_args = basic_format_args<wformat_context>;
#endif
 
  // [format.arguments], arguments
  // [format.arg], class template basic_format_arg
  template<typename _Context>
    class basic_format_arg;
 
  /** A compile-time checked format string for the specified argument types.
   *
   * @since C++23 but available as an extension in C++20.
   */
  template<typename _CharT, typename... _Args>
    struct basic_format_string
    {
      template<typename _Tp>
        requires convertible_to<const _Tp&, basic_string_view<_CharT>>
        consteval
        basic_format_string(const _Tp& __s);
 
      [[__gnu__::__always_inline__]]
      basic_format_string(__format::_Runtime_format_string<_CharT> __s) noexcept
      : _M_str(__s._M_str)
      { }
 
      [[__gnu__::__always_inline__]]
      constexpr basic_string_view<_CharT>
      get() const noexcept
      { return _M_str; }
 
    private:
      basic_string_view<_CharT> _M_str;
    };
 
  template<typename... _Args>
    using format_string = basic_format_string<char, type_identity_t<_Args>...>;
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename... _Args>
    using wformat_string
      = basic_format_string<wchar_t, type_identity_t<_Args>...>;
#endif
 
#if __cpp_lib_format >= 202311L // >= C++26
  [[__gnu__::__always_inline__]]
  inline __format::_Runtime_format_string<char>
  runtime_format(string_view __fmt) noexcept
  { return __fmt; }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  [[__gnu__::__always_inline__]]
  inline __format::_Runtime_format_string<wchar_t>
  runtime_format(wstring_view __fmt) noexcept
  { return __fmt; }
#endif
#endif // C++26
 
  // [format.formatter], formatter
 
  /// The primary template of std::formatter is disabled.
  template<typename _Tp, typename _CharT>
    struct formatter
    {
      formatter() = delete; // No std::formatter specialization for this type.
      formatter(const formatter&) = delete;
      formatter& operator=(const formatter&) = delete;
    };
 
  // [format.error], class format_error
  class format_error : public runtime_error
  {
  public:
    explicit format_error(const string& __what) : runtime_error(__what) { }
    explicit format_error(const char* __what) : runtime_error(__what) { }
  };
 
  /// @cond undocumented
  [[noreturn]]
  inline void
  __throw_format_error(const char* __what)
  { _GLIBCXX_THROW_OR_ABORT(format_error(__what)); }
 
namespace __format
{
  // XXX use named functions for each constexpr error?
 
  [[noreturn]]
  inline void
  __unmatched_left_brace_in_format_string()
  { __throw_format_error("format error: unmatched '{' in format string"); }
 
  [[noreturn]]
  inline void
  __unmatched_right_brace_in_format_string()
  { __throw_format_error("format error: unmatched '}' in format string"); }
 
  [[noreturn]]
  inline void
  __conflicting_indexing_in_format_string()
  { __throw_format_error("format error: conflicting indexing style in format string"); }
 
  [[noreturn]]
  inline void
  __invalid_arg_id_in_format_string()
  { __throw_format_error("format error: invalid arg-id in format string"); }
 
  [[noreturn]]
  inline void
  __failed_to_parse_format_spec()
  { __throw_format_error("format error: failed to parse format-spec"); }
 
  template<typename _CharT> class _Scanner;
 
  enum _Arg_t : unsigned char {
    _Arg_none, _Arg_bool, _Arg_c, _Arg_i, _Arg_u, _Arg_ll, _Arg_ull,
    _Arg_flt, _Arg_dbl, _Arg_ldbl, _Arg_str, _Arg_sv, _Arg_ptr, _Arg_handle,
    _Arg_i128, _Arg_u128,
    _Arg_bf16, _Arg_f16, _Arg_f32, _Arg_f64, // These are unused.
#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
    _Arg_next_value_,
    _Arg_f128 = _Arg_ldbl,
    _Arg_ibm128 = _Arg_next_value_,
#else
    _Arg_f128,
#endif
    _Arg_max_
  };
 
  template<typename _CharT, typename _Tp>
    consteval _Arg_t
    __to_arg_t_enum() noexcept;
 
} // namespace __format
  /// @endcond
 
  // [format.parse.ctx], class template basic_format_parse_context
  template<typename _CharT> class basic_format_parse_context;
  using format_parse_context = basic_format_parse_context<char>;
#ifdef _GLIBCXX_USE_WCHAR_T
  using wformat_parse_context = basic_format_parse_context<wchar_t>;
#endif
 
  template<typename _CharT>
    class basic_format_parse_context
    {
    public:
      using char_type = _CharT;
      using const_iterator = typename basic_string_view<_CharT>::const_iterator;
      using iterator = const_iterator;
 
      constexpr explicit
      basic_format_parse_context(basic_string_view<_CharT> __fmt) noexcept
      : _M_begin(__fmt.begin()), _M_end(__fmt.end())
      { }
 
      basic_format_parse_context(const basic_format_parse_context&) = delete;
      void operator=(const basic_format_parse_context&) = delete;
 
      constexpr const_iterator begin() const noexcept { return _M_begin; }
      constexpr const_iterator end() const noexcept { return _M_end; }
 
      constexpr void
      advance_to(const_iterator __it) noexcept
      { _M_begin = __it; }
 
      constexpr size_t
      next_arg_id()
      {
        if (_M_indexing == _Manual)
          __format::__conflicting_indexing_in_format_string();
        _M_indexing = _Auto;
 
        // _GLIBCXX_RESOLVE_LIB_DEFECTS
        // 3825. Missing compile-time argument id check in next_arg_id
        if (std::is_constant_evaluated())
          if (_M_next_arg_id == _M_num_args)
            __format::__invalid_arg_id_in_format_string();
        return _M_next_arg_id++;
      }
 
      constexpr void
      check_arg_id(size_t __id)
      {
        if (_M_indexing == _Auto)
          __format::__conflicting_indexing_in_format_string();
        _M_indexing = _Manual;
 
        if (std::is_constant_evaluated())
          if (__id >= _M_num_args)
            __format::__invalid_arg_id_in_format_string();
      }
 
#if __cpp_lib_format >= 202305L // >= C++26
      template<typename... _Ts>
        constexpr void
        check_dynamic_spec(size_t __id) noexcept
        {
          static_assert(__valid_types_for_check_dynamic_spec<_Ts...>(),
                        "template arguments for check_dynamic_spec<Ts...>(id) "
                        "must be unique and must be one of the allowed types");
          if constexpr (sizeof...(_Ts))
            if consteval {
              const __format::_Arg_t __t[] = {
                __format::__to_arg_t_enum<_CharT, _Ts>()...
              };
              __check_dynamic_spec(__id, __t);
            }
        }
 
      constexpr void
      check_dynamic_spec_integral(size_t __id) noexcept
      {
        if consteval {
          using enum __format::_Arg_t;
          const __format::_Arg_t __t[] = { _Arg_i, _Arg_u, _Arg_ll, _Arg_ull };
          __check_dynamic_spec(__id, __t);
        }
      }
 
      constexpr void
      check_dynamic_spec_string(size_t __id) noexcept
      {
        if consteval {
          using enum __format::_Arg_t;
          const __format::_Arg_t __t[] = { _Arg_str, _Arg_sv };
          __check_dynamic_spec(__id, __t);
        }
      }
 
    private:
      // True if _Tp occurs exactly once in _Ts.
      template<typename _Tp, typename... _Ts>
        static constexpr bool __once = (is_same_v<_Tp, _Ts> + ...) == 1;
 
      template<typename... _Ts>
        static consteval bool
        __valid_types_for_check_dynamic_spec()
        {
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 4142. check_dynamic_spec should require at least one type
          if constexpr (sizeof...(_Ts) == 0)
            return false;
          else
            {
              // The types in Ts... are unique. Each type in Ts... is one of
              // bool, char_type, int, unsigned int, long long int,
              // unsigned long long int, float, double, long double,
              // const char_type*, basic_string_view<char_type>, or const void*.
              unsigned __sum
                = __once<bool, _Ts...>
                + __once<char_type, _Ts...>
                + __once<int, _Ts...>
                + __once<unsigned int, _Ts...>
                + __once<long long int, _Ts...>
                + __once<unsigned long long int, _Ts...>
                + __once<float, _Ts...>
                + __once<double, _Ts...>
                + __once<long double, _Ts...>
                + __once<const char_type*, _Ts...>
                + __once<basic_string_view<char_type>, _Ts...>
                + __once<const void*, _Ts...>;
              return __sum == sizeof...(_Ts);
            }
        }
 
      // Common implementation of check_dynamic_spec{,_string,_integral}
      consteval void
      __check_dynamic_spec(size_t __id,
                           span<const __format::_Arg_t> __types) noexcept
      {
        if (__id >= _M_num_args)
          __format::__invalid_arg_id_in_format_string();
 
        // This static_cast is safe because all parse contexts created by the
        // library are _Scan_parse_context (or a type derived from that).
        // User code can only create basic_format_parse_context objects with
        // _M_num_args == 0 and those will be rejected by the condition above.
        if (auto* __args = static_cast<_Scan_parse_context*>(this)->_M_types)
          {
            for (auto __t : __types)
              if (__args[__id] == __t)
                return;
 
            __invalid_dynamic_spec("arg(id) type does not match");
          }
        // else this is a formatting scanner, do not do any type checks.
      }
 
      // This must not be constexpr.
      static void __invalid_dynamic_spec(const char*);
#endif
 
      // This constructor should only be used by the implementation.
      constexpr explicit
      basic_format_parse_context(basic_string_view<_CharT> __fmt,
                                 size_t __num_args) noexcept
      : _M_begin(__fmt.begin()), _M_end(__fmt.end()), _M_num_args(__num_args)
      { }
 
    private:
      iterator _M_begin;
      iterator _M_end;
      enum _Indexing { _Unknown, _Manual, _Auto };
      _Indexing _M_indexing = _Unknown;
      size_t _M_next_arg_id = 0;
      size_t _M_num_args = 0;
 
      // Derived parse context used by Scanner when checking format strings.
      struct _Scan_parse_context;
      friend __format::_Scanner<_CharT>;
    };
 
  template<typename _CharT>
    struct basic_format_parse_context<_CharT>::_Scan_parse_context
    : basic_format_parse_context<_CharT>
    {
      using basic_format_parse_context<_CharT>::basic_format_parse_context;
      const __format::_Arg_t* _M_types = nullptr;
    };
 
/// @cond undocumented
  template<typename _Tp, template<typename...> class _Class>
    constexpr bool __is_specialization_of = false;
  template<template<typename...> class _Class, typename... _Args>
    constexpr bool __is_specialization_of<_Class<_Args...>, _Class> = true;
 
namespace __format
{
  // pre: first != last
  template<typename _CharT>
    constexpr pair<unsigned short, const _CharT*>
    __parse_integer(const _CharT* __first, const _CharT* __last)
    {
      if (__first == __last)
        __builtin_unreachable();
 
      if constexpr (is_same_v<_CharT, char>)
        {
          const auto __start = __first;
          unsigned short __val = 0;
          // N.B. std::from_chars is not constexpr in C++20.
          if (__detail::__from_chars_alnum<true>(__first, __last, __val, 10)
                && __first != __start) [[likely]]
            return {__val, __first};
        }
      else
        {
          constexpr int __n = 32;
          char __buf[__n]{};
          for (int __i = 0; __i < __n && (__first + __i) != __last; ++__i)
            __buf[__i] = __first[__i];
          auto [__v, __ptr] = __format::__parse_integer(__buf, __buf + __n);
          if (__ptr) [[likely]]
            return {__v, __first + (__ptr - __buf)};
        }
      return {0, nullptr};
    }
 
  template<typename _CharT>
    constexpr pair<unsigned short, const _CharT*>
    __parse_arg_id(const _CharT* __first, const _CharT* __last)
    {
      if (__first == __last)
        __builtin_unreachable();
 
      if (*__first == '0')
        return {0, __first + 1}; // No leading zeros allowed, so '0...' == 0
 
      if ('1' <= *__first && *__first <= '9')
        {
          const unsigned short __id = *__first - '0';
          const auto __next = __first + 1;
          // Optimize for most likely case of single digit arg-id.
          if (__next == __last || !('0' <= *__next && *__next <= '9'))
            return {__id, __next};
          else
            return __format::__parse_integer(__first, __last);
        }
      return {0, nullptr};
    }
 
  enum _Pres_type {
    _Pres_none = 0, // Default type (not valid for integer presentation types).
    // Presentation types for integral types (including bool and charT).
    _Pres_d = 1, _Pres_b, _Pres_B, _Pres_o, _Pres_x, _Pres_X, _Pres_c,
    // Presentation types for floating-point types.
    _Pres_a = 1, _Pres_A, _Pres_e, _Pres_E, _Pres_f, _Pres_F, _Pres_g, _Pres_G,
    _Pres_p = 0, _Pres_P,     // For pointers.
    _Pres_s = 0,              // For strings, bool
    _Pres_seq = 0, _Pres_str, // For ranges
    _Pres_esc = 0xf,          // For strings, charT and ranges
  };
 
  enum _Align {
    _Align_default,
    _Align_left,
    _Align_right,
    _Align_centre,
  };
 
  enum _Sign {
    _Sign_default,
    _Sign_plus,
    _Sign_minus,  // XXX does this need to be distinct from _Sign_default?
    _Sign_space,
  };
 
  enum _WidthPrec {
    _WP_none,    // No width/prec specified.
    _WP_value,   // Fixed width/prec specified.
    _WP_from_arg // Use a formatting argument for width/prec.
  };
 
  template<typename _Context>
    size_t
    __int_from_arg(const basic_format_arg<_Context>& __arg);
 
  constexpr bool __is_digit(char __c)
  { return std::__detail::__from_chars_alnum_to_val(__c) < 10; }
 
  constexpr bool __is_xdigit(char __c)
  { return std::__detail::__from_chars_alnum_to_val(__c) < 16; }
 
  template<typename _CharT>
    struct _Spec
    {
      _Align     _M_align : 2;
      _Sign      _M_sign : 2;
      unsigned   _M_alt : 1;
      unsigned   _M_localized : 1;
      unsigned   _M_zero_fill : 1;
      _WidthPrec _M_width_kind : 2;
      _WidthPrec _M_prec_kind : 2;
      _Pres_type _M_type : 4;
      unsigned   _M_reserved : 1;
      unsigned   _M_reserved2 : 16;
      unsigned short _M_width;
      unsigned short _M_prec;
      char32_t _M_fill = ' ';
 
      using iterator = typename basic_string_view<_CharT>::iterator;
 
      static constexpr _Align
      _S_align(_CharT __c) noexcept
      {
        switch (__c)
        {
          case '<': return _Align_left;
          case '>': return _Align_right;
          case '^': return _Align_centre;
          default: return _Align_default;
        }
      }
 
      // pre: __first != __last
      constexpr iterator
      _M_parse_fill_and_align(iterator __first, iterator __last) noexcept
      { return _M_parse_fill_and_align(__first, __last, "{"); }
 
      // pre: __first != __last
      constexpr iterator
      _M_parse_fill_and_align(iterator __first, iterator __last, string_view __not_fill) noexcept
      {
        for (char __c : __not_fill)
          if (*__first == static_cast<_CharT>(__c))
            return __first;
 
        using namespace __unicode;
        if constexpr (__literal_encoding_is_unicode<_CharT>())
          {
            // Accept any UCS scalar value as fill character.
            _Utf32_view<ranges::subrange<iterator>> __uv({__first, __last});
            if (!__uv.empty())
            {
              auto __beg = __uv.begin();
              char32_t __c = *__beg++;
              if (__is_scalar_value(__c))
                if (auto __next = __beg.base(); __next != __last)
                  if (_Align __align = _S_align(*__next))
                    {
                      _M_fill = __c;
                      _M_align = __align;
                      return ++__next;
                    }
             }
          }
        else if (__last - __first >= 2)
          if (_Align __align = _S_align(__first[1]))
            {
              _M_fill = *__first;
              _M_align = __align;
              return __first + 2;
            }
 
        if (_Align __align = _S_align(__first[0]))
          {
            _M_fill = ' ';
            _M_align = __align;
            return __first + 1;
          }
        return __first;
      }
 
      static constexpr _Sign
      _S_sign(_CharT __c) noexcept
      {
        switch (__c)
        {
          case '+': return _Sign_plus;
          case '-': return _Sign_minus;
          case ' ': return _Sign_space;
          default:  return _Sign_default;
        }
      }
 
      // pre: __first != __last
      constexpr iterator
      _M_parse_sign(iterator __first, iterator) noexcept
      {
        if (_Sign __sign = _S_sign(*__first))
          {
            _M_sign = __sign;
            return __first + 1;
          }
        return __first;
      }
 
      // pre: *__first is valid
      constexpr iterator
      _M_parse_alternate_form(iterator __first, iterator) noexcept
      {
        if (*__first == '#')
          {
            _M_alt = true;
            ++__first;
          }
        return __first;
      }
 
      // pre: __first != __last
      constexpr iterator
      _M_parse_zero_fill(iterator __first, iterator /* __last */) noexcept
      {
        if (*__first == '0')
          {
            _M_zero_fill = true;
            ++__first;
          }
        return __first;
      }
 
      // pre: __first != __last
      static constexpr iterator
      _S_parse_width_or_precision(iterator __first, iterator __last,
                                  unsigned short& __val, bool& __arg_id,
                                  basic_format_parse_context<_CharT>& __pc)
      {
        if (__format::__is_digit(*__first))
          {
            auto [__v, __ptr] = __format::__parse_integer(__first, __last);
            if (!__ptr)
              __throw_format_error("format error: invalid width or precision "
                                   "in format-spec");
            __first = __ptr;
            __val = __v;
          }
        else if (*__first == '{')
          {
            __arg_id = true;
            ++__first;
            if (__first == __last)
              __format::__unmatched_left_brace_in_format_string();
            if (*__first == '}')
              __val = __pc.next_arg_id();
            else
              {
                auto [__v, __ptr] = __format::__parse_arg_id(__first, __last);
                if (__ptr == nullptr || __ptr == __last || *__ptr != '}')
                  __format::__invalid_arg_id_in_format_string();
                __first = __ptr;
                __pc.check_arg_id(__v);
                __val = __v;
              }
#if __cpp_lib_format >= 202305L
            __pc.check_dynamic_spec_integral(__val);
#endif
            ++__first; // past the '}'
          }
        return __first;
      }
 
      // pre: __first != __last
      constexpr iterator
      _M_parse_width(iterator __first, iterator __last,
                     basic_format_parse_context<_CharT>& __pc)
      {
        bool __arg_id = false;
        if (*__first == '0')
          __throw_format_error("format error: width must be non-zero in "
                               "format string");
        auto __next = _S_parse_width_or_precision(__first, __last, _M_width,
                                                  __arg_id, __pc);
        if (__next != __first)
          _M_width_kind = __arg_id ? _WP_from_arg : _WP_value;
        return __next;
      }
 
      // pre: __first != __last
      constexpr iterator
      _M_parse_precision(iterator __first, iterator __last,
                         basic_format_parse_context<_CharT>& __pc)
      {
        if (__first[0] != '.')
          return __first;
 
        iterator __next = ++__first;
        bool __arg_id = false;
        if (__next != __last)
          __next = _S_parse_width_or_precision(__first, __last, _M_prec,
                                               __arg_id, __pc);
        if (__next == __first)
          __throw_format_error("format error: missing precision after '.' in "
                               "format string");
        _M_prec_kind = __arg_id ? _WP_from_arg : _WP_value;
        return __next;
      }
 
      // pre: __first != __last
      constexpr iterator
      _M_parse_locale(iterator __first, iterator /* __last */) noexcept
      {
        if (*__first == 'L')
          {
            _M_localized = true;
            ++__first;
          }
        return __first;
      }
 
      template<typename _Context>
        size_t
        _M_get_width(_Context& __ctx) const
        {
          size_t __width = 0;
          if (_M_width_kind == _WP_value)
            __width = _M_width;
          else if (_M_width_kind == _WP_from_arg)
            __width = __format::__int_from_arg(__ctx.arg(_M_width));
          return __width;
        }
 
      template<typename _Context>
        size_t
        _M_get_precision(_Context& __ctx) const
        {
          size_t __prec = -1;
          if (_M_prec_kind == _WP_value)
            __prec = _M_prec;
          else if (_M_prec_kind == _WP_from_arg)
            __prec = __format::__int_from_arg(__ctx.arg(_M_prec));
          return __prec;
        }
    };
 
  template<typename _Int>
    inline char*
    __put_sign(_Int __i, _Sign __sign, char* __dest) noexcept
    {
      if (__i < 0)
        *__dest = '-';
      else if (__sign == _Sign_plus)
        *__dest = '+';
      else if (__sign == _Sign_space)
        *__dest = ' ';
      else
        ++__dest;
      return __dest;
    }
 
  // Write STR to OUT (and do so efficiently if OUT is a _Sink_iter).
  template<typename _Out, typename _CharT>
    requires output_iterator<_Out, const _CharT&>
    inline _Out
    __write(_Out __out, basic_string_view<_CharT> __str)
    {
      if constexpr (is_same_v<_Out, _Sink_iter<_CharT>>)
        {
          if (__str.size())
            __out = __str;
        }
      else
        for (_CharT __c : __str)
          *__out++ = __c;
      return __out;
    }
 
