libstdc++
ranges
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1 // <ranges> -*- C++ -*-
2 
3 // Copyright (C) 2019-2021 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24 
25 /** @file include/ranges
26  * This is a Standard C++ Library header.
27  * @ingroup concepts
28  */
29 
30 #ifndef _GLIBCXX_RANGES
31 #define _GLIBCXX_RANGES 1
32 
33 #if __cplusplus > 201703L
34 
35 #pragma GCC system_header
36 
37 #include <concepts>
38 
39 #if __cpp_lib_concepts
40 
41 #include <compare>
42 #include <initializer_list>
43 #include <iterator>
44 #include <optional>
45 #include <tuple>
46 #include <bits/ranges_util.h>
47 #include <bits/refwrap.h>
48 
49 /**
50  * @defgroup ranges Ranges
51  *
52  * Components for dealing with ranges of elements.
53  */
54 
55 namespace std _GLIBCXX_VISIBILITY(default)
56 {
57 _GLIBCXX_BEGIN_NAMESPACE_VERSION
58 namespace ranges
59 {
60  // [range.access] customization point objects
61  // [range.req] range and view concepts
62  // [range.dangling] dangling iterator handling
63  // Defined in <bits/ranges_base.h>
64 
65  // [view.interface] View interface
66  // [range.subrange] Sub-ranges
67  // Defined in <bits/ranges_util.h>
68 
69  // C++20 24.6 [range.factories] Range factories
70 
71  /// A view that contains no elements.
72  template<typename _Tp> requires is_object_v<_Tp>
73  class empty_view
74  : public view_interface<empty_view<_Tp>>
75  {
76  public:
77  static constexpr _Tp* begin() noexcept { return nullptr; }
78  static constexpr _Tp* end() noexcept { return nullptr; }
79  static constexpr _Tp* data() noexcept { return nullptr; }
80  static constexpr size_t size() noexcept { return 0; }
81  static constexpr bool empty() noexcept { return true; }
82  };
83 
84  template<typename _Tp>
85  inline constexpr bool enable_borrowed_range<empty_view<_Tp>> = true;
86 
87  namespace __detail
88  {
89  template<typename _Tp>
90  concept __boxable = copy_constructible<_Tp> && is_object_v<_Tp>;
91 
92  template<__boxable _Tp>
93  struct __box : std::optional<_Tp>
94  {
95  using std::optional<_Tp>::optional;
96 
97  constexpr
98  __box()
99  noexcept(is_nothrow_default_constructible_v<_Tp>)
100  requires default_initializable<_Tp>
101  : std::optional<_Tp>{std::in_place}
102  { }
103 
104  __box(const __box&) = default;
105  __box(__box&&) = default;
106 
107  using std::optional<_Tp>::operator=;
108 
109  // _GLIBCXX_RESOLVE_LIB_DEFECTS
110  // 3477. Simplify constraints for semiregular-box
111  __box&
112  operator=(const __box& __that)
113  noexcept(is_nothrow_copy_constructible_v<_Tp>)
114  requires (!copyable<_Tp>)
115  {
116  if ((bool)__that)
117  this->emplace(*__that);
118  else
119  this->reset();
120  return *this;
121  }
122 
123  __box&
124  operator=(__box&& __that)
125  noexcept(is_nothrow_move_constructible_v<_Tp>)
126  requires (!movable<_Tp>)
127  {
128  if ((bool)__that)
129  this->emplace(std::move(*__that));
130  else
131  this->reset();
132  return *this;
133  }
134  };
135 
136  // For types which are already semiregular, this specialization of the
137  // semiregular wrapper stores the object directly without going through
138  // std::optional. It provides just the subset of the primary template's
139  // API that we currently use.
140  template<__boxable _Tp> requires semiregular<_Tp>
141  struct __box<_Tp>
142  {
143  private:
144  [[no_unique_address]] _Tp _M_value = _Tp();
145 
146  public:
147  __box() = default;
148 
149  constexpr explicit
150  __box(const _Tp& __t)
151  noexcept(is_nothrow_copy_constructible_v<_Tp>)
152  : _M_value{__t}
153  { }
154 
155  constexpr explicit
156  __box(_Tp&& __t)
157  noexcept(is_nothrow_move_constructible_v<_Tp>)
158  : _M_value{std::move(__t)}
159  { }
160 
161  template<typename... _Args>
162  requires constructible_from<_Tp, _Args...>
163  constexpr explicit
164  __box(in_place_t, _Args&&... __args)
165  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
166  : _M_value{std::forward<_Args>(__args)...}
167  { }
168 
169  constexpr bool
170  has_value() const noexcept
171  { return true; };
172 
173  constexpr _Tp&
174  operator*() noexcept
175  { return _M_value; }
176 
177  constexpr const _Tp&
178  operator*() const noexcept
179  { return _M_value; }
180 
181  constexpr _Tp*
182  operator->() noexcept
183  { return &_M_value; }
184 
185  constexpr const _Tp*
186  operator->() const noexcept
187  { return &_M_value; }
188  };
189  } // namespace __detail
190 
191  /// A view that contains exactly one element.