  // Write STR to OUT with NFILL copies of FILL_CHAR specified by ALIGN.
  // pre: __align != _Align_default
  template<typename _Out, typename _CharT>
    _Out
    __write_padded(_Out __out, basic_string_view<_CharT> __str,
                   _Align __align, size_t __nfill, char32_t __fill_char)
    {
      const size_t __buflen = 0x20;
      _CharT __padding_chars[__buflen];
      __padding_chars[0] = _CharT();
      basic_string_view<_CharT> __padding{__padding_chars, __buflen};
 
      auto __pad = [&__padding] (size_t __n, _Out& __o) {
        if (__n == 0)
          return;
        while (__n > __padding.size())
          {
            __o = __format::__write(std::move(__o), __padding);
            __n -= __padding.size();
          }
        if (__n != 0)
          __o = __format::__write(std::move(__o), __padding.substr(0, __n));
      };
 
      size_t __l, __r, __max;
      if (__align == _Align_centre)
        {
          __l = __nfill / 2;
          __r = __l + (__nfill & 1);
          __max = __r;
        }
      else if (__align == _Align_right)
        {
          __l = __nfill;
          __r = 0;
          __max = __l;
        }
      else
        {
          __l = 0;
          __r = __nfill;
          __max = __r;
        }
 
      using namespace __unicode;
      if constexpr (__literal_encoding_is_unicode<_CharT>())
        if (!__is_single_code_unit<_CharT>(__fill_char)) [[unlikely]]
          {
            // Encode fill char as multiple code units of type _CharT.
            const char32_t __arr[1]{ __fill_char };
            _Utf_view<_CharT, const char32_t(&)[1]> __v(__arr);
            basic_string<_CharT> __padstr(__v.begin(), __v.end());
            __padding = __padstr;
            while (__l-- > 0)
              __out = __format::__write(std::move(__out), __padding);
            __out = __format::__write(std::move(__out), __str);
            while (__r-- > 0)
              __out = __format::__write(std::move(__out), __padding);
            return __out;
          }
 
      if (__max < __buflen)
        __padding.remove_suffix(__buflen - __max);
      else
        __max = __buflen;
 
      char_traits<_CharT>::assign(__padding_chars, __max, __fill_char);
      __pad(__l, __out);
      __out = __format::__write(std::move(__out), __str);
      __pad(__r, __out);
 
      return __out;
    }
 
  // Write STR to OUT, with alignment and padding as determined by SPEC.
  // pre: __spec._M_align != _Align_default || __align != _Align_default
  template<typename _CharT, typename _Out>
    _Out
    __write_padded_as_spec(basic_string_view<type_identity_t<_CharT>> __str,
                           size_t __estimated_width,
                           basic_format_context<_Out, _CharT>& __fc,
                           const _Spec<_CharT>& __spec,
                           _Align __align = _Align_left)
    {
      size_t __width = __spec._M_get_width(__fc);
 
      if (__width <= __estimated_width)
        return __format::__write(__fc.out(), __str);
 
      const size_t __nfill = __width - __estimated_width;
 
      if (__spec._M_align)
        __align = __spec._M_align;
 
      return __format::__write_padded(__fc.out(), __str, __align, __nfill,
                                      __spec._M_fill);
    }
 
  // Values are indices into _Escapes::all.
  enum class _Term_char : unsigned char {
    _Tc_quote = 12,
    _Tc_apos = 15
  };
 
  template<typename _CharT>
    struct _Escapes
    {
      using _Str_view = basic_string_view<_CharT>;
 
      static consteval
      _Str_view _S_all()
      { return _GLIBCXX_WIDEN("\t\\t\n\\n\r\\r\\\\\\\"\\\"'\\'\\u\\x"); }
 
      static constexpr
      _CharT _S_term(_Term_char __term)
      { return _S_all()[static_cast<unsigned char>(__term)]; }
 
      static consteval
      _Str_view _S_tab()
      { return _S_all().substr(0, 3); }
 
      static consteval
      _Str_view _S_newline()
      { return _S_all().substr(3, 3); }
 
      static consteval
      _Str_view _S_return()
      { return _S_all().substr(6, 3); }
 
      static consteval
      _Str_view _S_bslash()
      { return _S_all().substr(9, 3); }
 
      static consteval
      _Str_view _S_quote()
      { return _S_all().substr(12, 3); }
 
      static consteval
      _Str_view _S_apos()
      { return _S_all().substr(15, 3); }
 
      static consteval
      _Str_view _S_u()
      { return _S_all().substr(18, 2); }
 
      static consteval
      _Str_view _S_x()
      { return _S_all().substr(20, 2); }
    };
 
  template<typename _CharT>
    struct _Separators
    {
      using _Str_view = basic_string_view<_CharT>;
 
      static consteval
      _Str_view _S_all()
      { return _GLIBCXX_WIDEN("[]{}(), : "); }
 
      static consteval
      _Str_view _S_squares()
      { return _S_all().substr(0, 2); }
 
      static consteval
      _Str_view _S_braces()
      { return _S_all().substr(2, 2); }
 
      static consteval
      _Str_view _S_parens()
      { return _S_all().substr(4, 2); }
 
      static consteval
      _Str_view _S_comma()
      { return _S_all().substr(6, 2); }
 
      static consteval
      _Str_view _S_colon()
      { return _S_all().substr(8, 2); }
    };
 
  template<typename _CharT>
    constexpr bool __should_escape_ascii(_CharT __c, _Term_char __term)
    {
      using _Esc = _Escapes<_CharT>;
      switch (__c)
        {
          case _Esc::_S_tab()[0]:
          case _Esc::_S_newline()[0]:
          case _Esc::_S_return()[0]:
          case _Esc::_S_bslash()[0]:
            return true;
          case _Esc::_S_quote()[0]:
            return __term == _Term_char::_Tc_quote;
          case _Esc::_S_apos()[0]:
            return __term == _Term_char::_Tc_apos;
          default:
            return (__c >= 0 && __c < 0x20) || __c == 0x7f;
        };
  }
 
  // @pre __c <= 0x10FFFF
  constexpr bool __should_escape_unicode(char32_t __c, bool __prev_esc)
  {
    if (__unicode::__should_escape_category(__c))
      return __c != U' ';
    if (!__prev_esc)
      return false;
    return __unicode::__grapheme_cluster_break_property(__c)
             == __unicode::_Gcb_property::_Gcb_Extend;
  }
 
  using uint_least32_t = __UINT_LEAST32_TYPE__;
  template<typename _Out, typename _CharT>
    _Out
    __write_escape_seq(_Out __out, uint_least32_t __val,
                       basic_string_view<_CharT> __prefix)
    {
      constexpr size_t __max = 8;
      char __buf[__max];
      const string_view __narrow(
        __buf,
        std::__to_chars_i<uint_least32_t>(__buf, __buf + __max, __val, 16).ptr);
 
      __out = __format::__write(__out, __prefix);
      *__out = _Separators<_CharT>::_S_braces()[0];
      ++__out;
      if constexpr (is_same_v<char, _CharT>)
        __out = __format::__write(__out, __narrow);
#ifdef _GLIBCXX_USE_WCHAR_T
      else
        {
          wchar_t __wbuf[__max];
          const size_t __n = __narrow.size();
          std::__to_wstring_numeric(__narrow.data(), __n, __wbuf);
          __out = __format::__write(__out, wstring_view(__wbuf, __n));
        }
#endif
      *__out = _Separators<_CharT>::_S_braces()[1];
      return ++__out;
    }
 
  template<typename _Out, typename _CharT>
    _Out
    __write_escaped_char(_Out __out, _CharT __c)
    {
      using _UChar = make_unsigned_t<_CharT>;
      using _Esc = _Escapes<_CharT>;
      switch (__c)
        {
          case _Esc::_S_tab()[0]:
            return __format::__write(__out, _Esc::_S_tab().substr(1, 2));
          case _Esc::_S_newline()[0]:
            return __format::__write(__out, _Esc::_S_newline().substr(1, 2));
          case _Esc::_S_return()[0]:
            return __format::__write(__out, _Esc::_S_return().substr(1, 2));
          case _Esc::_S_bslash()[0]:
            return __format::__write(__out, _Esc::_S_bslash().substr(1, 2));
          case _Esc::_S_quote()[0]:
            return __format::__write(__out, _Esc::_S_quote().substr(1, 2));
          case _Esc::_S_apos()[0]:
            return __format::__write(__out, _Esc::_S_apos().substr(1, 2));
          default:
            return __format::__write_escape_seq(__out,
                                static_cast<_UChar>(__c),
                                                _Esc::_S_u());
        }
    }
 
  template<typename _CharT, typename _Out>
    _Out
    __write_escaped_ascii(_Out __out,
                          basic_string_view<_CharT> __str,
                          _Term_char __term)
    {
      using _Str_view = basic_string_view<_CharT>;
      auto __first = __str.begin();
      auto const __last = __str.end();
      while (__first != __last)
      {
        auto __print = __first;
        // assume anything outside ASCII is printable
        while (__print != __last
                 && !__format::__should_escape_ascii(*__print, __term))
          ++__print;
 
        if (__print != __first)
          __out = __format::__write(__out, _Str_view(__first, __print));
 
        if (__print == __last)
          return __out;
 
        __first = __print;
        __out = __format::__write_escaped_char(__out, *__first);
        ++__first;
      }
      return __out;
    }
 
  template<typename _CharT, typename _Out>
    _Out
    __write_escaped_unicode(_Out __out,
                            basic_string_view<_CharT> __str,
                            _Term_char __term)
    {
      using _Str_view = basic_string_view<_CharT>;
      using _UChar = make_unsigned_t<_CharT>;
      using _Esc = _Escapes<_CharT>;
 
      static constexpr char32_t __replace = U'\uFFFD';
      static constexpr _Str_view __replace_rep = []
        {
          // N.B. "\uFFFD" is ill-formed if encoding is not unicode.
          if constexpr (is_same_v<char, _CharT>)
            return "\xEF\xBF\xBD";
          else
            return L"\xFFFD";
        }();
 
      __unicode::_Utf_view<char32_t, _Str_view> __v(std::move(__str));
      auto __first = __v.begin();
      auto const __last = __v.end();
 
      bool __prev_esc = true;
      while (__first != __last)
        {
          bool __esc_ascii = false;
          bool __esc_unicode = false;
          bool __esc_replace = false;
          auto __should_escape = [&](auto const& __it)
            {
              if (*__it <= 0x7f)
                return __esc_ascii
                         = __format::__should_escape_ascii(*__it.base(), __term);
              if (__format::__should_escape_unicode(*__it, __prev_esc))
                return __esc_unicode = true;
              if (*__it == __replace)
                {
                  _Str_view __units(__it.base(), __it._M_units());
                  return __esc_replace = (__units != __replace_rep);
                }
              return false;
            };
 
          auto __print = __first;
          while (__print != __last && !__should_escape(__print))
            {
              __prev_esc = false;
              ++__print;
            }
 
          if (__print != __first)
            __out = __format::__write(__out, _Str_view(__first.base(), __print.base()));
 
          if (__print == __last)
            return __out;
 
          __first = __print;
          if (__esc_ascii)
            __out = __format::__write_escaped_char(__out, *__first.base());
          else if (__esc_unicode)
            __out = __format::__write_escape_seq(__out, *__first, _Esc::_S_u());
          else // __esc_replace
            for (_CharT __c : _Str_view(__first.base(), __first._M_units()))
              __out = __format::__write_escape_seq(__out,
                                                   static_cast<_UChar>(__c),
                                                   _Esc::_S_x());
          __prev_esc = true;
          ++__first;
 
        }
      return __out;
    }
 
  template<typename _CharT, typename _Out>
    _Out
    __write_escaped(_Out __out,  basic_string_view<_CharT> __str, _Term_char __term)
    {
      *__out = _Escapes<_CharT>::_S_term(__term);
      ++__out;
 
      if constexpr (__unicode::__literal_encoding_is_unicode<_CharT>())
        __out = __format::__write_escaped_unicode(__out, __str, __term);
      else if constexpr (is_same_v<char, _CharT>
                          && __unicode::__literal_encoding_is_extended_ascii())
        __out = __format::__write_escaped_ascii(__out, __str, __term);
      else
        // TODO Handle non-ascii extended encoding
        __out = __format::__write_escaped_ascii(__out, __str, __term);
 
      *__out = _Escapes<_CharT>::_S_term(__term);
      return ++__out;
    }
 
  // A lightweight optional<locale>.
  struct _Optional_locale
  {
    [[__gnu__::__always_inline__]]
    _Optional_locale() : _M_dummy(), _M_hasval(false) { }
 
    _Optional_locale(const locale& __loc) noexcept
    : _M_loc(__loc), _M_hasval(true)
    { }
 
    _Optional_locale(const _Optional_locale& __l) noexcept
    : _M_dummy(), _M_hasval(__l._M_hasval)
    {
      if (_M_hasval)
        std::construct_at(&_M_loc, __l._M_loc);
    }
 
    _Optional_locale&
    operator=(const _Optional_locale& __l) noexcept
    {
      if (_M_hasval)
        {
          if (__l._M_hasval)
            _M_loc = __l._M_loc;
          else
            {
              _M_loc.~locale();
              _M_hasval = false;
            }
        }
      else if (__l._M_hasval)
        {
          std::construct_at(&_M_loc, __l._M_loc);
          _M_hasval = true;
        }
      return *this;
    }
 
    ~_Optional_locale() { if (_M_hasval) _M_loc.~locale(); }
 
    _Optional_locale&
    operator=(locale&& __loc) noexcept
    {
      if (_M_hasval)
        _M_loc = std::move(__loc);
      else
        {
          std::construct_at(&_M_loc, std::move(__loc));
          _M_hasval = true;
        }
      return *this;
    }
 
    const locale&
    value() noexcept
    {
      if (!_M_hasval)
        {
          std::construct_at(&_M_loc);
          _M_hasval = true;
        }
      return _M_loc;
    }
 
    bool has_value() const noexcept { return _M_hasval; }
 
    union {
      char _M_dummy = '\0';
      std::locale _M_loc;
    };
    bool _M_hasval = false;
  };
 
  template<__char _CharT>
    struct __formatter_str
    {
      __formatter_str() = default;
 
      constexpr
      __formatter_str(_Spec<_CharT> __spec) noexcept
       : _M_spec(__spec)
      { }
 
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      {
        auto __first = __pc.begin();
        const auto __last = __pc.end();
        _Spec<_CharT> __spec{};
 
        auto __finalize = [this, &__spec] {
          _M_spec = __spec;
        };
 
        auto __finished = [&] {
          if (__first == __last || *__first == '}')
            {
              __finalize();
              return true;
            }
          return false;
        };
 
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_fill_and_align(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_width(__first, __last, __pc);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_precision(__first, __last, __pc);
        if (__finished())
          return __first;
 
        if (*__first == 's')
          ++__first;
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
        else if (*__first == '?')
          {
            __spec._M_type = _Pres_esc;
            ++__first;
          }
#endif
 
        if (__finished())
          return __first;
 
        __format::__failed_to_parse_format_spec();
      }
 
      template<typename _Out>
        _Out
        format(basic_string_view<_CharT> __s,
               basic_format_context<_Out, _CharT>& __fc) const
        {
          constexpr auto __term = __format::_Term_char::_Tc_quote;
          const auto __write_direct = [&]
            {
              if (_M_spec._M_type == _Pres_esc)
                return __format::__write_escaped(__fc.out(), __s, __term);
              else
                return __format::__write(__fc.out(), __s);
            };
 
          if (_M_spec._M_width_kind == _WP_none
                && _M_spec._M_prec_kind == _WP_none)
            return __write_direct();
 
          const size_t __prec =
            _M_spec._M_prec_kind != _WP_none
              ? _M_spec._M_get_precision(__fc)
              : basic_string_view<_CharT>::npos;
 
          const size_t __estimated_width = _S_trunc(__s, __prec);
          // N.B. Escaping only increases width
          if (_M_spec._M_get_width(__fc) <= __estimated_width
                && _M_spec._M_prec_kind == _WP_none)
             return __write_direct();
 
          if (_M_spec._M_type != _Pres_esc)
            return __format::__write_padded_as_spec(__s, __estimated_width,
                                                    __fc, _M_spec);
 
          __format::_Str_sink<_CharT> __sink;
          __format::__write_escaped(__sink.out(), __s, __term);
          basic_string_view<_CharT> __escaped(__sink.view().data(),
                                              __sink.view().size());
          const size_t __escaped_width = _S_trunc(__escaped, __prec);
          // N.B. [tab:format.type.string] defines '?' as
          // Copies the escaped string ([format.string.escaped]) to the output,
          // so precision seem to appy to escaped string.
          return __format::__write_padded_as_spec(__escaped, __escaped_width,
                                                  __fc, _M_spec);
        }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      template<ranges::input_range _Rg, typename _Out>
        requires same_as<remove_cvref_t<ranges::range_reference_t<_Rg>>, _CharT>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format_range(_Rg&& __rg, basic_format_context<_Out, _CharT>& __fc) const
        {
          using _String = basic_string<_CharT>;
          using _String_view = basic_string_view<_CharT>;
          if constexpr (ranges::forward_range<_Rg> || ranges::sized_range<_Rg>)
            {
              const size_t __n(ranges::distance(__rg));
              if constexpr (ranges::contiguous_range<_Rg>)
                return format(_String_view(ranges::data(__rg), __n), __fc);
              else if (__n <= __format::__stackbuf_size<_CharT>)
                {
                  _CharT __buf[__format::__stackbuf_size<_CharT>];
                  ranges::copy(__rg, __buf);
                  return format(_String_view(__buf, __n), __fc);
                }
              else if constexpr (ranges::sized_range<_Rg>)
                return format(_String(from_range, __rg), __fc);
              else if constexpr (ranges::random_access_range<_Rg>)
                {
                  ranges::iterator_t<_Rg> __first = ranges::begin(__rg);
                  ranges::subrange __sub(__first, __first + __n);
                  return format(_String(from_range, __sub), __fc);
                }
              else
                {
                  // N.B. preserve the computed size
                  ranges::subrange __sub(__rg, __n);
                  return format(_String(from_range, __sub), __fc);
                }
            }
          else
            return format(_String(from_range, __rg), __fc);
        }
 
      constexpr void
      set_debug_format() noexcept
      { _M_spec._M_type = _Pres_esc; }
#endif
 
    private:
      static size_t
      _S_trunc(basic_string_view<_CharT>& __s, size_t __prec)
      {
        if constexpr (__unicode::__literal_encoding_is_unicode<_CharT>())
         {
           if (__prec != basic_string_view<_CharT>::npos)
             return __unicode::__truncate(__s, __prec);
           else
             return __unicode::__field_width(__s);
         }
       else
        {
          __s = __s.substr(0, __prec);
          return __s.size();
        }
      }
 
      _Spec<_CharT> _M_spec{};
    };
 
  template<__char _CharT>
    struct __formatter_int
    {
      // If no presentation type is specified, meaning of "none" depends
      // whether we are formatting an integer or a char or a bool.
      static constexpr _Pres_type _AsInteger = _Pres_d;
      static constexpr _Pres_type _AsBool = _Pres_s;
      static constexpr _Pres_type _AsChar = _Pres_c;
 
      constexpr typename basic_format_parse_context<_CharT>::iterator
      _M_do_parse(basic_format_parse_context<_CharT>& __pc, _Pres_type __type)
      {
        _Spec<_CharT> __spec{};
        __spec._M_type = __type;
 
        const auto __last = __pc.end();
        auto __first = __pc.begin();
 
        auto __finalize = [this, &__spec] {
          _M_spec = __spec;
        };
 
        auto __finished = [&] {
          if (__first == __last || *__first == '}')
            {
              __finalize();
              return true;
            }
          return false;
        };
 
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_fill_and_align(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_sign(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_alternate_form(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_zero_fill(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_width(__first, __last, __pc);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_locale(__first, __last);
        if (__finished())
          return __first;
 
        switch (*__first)
        {
          case 'b':
            __spec._M_type = _Pres_b;
            ++__first;
            break;
          case 'B':
            __spec._M_type = _Pres_B;
            ++__first;
            break;
          case 'c':
            // _GLIBCXX_RESOLVE_LIB_DEFECTS
            // 3586. format should not print bool with 'c'
            if (__type != _AsBool)
              {
                __spec._M_type = _Pres_c;
                ++__first;
              }
            break;
          case 'd':
            __spec._M_type = _Pres_d;
            ++__first;
            break;
          case 'o':
            __spec._M_type = _Pres_o;
            ++__first;
            break;
          case 'x':
            __spec._M_type = _Pres_x;
            ++__first;
            break;
          case 'X':
            __spec._M_type = _Pres_X;
            ++__first;
            break;
          case 's':
            if (__type == _AsBool)
              {
                __spec._M_type = _Pres_s; // same value (and meaning) as "none"
                ++__first;
              }
            break;
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
          case '?':
            if (__type == _AsChar)
              {
                __spec._M_type = _Pres_esc;
                ++__first;
              }
#endif
            break;
          }
 
        if (__finished())
          return __first;
 
        __format::__failed_to_parse_format_spec();
      }
 
      template<typename _Tp>
        constexpr typename basic_format_parse_context<_CharT>::iterator
        _M_parse(basic_format_parse_context<_CharT>& __pc)
        {
          if constexpr (is_same_v<_Tp, bool>)
            {
              auto __end = _M_do_parse(__pc, _AsBool);
              if (_M_spec._M_type == _Pres_s)
                if (_M_spec._M_sign || _M_spec._M_alt || _M_spec._M_zero_fill)
                  __throw_format_error("format error: format-spec contains "
                                       "invalid formatting options for "
                                       "'bool'");
              return __end;
            }
          else if constexpr (__char<_Tp>)
            {
              auto __end = _M_do_parse(__pc, _AsChar);
              if (_M_spec._M_type == _Pres_c || _M_spec._M_type == _Pres_esc)
                if (_M_spec._M_sign || _M_spec._M_alt || _M_spec._M_zero_fill
                      /* XXX should be invalid? || _M_spec._M_localized */)
                  __throw_format_error("format error: format-spec contains "
                                       "invalid formatting options for "
                                       "'charT'");
              return __end;
            }
          else
            return _M_do_parse(__pc, _AsInteger);
        }
 
      template<typename _Int, typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Int __i, basic_format_context<_Out, _CharT>& __fc) const
        {
          if (_M_spec._M_type == _Pres_c)
            return _M_format_character(_S_to_character(__i), __fc);
 
          char __buf[sizeof(_Int) * __CHAR_BIT__ + 3];
          to_chars_result __res{};
 
          string_view __base_prefix;
          make_unsigned_t<_Int> __u;
          if (__i < 0)
            __u = -static_cast<make_unsigned_t<_Int>>(__i);
          else
            __u = __i;
 
          char* __start = __buf + 3;
          char* const __end = __buf + sizeof(__buf);
          char* const __start_digits = __start;
 
          switch (_M_spec._M_type)
          {
            case _Pres_b:
            case _Pres_B:
              __base_prefix = _M_spec._M_type == _Pres_b ? "0b" : "0B";
              __res = to_chars(__start, __end, __u, 2);
              break;
#if 0
            case _Pres_c:
              return _M_format_character(_S_to_character(__i), __fc);
#endif
            case _Pres_none:
              // Should not reach here with _Pres_none for bool or charT, so:
              [[fallthrough]];
            case _Pres_d:
              __res = to_chars(__start, __end, __u, 10);
              break;
            case _Pres_o:
              if (__i != 0)
                __base_prefix = "0";
              __res = to_chars(__start, __end, __u, 8);
              break;
            case _Pres_x:
            case _Pres_X:
              __base_prefix = _M_spec._M_type == _Pres_x ? "0x" : "0X";
              __res = to_chars(__start, __end, __u, 16);
              if (_M_spec._M_type == _Pres_X)
                for (auto __p = __start; __p != __res.ptr; ++__p)
#if __has_builtin(__builtin_toupper)
                  *__p = __builtin_toupper(*__p);
#else
                  *__p = std::toupper(*__p);
#endif
              break;
            default:
              __builtin_unreachable();
          }
 
          if (_M_spec._M_alt && __base_prefix.size())
            {
              __start -= __base_prefix.size();
              __builtin_memcpy(__start, __base_prefix.data(),
                               __base_prefix.size());
            }
          __start = __format::__put_sign(__i, _M_spec._M_sign, __start - 1);
 
          return _M_format_int(string_view(__start, __res.ptr - __start),
                               __start_digits - __start, __fc);
        }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(bool __i, basic_format_context<_Out, _CharT>& __fc) const
        {
          if (_M_spec._M_type == _Pres_c)
            return _M_format_character(static_cast<unsigned char>(__i), __fc);
          if (_M_spec._M_type != _Pres_s)
            return format(static_cast<unsigned char>(__i), __fc);
 
          basic_string<_CharT> __s;
          size_t __est_width;
          if (_M_spec._M_localized) [[unlikely]]
            {
              auto& __np = std::use_facet<numpunct<_CharT>>(__fc.locale());
              __s = __i ? __np.truename() : __np.falsename();
              __est_width = __s.size(); // TODO Unicode-aware estimate
            }
          else
            {
              if constexpr (is_same_v<char, _CharT>)
                __s = __i ? "true" : "false";
              else
                __s = __i ? L"true" : L"false";
              __est_width = __s.size();
            }
 
          return __format::__write_padded_as_spec(__s, __est_width, __fc,
                                                  _M_spec);
        }
 
      [[__gnu__::__always_inline__]]
      static size_t
      _S_character_width(_CharT __c)
      {
        // N.B. single byte cannot encode charcter of width greater than 1
        if constexpr (sizeof(_CharT) > 1u &&
                        __unicode::__literal_encoding_is_unicode<_CharT>())
          return __unicode::__field_width(__c);
        else
          return 1u;
      }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format_character(_CharT __c,
                      basic_format_context<_Out, _CharT>& __fc) const
        {
          return __format::__write_padded_as_spec({&__c, 1u},
                                                  _S_character_width(__c),
                                                  __fc, _M_spec);
        }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format_character_escaped(_CharT __c,
                                    basic_format_context<_Out, _CharT>& __fc) const
        {
          using _Esc = _Escapes<_CharT>;
          constexpr auto __term = __format::_Term_char::_Tc_apos;
          const basic_string_view<_CharT> __in(&__c, 1u);
          if (_M_spec._M_get_width(__fc) <= 3u)
            return __format::__write_escaped(__fc.out(), __in, __term);
 
          _CharT __buf[12];
          __format::_Fixedbuf_sink<_CharT> __sink(__buf);
          __format::__write_escaped(__sink.out(), __in, __term);
 
          const basic_string_view<_CharT> __escaped = __sink.view();
          size_t __estimated_width;
          if (__escaped[1] == _Esc::_S_bslash()[0]) // escape sequence
            __estimated_width = __escaped.size();
          else
            __estimated_width = 2 + _S_character_width(__c);
          return __format::__write_padded_as_spec(__escaped,
                                                  __estimated_width,
                                                  __fc, _M_spec);
        }
 
      template<typename _Int>
        static _CharT
        _S_to_character(_Int __i)
        {
          using _Traits = __gnu_cxx::__int_traits<_CharT>;
          if constexpr (is_signed_v<_Int> == is_signed_v<_CharT>)
            {
              if (_Traits::__min <= __i && __i <= _Traits::__max)
                return static_cast<_CharT>(__i);
            }
          else if constexpr (is_signed_v<_Int>)
            {
              if (__i >= 0 && make_unsigned_t<_Int>(__i) <= _Traits::__max)
                return static_cast<_CharT>(__i);
            }
          else if (__i <= make_unsigned_t<_CharT>(_Traits::__max))
            return static_cast<_CharT>(__i);
          __throw_format_error("format error: integer not representable as "
                               "character");
        }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format_int(string_view __narrow_str, size_t __prefix_len,
                      basic_format_context<_Out, _CharT>& __fc) const
        {
          size_t __width = _M_spec._M_get_width(__fc);
 
          basic_string_view<_CharT> __str;
          if constexpr (is_same_v<char, _CharT>)
            __str = __narrow_str;
#ifdef _GLIBCXX_USE_WCHAR_T
          else
            {
              size_t __n = __narrow_str.size();
              auto __p = (_CharT*)__builtin_alloca(__n * sizeof(_CharT));
              std::__to_wstring_numeric(__narrow_str.data(), __n, __p);
              __str = {__p, __n};
            }
#endif
 
          if (_M_spec._M_localized)
            {
              const auto& __l = __fc.locale();
              if (__l.name() != "C")
                {
                  auto& __np = use_facet<numpunct<_CharT>>(__l);
                  string __grp = __np.grouping();
                  if (!__grp.empty())
                    {
                      size_t __n = __str.size() - __prefix_len;
                      auto __p = (_CharT*)__builtin_alloca(2 * __n
                                                             * sizeof(_CharT)
                                                             + __prefix_len);
                      auto __s = __str.data();
                      char_traits<_CharT>::copy(__p, __s, __prefix_len);
                      __s += __prefix_len;
                      auto __end = std::__add_grouping(__p + __prefix_len,
                                                       __np.thousands_sep(),
                                                       __grp.data(),
                                                       __grp.size(),
                                                       __s, __s + __n);
                      __str = {__p, size_t(__end - __p)};
                    }
                }
            }
 
          if (__width <= __str.size())
            return __format::__write(__fc.out(), __str);
 
          char32_t __fill_char = _M_spec._M_fill;
          _Align __align = _M_spec._M_align;
 
          size_t __nfill = __width - __str.size();
          auto __out = __fc.out();
          if (__align == _Align_default)
            {
              __align = _Align_right;
              if (_M_spec._M_zero_fill)
                {
                  __fill_char = _CharT('0');
                  // Write sign and base prefix before zero filling.
                  if (__prefix_len != 0)
                    {
                      __out = __format::__write(std::move(__out),
                                                __str.substr(0, __prefix_len));
                      __str.remove_prefix(__prefix_len);
                    }
                }
              else
                __fill_char = _CharT(' ');
            }
          return __format::__write_padded(std::move(__out), __str,
                                          __align, __nfill, __fill_char);
        }
 
#if defined __SIZEOF_INT128__ && defined __STRICT_ANSI__
      template<typename _Tp>
        using make_unsigned_t
          = typename __conditional_t<(sizeof(_Tp) <= sizeof(long long)),
                                     std::make_unsigned<_Tp>,
                                     type_identity<unsigned __int128>>::type;
 
      // std::to_chars is not overloaded for int128 in strict mode.
      template<typename _Int>
        static to_chars_result
        to_chars(char* __first, char* __last, _Int __value, int __base)
        { return std::__to_chars_i<_Int>(__first, __last, __value, __base); }
#endif
 
      _Spec<_CharT> _M_spec{};
    };
 
  // Decide how 128-bit floating-point types should be formatted (or not).
  // When supported, the typedef __format::__float128_t is the type that
  // format arguments should be converted to for storage in basic_format_arg.
  // Define the macro _GLIBCXX_FORMAT_F128 to say they're supported.
  // _GLIBCXX_FORMAT_F128=1 means __float128, _Float128 etc. will be formatted
  // by converting them to long double (or __ieee128 for powerpc64le).
  // _GLIBCXX_FORMAT_F128=2 means basic_format_arg needs to enable explicit
  // support for _Float128, rather than formatting it as another type.
#undef _GLIBCXX_FORMAT_F128
 
#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
 
  // Format 128-bit floating-point types using __ieee128.
  using __float128_t = __ieee128;
# define _GLIBCXX_FORMAT_F128 1
 
#ifdef __LONG_DOUBLE_IEEE128__
  // These overloads exist in the library, but are not declared.
  // Make them available as std::__format::to_chars.
  to_chars_result
  to_chars(char*, char*, __ibm128) noexcept
    __asm("_ZSt8to_charsPcS_e");
 
  to_chars_result
  to_chars(char*, char*, __ibm128, chars_format) noexcept
    __asm("_ZSt8to_charsPcS_eSt12chars_format");
 
  to_chars_result
  to_chars(char*, char*, __ibm128, chars_format, int) noexcept
    __asm("_ZSt8to_charsPcS_eSt12chars_formati");
#elif __cplusplus == 202002L
  to_chars_result
  to_chars(char*, char*, __ieee128) noexcept
    __asm("_ZSt8to_charsPcS_u9__ieee128");
 
  to_chars_result
  to_chars(char*, char*, __ieee128, chars_format) noexcept
    __asm("_ZSt8to_charsPcS_u9__ieee128St12chars_format");
 
  to_chars_result
  to_chars(char*, char*, __ieee128, chars_format, int) noexcept
    __asm("_ZSt8to_charsPcS_u9__ieee128St12chars_formati");
#endif
 
#elif defined _GLIBCXX_LDOUBLE_IS_IEEE_BINARY128
 
  // Format 128-bit floating-point types using long double.
  using __float128_t = long double;
# define _GLIBCXX_FORMAT_F128 1
 
#elif __FLT128_DIG__ && defined(_GLIBCXX_HAVE_FLOAT128_MATH)
 
  // Format 128-bit floating-point types using _Float128.
  using __float128_t = _Float128;
# define _GLIBCXX_FORMAT_F128 2
 
# if __cplusplus == 202002L
  // These overloads exist in the library, but are not declared for C++20.
  // Make them available as std::__format::to_chars.
  to_chars_result
  to_chars(char*, char*, _Float128) noexcept
#  if _GLIBCXX_INLINE_VERSION
    __asm("_ZNSt3__88to_charsEPcS0_DF128_");
#  else
    __asm("_ZSt8to_charsPcS_DF128_");
#  endif
 
  to_chars_result
  to_chars(char*, char*, _Float128, chars_format) noexcept
#  if _GLIBCXX_INLINE_VERSION
    __asm("_ZNSt3__88to_charsEPcS0_DF128_NS_12chars_formatE");
#  else
    __asm("_ZSt8to_charsPcS_DF128_St12chars_format");
#  endif
 
  to_chars_result
  to_chars(char*, char*, _Float128, chars_format, int) noexcept
#  if _GLIBCXX_INLINE_VERSION
    __asm("_ZNSt3__88to_charsEPcS0_DF128_NS_12chars_formatEi");
#  else
    __asm("_ZSt8to_charsPcS_DF128_St12chars_formati");
#  endif
# endif
#endif
 
  using std::to_chars;
 
  // We can format a floating-point type iff it is usable with to_chars.
  template<typename _Tp>
    concept __formattable_float
      = is_same_v<remove_cv_t<_Tp>, _Tp> && requires (_Tp __t, char* __p)
      { __format::to_chars(__p, __p, __t, chars_format::scientific, 6); };
 
  template<__char _CharT>
    struct __formatter_fp
    {
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      {
        _Spec<_CharT> __spec{};
        const auto __last = __pc.end();
        auto __first = __pc.begin();
 
        auto __finalize = [this, &__spec] {
          _M_spec = __spec;
        };
 
        auto __finished = [&] {
          if (__first == __last || *__first == '}')
            {
              __finalize();
              return true;
            }
          return false;
        };
 
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_fill_and_align(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_sign(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_alternate_form(__first, __last);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_zero_fill(__first, __last);
        if (__finished())
          return __first;
 
        if (__first[0] != '.')
          {
            __first = __spec._M_parse_width(__first, __last, __pc);
            if (__finished())
              return __first;
          }
 
        __first = __spec._M_parse_precision(__first, __last, __pc);
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_locale(__first, __last);
        if (__finished())
          return __first;
 
        switch (*__first)
        {
          case 'a':
            __spec._M_type = _Pres_a;
            ++__first;
            break;
          case 'A':
            __spec._M_type = _Pres_A;
            ++__first;
            break;
          case 'e':
            __spec._M_type = _Pres_e;
            ++__first;
            break;
          case 'E':
            __spec._M_type = _Pres_E;
            ++__first;
            break;
          case 'f':
            __spec._M_type = _Pres_f;
            ++__first;
            break;
          case 'F':
            __spec._M_type = _Pres_F;
            ++__first;
            break;
          case 'g':
            __spec._M_type = _Pres_g;
            ++__first;
            break;
          case 'G':
            __spec._M_type = _Pres_G;
            ++__first;
            break;
          }
 
        if (__finished())
          return __first;
 
        __format::__failed_to_parse_format_spec();
      }
 
      template<typename _Fp, typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Fp __v, basic_format_context<_Out, _CharT>& __fc) const
        {
          std::string __dynbuf;
          char __buf[128];
          to_chars_result __res{};
 
          size_t __prec = 6;
          bool __use_prec = _M_spec._M_prec_kind != _WP_none;
          if (__use_prec)
            __prec = _M_spec._M_get_precision(__fc);
 
          char* __start = __buf + 1; // reserve space for sign
          char* __end = __buf + sizeof(__buf);
 
          chars_format __fmt{};
          bool __upper = false;
          bool __trailing_zeros = false;
          char __expc = 'e';
 
          switch (_M_spec._M_type)
          {
            case _Pres_A:
              __upper = true;
              __expc = 'P';
              [[fallthrough]];
            case _Pres_a:
              if (_M_spec._M_type != _Pres_A)
                __expc = 'p';
              __fmt = chars_format::hex;
              break;
            case _Pres_E:
              __upper = true;
              __expc = 'E';
              [[fallthrough]];
            case _Pres_e:
              __use_prec = true;
              __fmt = chars_format::scientific;
              break;
            case _Pres_F:
              __upper = true;
              [[fallthrough]];
            case _Pres_f:
              __use_prec = true;
              __fmt = chars_format::fixed;
              break;
            case _Pres_G:
              __upper = true;
              __expc = 'E';
              [[fallthrough]];
            case _Pres_g:
              __trailing_zeros = true;
              __use_prec = true;
              __fmt = chars_format::general;
              break;
            case _Pres_none:
              if (__use_prec)
                __fmt = chars_format::general;
              break;
            default:
              __builtin_unreachable();
          }
 
          // Write value into buffer using std::to_chars.
          auto __to_chars = [&](char* __b, char* __e) {
            if (__use_prec)
              return __format::to_chars(__b, __e, __v, __fmt, __prec);
            else if (__fmt != chars_format{})
              return __format::to_chars(__b, __e, __v, __fmt);
            else
              return __format::to_chars(__b, __e, __v);
          };
 
          // First try using stack buffer.
          __res = __to_chars(__start, __end);
 
          if (__builtin_expect(__res.ec == errc::value_too_large, 0))
            {
              // If the buffer is too small it's probably because of a large
              // precision, or a very large value in fixed format.
              size_t __guess = 8 + __prec;
              if (__fmt == chars_format::fixed) // +ddd.prec
                {
                  if constexpr (is_same_v<_Fp, float> || is_same_v<_Fp, double>
                                  || is_same_v<_Fp, long double>)
                    {
                      // The number of digits to the left of the decimal point
                      // is floor(log10(max(abs(__v),1)))+1
                      int __exp{};
                      if constexpr (is_same_v<_Fp, float>)
                        __builtin_frexpf(__v, &__exp);
                      else if constexpr (is_same_v<_Fp, double>)
                        __builtin_frexp(__v, &__exp);
                      else if constexpr (is_same_v<_Fp, long double>)
                        __builtin_frexpl(__v, &__exp);
                      if (__exp > 0)
                        __guess += 1U + __exp * 4004U / 13301U; // log10(2) approx.
                    }
                  else
                    __guess += numeric_limits<_Fp>::max_exponent10;
                }
              if (__guess <= sizeof(__buf)) [[unlikely]]
                __guess = sizeof(__buf) * 2;
              __dynbuf.reserve(__guess);
 
              do
                {
                  // Mangling of this lambda, and thus resize_and_overwrite
                  // instantiated with it, was fixed in ABI 18 (G++ 13).  Since
                  // <format> was new in G++ 13, and is experimental, that
                  // isn't a problem.
                  auto __overwrite = [&__to_chars, &__res] (char* __p, size_t __n)
                  {
                    __res = __to_chars(__p + 1, __p + __n - 1);
                    return __res.ec == errc{} ? __res.ptr - __p : 0;
                  };
 
                  __dynbuf.__resize_and_overwrite(__dynbuf.capacity() * 2,
                                                  __overwrite);
                  __start = __dynbuf.data() + 1; // reserve space for sign
                  __end = __dynbuf.data() + __dynbuf.size();
                }
              while (__builtin_expect(__res.ec == errc::value_too_large, 0));
          }
 
          // Use uppercase for 'A', 'E', and 'G' formats.
          if (__upper)
            {
              for (char* __p = __start; __p != __res.ptr; ++__p)
                *__p = std::toupper(*__p);
            }
 
          bool __have_sign = true;
          // Add sign for non-negative values.
          if (!__builtin_signbit(__v))
            {
              if (_M_spec._M_sign == _Sign_plus)
                *--__start = '+';
              else if (_M_spec._M_sign == _Sign_space)
                *--__start = ' ';
              else
                __have_sign = false;
            }
 
          string_view __narrow_str(__start, __res.ptr - __start);
 
          // Use alternate form. Ensure decimal point is always present,
          // and add trailing zeros (up to precision) for g and G forms.
          if (_M_spec._M_alt && __builtin_isfinite(__v))
            {
              string_view __s = __narrow_str;
              size_t __sigfigs; // Number of significant figures.
              size_t __z = 0;   // Number of trailing zeros to add.
              size_t __p;       // Position of the exponent character (if any).
              size_t __d = __s.find('.'); // Position of decimal point.
              if (__d != __s.npos) // Found decimal point.
                {
                  __p = __s.find(__expc, __d + 1);
                  if (__p == __s.npos)
                    __p = __s.size();
 