192  template<copy_constructible _Tp> requires is_object_v<_Tp>
193  class single_view : public view_interface<single_view<_Tp>>
194  {
195  public:
196  single_view() = default;
197 
198  constexpr explicit
199  single_view(const _Tp& __t)
200  : _M_value(__t)
201  { }
202 
203  constexpr explicit
204  single_view(_Tp&& __t)
205  : _M_value(std::move(__t))
206  { }
207 
208  // _GLIBCXX_RESOLVE_LIB_DEFECTS
209  // 3428. single_view's in place constructor should be explicit
210  template<typename... _Args>
211  requires constructible_from<_Tp, _Args...>
212  constexpr explicit
213  single_view(in_place_t, _Args&&... __args)
214  : _M_value{in_place, std::forward<_Args>(__args)...}
215  { }
216 
217  constexpr _Tp*
218  begin() noexcept
219  { return data(); }
220 
221  constexpr const _Tp*
222  begin() const noexcept
223  { return data(); }
224 
225  constexpr _Tp*
226  end() noexcept
227  { return data() + 1; }
228 
229  constexpr const _Tp*
230  end() const noexcept
231  { return data() + 1; }
232 
233  static constexpr size_t
234  size() noexcept
235  { return 1; }
236 
237  constexpr _Tp*
238  data() noexcept
239  { return _M_value.operator->(); }
240 
241  constexpr const _Tp*
242  data() const noexcept
243  { return _M_value.operator->(); }
244 
245  private:
246  [[no_unique_address]] __detail::__box<_Tp> _M_value;
247  };
248 
249  namespace __detail
250  {
251  template<typename _Wp>
252  constexpr auto __to_signed_like(_Wp __w) noexcept
253  {
254  if constexpr (!integral<_Wp>)
255  return iter_difference_t<_Wp>();
256  else if constexpr (sizeof(iter_difference_t<_Wp>) > sizeof(_Wp))
257  return iter_difference_t<_Wp>(__w);
258  else if constexpr (sizeof(ptrdiff_t) > sizeof(_Wp))
259  return ptrdiff_t(__w);
260  else if constexpr (sizeof(long long) > sizeof(_Wp))
261  return (long long)(__w);
262 #ifdef __SIZEOF_INT128__
263  else if constexpr (__SIZEOF_INT128__ > sizeof(_Wp))
264  return __int128(__w);
265 #endif
266  else
267  return __max_diff_type(__w);
268  }
269 
270  template<typename _Wp>
271  using __iota_diff_t = decltype(__to_signed_like(std::declval<_Wp>()));
272 
273  template<typename _It>
274  concept __decrementable = incrementable<_It>
275  && requires(_It __i)
276  {
277  { --__i } -> same_as<_It&>;
278  { __i-- } -> same_as<_It>;
279  };
280 
281  template<typename _It>
282  concept __advanceable = __decrementable<_It> && totally_ordered<_It>
283  && requires( _It __i, const _It __j, const __iota_diff_t<_It> __n)
284  {
285  { __i += __n } -> same_as<_It&>;
286  { __i -= __n } -> same_as<_It&>;
287  _It(__j + __n);
288  _It(__n + __j);
289  _It(__j - __n);
290  { __j - __j } -> convertible_to<__iota_diff_t<_It>>;
291  };
292 
293  template<typename _Winc>
294  struct __iota_view_iter_cat
295  { };
296 
297  template<incrementable _Winc>
298  struct __iota_view_iter_cat<_Winc>
299  { using iterator_category = input_iterator_tag; };
300  } // namespace __detail
301 
302  template<weakly_incrementable _Winc,
303  semiregular _Bound = unreachable_sentinel_t>
304  requires std::__detail::__weakly_eq_cmp_with<_Winc, _Bound>
305  && semiregular<_Winc>
306  class iota_view : public view_interface<iota_view<_Winc, _Bound>>
307  {
308  private:
309  struct _Sentinel;
310 
311  struct _Iterator : __detail::__iota_view_iter_cat<_Winc>
312  {
313  private:
314  static auto
315  _S_iter_concept()
316  {
317  using namespace __detail;
318  if constexpr (__advanceable<_Winc>)
319  return random_access_iterator_tag{};
320  else if constexpr (__decrementable<_Winc>)
321  return bidirectional_iterator_tag{};
322  else if constexpr (incrementable<_Winc>)
323  return forward_iterator_tag{};
324  else
325  return input_iterator_tag{};
326  }
327 
328  public:
329  using iterator_concept = decltype(_S_iter_concept());
330  // iterator_category defined in __iota_view_iter_cat
331  using value_type = _Winc;
332  using difference_type = __detail::__iota_diff_t<_Winc>;
333 
334  _Iterator() = default;
335 