                  // If presentation type is g or G we might need to add zeros.
                  if (__trailing_zeros)
                    {
                      // Find number of digits after first significant figure.
                      if (__s[__have_sign] != '0')
                        // A string like "D.D" or "-D.DDD"
                        __sigfigs = __p - __have_sign - 1;
                      else
                        // A string like "0.D" or "-0.0DD".
                        // Safe to assume there is a non-zero digit, because
                        // otherwise there would be no decimal point.
                        __sigfigs = __p - __s.find_first_not_of('0', __d + 1);
                    }
                }
              else // No decimal point, we need to insert one.
                {
                  __p = __s.find(__expc); // Find the exponent, if present.
                  if (__p == __s.npos)
                    __p = __s.size();
                  __d = __p; // Position where '.' should be inserted.
                  __sigfigs = __d - __have_sign;
                }
 
              if (__trailing_zeros && __prec != 0)
                {
                  // For g and G presentation types std::to_chars produces
                  // no more than prec significant figures. Insert this many
                  // zeros so the result has exactly prec significant figures.
                  __z = __prec - __sigfigs;
                }
 
              if (size_t __extras = int(__d == __p) + __z) // How many to add.
                {
                  if (__dynbuf.empty() && __extras <= size_t(__end - __res.ptr))
                    {
                      // The stack buffer is large enough for the result.
                      // Move exponent to make space for extra chars.
                      __builtin_memmove(__start + __p + __extras,
                                        __start + __p,
                                        __s.size() - __p);
                      if (__d == __p)
                        __start[__p++] = '.';
                      __builtin_memset(__start + __p, '0', __z);
                      __narrow_str = {__s.data(), __s.size() + __extras};
                    }
                  else // Need to switch to the dynamic buffer.
                    {
                      __dynbuf.reserve(__s.size() + __extras);
                      if (__dynbuf.empty())
                        {
                          __dynbuf = __s.substr(0, __p);
                          if (__d == __p)
                            __dynbuf += '.';
                          if (__z)
                            __dynbuf.append(__z, '0');
                          __dynbuf.append(__s.substr(__p));
                        }
                      else
                        {
                          __dynbuf.insert(__p, __extras, '0');
                          if (__d == __p)
                            __dynbuf[__p] = '.';
                        }
                      __narrow_str = __dynbuf;
                    }
                }
            }
 
          basic_string<_CharT> __wstr;
          basic_string_view<_CharT> __str;
          if constexpr (is_same_v<_CharT, char>)
            __str = __narrow_str;
#ifdef _GLIBCXX_USE_WCHAR_T
          else
            {
              __wstr = std::__to_wstring_numeric(__narrow_str);
              __str = __wstr;
            }
#endif
 
          if (_M_spec._M_localized && __builtin_isfinite(__v))
            {
              auto __s = _M_localize(__str, __expc, __fc.locale());
              if (!__s.empty())
                __str = __wstr = std::move(__s);
            }
 
          size_t __width = _M_spec._M_get_width(__fc);
 
          if (__width <= __str.size())
            return __format::__write(__fc.out(), __str);
 
          char32_t __fill_char = _M_spec._M_fill;
          _Align __align = _M_spec._M_align;
 
          size_t __nfill = __width - __str.size();
          auto __out = __fc.out();
          if (__align == _Align_default)
            {
              __align = _Align_right;
              if (_M_spec._M_zero_fill && __builtin_isfinite(__v))
                {
                  __fill_char = _CharT('0');
                  // Write sign before zero filling.
                  if (!__format::__is_xdigit(__narrow_str[0]))
                    {
                      *__out++ = __str[0];
                      __str.remove_prefix(1);
                    }
                }
              else
                __fill_char = _CharT(' ');
            }
          return __format::__write_padded(std::move(__out), __str,
                                          __align, __nfill, __fill_char);
        }
 
      // Locale-specific format.
      basic_string<_CharT>
      _M_localize(basic_string_view<_CharT> __str, char __expc,
                  const locale& __loc) const
      {
        basic_string<_CharT> __lstr;
 
        if (__loc == locale::classic())
          return __lstr; // Nothing to do.
 
        const auto& __np = use_facet<numpunct<_CharT>>(__loc);
        const _CharT __point = __np.decimal_point();
        const string __grp = __np.grouping();
 
        _CharT __dot, __exp;
        if constexpr (is_same_v<_CharT, char>)
          {
            __dot = '.';
            __exp = __expc;
          }
        else
          {
            __dot = L'.';
            switch (__expc)
            {
              case 'e':
                __exp = L'e';
                break;
              case 'E':
                __exp = L'E';
                break;
              case 'p':
                __exp = L'p';
                break;
              case 'P':
                __exp = L'P';
                break;
              default:
                __builtin_unreachable();
            }
          }
 
        if (__grp.empty() && __point == __dot)
          return __lstr; // Locale uses '.' and no grouping.
 
        size_t __d = __str.find(__dot); // Index of radix character (if any).
        size_t __e = min(__d, __str.find(__exp)); // First of radix or exponent
        if (__e == __str.npos)
          __e = __str.size();
        const size_t __r = __str.size() - __e; // Length of remainder.
        auto __overwrite = [&](_CharT* __p, size_t) {
          // Apply grouping to the digits before the radix or exponent.
          int __off = 0;
          if (auto __c = __str.front(); __c == '-' || __c == '+' || __c == ' ')
            {
              *__p = __c;
              __off = 1;
            }
          auto __end = std::__add_grouping(__p + __off, __np.thousands_sep(),
                                           __grp.data(), __grp.size(),
                                           __str.data() + __off,
                                           __str.data() + __e);
          if (__r) // If there's a fractional part or exponent
            {
              if (__d != __str.npos)
                {
                  *__end = __point; // Add the locale's radix character.
                  ++__end;
                  ++__e;
                }
              const size_t __rlen = __str.size() - __e;
              // Append fractional digits and/or exponent:
              char_traits<_CharT>::copy(__end, __str.data() + __e, __rlen);
              __end += __rlen;
            }
          return (__end - __p);
        };
        __lstr.__resize_and_overwrite(__e * 2 + __r, __overwrite);
        return __lstr;
      }
 
      _Spec<_CharT> _M_spec{};
    };
 
} // namespace __format
/// @endcond
 
  /// Format a character.
  template<__format::__char _CharT>
    struct formatter<_CharT, _CharT>
    {
      formatter() = default;
 
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      {
        return _M_f.template _M_parse<_CharT>(__pc);
      }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_CharT __u, basic_format_context<_Out, _CharT>& __fc) const
        {
          if (_M_f._M_spec._M_type == __format::_Pres_none
              || _M_f._M_spec._M_type == __format::_Pres_c)
            return _M_f._M_format_character(__u, __fc);
          else if (_M_f._M_spec._M_type == __format::_Pres_esc)
            return _M_f._M_format_character_escaped(__u, __fc);
          else
            return _M_f.format(static_cast<make_unsigned_t<_CharT>>(__u), __fc);
        }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void
      set_debug_format() noexcept
      { _M_f._M_spec._M_type = __format::_Pres_esc; }
#endif
 
    private:
      __format::__formatter_int<_CharT> _M_f;
    };
 
#ifdef _GLIBCXX_USE_WCHAR_T
  /// Format a char value for wide character output.
  template<>
    struct formatter<char, wchar_t>
    {
      formatter() = default;
 
      constexpr typename basic_format_parse_context<wchar_t>::iterator
      parse(basic_format_parse_context<wchar_t>& __pc)
      {
        return _M_f._M_parse<char>(__pc);
      }
 
      template<typename _Out>
        typename basic_format_context<_Out, wchar_t>::iterator
        format(char __u, basic_format_context<_Out, wchar_t>& __fc) const
        {
          if (_M_f._M_spec._M_type == __format::_Pres_none
              || _M_f._M_spec._M_type == __format::_Pres_c)
            return _M_f._M_format_character(__u, __fc);
          else if (_M_f._M_spec._M_type == __format::_Pres_esc)
            return _M_f._M_format_character_escaped(__u, __fc);
          else
            return _M_f.format(static_cast<unsigned char>(__u), __fc);
        }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void
      set_debug_format() noexcept
      { _M_f._M_spec._M_type = __format::_Pres_esc; }
#endif
 
    private:
      __format::__formatter_int<wchar_t> _M_f;
    };
#endif // USE_WCHAR_T
 
  /** Format a string.
   * @{
   */
  template<__format::__char _CharT>
    struct formatter<_CharT*, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        [[__gnu__::__nonnull__]]
        typename basic_format_context<_Out, _CharT>::iterator
        format(_CharT* __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format(__u, __fc); }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
#endif
 
    private:
      __format::__formatter_str<_CharT> _M_f;
    };
 
  template<__format::__char _CharT>
    struct formatter<const _CharT*, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        [[__gnu__::__nonnull__]]
        typename basic_format_context<_Out, _CharT>::iterator
        format(const _CharT* __u,
               basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format(__u, __fc); }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
#endif
 
    private:
      __format::__formatter_str<_CharT> _M_f;
    };
 
  template<__format::__char _CharT, size_t _Nm>
    struct formatter<_CharT[_Nm], _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(const _CharT (&__u)[_Nm],
               basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format({__u, _Nm}, __fc); }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
#endif
 
    private:
      __format::__formatter_str<_CharT> _M_f;
    };
 
  template<typename _Traits, typename _Alloc>
    struct formatter<basic_string<char, _Traits, _Alloc>, char>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<char>::iterator
      parse(basic_format_parse_context<char>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, char>::iterator
        format(const basic_string<char, _Traits, _Alloc>& __u,
               basic_format_context<_Out, char>& __fc) const
        { return _M_f.format(__u, __fc); }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
#endif
 
    private:
      __format::__formatter_str<char> _M_f;
    };
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Traits, typename _Alloc>
    struct formatter<basic_string<wchar_t, _Traits, _Alloc>, wchar_t>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<wchar_t>::iterator
      parse(basic_format_parse_context<wchar_t>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, wchar_t>::iterator
        format(const basic_string<wchar_t, _Traits, _Alloc>& __u,
               basic_format_context<_Out, wchar_t>& __fc) const
        { return _M_f.format(__u, __fc); }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
#endif
 
    private:
      __format::__formatter_str<wchar_t> _M_f;
    };
#endif // USE_WCHAR_T
 
  template<typename _Traits>
    struct formatter<basic_string_view<char, _Traits>, char>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<char>::iterator
      parse(basic_format_parse_context<char>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, char>::iterator
        format(basic_string_view<char, _Traits> __u,
               basic_format_context<_Out, char>& __fc) const
        { return _M_f.format(__u, __fc); }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
#endif
 
    private:
      __format::__formatter_str<char> _M_f;
    };
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Traits>
    struct formatter<basic_string_view<wchar_t, _Traits>, wchar_t>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<wchar_t>::iterator
      parse(basic_format_parse_context<wchar_t>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, wchar_t>::iterator
        format(basic_string_view<wchar_t, _Traits> __u,
               basic_format_context<_Out, wchar_t>& __fc) const
        { return _M_f.format(__u, __fc); }
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
      constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
#endif
 
    private:
      __format::__formatter_str<wchar_t> _M_f;
    };
#endif // USE_WCHAR_T
  /// @}
 
/// @cond undocumented
namespace __format
{
  // each cv-unqualified arithmetic type ArithmeticT other than
  // char, wchar_t, char8_t, char16_t, or char32_t
  template<typename _Tp>
    constexpr bool __is_formattable_integer = __is_integer<_Tp>::__value;
 
#if defined __SIZEOF_INT128__
  template<> inline constexpr bool __is_formattable_integer<__int128>  = true;
  template<> inline constexpr bool __is_formattable_integer<unsigned __int128>
      = true;
#endif
 
  template<> inline constexpr bool __is_formattable_integer<char> = false;
  template<> inline constexpr bool __is_formattable_integer<wchar_t> = false;
#ifdef _GLIBCXX_USE_CHAR8_T
  template<> inline constexpr bool __is_formattable_integer<char8_t> = false;
#endif
  template<> inline constexpr bool __is_formattable_integer<char16_t> = false;
  template<> inline constexpr bool __is_formattable_integer<char32_t> = false;
}
/// @endcond
 
  /// Format an integer.
  template<typename _Tp, __format::__char _CharT>
    requires __format::__is_formattable_integer<_Tp>
    struct formatter<_Tp, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      {
        return _M_f.template _M_parse<_Tp>(__pc);
      }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Tp __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format(__u, __fc); }
 
    private:
      __format::__formatter_int<_CharT> _M_f;
    };
 
#if defined __glibcxx_to_chars
  /// Format a floating-point value.
  template<__format::__formattable_float _Tp, __format::__char _CharT>
    struct formatter<_Tp, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Tp __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format(__u, __fc); }
 
    private:
      __format::__formatter_fp<_CharT> _M_f;
    };
 
#if __LDBL_MANT_DIG__ == __DBL_MANT_DIG__
  // Reuse __formatter_fp<C>::format<double, Out> for long double.
  template<__format::__char _CharT>
    struct formatter<long double, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(long double __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format((double)__u, __fc); }
 
    private:
      __format::__formatter_fp<_CharT> _M_f;
    };
#endif
 
#if defined(__STDCPP_FLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
  // Reuse __formatter_fp<C>::format<float, Out> for _Float16.
  template<__format::__char _CharT>
    struct formatter<_Float16, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Float16 __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format((float)__u, __fc); }
 
    private:
      __format::__formatter_fp<_CharT> _M_f;
    };
#endif
 
#if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
  // Reuse __formatter_fp<C>::format<float, Out> for _Float32.
  template<__format::__char _CharT>
    struct formatter<_Float32, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Float32 __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format((float)__u, __fc); }
 
    private:
      __format::__formatter_fp<_CharT> _M_f;
    };
#endif
 
#if defined(__FLT64_DIG__) && defined(_GLIBCXX_DOUBLE_IS_IEEE_BINARY64)
  // Reuse __formatter_fp<C>::format<double, Out> for _Float64.
  template<__format::__char _CharT>
    struct formatter<_Float64, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Float64 __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format((double)__u, __fc); }
 
    private:
      __format::__formatter_fp<_CharT> _M_f;
    };
#endif
 
#if defined(__FLT128_DIG__) && _GLIBCXX_FORMAT_F128 == 1
  // Reuse __formatter_fp<C>::format<__float128_t, Out> for _Float128.
  template<__format::__char _CharT>
    struct formatter<_Float128, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Float128 __u, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format((__format::__float128_t)__u, __fc); }
 
    private:
      __format::__formatter_fp<_CharT> _M_f;
    };
#endif
 
#if defined(__STDCPP_BFLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
  // Reuse __formatter_fp<C>::format<float, Out> for bfloat16_t.
  template<__format::__char _CharT>
    struct formatter<__gnu_cxx::__bfloat16_t, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(__gnu_cxx::__bfloat16_t __u,
               basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format((float)__u, __fc); }
 
    private:
      __format::__formatter_fp<_CharT> _M_f;
    };
#endif
#endif // __cpp_lib_to_chars
 
  /** Format a pointer.
   * @{
   */
  template<__format::__char _CharT>
    struct formatter<const void*, _CharT>
    {
      formatter() = default;
 
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      {
        __format::_Spec<_CharT> __spec{};
        const auto __last = __pc.end();
        auto __first = __pc.begin();
 
        auto __finalize = [this, &__spec] {
          _M_spec = __spec;
        };
 
        auto __finished = [&] {
          if (__first == __last || *__first == '}')
            {
              __finalize();
              return true;
            }
          return false;
        };
 
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_fill_and_align(__first, __last);
        if (__finished())
          return __first;
 
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// P2510R3 Formatting pointers
#if __glibcxx_format >= 202304L
        __first = __spec._M_parse_zero_fill(__first, __last);
        if (__finished())
          return __first;
#endif
 
        __first = __spec._M_parse_width(__first, __last, __pc);
 
        if (__first != __last)
          {
            if (*__first == 'p')
              ++__first;
#if __glibcxx_format >= 202304L
            else if (*__first == 'P')
            {
              __spec._M_type = __format::_Pres_P;
              ++__first;
            }
#endif
          }
 
        if (__finished())
          return __first;
 
        __format::__failed_to_parse_format_spec();
      }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(const void* __v, basic_format_context<_Out, _CharT>& __fc) const
        {
          auto __u = reinterpret_cast<__UINTPTR_TYPE__>(__v);
          char __buf[2 + sizeof(__v) * 2];
          auto [__ptr, __ec] = std::to_chars(__buf + 2, std::end(__buf),
                                             __u, 16);
          int __n = __ptr - __buf;
          __buf[0] = '0';
          __buf[1] = 'x';
#if __glibcxx_format >= 202304L
          if (_M_spec._M_type == __format::_Pres_P)
            {
              __buf[1] = 'X';
              for (auto __p = __buf + 2; __p != __ptr; ++__p)
#if __has_builtin(__builtin_toupper)
                *__p = __builtin_toupper(*__p);
#else
                *__p = std::toupper(*__p);
#endif
            }
#endif
 
          basic_string_view<_CharT> __str;
          if constexpr (is_same_v<_CharT, char>)
            __str = string_view(__buf, __n);
#ifdef _GLIBCXX_USE_WCHAR_T
          else
            {
              auto __p = (_CharT*)__builtin_alloca(__n * sizeof(_CharT));
              std::__to_wstring_numeric(__buf, __n, __p);
              __str = wstring_view(__p, __n);
            }
#endif
 
#if __glibcxx_format >= 202304L
          if (_M_spec._M_zero_fill)
            {
              size_t __width = _M_spec._M_get_width(__fc);
              if (__width <= __str.size())
                return __format::__write(__fc.out(), __str);
 
              auto __out = __fc.out();
              // Write "0x" or "0X" prefix before zero-filling.
              __out = __format::__write(std::move(__out), __str.substr(0, 2));
              __str.remove_prefix(2);
              size_t __nfill = __width - __n;
              return __format::__write_padded(std::move(__out), __str,
                                              __format::_Align_right,
                                              __nfill, _CharT('0'));
            }
#endif
 
          return __format::__write_padded_as_spec(__str, __n, __fc, _M_spec,
                                                  __format::_Align_right);
        }
 
    private:
      __format::_Spec<_CharT> _M_spec{};
    };
 
  template<__format::__char _CharT>
    struct formatter<void*, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(void* __v, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format(__v, __fc); }
 
    private:
      formatter<const void*, _CharT> _M_f;
    };
 
  template<__format::__char _CharT>
    struct formatter<nullptr_t, _CharT>
    {
      formatter() = default;
 
      [[__gnu__::__always_inline__]]
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      { return _M_f.parse(__pc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(nullptr_t, basic_format_context<_Out, _CharT>& __fc) const
        { return _M_f.format(nullptr, __fc); }
 
    private:
      formatter<const void*, _CharT> _M_f;
    };
  /// @}
 
#if defined _GLIBCXX_USE_WCHAR_T && __glibcxx_format_ranges
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 3944. Formatters converting sequences of char to sequences of wchar_t
 
  struct __formatter_disabled
  {
    __formatter_disabled() = delete; // Cannot format char sequence to wchar_t
    __formatter_disabled(const __formatter_disabled&) = delete;
    __formatter_disabled& operator=(const __formatter_disabled&) = delete;
  };
 
  template<>
    struct formatter<char*, wchar_t>
    : private __formatter_disabled { };
  template<>
    struct formatter<const char*, wchar_t>
    : private __formatter_disabled { };
  template<size_t _Nm>
    struct formatter<char[_Nm], wchar_t>
    : private __formatter_disabled { };
  template<class _Traits, class _Allocator>
    struct formatter<basic_string<char, _Traits, _Allocator>, wchar_t>
    : private __formatter_disabled { };
  template<class _Traits>
    struct formatter<basic_string_view<char, _Traits>, wchar_t>
    : private __formatter_disabled { };
#endif
 
  /// An iterator after the last character written, and the number of
  /// characters that would have been written.
  template<typename _Out>
    struct format_to_n_result
    {
      _Out out;
      iter_difference_t<_Out> size;
    };
 
_GLIBCXX_BEGIN_NAMESPACE_CONTAINER
template<typename, typename> class vector;
_GLIBCXX_END_NAMESPACE_CONTAINER
 
/// @cond undocumented
namespace __format
{
  template<typename _CharT>
    class _Sink_iter
    {
      _Sink<_CharT>* _M_sink = nullptr;
 
    public:
      using iterator_category = output_iterator_tag;
      using value_type = void;
      using difference_type = ptrdiff_t;
      using pointer = void;
      using reference = void;
 
      _Sink_iter() = default;
      _Sink_iter(const _Sink_iter&) = default;
      _Sink_iter& operator=(const _Sink_iter&) = default;
 
      [[__gnu__::__always_inline__]]
      explicit constexpr
      _Sink_iter(_Sink<_CharT>& __sink) : _M_sink(std::addressof(__sink)) { }
 
      [[__gnu__::__always_inline__]]
      constexpr _Sink_iter&
      operator=(_CharT __c)
      {
        _M_sink->_M_write(__c);
        return *this;
      }
 
      [[__gnu__::__always_inline__]]
      constexpr _Sink_iter&
      operator=(basic_string_view<_CharT> __s)
      {
        _M_sink->_M_write(__s);
        return *this;
      }
 
      [[__gnu__::__always_inline__]]
      constexpr _Sink_iter&
      operator*() { return *this; }
 
      [[__gnu__::__always_inline__]]
      constexpr _Sink_iter&
      operator++() { return *this; }
 
      [[__gnu__::__always_inline__]]
      constexpr _Sink_iter
      operator++(int) { return *this; }
 
      auto
      _M_reserve(size_t __n) const
      { return _M_sink->_M_reserve(__n); }
    };
 
  // Abstract base class for type-erased character sinks.
  // All formatting and output is done via this type's iterator,
  // to reduce the number of different template instantiations.
  template<typename _CharT>
    class _Sink
    {
      friend class _Sink_iter<_CharT>;
 
      span<_CharT> _M_span;
      typename span<_CharT>::iterator _M_next;
 
      // Called when the span is full, to make more space available.
      // Precondition: _M_next != _M_span.begin()
      // Postcondition: _M_next != _M_span.end()
      // TODO: remove the precondition? could make overflow handle it.
      virtual void _M_overflow() = 0;
 
    protected:
      // Precondition: __span.size() != 0
      [[__gnu__::__always_inline__]]
      explicit constexpr
      _Sink(span<_CharT> __span) noexcept
      : _M_span(__span), _M_next(__span.begin())
      { }
 
      // The portion of the span that has been written to.
      [[__gnu__::__always_inline__]]
      span<_CharT>
      _M_used() const noexcept
      { return _M_span.first(_M_next - _M_span.begin()); }
 
      // The portion of the span that has not been written to.
      [[__gnu__::__always_inline__]]
      constexpr span<_CharT>
      _M_unused() const noexcept
      { return _M_span.subspan(_M_next - _M_span.begin()); }
 
      // Use the start of the span as the next write position.
      [[__gnu__::__always_inline__]]
      constexpr void
      _M_rewind() noexcept
      { _M_next = _M_span.begin(); }
 
      // Replace the current output range.
      void
      _M_reset(span<_CharT> __s, size_t __pos = 0) noexcept
      {
        _M_span = __s;
        _M_next = __s.begin() + __pos;
      }
 
      // Called by the iterator for *it++ = c
      constexpr void
      _M_write(_CharT __c)
      {
        *_M_next++ = __c;
        if (_M_next - _M_span.begin() == std::ssize(_M_span)) [[unlikely]]
          _M_overflow();
      }
 
      constexpr void
      _M_write(basic_string_view<_CharT> __s)
      {
        span __to = _M_unused();
        while (__to.size() <= __s.size())
          {
            __s.copy(__to.data(), __to.size());
            _M_next += __to.size();
            __s.remove_prefix(__to.size());
            _M_overflow();
            __to = _M_unused();
          }
        if (__s.size())
          {
            __s.copy(__to.data(), __s.size());
            _M_next += __s.size();
          }
      }
 
      // A successful _Reservation can be used to directly write
      // up to N characters to the sink to avoid unwanted buffering.
      struct _Reservation
      {
        // True if the reservation was successful, false otherwise.
        explicit operator bool() const noexcept { return _M_sink; }
        // A pointer to write directly to the sink.
        _CharT* get() const noexcept { return _M_sink->_M_next.operator->(); }
        // Add n to the _M_next iterator for the sink.
        void _M_bump(size_t __n) { _M_sink->_M_bump(__n); }
        _Sink* _M_sink;
      };
 
      // Attempt to reserve space to write n characters to the sink.
      // If anything is written to the reservation then there must be a call
      // to _M_bump(N2) before any call to another member function of *this,
      // where N2 is the number of characters written.
      virtual _Reservation
      _M_reserve(size_t __n)
      {
        if (__n <= _M_unused().size())
          return { this };
 
        if (__n <= _M_span.size()) // Cannot meet the request.
          {
            _M_overflow(); // Make more space available.
            if (__n <= _M_unused().size())
              return { this };
          }
        return { nullptr };
      }
 
      // Update the next output position after writing directly to the sink.
      // pre: no calls to _M_write or _M_overflow since _M_reserve.
      virtual void
      _M_bump(size_t __n)
      { _M_next += __n; }
 
    public:
      _Sink(const _Sink&) = delete;
      _Sink& operator=(const _Sink&) = delete;
 
      [[__gnu__::__always_inline__]]
      constexpr _Sink_iter<_CharT>
      out() noexcept
      { return _Sink_iter<_CharT>(*this); }
    };
 
 
  template<typename _CharT>
    class _Fixedbuf_sink final : public _Sink<_CharT>
    {
      void
      _M_overflow() override
      {
        __glibcxx_assert(false);
        this->_M_rewind();
      }
 
    public:
      [[__gnu__::__always_inline__]]
      constexpr explicit
      _Fixedbuf_sink(span<_CharT> __buf)
        : _Sink<_CharT>(__buf)
      { }
 
      constexpr basic_string_view<_CharT>
      view() const
      {
        auto __s = this->_M_used();
        return basic_string_view<_CharT>(__s.data(), __s.size());
      }
    };
 
  // A sink with an internal buffer. This is used to implement concrete sinks.
  template<typename _CharT>
    class _Buf_sink : public _Sink<_CharT>
    {
    protected:
      _CharT _M_buf[__stackbuf_size<_CharT>];
 
      [[__gnu__::__always_inline__]]
      constexpr
      _Buf_sink() noexcept
      : _Sink<_CharT>(_M_buf)
      { }
    };
 
  using _GLIBCXX_STD_C::vector;
 
  // A sink that fills a sequence (e.g. std::string, std::vector, std::deque).
  // Writes to a buffer then appends that to the sequence when it fills up.
  template<typename _Seq>
    class _Seq_sink final : public _Buf_sink<typename _Seq::value_type>
    {
      using _CharT = typename _Seq::value_type;
 
      _Seq _M_seq;
 
      // Transfer buffer contents to the sequence, so buffer can be refilled.
      void
      _M_overflow() override
      {
        auto __s = this->_M_used();
        if (__s.empty()) [[unlikely]]
          return; // Nothing in the buffer to transfer to _M_seq.
 
        // If _M_reserve was called then _M_bump must have been called too.
        _GLIBCXX_DEBUG_ASSERT(__s.data() != _M_seq.data());
 
        if constexpr (__is_specialization_of<_Seq, basic_string>)
          _M_seq.append(__s.data(), __s.size());
        else
          _M_seq.insert(_M_seq.end(), __s.begin(), __s.end());
 
        // Make the whole of _M_buf available for the next write:
        this->_M_rewind();
      }
 
      typename _Sink<_CharT>::_Reservation
      _M_reserve(size_t __n) override
      {
        // We might already have n characters available in this->_M_unused(),
        // but the whole point of this function is to be an optimization for
        // the std::format("{}", x) case. We want to avoid writing to _M_buf
        // and then copying that into a basic_string if possible, so this
        // function prefers to create space directly in _M_seq rather than
        // using _M_buf.
 
        if constexpr (__is_specialization_of<_Seq, basic_string>
                        || __is_specialization_of<_Seq, vector>)
          {
            // Flush the buffer to _M_seq first (should not be needed).
            if (this->_M_used().size()) [[unlikely]]
              _Seq_sink::_M_overflow();
 
            // Expand _M_seq to make __n new characters available:
            const auto __sz = _M_seq.size();
            if constexpr (is_same_v<string, _Seq> || is_same_v<wstring, _Seq>)
              _M_seq.__resize_and_overwrite(__sz + __n,
                                            [](auto, auto __n2) {
                                              return __n2;
                                            });
            else
              _M_seq.resize(__sz + __n);
 
            // Set _M_used() to be a span over the original part of _M_seq
            // and _M_unused() to be the extra capacity we just created:
            this->_M_reset(_M_seq, __sz);
            return { this };
          }
        else // Try to use the base class' buffer.
          return _Sink<_CharT>::_M_reserve(__n);
      }
 
      void
      _M_bump(size_t __n) override
      {
        if constexpr (__is_specialization_of<_Seq, basic_string>
                        || __is_specialization_of<_Seq, vector>)
          {
            auto __s = this->_M_used();
            _GLIBCXX_DEBUG_ASSERT(__s.data() == _M_seq.data());
            // Truncate the sequence to the part that was actually written to:
            _M_seq.resize(__s.size() + __n);
            // Switch back to using buffer:
            this->_M_reset(this->_M_buf);
          }
      }
 
    public:
      // TODO: for SSO string, use SSO buffer as initial span, then switch
      // to _M_buf if it overflows? Or even do that for all unused capacity?
 
      [[__gnu__::__always_inline__]]
      _Seq_sink() noexcept(is_nothrow_default_constructible_v<_Seq>)
      { }
 
      _Seq_sink(_Seq&& __s) noexcept(is_nothrow_move_constructible_v<_Seq>)
      : _M_seq(std::move(__s))
      { }
 
      using _Sink<_CharT>::out;
 
      _Seq
      get() &&
      {
        if (this->_M_used().size() != 0)
          _Seq_sink::_M_overflow();
        return std::move(_M_seq);
      }
 
      // A writable span that views everything written to the sink.
      // Will be either a view over _M_seq or the used part of _M_buf.
      span<_CharT>
      view()
      {
        auto __s = this->_M_used();
        if (_M_seq.size())
          {
            if (__s.size() != 0)
              _Seq_sink::_M_overflow();
            return _M_seq;
          }
        return __s;
      }
    };
 
  // A sink that writes to an output iterator.
  // Writes to a fixed-size buffer and then flushes to the output iterator
  // when the buffer fills up.
  template<typename _CharT, typename _OutIter>
    class _Iter_sink : public _Buf_sink<_CharT>
    {
      _OutIter _M_out;
      iter_difference_t<_OutIter> _M_max;
 
    protected:
      size_t _M_count = 0;
 
      void
      _M_overflow() override
      {
        auto __s = this->_M_used();
        if (_M_max < 0) // No maximum.
          _M_out = ranges::copy(__s, std::move(_M_out)).out;
        else if (_M_count < static_cast<size_t>(_M_max))
          {
            auto __max = _M_max - _M_count;
            span<_CharT> __first;
            if (__max < __s.size())
              __first = __s.first(static_cast<size_t>(__max));
            else
              __first = __s;
            _M_out = ranges::copy(__first, std::move(_M_out)).out;
          }
        this->_M_rewind();
        _M_count += __s.size();
      }
 
    public:
      [[__gnu__::__always_inline__]]
      explicit
      _Iter_sink(_OutIter __out, iter_difference_t<_OutIter> __max = -1)
      : _M_out(std::move(__out)), _M_max(__max)
      { }
 
      using _Sink<_CharT>::out;
 
      format_to_n_result<_OutIter>
      _M_finish() &&
      {
        if (this->_M_used().size() != 0)
          _Iter_sink::_M_overflow();
        iter_difference_t<_OutIter> __count(_M_count);
        return { std::move(_M_out), __count };
      }
    };
 
  // Partial specialization for contiguous iterators.
  // No buffer is used, characters are written straight to the iterator.
  // We do not know the size of the output range, so the span size just grows
  // as needed. The end of the span might be an invalid pointer outside the
  // valid range, but we never actually call _M_span.end(). This class does
  // not introduce any invalid pointer arithmetic or overflows that would not
  // have happened anyway.
  template<typename _CharT, contiguous_iterator _OutIter>
    requires same_as<iter_value_t<_OutIter>, _CharT>
    class _Iter_sink<_CharT, _OutIter> : public _Sink<_CharT>
    {
      _OutIter _M_first;
      iter_difference_t<_OutIter> _M_max = -1;
    protected:
      size_t _M_count = 0;
    private:
      _CharT _M_buf[64]; // Write here after outputting _M_max characters.
 
    protected:
      void
      _M_overflow() override
      {
        if (this->_M_unused().size() != 0)
          return; // No need to switch to internal buffer yet.
 
        auto __s = this->_M_used();
 
        if (_M_max >= 0)
          {
            _M_count += __s.size();
            // Span was already sized for the maximum character count,
            // if it overflows then any further output must go to the
            // internal buffer, to be discarded.
            this->_M_reset(this->_M_buf);
          }
        else
          {
            // No maximum character count. Just extend the span to allow
            // writing more characters to it.
            this->_M_reset({__s.data(), __s.size() + 1024}, __s.size());
          }
      }
 
      typename _Sink<_CharT>::_Reservation
      _M_reserve(size_t __n) final
      {
        auto __avail = this->_M_unused();
        if (__n > __avail.size())
          {
            if (_M_max >= 0)
              return {}; // cannot grow
 
            auto __s = this->_M_used();
            this->_M_reset({__s.data(), __s.size() + __n}, __s.size());
          }
        return { this };
      }
 
    private:
      static span<_CharT>
      _S_make_span(_CharT* __ptr, iter_difference_t<_OutIter> __n,
                   span<_CharT> __buf) noexcept
      {
        if (__n == 0)
          return __buf; // Only write to the internal buffer.
 
        if (__n > 0)
          {
            if constexpr (!is_integral_v<iter_difference_t<_OutIter>>
                            || sizeof(__n) > sizeof(size_t))
              {
                // __int128 or __detail::__max_diff_type
                auto __m = iter_difference_t<_OutIter>((size_t)-1);
                if (__n > __m)
                  __n = __m;
              }
            return {__ptr, (size_t)__n};
          }
 
#if __has_builtin(__builtin_dynamic_object_size)
        if (size_t __bytes = __builtin_dynamic_object_size(__ptr, 2))
          return {__ptr, __bytes / sizeof(_CharT)};
#endif
        // Avoid forming a pointer to a different memory page.
        const auto __off = reinterpret_cast<__UINTPTR_TYPE__>(__ptr) % 1024;
        __n = (1024 - __off) / sizeof(_CharT);
        if (__n > 0) [[likely]]
          return {__ptr, static_cast<size_t>(__n)};
        else // Misaligned/packed buffer of wchar_t?
          return {__ptr, 1};
      }
 
    public:
      explicit
      _Iter_sink(_OutIter __out, iter_difference_t<_OutIter> __n = -1) noexcept
      : _Sink<_CharT>(_S_make_span(std::to_address(__out), __n, _M_buf)),
        _M_first(__out), _M_max(__n)
      { }
 
      format_to_n_result<_OutIter>
      _M_finish() &&
      {
        auto __s = this->_M_used();
        if (__s.data() == _M_buf)
          {
            // Switched to internal buffer, so must have written _M_max.
            iter_difference_t<_OutIter> __count(_M_count + __s.size());
            return { _M_first + _M_max, __count };
          }
        else // Not using internal buffer yet
          {
            iter_difference_t<_OutIter> __count(__s.size());
            return { _M_first + __count, __count };
          }
      }
    };
 
  template<typename _Context>
    struct _Arg_value
    {
      using _CharT = typename _Context::char_type;
 
      struct _HandleBase
      {
        const void* _M_ptr;
        void (*_M_func)();
      };
 
      union
      {
        monostate _M_none;
        bool _M_bool;
        _CharT _M_c;
        int _M_i;
        unsigned _M_u;
        long long _M_ll;
        unsigned long long _M_ull;
        float _M_flt;
        double _M_dbl;
#ifndef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT // No long double if it's ambiguous.
        long double _M_ldbl;
#endif
        const _CharT* _M_str;
        basic_string_view<_CharT> _M_sv;
        const void* _M_ptr;
        _HandleBase _M_handle;
#ifdef __SIZEOF_INT128__
        __int128 _M_i128;
        unsigned __int128 _M_u128;
#endif
#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
        __ieee128 _M_f128;
        __ibm128  _M_ibm128;
#elif _GLIBCXX_FORMAT_F128 == 2
        __float128_t _M_f128;
#endif
      };
 
      [[__gnu__::__always_inline__]]
      _Arg_value() : _M_none() { }
 
#if 0
      template<typename _Tp>
        _Arg_value(in_place_type_t<_Tp>, _Tp __val)
        { _S_get<_Tp>() = __val; }
#endif
 
      template<typename _Tp, typename _Self>
        [[__gnu__::__always_inline__]]
        static auto&
        _S_get(_Self& __u) noexcept
        {
          if constexpr (is_same_v<_Tp, bool>)
            return __u._M_bool;
          else if constexpr (is_same_v<_Tp, _CharT>)
            return __u._M_c;
          else if constexpr (is_same_v<_Tp, int>)
            return __u._M_i;
          else if constexpr (is_same_v<_Tp, unsigned>)
            return __u._M_u;
          else if constexpr (is_same_v<_Tp, long long>)
            return __u._M_ll;
          else if constexpr (is_same_v<_Tp, unsigned long long>)
            return __u._M_ull;
          else if constexpr (is_same_v<_Tp, float>)
            return __u._M_flt;
          else if constexpr (is_same_v<_Tp, double>)
            return __u._M_dbl;
#ifndef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
          else if constexpr (is_same_v<_Tp, long double>)
            return __u._M_ldbl;
#else
          else if constexpr (is_same_v<_Tp, __ieee128>)
            return __u._M_f128;
          else if constexpr (is_same_v<_Tp, __ibm128>)
            return __u._M_ibm128;
#endif
          else if constexpr (is_same_v<_Tp, const _CharT*>)
            return __u._M_str;
          else if constexpr (is_same_v<_Tp, basic_string_view<_CharT>>)
            return __u._M_sv;
          else if constexpr (is_same_v<_Tp, const void*>)
            return __u._M_ptr;
#ifdef __SIZEOF_INT128__
          else if constexpr (is_same_v<_Tp, __int128>)
            return __u._M_i128;
          else if constexpr (is_same_v<_Tp, unsigned __int128>)
            return __u._M_u128;
#endif
#if _GLIBCXX_FORMAT_F128 == 2
          else if constexpr (is_same_v<_Tp, __float128_t>)
            return __u._M_f128;
#endif
          else if constexpr (derived_from<_Tp, _HandleBase>)
            return static_cast<_Tp&>(__u._M_handle);
          // Otherwise, ill-formed.
        }
 
      template<typename _Tp>
        [[__gnu__::__always_inline__]]
        auto&
        _M_get() noexcept
        { return _S_get<_Tp>(*this); }
 
      template<typename _Tp>
        [[__gnu__::__always_inline__]]
        const auto&
        _M_get() const noexcept
        { return _S_get<_Tp>(*this); }
 
      template<typename _Tp>
        [[__gnu__::__always_inline__]]
        void
        _M_set(_Tp __v) noexcept
        {
          if constexpr (derived_from<_Tp, _HandleBase>)
            std::construct_at(&_M_handle, __v);
          else
            _S_get<_Tp>(*this) = __v;
        }
      };
 
  // [format.arg.store], class template format-arg-store
  template<typename _Context, typename... _Args>
    class _Arg_store;
 
  template<typename _Visitor, typename _Ctx>
    decltype(auto) __visit_format_arg(_Visitor&&, basic_format_arg<_Ctx>);
 
  template<typename _Ch, typename _Tp>
    consteval _Arg_t
    __to_arg_t_enum() noexcept;
} // namespace __format
/// @endcond
 
  template<typename _Context>
    class basic_format_arg
    {
      using _CharT = typename _Context::char_type;
 
      template<typename _Tp>
        static constexpr bool __formattable
          = __format::__formattable_with<_Tp, _Context>;
 
    public:
      class handle : public __format::_Arg_value<_Context>::_HandleBase
      {
        using _Base = typename __format::_Arg_value<_Context>::_HandleBase;
 
        // Format as const if possible, to reduce instantiations.
        template<typename _Tp>
          using __maybe_const_t
            = __conditional_t<__formattable<const _Tp>, const _Tp, _Tp>;
 
        template<typename _Tq>
          static void
          _S_format(basic_format_parse_context<_CharT>& __parse_ctx,
                    _Context& __format_ctx, const void* __ptr)
          {
            using _Td = remove_const_t<_Tq>;
            typename _Context::template formatter_type<_Td> __f;
            __parse_ctx.advance_to(__f.parse(__parse_ctx));
            _Tq& __val = *const_cast<_Tq*>(static_cast<const _Td*>(__ptr));
            __format_ctx.advance_to(__f.format(__val, __format_ctx));
          }
 
        template<typename _Tp>
          explicit
          handle(_Tp& __val) noexcept
          {
            this->_M_ptr = __builtin_addressof(__val);
            auto __func = _S_format<__maybe_const_t<_Tp>>;
            this->_M_func = reinterpret_cast<void(*)()>(__func);
          }
 
        friend class basic_format_arg<_Context>;
 
      public:
        handle(const handle&) = default;
        handle& operator=(const handle&) = default;
 
        [[__gnu__::__always_inline__]]
        void
        format(basic_format_parse_context<_CharT>& __pc, _Context& __fc) const
        {
          using _Func = void(*)(basic_format_parse_context<_CharT>&,
                                _Context&, const void*);
          auto __f = reinterpret_cast<_Func>(this->_M_func);
          __f(__pc, __fc, this->_M_ptr);
        }
      };
 
      [[__gnu__::__always_inline__]]
      basic_format_arg() noexcept : _M_type(__format::_Arg_none) { }
 
      [[nodiscard,__gnu__::__always_inline__]]
      explicit operator bool() const noexcept
      { return _M_type != __format::_Arg_none; }
 