336  constexpr explicit
337  _Iterator(_Winc __value)
338  : _M_value(__value) { }
339 
340  constexpr _Winc
341  operator*() const noexcept(is_nothrow_copy_constructible_v<_Winc>)
342  { return _M_value; }
343 
344  constexpr _Iterator&
345  operator++()
346  {
347  ++_M_value;
348  return *this;
349  }
350 
351  constexpr void
352  operator++(int)
353  { ++*this; }
354 
355  constexpr _Iterator
356  operator++(int) requires incrementable<_Winc>
357  {
358  auto __tmp = *this;
359  ++*this;
360  return __tmp;
361  }
362 
363  constexpr _Iterator&
364  operator--() requires __detail::__decrementable<_Winc>
365  {
366  --_M_value;
367  return *this;
368  }
369 
370  constexpr _Iterator
371  operator--(int) requires __detail::__decrementable<_Winc>
372  {
373  auto __tmp = *this;
374  --*this;
375  return __tmp;
376  }
377 
378  constexpr _Iterator&
379  operator+=(difference_type __n) requires __detail::__advanceable<_Winc>
380  {
381  using __detail::__is_integer_like;
382  using __detail::__is_signed_integer_like;
383  if constexpr (__is_integer_like<_Winc>
384  && !__is_signed_integer_like<_Winc>)
385  {
386  if (__n >= difference_type(0))
387  _M_value += static_cast<_Winc>(__n);
388  else
389  _M_value -= static_cast<_Winc>(-__n);
390  }
391  else
392  _M_value += __n;
393  return *this;
394  }
395 
396  constexpr _Iterator&
397  operator-=(difference_type __n) requires __detail::__advanceable<_Winc>
398  {
399  using __detail::__is_integer_like;
400  using __detail::__is_signed_integer_like;
401  if constexpr (__is_integer_like<_Winc>
402  && !__is_signed_integer_like<_Winc>)
403  {
404  if (__n >= difference_type(0))
405  _M_value -= static_cast<_Winc>(__n);
406  else
407  _M_value += static_cast<_Winc>(-__n);
408  }
409  else
410  _M_value -= __n;
411  return *this;
412  }
413 
414  constexpr _Winc
415  operator[](difference_type __n) const
416  requires __detail::__advanceable<_Winc>
417  { return _Winc(_M_value + __n); }
418 
419  friend constexpr bool
420  operator==(const _Iterator& __x, const _Iterator& __y)
421  requires equality_comparable<_Winc>
422  { return __x._M_value == __y._M_value; }
423 
424  friend constexpr bool
425  operator<(const _Iterator& __x, const _Iterator& __y)
426  requires totally_ordered<_Winc>
427  { return __x._M_value < __y._M_value; }
428 
429  friend constexpr bool
430  operator>(const _Iterator& __x, const _Iterator& __y)
431  requires totally_ordered<_Winc>
432  { return __y < __x; }
433 
434  friend constexpr bool
435  operator<=(const _Iterator& __x, const _Iterator& __y)
436  requires totally_ordered<_Winc>
437  { return !(__y < __x); }
438 
439  friend constexpr bool
440  operator>=(const _Iterator& __x, const _Iterator& __y)
441  requires totally_ordered<_Winc>
442  { return !(__x < __y); }
443 
444 #ifdef __cpp_lib_three_way_comparison
445  friend constexpr auto
446  operator<=>(const _Iterator& __x, const _Iterator& __y)
447  requires totally_ordered<_Winc> && three_way_comparable<_Winc>
448  { return __x._M_value <=> __y._M_value; }
449 #endif
450 
451  friend constexpr _Iterator
452  operator+(_Iterator __i, difference_type __n)
453  requires __detail::__advanceable<_Winc>
454  { return __i += __n; }
455 
456  friend constexpr _Iterator
457  operator+(difference_type __n, _Iterator __i)
458  requires __detail::__advanceable<_Winc>
459  { return __i += __n; }
460 
461  friend constexpr _Iterator
462  operator-(_Iterator __i, difference_type __n)
463  requires __detail::__advanceable<_Winc>
464  { return __i -= __n; }
465 
466  friend constexpr difference_type
467  operator-(const _Iterator& __x, const _Iterator& __y)
468  requires __detail::__advanceable<_Winc>
469  {
470  using __detail::__is_integer_like;
471  using __detail::__is_signed_integer_like;
472  using _Dt = difference_type;
473  if constexpr (__is_integer_like<_Winc>)
474  {
475  if constexpr (__is_signed_integer_like<_Winc>)
476  return _Dt(_Dt(__x._M_value) - _Dt(__y._M_value));
477  else