#if __cpp_lib_format >= 202306L // >= C++26
      template<typename _Visitor>
        decltype(auto)
        visit(this basic_format_arg __arg, _Visitor&& __vis)
        { return __arg._M_visit_user(std::forward<_Visitor>(__vis), __arg._M_type); }
 
      template<typename _Res, typename _Visitor>
        _Res
        visit(this basic_format_arg __arg, _Visitor&& __vis)
        { return __arg._M_visit_user(std::forward<_Visitor>(__vis), __arg._M_type); }
#endif
 
    private:
      template<typename _Ctx>
        friend class basic_format_args;
 
      template<typename _Ctx, typename... _Args>
        friend class __format::_Arg_store;
 
      static_assert(is_trivially_copyable_v<__format::_Arg_value<_Context>>);
 
      __format::_Arg_value<_Context> _M_val;
      __format::_Arg_t _M_type;
 
      // Transform incoming argument type to the type stored in _Arg_value.
      // e.g. short -> int, std::string -> std::string_view,
      // char[3] -> const char*.
      template<typename _Tp>
        static consteval auto
        _S_to_arg_type()
        {
          using _Td = remove_const_t<_Tp>;
          if constexpr (is_same_v<_Td, bool>)
            return type_identity<bool>();
          else if constexpr (is_same_v<_Td, _CharT>)
            return type_identity<_CharT>();
          else if constexpr (is_same_v<_Td, char> && is_same_v<_CharT, wchar_t>)
            return type_identity<_CharT>();
#ifdef __SIZEOF_INT128__ // Check before signed/unsigned integer
          else if constexpr (is_same_v<_Td, __int128>)
            return type_identity<__int128>();
          else if constexpr (is_same_v<_Td, unsigned __int128>)
            return type_identity<unsigned __int128>();
#endif
          else if constexpr (__is_signed_integer<_Td>::value)
            {
              if constexpr (sizeof(_Td) <= sizeof(int))
                return type_identity<int>();
              else if constexpr (sizeof(_Td) <= sizeof(long long))
                return type_identity<long long>();
            }
          else if constexpr (__is_unsigned_integer<_Td>::value)
            {
              if constexpr (sizeof(_Td) <= sizeof(unsigned))
                return type_identity<unsigned>();
              else if constexpr (sizeof(_Td) <= sizeof(unsigned long long))
                return type_identity<unsigned long long>();
            }
          else if constexpr (is_same_v<_Td, float>)
            return type_identity<float>();
          else if constexpr (is_same_v<_Td, double>)
            return type_identity<double>();
#ifndef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
          else if constexpr (is_same_v<_Td, long double>)
            return type_identity<long double>();
#else
          else if constexpr (is_same_v<_Td, __ibm128>)
            return type_identity<__ibm128>();
          else if constexpr (is_same_v<_Td, __ieee128>)
            return type_identity<__ieee128>();
#endif
 
#if defined(__FLT16_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
          else if constexpr (is_same_v<_Td, _Float16>)
            return type_identity<float>();
#endif
 
#if defined(__BFLT16_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
          else if constexpr (is_same_v<_Td, decltype(0.0bf16)>)
            return type_identity<float>();
#endif
 
#if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
          else if constexpr (is_same_v<_Td, _Float32>)
            return type_identity<float>();
#endif
 
#if defined(__FLT64_DIG__) && defined(_GLIBCXX_DOUBLE_IS_IEEE_BINARY64)
          else if constexpr (is_same_v<_Td, _Float64>)
            return type_identity<double>();
#endif
 
#if _GLIBCXX_FORMAT_F128
# if __FLT128_DIG__
          else if constexpr (is_same_v<_Td, _Float128>)
            return type_identity<__format::__float128_t>();
# endif
# if __SIZEOF_FLOAT128__
          else if constexpr (is_same_v<_Td, __float128>)
            return type_identity<__format::__float128_t>();
# endif
#endif
          else if constexpr (__is_specialization_of<_Td, basic_string_view>
                            || __is_specialization_of<_Td, basic_string>)
            {
              if constexpr (is_same_v<typename _Td::value_type, _CharT>)
                return type_identity<basic_string_view<_CharT>>();
              else
                return type_identity<handle>();
            }
          else if constexpr (is_same_v<decay_t<_Td>, const _CharT*>)
            return type_identity<const _CharT*>();
          else if constexpr (is_same_v<decay_t<_Td>, _CharT*>)
            return type_identity<const _CharT*>();
          else if constexpr (is_void_v<remove_pointer_t<_Td>>)
            return type_identity<const void*>();
          else if constexpr (is_same_v<_Td, nullptr_t>)
            return type_identity<const void*>();
          else
            return type_identity<handle>();
        }
 
      // Transform a formattable type to the appropriate storage type.
      template<typename _Tp>
        using _Normalize = typename decltype(_S_to_arg_type<_Tp>())::type;
 
      // Get the _Arg_t value corresponding to a normalized type.
      template<typename _Tp>
        static consteval __format::_Arg_t
        _S_to_enum()
        {
          using namespace __format;
          if constexpr (is_same_v<_Tp, bool>)
            return _Arg_bool;
          else if constexpr (is_same_v<_Tp, _CharT>)
            return _Arg_c;
          else if constexpr (is_same_v<_Tp, int>)
            return _Arg_i;
          else if constexpr (is_same_v<_Tp, unsigned>)
            return _Arg_u;
          else if constexpr (is_same_v<_Tp, long long>)
            return _Arg_ll;
          else if constexpr (is_same_v<_Tp, unsigned long long>)
            return _Arg_ull;
          else if constexpr (is_same_v<_Tp, float>)
            return _Arg_flt;
          else if constexpr (is_same_v<_Tp, double>)
            return _Arg_dbl;
#ifndef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
          else if constexpr (is_same_v<_Tp, long double>)
            return _Arg_ldbl;
#else
          // Don't use _Arg_ldbl for this target, it's ambiguous.
          else if constexpr (is_same_v<_Tp, __ibm128>)
            return _Arg_ibm128;
          else if constexpr (is_same_v<_Tp, __ieee128>)
            return _Arg_f128;
#endif
          else if constexpr (is_same_v<_Tp, const _CharT*>)
            return _Arg_str;
          else if constexpr (is_same_v<_Tp, basic_string_view<_CharT>>)
            return _Arg_sv;
          else if constexpr (is_same_v<_Tp, const void*>)
            return _Arg_ptr;
#ifdef __SIZEOF_INT128__
          else if constexpr (is_same_v<_Tp, __int128>)
            return _Arg_i128;
          else if constexpr (is_same_v<_Tp, unsigned __int128>)
            return _Arg_u128;
#endif
 
#if _GLIBCXX_FORMAT_F128 == 2
          else if constexpr (is_same_v<_Tp, __format::__float128_t>)
            return _Arg_f128;
#endif
          else if constexpr (is_same_v<_Tp, handle>)
            return _Arg_handle;
        }
 
      template<typename _Tp>
        void
        _M_set(_Tp __v) noexcept
        {
          _M_type = _S_to_enum<_Tp>();
          _M_val._M_set(__v);
        }
 
      template<typename _Tp>
        requires __format::__formattable_with<_Tp, _Context>
        explicit
        basic_format_arg(_Tp& __v) noexcept
        {
          using _Td = _Normalize<_Tp>;
          if constexpr (is_same_v<_Td, basic_string_view<_CharT>>)
            _M_set(_Td{__v.data(), __v.size()});
          else if constexpr (is_same_v<remove_const_t<_Tp>, char>
                               && is_same_v<_CharT, wchar_t>)
            _M_set(static_cast<_Td>(static_cast<unsigned char>(__v)));
          else
            _M_set(static_cast<_Td>(__v));
        }
 
      template<typename _Ctx, typename... _Argz>
        friend auto
        make_format_args(_Argz&...) noexcept;
 
      template<typename _Visitor, typename _Ctx>
        friend decltype(auto)
        visit_format_arg(_Visitor&& __vis, basic_format_arg<_Ctx>);
 
      template<typename _Visitor, typename _Ctx>
        friend decltype(auto)
        __format::__visit_format_arg(_Visitor&&, basic_format_arg<_Ctx>);
 
      template<typename _Ch, typename _Tp>
        friend consteval __format::_Arg_t
        __format::__to_arg_t_enum() noexcept;
 
      template<typename _Visitor>
        decltype(auto)
        _M_visit(_Visitor&& __vis, __format::_Arg_t __type)
        {
          using namespace __format;
          switch (__type)
          {
            case _Arg_none:
              return std::forward<_Visitor>(__vis)(_M_val._M_none);
            case _Arg_bool:
              return std::forward<_Visitor>(__vis)(_M_val._M_bool);
            case _Arg_c:
              return std::forward<_Visitor>(__vis)(_M_val._M_c);
            case _Arg_i:
              return std::forward<_Visitor>(__vis)(_M_val._M_i);
            case _Arg_u:
              return std::forward<_Visitor>(__vis)(_M_val._M_u);
            case _Arg_ll:
              return std::forward<_Visitor>(__vis)(_M_val._M_ll);
            case _Arg_ull:
              return std::forward<_Visitor>(__vis)(_M_val._M_ull);
#if __glibcxx_to_chars // FIXME: need to be able to format these types!
            case _Arg_flt:
              return std::forward<_Visitor>(__vis)(_M_val._M_flt);
            case _Arg_dbl:
              return std::forward<_Visitor>(__vis)(_M_val._M_dbl);
#ifndef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
            case _Arg_ldbl:
              return std::forward<_Visitor>(__vis)(_M_val._M_ldbl);
#else
            case _Arg_f128:
              return std::forward<_Visitor>(__vis)(_M_val._M_f128);
            case _Arg_ibm128:
              return std::forward<_Visitor>(__vis)(_M_val._M_ibm128);
#endif
#endif
            case _Arg_str:
              return std::forward<_Visitor>(__vis)(_M_val._M_str);
            case _Arg_sv:
              return std::forward<_Visitor>(__vis)(_M_val._M_sv);
            case _Arg_ptr:
              return std::forward<_Visitor>(__vis)(_M_val._M_ptr);
            case _Arg_handle:
            {
              auto& __h = static_cast<handle&>(_M_val._M_handle);
              return std::forward<_Visitor>(__vis)(__h);
            }
#ifdef __SIZEOF_INT128__
            case _Arg_i128:
              return std::forward<_Visitor>(__vis)(_M_val._M_i128);
            case _Arg_u128:
              return std::forward<_Visitor>(__vis)(_M_val._M_u128);
#endif
 
#if _GLIBCXX_FORMAT_F128 == 2
            case _Arg_f128:
              return std::forward<_Visitor>(__vis)(_M_val._M_f128);
#endif
 
            default:
              // _Arg_f16 etc.
              __builtin_unreachable();
          }
        }
 
      template<typename _Visitor>
        decltype(auto)
        _M_visit_user(_Visitor&& __vis, __format::_Arg_t __type)
        {
          return _M_visit([&__vis]<typename _Tp>(_Tp& __val) -> decltype(auto)
            {
              constexpr bool __user_facing = __is_one_of<_Tp,
                monostate, bool, _CharT,
                int, unsigned int, long long int, unsigned long long int,
                float, double, long double,
                const _CharT*, basic_string_view<_CharT>,
                const void*, handle>::value;
             if constexpr (__user_facing)
               return std::forward<_Visitor>(__vis)(__val);
             else
               {
                 handle __h(__val);
                 return std::forward<_Visitor>(__vis)(__h);
               }
           }, __type);
        }
    };
 
  template<typename _Visitor, typename _Context>
    _GLIBCXX26_DEPRECATED_SUGGEST("std::basic_format_arg::visit")
    inline decltype(auto)
    visit_format_arg(_Visitor&& __vis, basic_format_arg<_Context> __arg)
    {
      return __arg._M_visit_user(std::forward<_Visitor>(__vis), __arg._M_type);
    }
 
/// @cond undocumented
namespace __format
{
  template<typename _Visitor, typename _Ctx>
    inline decltype(auto)
    __visit_format_arg(_Visitor&& __vis, basic_format_arg<_Ctx> __arg)
    {
      return __arg._M_visit(std::forward<_Visitor>(__vis), __arg._M_type);
    }
 
  struct _WidthPrecVisitor
  {
    template<typename _Tp>
      size_t
      operator()(_Tp& __arg) const
      {
        if constexpr (is_same_v<_Tp, monostate>)
          __format::__invalid_arg_id_in_format_string();
        // _GLIBCXX_RESOLVE_LIB_DEFECTS
        // 3720. Restrict the valid types of arg-id for width and precision
        // 3721. Allow an arg-id with a value of zero for width
        else if constexpr (sizeof(_Tp) <= sizeof(long long))
          {
            // _GLIBCXX_RESOLVE_LIB_DEFECTS
            // 3720. Restrict the valid types of arg-id for width and precision
            if constexpr (__is_unsigned_integer<_Tp>::value)
              return __arg;
            else if constexpr (__is_signed_integer<_Tp>::value)
              if (__arg >= 0)
                return __arg;
          }
        __throw_format_error("format error: argument used for width or "
                             "precision must be a non-negative integer");
      }
  };
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
  template<typename _Context>
    inline size_t
    __int_from_arg(const basic_format_arg<_Context>& __arg)
    { return __format::__visit_format_arg(_WidthPrecVisitor(), __arg); }
 
  // Pack _Arg_t enum values into a single 60-bit integer.
  template<int _Bits, size_t _Nm>
    constexpr auto
    __pack_arg_types(const array<_Arg_t, _Nm>& __types)
    {
      __UINT64_TYPE__ __packed_types = 0;
      for (auto __i = __types.rbegin(); __i != __types.rend(); ++__i)
        __packed_types = (__packed_types << _Bits) | *__i;
      return __packed_types;
    }
} // namespace __format
/// @endcond
 
  template<typename _Context>
    class basic_format_args
    {
      static constexpr int _S_packed_type_bits = 5; // _Arg_t values [0,20]
      static constexpr int _S_packed_type_mask = 0b11111;
      static constexpr int _S_max_packed_args = 12;
 
      static_assert( __format::_Arg_max_ <= (1 << _S_packed_type_bits) );
 
      template<typename... _Args>
        using _Store = __format::_Arg_store<_Context, _Args...>;
 
      template<typename _Ctx, typename... _Args>
        friend class __format::_Arg_store;
 
      using uint64_t = __UINT64_TYPE__;
      using _Format_arg = basic_format_arg<_Context>;
      using _Format_arg_val = __format::_Arg_value<_Context>;
 
      // If args are packed then the number of args is in _M_packed_size and
      // the packed types are in _M_unpacked_size, accessed via _M_type(i).
      // If args are not packed then the number of args is in _M_unpacked_size
      // and _M_packed_size is zero.
      uint64_t _M_packed_size : 4;
      uint64_t _M_unpacked_size : 60;
 
      union {
        const _Format_arg_val* _M_values; // Active when _M_packed_size != 0
        const _Format_arg* _M_args;       // Active when _M_packed_size == 0
      };
 
      size_t
      _M_size() const noexcept
      { return _M_packed_size ? _M_packed_size : _M_unpacked_size; }
 
      typename __format::_Arg_t
      _M_type(size_t __i) const noexcept
      {
        uint64_t __t = _M_unpacked_size >> (__i * _S_packed_type_bits);
        return static_cast<__format::_Arg_t>(__t & _S_packed_type_mask);
      }
 
      template<typename _Ctx, typename... _Args>
        friend auto
        make_format_args(_Args&...) noexcept;
 
      // An array of _Arg_t enums corresponding to _Args...
      template<typename... _Args>
        static consteval array<__format::_Arg_t, sizeof...(_Args)>
        _S_types_to_pack()
        { return {_Format_arg::template _S_to_enum<_Args>()...}; }
 
    public:
      template<typename... _Args>
        basic_format_args(const _Store<_Args...>& __store) noexcept;
 
      [[nodiscard,__gnu__::__always_inline__]]
      basic_format_arg<_Context>
      get(size_t __i) const noexcept
      {
        basic_format_arg<_Context> __arg;
        if (__i < _M_packed_size)
          {
            __arg._M_type = _M_type(__i);
            __arg._M_val = _M_values[__i];
          }
        else if (_M_packed_size == 0 && __i < _M_unpacked_size)
          __arg = _M_args[__i];
        return __arg;
      }
    };
 
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 3810. CTAD for std::basic_format_args
  template<typename _Context, typename... _Args>
    basic_format_args(__format::_Arg_store<_Context, _Args...>)
      -> basic_format_args<_Context>;
 
  template<typename _Context, typename... _Args>
    auto
    make_format_args(_Args&... __fmt_args) noexcept;
 
  // An array of type-erased formatting arguments.
  template<typename _Context, typename... _Args>
    class __format::_Arg_store
    {
      friend std::basic_format_args<_Context>;
 
      template<typename _Ctx, typename... _Argz>
        friend auto std::
#if _GLIBCXX_INLINE_VERSION
        __8:: // Needed for PR c++/59256
#endif
        make_format_args(_Argz&...) noexcept;
 
      // For a sufficiently small number of arguments we only store values.
      // basic_format_args can get the types from the _Args pack.
      static constexpr bool _S_values_only
        = sizeof...(_Args) <= basic_format_args<_Context>::_S_max_packed_args;
 
      using _Element_t
        = __conditional_t<_S_values_only,
                          __format::_Arg_value<_Context>,
                          basic_format_arg<_Context>>;
 
      _Element_t _M_args[sizeof...(_Args)];
 
      template<typename _Tp>
        static _Element_t
        _S_make_elt(_Tp& __v)
        {
          using _Tq = remove_const_t<_Tp>;
          using _CharT = typename _Context::char_type;
          static_assert(is_default_constructible_v<formatter<_Tq, _CharT>>,
                        "std::formatter must be specialized for the type "
                        "of each format arg");
          using __format::__formattable_with;
          if constexpr (is_const_v<_Tp>)
            if constexpr (!__formattable_with<_Tp, _Context>)
              if constexpr (__formattable_with<_Tq, _Context>)
                static_assert(__formattable_with<_Tp, _Context>,
                              "format arg must be non-const because its "
                              "std::formatter specialization has a "
                              "non-const reference parameter");
          basic_format_arg<_Context> __arg(__v);
          if constexpr (_S_values_only)
            return __arg._M_val;
          else
            return __arg;
        }
 
      template<typename... _Tp>
        requires (sizeof...(_Tp) == sizeof...(_Args))
        [[__gnu__::__always_inline__]]
        _Arg_store(_Tp&... __a) noexcept
        : _M_args{_S_make_elt(__a)...}
        { }
    };
 
  template<typename _Context>
    class __format::_Arg_store<_Context>
    { };
 
  template<typename _Context>
    template<typename... _Args>
      inline
      basic_format_args<_Context>::
      basic_format_args(const _Store<_Args...>& __store) noexcept
      {
        if constexpr (sizeof...(_Args) == 0)
          {
            _M_packed_size = 0;
            _M_unpacked_size = 0;
            _M_args = nullptr;
          }
        else if constexpr (sizeof...(_Args) <= _S_max_packed_args)
          {
            // The number of packed arguments:
            _M_packed_size = sizeof...(_Args);
            // The packed type enums:
            _M_unpacked_size
              = __format::__pack_arg_types<_S_packed_type_bits>(_S_types_to_pack<_Args...>());
            // The _Arg_value objects.
            _M_values = __store._M_args;
          }
        else
          {
            // No packed arguments:
            _M_packed_size = 0;
            // The number of unpacked arguments:
            _M_unpacked_size = sizeof...(_Args);
            // The basic_format_arg objects:
            _M_args = __store._M_args;
          }
      }
 
  /// Capture formatting arguments for use by `std::vformat`.
  template<typename _Context = format_context, typename... _Args>
    [[nodiscard,__gnu__::__always_inline__]]
    inline auto
    make_format_args(_Args&... __fmt_args) noexcept
    {
      using _Fmt_arg = basic_format_arg<_Context>;
      using _Store = __format::_Arg_store<_Context, typename _Fmt_arg::template
                     _Normalize<_Args>...>;
      return _Store(__fmt_args...);
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  /// Capture formatting arguments for use by `std::vformat` (for wide output).
  template<typename... _Args>
    [[nodiscard,__gnu__::__always_inline__]]
    inline auto
    make_wformat_args(_Args&... __args) noexcept
    { return std::make_format_args<wformat_context>(__args...); }
#endif
 
/// @cond undocumented
namespace __format
{
  template<typename _Out, typename _CharT, typename _Context>
    _Out
    __do_vformat_to(_Out, basic_string_view<_CharT>,
                    const basic_format_args<_Context>&,
                    const locale* = nullptr);
 
  template<typename _CharT> struct __formatter_chrono;
 
} // namespace __format
/// @endcond
 
  /** Context for std::format and similar functions.
   *
   * A formatting context contains an output iterator and locale to use
   * for the formatting operations. Most programs will never need to use
   * this class template explicitly. For typical uses of `std::format` the
   * library will use the specializations `std::format_context` (for `char`)
   * and `std::wformat_context` (for `wchar_t`).
   *
   * You are not allowed to define partial or explicit specializations of
   * this class template.
   *
   * @since C++20
   */
  template<typename _Out, typename _CharT>
    class basic_format_context
    {
      static_assert( output_iterator<_Out, const _CharT&> );
 
      basic_format_args<basic_format_context> _M_args;
      _Out _M_out;
      __format::_Optional_locale _M_loc;
 
      basic_format_context(basic_format_args<basic_format_context> __args,
                           _Out __out)
      : _M_args(__args), _M_out(std::move(__out))
      { }
 
      basic_format_context(basic_format_args<basic_format_context> __args,
                           _Out __out, const std::locale& __loc)
      : _M_args(__args), _M_out(std::move(__out)), _M_loc(__loc)
      { }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 4061. Should std::basic_format_context be
      //       default-constructible/copyable/movable?
      basic_format_context(const basic_format_context&) = delete;
      basic_format_context& operator=(const basic_format_context&) = delete;
 
      template<typename _Out2, typename _CharT2, typename _Context2>
        friend _Out2
        __format::__do_vformat_to(_Out2, basic_string_view<_CharT2>,
                                  const basic_format_args<_Context2>&,
                                  const locale*);
 
      friend __format::__formatter_chrono<_CharT>;
 
    public:
      ~basic_format_context() = default;
 
      using iterator = _Out;
      using char_type = _CharT;
      template<typename _Tp>
        using formatter_type = formatter<_Tp, _CharT>;
 
      [[nodiscard]]
      basic_format_arg<basic_format_context>
      arg(size_t __id) const noexcept
      { return _M_args.get(__id); }
 
      [[nodiscard]]
      std::locale locale() { return _M_loc.value(); }
 
      [[nodiscard]]
      iterator out() { return std::move(_M_out); }
 
      void advance_to(iterator __it) { _M_out = std::move(__it); }
    };
 
 
/// @cond undocumented
namespace __format
{
  // Abstract base class defining an interface for scanning format strings.
  // Scan the characters in a format string, dividing it up into strings of
  // ordinary characters, escape sequences, and replacement fields.
  // Call virtual functions for derived classes to parse format-specifiers
  // or write formatted output.
  template<typename _CharT>
    struct _Scanner
    {
      using iterator = typename basic_format_parse_context<_CharT>::iterator;
 
      typename basic_format_parse_context<_CharT>::_Scan_parse_context _M_pc;
 
      constexpr explicit
      _Scanner(basic_string_view<_CharT> __str, size_t __nargs = (size_t)-1)
      : _M_pc(__str, __nargs)
      { }
 
      constexpr iterator begin() const noexcept { return _M_pc.begin(); }
      constexpr iterator end() const noexcept { return _M_pc.end(); }
 
      constexpr void
      _M_scan()
      {
        basic_string_view<_CharT> __fmt = _M_fmt_str();
 
        if (__fmt.size() == 2 && __fmt[0] == '{' && __fmt[1] == '}')
          {
            _M_pc.advance_to(begin() + 1);
            _M_format_arg(_M_pc.next_arg_id());
            return;
          }
 
        size_t __lbr = __fmt.find('{');
        size_t __rbr = __fmt.find('}');
 
        while (__fmt.size())
          {
            auto __cmp = __lbr <=> __rbr;
            if (__cmp == 0)
              {
                _M_on_chars(end());
                _M_pc.advance_to(end());
                return;
              }
            else if (__cmp < 0)
              {
                if (__lbr + 1 == __fmt.size()
                      || (__rbr == __fmt.npos && __fmt[__lbr + 1] != '{'))
                  __format::__unmatched_left_brace_in_format_string();
                const bool __is_escape = __fmt[__lbr + 1] == '{';
                iterator __last = begin() + __lbr + int(__is_escape);
                _M_on_chars(__last);
                _M_pc.advance_to(__last + 1);
                __fmt = _M_fmt_str();
                if (__is_escape)
                  {
                    if (__rbr != __fmt.npos)
                      __rbr -= __lbr + 2;
                    __lbr = __fmt.find('{');
                  }
                else
                  {
                    _M_on_replacement_field();
                    __fmt = _M_fmt_str();
                    __lbr = __fmt.find('{');
                    __rbr = __fmt.find('}');
                  }
              }
            else
              {
                if (++__rbr == __fmt.size() || __fmt[__rbr] != '}')
                  __format::__unmatched_right_brace_in_format_string();
                iterator __last = begin() + __rbr;
                _M_on_chars(__last);
                _M_pc.advance_to(__last + 1);
                __fmt = _M_fmt_str();
                if (__lbr != __fmt.npos)
                  __lbr -= __rbr + 1;
                __rbr = __fmt.find('}');
              }
          }
      }
 
      constexpr basic_string_view<_CharT>
      _M_fmt_str() const noexcept
      { return {begin(), end()}; }
 
      constexpr virtual void _M_on_chars(iterator) { }
 
      constexpr void _M_on_replacement_field()
      {
        auto __next = begin();
 
        size_t __id;
        if (*__next == '}')
          __id = _M_pc.next_arg_id();
        else if (*__next == ':')
          {
            __id = _M_pc.next_arg_id();
            _M_pc.advance_to(++__next);
          }
        else
          {
            auto [__i, __ptr] = __format::__parse_arg_id(begin(), end());
            if (!__ptr || !(*__ptr == '}' || *__ptr == ':'))
              __format::__invalid_arg_id_in_format_string();
            _M_pc.check_arg_id(__id = __i);
            if (*__ptr == ':')
              {
                _M_pc.advance_to(++__ptr);
              }
            else
              _M_pc.advance_to(__ptr);
          }
        _M_format_arg(__id);
        if (begin() == end() || *begin() != '}')
          __format::__unmatched_left_brace_in_format_string();
        _M_pc.advance_to(begin() + 1); // Move past '}'
      }
 
      constexpr virtual void _M_format_arg(size_t __id) = 0;
    };
 
  // Process a format string and format the arguments in the context.
  template<typename _Out, typename _CharT>
    class _Formatting_scanner : public _Scanner<_CharT>
    {
    public:
      _Formatting_scanner(basic_format_context<_Out, _CharT>& __fc,
                          basic_string_view<_CharT> __str)
      : _Scanner<_CharT>(__str), _M_fc(__fc)
      { }
 
    private:
      basic_format_context<_Out, _CharT>& _M_fc;
 
      using iterator = typename _Scanner<_CharT>::iterator;
 
      constexpr void
      _M_on_chars(iterator __last) override
      {
        basic_string_view<_CharT> __str(this->begin(), __last);
        _M_fc.advance_to(__format::__write(_M_fc.out(), __str));
      }
 
      constexpr void
      _M_format_arg(size_t __id) override
      {
        using _Context = basic_format_context<_Out, _CharT>;
        using handle = typename basic_format_arg<_Context>::handle;
 
        __format::__visit_format_arg([this](auto& __arg) {
          using _Type = remove_reference_t<decltype(__arg)>;
          using _Formatter = typename _Context::template formatter_type<_Type>;
          if constexpr (is_same_v<_Type, monostate>)
            __format::__invalid_arg_id_in_format_string();
          else if constexpr (is_same_v<_Type, handle>)
            __arg.format(this->_M_pc, this->_M_fc);
          else if constexpr (is_default_constructible_v<_Formatter>)
            {
              _Formatter __f;
              this->_M_pc.advance_to(__f.parse(this->_M_pc));
              this->_M_fc.advance_to(__f.format(__arg, this->_M_fc));
            }
          else
            static_assert(__format::__formattable_with<_Type, _Context>);
        }, _M_fc.arg(__id));
      }
    };
 
  template<typename _CharT, typename _Tp>
    consteval _Arg_t
    __to_arg_t_enum() noexcept
    {
      using _Context = __format::__format_context<_CharT>;
      using _Fmt_arg = basic_format_arg<_Context>;
      using _NormalizedTp = typename _Fmt_arg::template _Normalize<_Tp>;
      return _Fmt_arg::template _S_to_enum<_NormalizedTp>();
    }
 
  // Validate a format string for Args.
  template<typename _CharT, typename... _Args>
    class _Checking_scanner : public _Scanner<_CharT>
    {
      static_assert(
        (is_default_constructible_v<formatter<_Args, _CharT>> && ...),
        "std::formatter must be specialized for each type being formatted");
 
    public:
      consteval
      _Checking_scanner(basic_string_view<_CharT> __str)
      : _Scanner<_CharT>(__str, sizeof...(_Args))
      {
#if __cpp_lib_format >= 202305L
        this->_M_pc._M_types = _M_types.data();
#endif
      }
 
    private:
      constexpr void
      _M_format_arg(size_t __id) override
      {
        if constexpr (sizeof...(_Args) != 0)
          {
            if (__id < sizeof...(_Args))
              {
                _M_parse_format_spec<_Args...>(__id);
                return;
              }
          }
        __builtin_unreachable();
      }
 
      template<typename _Tp, typename... _OtherArgs>
        constexpr void
        _M_parse_format_spec(size_t __id)
        {
          if (__id == 0)
            {
              formatter<_Tp, _CharT> __f;
              this->_M_pc.advance_to(__f.parse(this->_M_pc));
            }
          else if constexpr (sizeof...(_OtherArgs) != 0)
            _M_parse_format_spec<_OtherArgs...>(__id - 1);
          else
            __builtin_unreachable();
        }
 
#if __cpp_lib_format >= 202305L
      array<_Arg_t, sizeof...(_Args)>
        _M_types{ { __format::__to_arg_t_enum<_CharT, _Args>()... } };
#endif
    };
 
  template<typename _Out, typename _CharT, typename _Context>
    inline _Out
    __do_vformat_to(_Out __out, basic_string_view<_CharT> __fmt,
                    const basic_format_args<_Context>& __args,
                    const locale* __loc)
    {
      _Iter_sink<_CharT, _Out> __sink(std::move(__out));
      _Sink_iter<_CharT> __sink_out;
 
      if constexpr (is_same_v<_Out, _Sink_iter<_CharT>>)
        __sink_out = __out; // Already a sink iterator, safe to use post-move.
      else
        __sink_out = __sink.out();
 
      if constexpr (is_same_v<_CharT, char>)
        // Fast path for "{}" format strings and simple format arg types.
        if (__fmt.size() == 2 && __fmt[0] == '{' && __fmt[1] == '}')
          {
            bool __done = false;
            __format::__visit_format_arg([&](auto& __arg) {
              using _Tp = remove_cvref_t<decltype(__arg)>;
              if constexpr (is_same_v<_Tp, bool>)
                {
                  size_t __len = 4 + !__arg;
                  const char* __chars[] = { "false", "true" };
                  if (auto __res = __sink_out._M_reserve(__len))
                    {
                      __builtin_memcpy(__res.get(), __chars[__arg], __len);
                      __res._M_bump(__len);
                      __done = true;
                    }
                }
              else if constexpr (is_same_v<_Tp, char>)
                {
                  if (auto __res = __sink_out._M_reserve(1))
                    {
                      *__res.get() = __arg;
                      __res._M_bump(1);
                      __done = true;
                    }
                }
              else if constexpr (is_integral_v<_Tp>)
                {
                  make_unsigned_t<_Tp> __uval;
                  const bool __neg = __arg < 0;
                  if (__neg)
                    __uval = make_unsigned_t<_Tp>(~__arg) + 1u;
                  else
                    __uval = __arg;
                  const auto __n = __detail::__to_chars_len(__uval);
                  if (auto __res = __sink_out._M_reserve(__n + __neg))
                    {
                      auto __ptr = __res.get();
                      *__ptr = '-';
                      __detail::__to_chars_10_impl(__ptr + (int)__neg, __n,
                                                   __uval);
                      __res._M_bump(__n + __neg);
                      __done = true;
                    }
                }
              else if constexpr (is_convertible_v<_Tp, string_view>)
                {
                  string_view __sv = __arg;
                  if (auto __res = __sink_out._M_reserve(__sv.size()))
                    {
                      __builtin_memcpy(__res.get(), __sv.data(), __sv.size());
                      __res._M_bump(__sv.size());
                      __done = true;
                    }
                }
            }, __args.get(0));
 
            if (__done)
              {
                if constexpr (is_same_v<_Out, _Sink_iter<_CharT>>)
                  return __sink_out;
                else
                  return std::move(__sink)._M_finish().out;
              }
          }
 
      auto __ctx = __loc == nullptr
                     ? _Context(__args, __sink_out)
                     : _Context(__args, __sink_out, *__loc);
      _Formatting_scanner<_Sink_iter<_CharT>, _CharT> __scanner(__ctx, __fmt);
      __scanner._M_scan();
 
      if constexpr (is_same_v<_Out, _Sink_iter<_CharT>>)
        return __ctx.out();
      else
        return std::move(__sink)._M_finish().out;
    }
#pragma GCC diagnostic pop
 
} // namespace __format
/// @endcond
 
  template<typename _CharT, typename... _Args>
    template<typename _Tp>
      requires convertible_to<const _Tp&, basic_string_view<_CharT>>
      consteval
      basic_format_string<_CharT, _Args...>::
      basic_format_string(const _Tp& __s)
      : _M_str(__s)
      {
        __format::_Checking_scanner<_CharT, remove_cvref_t<_Args>...>
          __scanner(_M_str);
        __scanner._M_scan();
      }
 
  // [format.functions], formatting functions
 
  template<typename _Out> requires output_iterator<_Out, const char&>
    [[__gnu__::__always_inline__]]
    inline _Out
    vformat_to(_Out __out, string_view __fmt, format_args __args)
    { return __format::__do_vformat_to(std::move(__out), __fmt, __args); }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Out> requires output_iterator<_Out, const wchar_t&>
    [[__gnu__::__always_inline__]]
    inline _Out
    vformat_to(_Out __out, wstring_view __fmt, wformat_args __args)
    { return __format::__do_vformat_to(std::move(__out), __fmt, __args); }
#endif
 
  template<typename _Out> requires output_iterator<_Out, const char&>
    [[__gnu__::__always_inline__]]
    inline _Out
    vformat_to(_Out __out, const locale& __loc, string_view __fmt,
               format_args __args)
    {
      return __format::__do_vformat_to(std::move(__out), __fmt, __args, &__loc);
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Out> requires output_iterator<_Out, const wchar_t&>
    [[__gnu__::__always_inline__]]
    inline _Out
    vformat_to(_Out __out, const locale& __loc, wstring_view __fmt,
               wformat_args __args)
    {
      return __format::__do_vformat_to(std::move(__out), __fmt, __args, &__loc);
    }
#endif
 
  [[nodiscard]]
  inline string
  vformat(string_view __fmt, format_args __args)
  {
    __format::_Str_sink<char> __buf;
    std::vformat_to(__buf.out(), __fmt, __args);
    return std::move(__buf).get();
  }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  [[nodiscard]]
  inline wstring
  vformat(wstring_view __fmt, wformat_args __args)
  {
    __format::_Str_sink<wchar_t> __buf;
    std::vformat_to(__buf.out(), __fmt, __args);
    return std::move(__buf).get();
  }
#endif
 
  [[nodiscard]]
  inline string
  vformat(const locale& __loc, string_view __fmt, format_args __args)
  {
    __format::_Str_sink<char> __buf;
    std::vformat_to(__buf.out(), __loc, __fmt, __args);
    return std::move(__buf).get();
  }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  [[nodiscard]]
  inline wstring
  vformat(const locale& __loc, wstring_view __fmt, wformat_args __args)
  {
    __format::_Str_sink<wchar_t> __buf;
    std::vformat_to(__buf.out(), __loc, __fmt, __args);
    return std::move(__buf).get();
  }
#endif
 
  template<typename... _Args>
    [[nodiscard]]
    inline string
    format(format_string<_Args...> __fmt, _Args&&... __args)
    { return std::vformat(__fmt.get(), std::make_format_args(__args...)); }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename... _Args>
    [[nodiscard]]
    inline wstring
    format(wformat_string<_Args...> __fmt, _Args&&... __args)
    { return std::vformat(__fmt.get(), std::make_wformat_args(__args...)); }
#endif
 
  template<typename... _Args>
    [[nodiscard]]
    inline string
    format(const locale& __loc, format_string<_Args...> __fmt,
           _Args&&... __args)
    {
      return std::vformat(__loc, __fmt.get(),
                          std::make_format_args(__args...));
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename... _Args>
    [[nodiscard]]
    inline wstring
    format(const locale& __loc, wformat_string<_Args...> __fmt,
           _Args&&... __args)
    {
      return std::vformat(__loc, __fmt.get(),
                          std::make_wformat_args(__args...));
    }
#endif
 
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const char&>
    inline _Out
    format_to(_Out __out, format_string<_Args...> __fmt, _Args&&... __args)
    {
      return std::vformat_to(std::move(__out), __fmt.get(),
                             std::make_format_args(__args...));
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const wchar_t&>
    inline _Out
    format_to(_Out __out, wformat_string<_Args...> __fmt, _Args&&... __args)
    {
      return std::vformat_to(std::move(__out), __fmt.get(),
                             std::make_wformat_args(__args...));
    }
#endif
 
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const char&>
    inline _Out
    format_to(_Out __out, const locale& __loc, format_string<_Args...> __fmt,
              _Args&&... __args)
    {
      return std::vformat_to(std::move(__out), __loc, __fmt.get(),
                             std::make_format_args(__args...));
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const wchar_t&>
    inline _Out
    format_to(_Out __out, const locale& __loc, wformat_string<_Args...> __fmt,
              _Args&&... __args)
    {
      return std::vformat_to(std::move(__out), __loc, __fmt.get(),
                             std::make_wformat_args(__args...));
    }
#endif
 
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const char&>
    inline format_to_n_result<_Out>
    format_to_n(_Out __out, iter_difference_t<_Out> __n,
                format_string<_Args...> __fmt, _Args&&... __args)
    {
      __format::_Iter_sink<char, _Out> __sink(std::move(__out), __n);
      std::vformat_to(__sink.out(), __fmt.get(),
                      std::make_format_args(__args...));
      return std::move(__sink)._M_finish();
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const wchar_t&>
    inline format_to_n_result<_Out>
    format_to_n(_Out __out, iter_difference_t<_Out> __n,
                wformat_string<_Args...> __fmt, _Args&&... __args)
    {
      __format::_Iter_sink<wchar_t, _Out> __sink(std::move(__out), __n);
      std::vformat_to(__sink.out(), __fmt.get(),
                      std::make_wformat_args(__args...));
      return std::move(__sink)._M_finish();
    }
#endif
 
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const char&>
    inline format_to_n_result<_Out>
    format_to_n(_Out __out, iter_difference_t<_Out> __n, const locale& __loc,
                format_string<_Args...> __fmt, _Args&&... __args)
    {
      __format::_Iter_sink<char, _Out> __sink(std::move(__out), __n);
      std::vformat_to(__sink.out(), __loc, __fmt.get(),
                      std::make_format_args(__args...));
      return std::move(__sink)._M_finish();
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename _Out, typename... _Args>
    requires output_iterator<_Out, const wchar_t&>
    inline format_to_n_result<_Out>
    format_to_n(_Out __out, iter_difference_t<_Out> __n, const locale& __loc,
                wformat_string<_Args...> __fmt, _Args&&... __args)
    {
      __format::_Iter_sink<wchar_t, _Out> __sink(std::move(__out), __n);
      std::vformat_to(__sink.out(), __loc, __fmt.get(),
                      std::make_wformat_args(__args...));
      return std::move(__sink)._M_finish();
    }
#endif
 
/// @cond undocumented
namespace __format
{
#if 1
  template<typename _CharT>
    class _Counting_sink final : public _Iter_sink<_CharT, _CharT*>
    {
    public:
      _Counting_sink() : _Iter_sink<_CharT, _CharT*>(nullptr, 0) { }
 
      [[__gnu__::__always_inline__]]
      size_t
      count() const
      { return this->_M_count + this->_M_used().size(); }
    };
#else
  template<typename _CharT>
    class _Counting_sink : public _Buf_sink<_CharT>
    {
      size_t _M_count = 0;
 
      void
      _M_overflow() override
      {
        if (!std::is_constant_evaluated())
          _M_count += this->_M_used().size();
        this->_M_rewind();
      }
 
    public:
      _Counting_sink() = default;
 
      [[__gnu__::__always_inline__]]
      size_t
      count() noexcept
      {
        _Counting_sink::_M_overflow();
        return _M_count;
      }
    };
#endif
} // namespace __format
/// @endcond
 
  template<typename... _Args>
    [[nodiscard]]
    inline size_t
    formatted_size(format_string<_Args...> __fmt, _Args&&... __args)
    {
      __format::_Counting_sink<char> __buf;
      std::vformat_to(__buf.out(), __fmt.get(),
                      std::make_format_args(__args...));
      return __buf.count();
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename... _Args>
    [[nodiscard]]
    inline size_t
    formatted_size(wformat_string<_Args...> __fmt, _Args&&... __args)
    {
      __format::_Counting_sink<wchar_t> __buf;
      std::vformat_to(__buf.out(), __fmt.get(),
                      std::make_wformat_args(__args...));
      return __buf.count();
    }
#endif
 
  template<typename... _Args>
    [[nodiscard]]
    inline size_t
    formatted_size(const locale& __loc, format_string<_Args...> __fmt,
                   _Args&&... __args)
    {
      __format::_Counting_sink<char> __buf;
      std::vformat_to(__buf.out(), __loc, __fmt.get(),
                      std::make_format_args(__args...));
      return __buf.count();
    }
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<typename... _Args>
    [[nodiscard]]
    inline size_t
    formatted_size(const locale& __loc, wformat_string<_Args...> __fmt,
                   _Args&&... __args)
    {
      __format::_Counting_sink<wchar_t> __buf;
      std::vformat_to(__buf.out(), __loc, __fmt.get(),
                      std::make_wformat_args(__args...));
      return __buf.count();
    }
#endif
 