478  return (__y._M_value > __x._M_value)
479  ? _Dt(-_Dt(__y._M_value - __x._M_value))
480  : _Dt(__x._M_value - __y._M_value);
481  }
482  else
483  return __x._M_value - __y._M_value;
484  }
485 
486  private:
487  _Winc _M_value = _Winc();
488 
489  friend _Sentinel;
490  };
491 
492  struct _Sentinel
493  {
494  private:
495  constexpr bool
496  _M_equal(const _Iterator& __x) const
497  { return __x._M_value == _M_bound; }
498 
499  _Bound _M_bound = _Bound();
500 
501  public:
502  _Sentinel() = default;
503 
504  constexpr explicit
505  _Sentinel(_Bound __bound)
506  : _M_bound(__bound) { }
507 
508  friend constexpr bool
509  operator==(const _Iterator& __x, const _Sentinel& __y)
510  { return __y._M_equal(__x); }
511 
512  friend constexpr iter_difference_t<_Winc>
513  operator-(const _Iterator& __x, const _Sentinel& __y)
514  requires sized_sentinel_for<_Bound, _Winc>
515  { return __x._M_value - __y._M_bound; }
516 
517  friend constexpr iter_difference_t<_Winc>
518  operator-(const _Sentinel& __x, const _Iterator& __y)
519  requires sized_sentinel_for<_Bound, _Winc>
520  { return -(__y - __x); }
521  };
522 
523  _Winc _M_value = _Winc();
524  [[no_unique_address]] _Bound _M_bound = _Bound();
525 
526  public:
527  iota_view() = default;
528 
529  constexpr explicit
530  iota_view(_Winc __value)
531  : _M_value(__value)
532  { }
533 
534  constexpr
535  iota_view(type_identity_t<_Winc> __value,
536  type_identity_t<_Bound> __bound)
537  : _M_value(__value), _M_bound(__bound)
538  {
539  if constexpr (totally_ordered_with<_Winc, _Bound>)
540  __glibcxx_assert( bool(__value <= __bound) );
541  }
542 
543  constexpr _Iterator
544  begin() const { return _Iterator{_M_value}; }
545 
546  constexpr auto
547  end() const
548  {
549  if constexpr (same_as<_Bound, unreachable_sentinel_t>)
550  return unreachable_sentinel;
551  else
552  return _Sentinel{_M_bound};
553  }
554 
555  constexpr _Iterator
556  end() const requires same_as<_Winc, _Bound>
557  { return _Iterator{_M_bound}; }
558 
559  constexpr auto
560  size() const
561  requires (same_as<_Winc, _Bound> && __detail::__advanceable<_Winc>)
562  || (integral<_Winc> && integral<_Bound>)
563  || sized_sentinel_for<_Bound, _Winc>
564  {
565  using __detail::__is_integer_like;
566  using __detail::__to_unsigned_like;
567  if constexpr (integral<_Winc> && integral<_Bound>)
568  {
569  using _Up = make_unsigned_t<decltype(_M_bound - _M_value)>;
570  return _Up(_M_bound) - _Up(_M_value);
571  }
572  else if constexpr (__is_integer_like<_Winc>)
573  return __to_unsigned_like(_M_bound) - __to_unsigned_like(_M_value);
574  else
575  return __to_unsigned_like(_M_bound - _M_value);
576  }
577  };
578 
579  template<typename _Winc, typename _Bound>
580  requires (!__detail::__is_integer_like<_Winc>
581  || !__detail::__is_integer_like<_Bound>
582  || (__detail::__is_signed_integer_like<_Winc>
583  == __detail::__is_signed_integer_like<_Bound>))
584  iota_view(_Winc, _Bound) -> iota_view<_Winc, _Bound>;
585 
586  template<weakly_incrementable _Winc, semiregular _Bound>
587  inline constexpr bool
588  enable_borrowed_range<iota_view<_Winc, _Bound>> = true;
589 
590 namespace views
591 {
592  template<typename _Tp>
593  inline constexpr empty_view<_Tp> empty{};
594 
595  struct _Single
596  {
597  template<typename _Tp>
598  constexpr auto
599  operator()(_Tp&& __e) const
600  { return single_view{std::forward<_Tp>(__e)}; }
601  };
602 
603  inline constexpr _Single single{};
604 
605  struct _Iota
606  {
607  template<typename _Tp>
608  constexpr auto
609  operator()(_Tp&& __e) const
610  { return iota_view{std::forward<_Tp>(__e)}; }
611 
612  template<typename _Tp, typename _Up>
613  constexpr auto
614  operator()(_Tp&& __e, _Up&& __f) const
615  { return iota_view{std::forward<_Tp>(__e), std::forward<_Up>(__f)}; }
616  };
617 
618  inline constexpr _Iota iota{};
619 } // namespace views
620 
621  namespace __detail
622  {
623  template<typename _Val, typename _CharT, typename _Traits>