#if __glibcxx_format_ranges // C++ >= 23 && HOSTED
  // [format.range], formatting of ranges
  // [format.range.fmtkind], variable template format_kind
  enum class range_format {
    disabled,
    map,
    set,
    sequence,
    string,
    debug_string
  };
 
  /** @brief A constant determining how a range should be formatted.
   *
   * The primary template of `std::format_kind` cannot be instantiated.
   * There is a partial specialization for input ranges and you can
   * specialize the variable template for your own cv-unqualified types
   * that satisfy the `ranges::input_range` concept.
   *
   * @since C++23
   */
  template<typename _Rg>
    constexpr auto format_kind = []{
      static_assert(false, "cannot use primary template of 'std::format_kind'");
      return type_identity<_Rg>{};
    }();
 
  /// @cond undocumented
  template<typename _Tp>
    consteval range_format
    __fmt_kind()
    {
      using _Ref = ranges::range_reference_t<_Tp>;
      if constexpr (is_same_v<remove_cvref_t<_Ref>, _Tp>)
        return range_format::disabled;
      else if constexpr (requires { typename _Tp::key_type; })
        {
          if constexpr (requires { typename _Tp::mapped_type; })
            {
              using _Up = remove_cvref_t<_Ref>;
              if constexpr (__is_pair<_Up>)
                return range_format::map;
              else if constexpr (__is_specialization_of<_Up, tuple>)
                if constexpr (tuple_size_v<_Up> == 2)
                  return range_format::map;
            }
          return range_format::set;
        }
      else
        return range_format::sequence;
    }
  /// @endcond
 
  /// A constant determining how a range should be formatted.
  template<ranges::input_range _Rg> requires same_as<_Rg, remove_cvref_t<_Rg>>
    constexpr range_format format_kind<_Rg> = __fmt_kind<_Rg>();
 
/// @cond undocumented
namespace __format
{
  template<typename _CharT, typename _Out, typename _Callback>
    typename basic_format_context<_Out, _CharT>::iterator
    __format_padded(basic_format_context<_Out, _CharT>& __fc,
                    const _Spec<_CharT>& __spec,
                    _Callback&& __call)
    {
      // This is required to implement formatting with padding,
      // as we need to format to temporary buffer, using the same iterator.
      static_assert(is_same_v<_Out, __format::_Sink_iter<_CharT>>);
 
      if (__spec._M_get_width(__fc) == 0)
        return __call(__fc);
 
      struct _Restore_out
      {
        _Restore_out(basic_format_context<_Sink_iter<_CharT>, _CharT>& __fc)
        : _M_ctx(std::addressof(__fc)), _M_out(__fc.out())
        { }
 
        void _M_trigger()
        {
          if (_M_ctx)
            _M_ctx->advance_to(_M_out);
          _M_ctx = nullptr;
        }
 
        ~_Restore_out()
        { _M_trigger(); }
 
      private:
        basic_format_context<_Sink_iter<_CharT>, _CharT>* _M_ctx;
        _Sink_iter<_CharT> _M_out;
      };
 
      _Restore_out __restore(__fc);
      // TODO Consider double sinking, first buffer of width
      // size and then original sink, if first buffer is overun
      // we do not need to align
      _Str_sink<_CharT> __buf;
      __fc.advance_to(__buf.out());
      __call(__fc);
      __restore._M_trigger();
 
      basic_string_view<_CharT> __str(__buf.view());
      size_t __width;
      if constexpr (__unicode::__literal_encoding_is_unicode<_CharT>())
        __width = __unicode::__field_width(__str);
      else
        __width = __str.size();
 
      return __format::__write_padded_as_spec(__str, __width, __fc, __spec);
    }
 
  // _Rg& and const _Rg& are both formattable and use same formatter
  // specialization for their references.
  template<typename _Rg, typename _CharT>
    concept __simply_formattable_range
      = __const_formattable_range<_Rg, _CharT>
          && same_as<remove_cvref_t<ranges::range_reference_t<_Rg>>,
                     remove_cvref_t<ranges::range_reference_t<const _Rg>>>;
 
  template<size_t _Pos, typename _Tp, typename _CharT>
    struct __indexed_formatter_storage
    {
      constexpr void
      _M_parse()
      {
        basic_format_parse_context<_CharT> __pc({});
        if (_M_formatter.parse(__pc) != __pc.end())
          __format::__failed_to_parse_format_spec();
      }
 
      template<typename _Out>
        void
        _M_format(__maybe_const<_Tp, _CharT>& __elem,
                  basic_format_context<_Out, _CharT>& __fc,
                  basic_string_view<_CharT> __sep) const
        {
          if constexpr (_Pos != 0)
            __fc.advance_to(__format::__write(__fc.out(), __sep));
          __fc.advance_to(_M_formatter.format(__elem, __fc));
        }
 
      [[__gnu__::__always_inline__]]
      constexpr void
      set_debug_format()
      {
        if constexpr (__has_debug_format<formatter<_Tp, _CharT>>)
          _M_formatter.set_debug_format();
      }
 
    private:
      formatter<_Tp, _CharT> _M_formatter;
    };
 
  template<typename _CharT, typename... _Tps>
    class __tuple_formatter
    {
      using _String_view = basic_string_view<_CharT>;
      using _Seps = __format::_Separators<_CharT>;
 
    public:
      constexpr void
      set_separator(basic_string_view<_CharT> __sep) noexcept
      { _M_sep = __sep; }
 
      constexpr void
      set_brackets(basic_string_view<_CharT> __open,
                   basic_string_view<_CharT> __close) noexcept
      {
        _M_open = __open;
        _M_close = __close;
      }
 
      // We deviate from standard, that declares this as template accepting
      // unconstrained ParseContext type, which seems unimplementable.
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      {
        auto __first = __pc.begin();
        const auto __last = __pc.end();
        __format::_Spec<_CharT> __spec{};
 
        auto __finished = [&]
          {
            if (__first != __last && *__first != '}')
              return false;
 
            _M_spec = __spec;
            _M_felems._M_parse();
            _M_felems.set_debug_format();
            return true;
          };
 
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_fill_and_align(__first, __last, "{:");
        if (__finished())
          return __first;
 
        __first = __spec._M_parse_width(__first, __last, __pc);
        if (__finished())
          return __first;
 
        if (*__first == 'n')
          {
            ++__first;
            _M_open = _M_close = _String_view();
          }
        else if (*__first == 'm')
          {
            ++__first;
            if constexpr (sizeof...(_Tps) == 2)
              {
                _M_sep = _Seps::_S_colon();
                _M_open = _M_close = _String_view();
              }
            else
              __throw_format_error("format error: 'm' specifier requires range"
                                   " of pair or tuple of two elements");
          }
 
        if (__finished())
          return __first;
 
        __format::__failed_to_parse_format_spec();
      }
 
    protected:
      template<typename _Tuple, typename _Out, size_t... _Ids>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format(_Tuple& __tuple, index_sequence<_Ids...>,
                  basic_format_context<_Out, _CharT>& __fc) const
        { return _M_format_elems(std::get<_Ids>(__tuple)..., __fc); }
 
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format_elems(__maybe_const<_Tps, _CharT>&... __elems,
                        basic_format_context<_Out, _CharT>& __fc) const
        {
          return __format::__format_padded(
                   __fc, _M_spec,
                   [this, &__elems...](basic_format_context<_Out, _CharT>& __nfc)
                     {
                       __nfc.advance_to(__format::__write(__nfc.out(), _M_open));
                       _M_felems._M_format(__elems..., __nfc, _M_sep);
                       return __format::__write(__nfc.out(), _M_close);
                     });
        }
 
    private:
      template<size_t... _Ids>
        struct __formatters_storage
          : __indexed_formatter_storage<_Ids, _Tps, _CharT>...
        {
          template<size_t _Id, typename _Up>
            using _Base = __indexed_formatter_storage<_Id, _Up, _CharT>;
 
          constexpr void
          _M_parse()
          {
            (_Base<_Ids, _Tps>::_M_parse(), ...);
          }
 
          template<typename _Out>
            void
            _M_format(__maybe_const<_Tps, _CharT>&... __elems,
                      basic_format_context<_Out, _CharT>& __fc,
                      _String_view __sep) const
            {
              (_Base<_Ids, _Tps>::_M_format(__elems, __fc, __sep), ...);
            }
 
          constexpr void
          set_debug_format()
          {
            (_Base<_Ids, _Tps>::set_debug_format(), ...);
          }
        };
 
      template<size_t... _Ids>
        static auto
        _S_create_storage(index_sequence<_Ids...>)
          -> __formatters_storage<_Ids...>;
      using _Formatters
        = decltype(_S_create_storage(index_sequence_for<_Tps...>()));
 
      _Spec<_CharT> _M_spec{};
      _String_view _M_open = _Seps::_S_parens().substr(0, 1);
      _String_view _M_close = _Seps::_S_parens().substr(1, 1);
      _String_view _M_sep = _Seps::_S_comma();
      _Formatters _M_felems;
    };
 
  template<typename _Tp>
    concept __is_map_formattable
      = __is_pair<_Tp> || (__is_tuple_v<_Tp> && tuple_size_v<_Tp> == 2);
 
} // namespace __format
/// @endcond
 
  // [format.tuple] Tuple formatter
  template<__format::__char _CharT, formattable<_CharT> _Fp,
           formattable<_CharT> _Sp>
    struct formatter<pair<_Fp, _Sp>, _CharT>
      : __format::__tuple_formatter<_CharT, remove_cvref_t<_Fp>,
                                    remove_cvref_t<_Sp>>
    {
    private:
      using __maybe_const_pair
        = __conditional_t<formattable<const _Fp, _CharT>
                          && formattable<const _Sp, _CharT>,
                          const pair<_Fp, _Sp>, pair<_Fp, _Sp>>;
    public:
      // We deviate from standard, that declares this as template accepting
      // unconstrained FormatContext type, which seems unimplementable.
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(__maybe_const_pair& __p,
               basic_format_context<_Out, _CharT>& __fc) const
        { return this->_M_format_elems(__p.first, __p.second, __fc); }
    };
 
  template<__format::__char _CharT, formattable<_CharT>... _Tps>
    struct formatter<tuple<_Tps...>, _CharT>
      : __format::__tuple_formatter<_CharT, remove_cvref_t<_Tps>...>
    {
    private:
      using __maybe_const_tuple
        = __conditional_t<(formattable<const _Tps, _CharT> && ...),
                          const tuple<_Tps...>, tuple<_Tps...>>;
    public:
      // We deviate from standard, that declares this as template accepting
      // unconstrained FormatContext type, which seems unimplementable.
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(__maybe_const_tuple& __t,
               basic_format_context<_Out, _CharT>& __fc) const
        { return this->_M_format(__t, index_sequence_for<_Tps...>(), __fc); }
    };
 
  // [format.range.formatter], class template range_formatter
  template<typename _Tp, __format::__char _CharT>
    requires same_as<remove_cvref_t<_Tp>, _Tp> && formattable<_Tp, _CharT>
    class range_formatter
    {
       using _String_view = basic_string_view<_CharT>;
       using _Seps = __format::_Separators<_CharT>;
 
    public:
       constexpr void
       set_separator(basic_string_view<_CharT> __sep) noexcept
       { _M_sep = __sep; }
 
       constexpr void
       set_brackets(basic_string_view<_CharT> __open,
                    basic_string_view<_CharT> __close) noexcept
       {
         _M_open = __open;
         _M_close = __close;
       }
 
       constexpr formatter<_Tp, _CharT>&
       underlying() noexcept
       { return _M_fval; }
 
       constexpr const formatter<_Tp, _CharT>&
       underlying() const noexcept
       { return _M_fval; }
 
       // We deviate from standard, that declares this as template accepting
       // unconstrained ParseContext type, which seems unimplementable.
       constexpr typename basic_format_parse_context<_CharT>::iterator
       parse(basic_format_parse_context<_CharT>& __pc)
       {
         auto __first = __pc.begin();
         const auto __last = __pc.end();
         __format::_Spec<_CharT> __spec{};
         bool __no_brace = false;
 
         auto __finished = [&]
           { return __first == __last || *__first == '}'; };
 
         auto __finalize = [&]
           {
             _M_spec = __spec;
             return __first;
           };
 
         auto __parse_val = [&](_String_view __nfs = _String_view())
           {
             basic_format_parse_context<_CharT> __npc(__nfs);
             if (_M_fval.parse(__npc) != __npc.end())
               __format::__failed_to_parse_format_spec();
             if constexpr (__format::__has_debug_format<formatter<_Tp, _CharT>>)
               _M_fval.set_debug_format();
             return __finalize();
           };
 
         if (__finished())
           return __parse_val();
 
         __first = __spec._M_parse_fill_and_align(__first, __last, "{:");
         if (__finished())
           return __parse_val();
 
         __first = __spec._M_parse_width(__first, __last, __pc);
         if (__finished())
           return __parse_val();
 
         if (*__first == '?')
           {
             ++__first;
             __spec._M_type = __format::_Pres_esc;
             if (__finished() || *__first != 's')
               __throw_format_error("format error: '?' is allowed only in"
                                    " combination with 's'");
           }
 
         if (*__first == 's')
           {
             ++__first;
             if constexpr (same_as<_Tp, _CharT>)
               {
                 if (__spec._M_type != __format::_Pres_esc)
                   __spec._M_type = __format::_Pres_str;
                 if (__finished())
                   return __finalize();
                 __throw_format_error("format error: element format specifier"
                                      " cannot be provided when 's' specifier is used");
               }
             else
               __throw_format_error("format error: 's' specifier requires"
                                    " range of character types");
           }
 
         if (__finished())
           return __parse_val();
 
         if (*__first == 'n')
           {
             ++__first;
             _M_open = _M_close = _String_view();
             __no_brace = true;
           }
 
         if (__finished())
           return __parse_val();
 
         if (*__first == 'm')
           {
             _String_view __m(__first, 1);
             ++__first;
             if constexpr (__format::__is_map_formattable<_Tp>)
               {
                 _M_sep = _Seps::_S_comma();
                 if (!__no_brace)
                   {
                     _M_open = _Seps::_S_braces().substr(0, 1);
                     _M_close = _Seps::_S_braces().substr(1, 1);
                   }
                 if (__finished())
                   return __parse_val(__m);
                 __throw_format_error("format error: element format specifier"
                                      " cannot be provided when 'm' specifier is used");
               }
             else
               __throw_format_error("format error: 'm' specifier requires"
                                    " range of pairs or tuples of two elements");
           }
 
        if (__finished())
          return __parse_val();
 
        if (*__first == ':')
          {
            __pc.advance_to(++__first);
            __first = _M_fval.parse(__pc);
          }
 
        if (__finished())
          return __finalize();
 
        __format::__failed_to_parse_format_spec();
      }
 
      // We deviate from standard, that declares this as template accepting
      // unconstrained FormatContext type, which seems unimplementable.
      template<ranges::input_range _Rg, typename _Out>
        requires formattable<ranges::range_reference_t<_Rg>, _CharT> &&
                 same_as<remove_cvref_t<ranges::range_reference_t<_Rg>>, _Tp>
        typename basic_format_context<_Out, _CharT>::iterator
        format(_Rg&& __rg, basic_format_context<_Out, _CharT>& __fc) const
        {
          using _Range = remove_reference_t<_Rg>;
          if constexpr (__format::__simply_formattable_range<_Range, _CharT>)
            return _M_format<const _Range>(__rg, __fc);
          else
            return _M_format(__rg, __fc);
        }
 
    private:
      template<ranges::input_range _Rg, typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format(_Rg& __rg, basic_format_context<_Out, _CharT>& __fc) const
        {
          if constexpr (same_as<_Tp, _CharT>)
            if (_M_spec._M_type == __format::_Pres_str
                  || _M_spec._M_type == __format::_Pres_esc)
              {
                __format::__formatter_str __fstr(_M_spec);
                return __fstr._M_format_range(__rg, __fc);
              }
          return __format::__format_padded(
                   __fc, _M_spec,
                   [this, &__rg](basic_format_context<_Out, _CharT>& __nfc)
                     { return _M_format_elems(__rg, __nfc); });
        }
 
 
      template<ranges::input_range _Rg, typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        _M_format_elems(_Rg& __rg,
                        basic_format_context<_Out, _CharT>& __fc) const
        {
          auto __out = __format::__write(__fc.out(), _M_open);
 
          auto __first = ranges::begin(__rg);
          auto const __last = ranges::end(__rg);
          if (__first == __last)
            return __format::__write(__out, _M_close);
 
          __fc.advance_to(__out);
          __out = _M_fval.format(*__first, __fc);
          for (++__first; __first != __last; ++__first)
            {
              __out = __format::__write(__out, _M_sep);
              __fc.advance_to(__out);
              __out = _M_fval.format(*__first, __fc);
            }
 
          return __format::__write(__out, _M_close);
        }
 
      __format::_Spec<_CharT> _M_spec{};
      _String_view _M_open = _Seps::_S_squares().substr(0, 1);
      _String_view _M_close = _Seps::_S_squares().substr(1, 1);
      _String_view _M_sep = _Seps::_S_comma();
      formatter<_Tp, _CharT> _M_fval;
    };
 
  // In standard this is shown as inheriting from specialization of
  // exposition only specialization for range-default-formatter for
  // each range_format. We opt for simpler implementation.
  // [format.range.fmtmap], [format.range.fmtset], [format.range.fmtstr],
  // specializations for maps, sets, and strings
  template<ranges::input_range _Rg, __format::__char _CharT>
    requires (format_kind<_Rg> != range_format::disabled)
      && formattable<ranges::range_reference_t<_Rg>, _CharT>
    struct formatter<_Rg, _CharT>
    {
    private:
      static const bool _S_range_format_is_string =
        (format_kind<_Rg> == range_format::string)
         || (format_kind<_Rg> == range_format::debug_string);
      using _Vt = remove_cvref_t<
                    ranges::range_reference_t<
                      __format::__maybe_const_range<_Rg, _CharT>>>;
 
      static consteval bool _S_is_correct()
        {
          if constexpr (_S_range_format_is_string)
            static_assert(same_as<_Vt, _CharT>);
          return true;
        }
 
      static_assert(_S_is_correct());
 
    public:
      constexpr formatter() noexcept
      {
        using _Seps = __format::_Separators<_CharT>;
        if constexpr (format_kind<_Rg> == range_format::map)
          {
            static_assert(__format::__is_map_formattable<_Vt>);
            _M_under.set_brackets(_Seps::_S_braces().substr(0, 1),
                                  _Seps::_S_braces().substr(1, 1));
            _M_under.underlying().set_brackets({}, {});
            _M_under.underlying().set_separator(_Seps::_S_colon());
          }
        else if constexpr (format_kind<_Rg> == range_format::set)
          _M_under.set_brackets(_Seps::_S_braces().substr(0, 1),
                                _Seps::_S_braces().substr(1, 1));
      }
 
      constexpr void
      set_separator(basic_string_view<_CharT> __sep) noexcept
        requires (format_kind<_Rg> == range_format::sequence)
      { _M_under.set_separator(__sep); }
 
      constexpr void
      set_brackets(basic_string_view<_CharT> __open,
                   basic_string_view<_CharT> __close) noexcept
        requires (format_kind<_Rg> == range_format::sequence)
      { _M_under.set_brackets(__open, __close); }
 
      // We deviate from standard, that declares this as template accepting
      // unconstrained ParseContext type, which seems unimplementable.
      constexpr typename basic_format_parse_context<_CharT>::iterator
      parse(basic_format_parse_context<_CharT>& __pc)
      {
        auto __res = _M_under.parse(__pc);
        if constexpr (format_kind<_Rg> == range_format::debug_string)
          _M_under.set_debug_format();
        return __res;
      }
 
      // We deviate from standard, that declares this as template accepting
      // unconstrained FormatContext type, which seems unimplementable.
      template<typename _Out>
        typename basic_format_context<_Out, _CharT>::iterator
        format(__format::__maybe_const_range<_Rg, _CharT>& __rg,
               basic_format_context<_Out, _CharT>& __fc) const
        {
          if constexpr (_S_range_format_is_string)
            return _M_under._M_format_range(__rg, __fc);
          else
            return _M_under.format(__rg, __fc);
        }
 
    private:
      using _Formatter_under
        = __conditional_t<_S_range_format_is_string,
                          __format::__formatter_str<_CharT>,
                          range_formatter<_Vt, _CharT>>;
      _Formatter_under _M_under;
    };
#endif // C++23 formatting ranges
#undef _GLIBCXX_WIDEN
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
#endif // __cpp_lib_format
#pragma GCC diagnostic pop
#endif // _GLIBCXX_FORMAT