624  concept __stream_extractable
625  = requires(basic_istream<_CharT, _Traits>& is, _Val& t) { is >> t; };
626  } // namespace __detail
627 
628  template<movable _Val, typename _CharT, typename _Traits>
629  requires default_initializable<_Val>
630  && __detail::__stream_extractable<_Val, _CharT, _Traits>
631  class basic_istream_view
632  : public view_interface<basic_istream_view<_Val, _CharT, _Traits>>
633  {
634  public:
635  basic_istream_view() = default;
636 
637  constexpr explicit
638  basic_istream_view(basic_istream<_CharT, _Traits>& __stream)
639  : _M_stream(std::__addressof(__stream))
640  { }
641 
642  constexpr auto
643  begin()
644  {
645  if (_M_stream != nullptr)
646  *_M_stream >> _M_object;
647  return _Iterator{this};
648  }
649 
650  constexpr default_sentinel_t
651  end() const noexcept
652  { return default_sentinel; }
653 
654  private:
655  basic_istream<_CharT, _Traits>* _M_stream = nullptr;
656  _Val _M_object = _Val();
657 
658  struct _Iterator
659  {
660  public:
661  using iterator_concept = input_iterator_tag;
662  using difference_type = ptrdiff_t;
663  using value_type = _Val;
664 
665  _Iterator() = default;
666 
667  constexpr explicit
668  _Iterator(basic_istream_view* __parent) noexcept
669  : _M_parent(__parent)
670  { }
671 
672  _Iterator(const _Iterator&) = delete;
673  _Iterator(_Iterator&&) = default;
674  _Iterator& operator=(const _Iterator&) = delete;
675  _Iterator& operator=(_Iterator&&) = default;
676 
677  _Iterator&
678  operator++()
679  {
680  __glibcxx_assert(_M_parent->_M_stream != nullptr);
681  *_M_parent->_M_stream >> _M_parent->_M_object;
682  return *this;
683  }
684 
685  void
686  operator++(int)
687  { ++*this; }
688 
689  _Val&
690  operator*() const
691  {
692  __glibcxx_assert(_M_parent->_M_stream != nullptr);
693  return _M_parent->_M_object;
694  }
695 
696  friend bool
697  operator==(const _Iterator& __x, default_sentinel_t)
698  { return __x._M_at_end(); }
699 
700  private:
701  basic_istream_view* _M_parent = nullptr;
702 
703  bool
704  _M_at_end() const
705  { return _M_parent == nullptr || !*_M_parent->_M_stream; }
706  };
707 
708  friend _Iterator;
709  };
710 
711  template<typename _Val, typename _CharT, typename _Traits>
712  basic_istream_view<_Val, _CharT, _Traits>
713  istream_view(basic_istream<_CharT, _Traits>& __s)
714  { return basic_istream_view<_Val, _CharT, _Traits>{__s}; }
715 
716  // C++20 24.7 [range.adaptors] Range adaptors
717 
718 namespace __detail
719 {
720  struct _Empty { };
721 
722  // Alias for a type that is conditionally present
723  // (and is an empty type otherwise).
724  // Data members using this alias should use [[no_unique_address]] so that
725  // they take no space when not needed.
726  template<bool _Present, typename _Tp>
727  using __maybe_present_t = conditional_t<_Present, _Tp, _Empty>;
728 
729  // Alias for a type that is conditionally const.
730  template<bool _Const, typename _Tp>
731  using __maybe_const_t = conditional_t<_Const, const _Tp, _Tp>;
732 
733 } // namespace __detail
734 
735 namespace views::__adaptor
736 {
737  // True if the range adaptor _Adaptor can be applied with _Args.
738  template<typename _Adaptor, typename... _Args>
739  concept __adaptor_invocable
740  = requires { std::declval<_Adaptor>()(declval<_Args>()...); };
741 
742  // True if the range adaptor non-closure _Adaptor can be partially applied
743  // with _Args.
744  template<typename _Adaptor, typename... _Args>
745  concept __adaptor_partial_app_viable = (_Adaptor::_S_arity > 1)
746  && (sizeof...(_Args) == _Adaptor::_S_arity - 1)
747  && (constructible_from<decay_t<_Args>, _Args> && ...);
748 
749  template<typename _Adaptor, typename... _Args>
750  struct _Partial;
751 
752  template<typename _Lhs, typename _Rhs>
753  struct _Pipe;
754 
755  // The base class of every range adaptor closure.
756  struct _RangeAdaptorClosure
757  {
758  // range | adaptor is equivalent to adaptor(range).
759  template<typename _Self, typename _Range>
760  requires derived_from<remove_cvref_t<_Self>, _RangeAdaptorClosure>
761  && __adaptor_invocable<_Self, _Range>
762  friend constexpr auto
763  operator|(_Range&& __r, _Self&& __self)
764  { return std::forward<_Self>(__self)(std::forward<_Range>(__r)); }
765 
766  // Compose the adaptors __lhs and __rhs into a pipeline, returning
767  // another range adaptor closure object.
768  template<typename _Lhs, typename _Rhs>
769  requires derived_from<_Lhs, _RangeAdaptorClosure>
770  && derived_from<_Rhs, _RangeAdaptorClosure>
771  friend constexpr auto
772  operator|(_Lhs __lhs, _Rhs __rhs)
773  { return _Pipe<_Lhs, _Rhs>{std::move(__lhs), std::move(__rhs)}; }
774  };
775 
776  // The base class of every range adaptor non-closure.
777  //
778  // The static data member _Derived::_S_arity must contain the total number of
779  // arguments that the adaptor takes, and the class _Derived must introduce
780  // _RangeAdaptor::operator() into the class scope via a using-declaration.
781  template<typename _Derived>
782  struct _RangeAdaptor
783  {
784  // Partially apply the arguments __args to the range adaptor _Derived,
785  // returning a range adaptor closure object.
786  template<typename... _Args>
787  requires __adaptor_partial_app_viable<_Derived, _Args...>
788  constexpr auto
789  operator()(_Args&&... __args) const
790  {
791  return _Partial<_Derived, decay_t<_Args>...>{std::forward<_Args>(__args)...};
792  }
793  };
794 
795  // A range adaptor closure that represents partial application of
796  // the range adaptor _Adaptor with arguments _Args.
797  template<typename _Adaptor, typename... _Args>
798  struct _Partial : _RangeAdaptorClosure
799  {
800  tuple<_Args...> _M_args;
801 
802  constexpr
803  _Partial(_Args... __args)
804  : _M_args(std::move(__args)...)
805  { }
806 
807  // Invoke _Adaptor with arguments __r, _M_args... according to the
808  // value category of the range adaptor closure object.
809  template<typename _Range>
810  requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
811  constexpr auto
812  operator()(_Range&& __r) const &
813  {
814  auto __forwarder = [&__r] (const auto&... __args) {
815  return _Adaptor{}(std::forward<_Range>(__r), __args...);
816  };
817  return std::apply(__forwarder, _M_args);
818  }
819 
820  template<typename _Range>
821  requires __adaptor_invocable<_Adaptor, _Range, _Args...>
822  constexpr auto
823  operator()(_Range&& __r) &&
824  {
825  auto __forwarder = [&__r] (auto&... __args) {
826  return _Adaptor{}(std::forward<_Range>(__r), std::move(__args)...);
827  };
828  return std::apply(__forwarder, _M_args);
829  }
830 
831  template<typename _Range>
832  constexpr auto
833  operator()(_Range&& __r) const && = delete;
834  };
835 
836  // A lightweight specialization of the above primary template for
837  // the common case where _Adaptor accepts a single extra argument.
838  template<typename _Adaptor, typename _Arg>
839  struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
840  {
841  _Arg _M_arg;
842 
843  constexpr
844  _Partial(_Arg __arg)
845  : _M_arg(std::move(__arg))
846  { }
847 
848  template<typename _Range>
849  requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
850  constexpr auto
851  operator()(_Range&& __r) const &
852  { return _Adaptor{}(std::forward<_Range>(__r), _M_arg); }
853 
854  template<typename _Range>
855  requires __adaptor_invocable<_Adaptor, _Range, _Arg>
856  constexpr auto
857  operator()(_Range&& __r) &&
858  { return _Adaptor{}(std::forward<_Range>(__r), std::move(_M_arg)); }
859 
860  template<typename _Range>
861  constexpr auto
862  operator()(_Range&& __r) const && = delete;
863  };
864 
865  template<typename _Lhs, typename _Rhs, typename _Range>
866  concept __pipe_invocable
867  = requires { std::declval<_Rhs>()(std::declval<_Lhs>()(std::declval<_Range>())); };
868 
869  // A range adaptor closure that represents composition of the range
870  // adaptor closures _Lhs and _Rhs.
871  template<typename _Lhs, typename _Rhs>
872  struct _Pipe : _RangeAdaptorClosure
873  {
874  [[no_unique_address]] _Lhs _M_lhs;
875  [[no_unique_address]] _Rhs _M_rhs;
876 
877  constexpr
878  _Pipe(_Lhs __lhs, _Rhs __rhs)
879  : _M_lhs(std::move(__lhs)), _M_rhs(std::move(__rhs))
880  { }
881 
882  // Invoke _M_rhs(_M_lhs(__r)) according to the value category of this
883  // range adaptor closure object.
884  template<typename _Range>
885  requires __pipe_invocable<const _Lhs&, const _Rhs&, _Range>
886  constexpr auto
887  operator()(_Range&& __r) const &
888  { return _M_rhs(_M_lhs(std::forward<_Range>(__r))); }
889 
890  template<typename _Range>
891  requires __pipe_invocable<_Lhs, _Rhs, _Range>
892  constexpr auto
893  operator()(_Range&& __r) &&
894  { return std::move(_M_rhs)(std::move(_M_lhs)(std::forward<_Range>(__r))); }
895 
896  template<typename _Range>
897  constexpr auto
898  operator()(_Range&& __r) const && = delete;
899  };
900 } // namespace views::__adaptor
901 
902  template<range _Range> requires is_object_v<_Range>
903  class ref_view : public view_interface<ref_view<_Range>>
904  {
905  private:
906  _Range* _M_r = nullptr;
907 
908  static void _S_fun(_Range&); // not defined
909  static void _S_fun(_Range&&) = delete;
910 
911  public:
912  constexpr
913  ref_view() noexcept = default;
914 
915  template<__detail::__not_same_as<ref_view> _Tp>
916  requires convertible_to<_Tp, _Range&>
917  && requires { _S_fun(declval<_Tp>()); }
918  constexpr
919  ref_view(_Tp&& __t)
920  : _M_r(std::__addressof(static_cast<_Range&>(std::forward<_Tp>(__t))))
921  { }
922 
923  constexpr _Range&
924  base() const
925  { return *_M_r; }
926 
927  constexpr iterator_t<_Range>
928  begin() const
929  { return ranges::begin(*_M_r); }
930 
931  constexpr sentinel_t<_Range>
932  end() const
933  { return ranges::end(*_M_r); }
934 
935  constexpr bool
936  empty() const requires requires { ranges::empty(*_M_r); }
937  { return ranges::empty(*_M_r); }
938 
939  constexpr auto
940  size() const requires sized_range<_Range>
941  { return ranges::size(*_M_r); }
942 
943  constexpr auto
944  data() const requires contiguous_range<_Range>
945  { return ranges::data(*_M_r); }
946  };
947 
948  template<typename _Range>
949  ref_view(_Range&) -> ref_view<_Range>;
950 
951  template<typename _Tp>
952  inline constexpr bool enable_borrowed_range<ref_view<_Tp>> = true;
953 
954  namespace views
955  {
956  namespace __detail
957  {
958  template<typename _Range>
959  concept __can_ref_view = requires { ref_view{std::declval<_Range>()}; };
960 
961  template<typename _Range>
962  concept __can_subrange = requires { subrange{std::declval<_Range>()}; };
963  } // namespace __detail
964 
965  struct _All : __adaptor::_RangeAdaptorClosure
966  {
967  template<viewable_range _Range>
968  requires view<decay_t<_Range>>
969  || __detail::__can_ref_view<_Range>
970  || __detail::__can_subrange<_Range>
971  constexpr auto
972  operator()(_Range&& __r) const
973  {
974  if constexpr (view<decay_t<_Range>>)
975  return std::forward<_Range>(__r);
976  else if constexpr (__detail::__can_ref_view<_Range>)
977  return ref_view{std::forward<_Range>(__r)};
978  else
979  return subrange{std::forward<_Range>(__r)};
980  }
981  };
982 
983  inline constexpr _All all;
984 
985  template<viewable_range _Range>
986  using all_t = decltype(all(std::declval<_Range>()));
987  } // namespace views
988 
989  // The following simple algos are transcribed from ranges_algo.h to avoid
990  // having to include that entire header.
991  namespace __detail
992  {
993  template<typename _Iter, typename _Sent, typename _Tp>
994  constexpr _Iter
995  find(_Iter __first, _Sent __last, const _Tp& __value)
996  {
997  while (__first != __last
998  && !(bool)(*__first == __value))
999  ++__first;
1000  return __first;
1001  }
1002 
1003  template<typename _Iter, typename _Sent, typename _Pred>
1004  constexpr _Iter
1005  find_if(_Iter __first, _Sent __last, _Pred __pred)
1006  {
1007  while (__first != __last
1008  && !(bool)std::__invoke(__pred, *__first))
1009  ++__first;
1010  return __first;
1011  }
1012 
1013  template<typename _Iter, typename _Sent, typename _Pred>
1014  constexpr _Iter
1015  find_if_not(_Iter __first, _Sent __last, _Pred __pred)
1016  {
1017  while (__first != __last
1018  && (bool)std::__invoke(__pred, *__first))
1019  ++__first;
1020  return __first;
1021  }
1022 
1023  template<typename _Iter1, typename _Sent1, typename _Iter2, typename _Sent2>
1024  constexpr pair<_Iter1, _Iter2>
1025  mismatch(_Iter1 __first1, _Sent1 __last1, _Iter2 __first2, _Sent2 __last2)
1026  {
1027  while (__first1 != __last1 && __first2 != __last2
1028  && (bool)ranges::equal_to{}(*__first1, *__first2))
1029  {
1030  ++__first1;
1031  ++__first2;
1032  }
1033  return { std::move(__first1), std::move(__first2) };
1034  }
1035  } // namespace __detail
1036 
1037  namespace __detail
1038  {
1039  template<range _Range>
1040  struct _CachedPosition
1041  {
1042  constexpr bool
1043  _M_has_value() const
1044  { return false; }
1045 
1046  constexpr iterator_t<_Range>
1047  _M_get(const _Range&) const
1048  {
1049  __glibcxx_assert(false);
1050  return {};
1051  }
1052 
1053  constexpr void
1054  _M_set(const _Range&, const iterator_t<_Range>&) const
1055  { }
1056  };
1057 
1058  template<forward_range _Range>
1059  struct _CachedPosition<_Range>
1060  {
1061  private:
1062  iterator_t<_Range> _M_iter{};
1063 
1064  public:
1065  constexpr bool
1066  _M_has_value() const
1067  { return _M_iter != iterator_t<_Range>{}; }
1068 
1069  constexpr iterator_t<_Range>
1070  _M_get(const _Range&) const
1071  {
1072  __glibcxx_assert(_M_has_value());
1073  return _M_iter;
1074  }
1075 
1076  constexpr void
1077  _M_set(const _Range&, const iterator_t<_Range>& __it)
1078  {
1079  __glibcxx_assert(!_M_has_value());
1080  _M_iter = __it;
1081  }
1082  };
1083 
1084  template<random_access_range _Range>
1085  requires (sizeof(range_difference_t<_Range>)
1086  <= sizeof(iterator_t<_Range>))
1087  struct _CachedPosition<_Range>
1088  {
1089  private:
1090  range_difference_t<_Range> _M_offset = -1;
1091 
1092  public:
1093  constexpr bool
1094  _M_has_value() const
1095  { return _M_offset >= 0; }
1096 
1097  constexpr iterator_t<_Range>
1098  _M_get(_Range& __r) const
1099  {
1100  __glibcxx_assert(_M_has_value());
1101  return ranges::begin(__r) + _M_offset;
1102  }
1103 
1104  constexpr void
1105  _M_set(_Range& __r, const iterator_t<_Range>& __it)
1106  {
1107  __glibcxx_assert(!_M_has_value());
1108  _M_offset = __it - ranges::begin(__r);
1109  }
1110  };
1111  } // namespace __detail
1112 
1113  namespace __detail
1114  {
1115  template<typename _Base>
1116  struct __filter_view_iter_cat
1117  { };
1118 
1119  template<forward_range _Base>
1120  struct __filter_view_iter_cat<_Base>
1121  {
1122  private:
1123  static auto
1124  _S_iter_cat()
1125  {
1126  using _Cat = typename iterator_traits<iterator_t<_Base>>::iterator_category;
1127  if constexpr (derived_from<_Cat, bidirectional_iterator_tag>)
1128  return bidirectional_iterator_tag{};
1129  else if constexpr (derived_from<_Cat, forward_iterator_tag>)
1130  return forward_iterator_tag{};
1131  else
1132  return _Cat{};
1133  }
1134  public:
1135  using iterator_category = decltype(_S_iter_cat());
1136  };
1137  } // namespace __detail
1138 
1139  template<input_range _Vp,
1140  indirect_unary_predicate<iterator_t<_Vp>> _Pred>
1141  requires view<_Vp> && is_object_v<_Pred>
1142  class filter_view : public view_interface<filter_view<_Vp, _Pred>>
1143  {
1144  private:
1145  struct _Sentinel;
1146 
1147  struct _Iterator : __detail::__filter_view_iter_cat<_Vp>
1148  {
1149  private:
1150  static constexpr auto
1151  _S_iter_concept()
1152  {
1153  if constexpr (bidirectional_range<_Vp>)
1154  return bidirectional_iterator_tag{};
1155  else if constexpr (forward_range<_Vp>)
1156  return forward_iterator_tag{};
1157  else
1158  return input_iterator_tag{};
1159  }
1160 
1161  friend filter_view;
1162 
1163  using _Vp_iter = iterator_t<_Vp>;
1164 
1165  _Vp_iter _M_current = _Vp_iter();
1166  filter_view* _M_parent = nullptr;
1167 
1168  public:
1169  using iterator_concept = decltype(_S_iter_concept());
1170  // iterator_category defined in __filter_view_iter_cat
1171  using value_type = range_value_t<_Vp>;
1172  using difference_type = range_difference_t<_Vp>;
1173 
1174  _Iterator() = default;
1175 
1176  constexpr
1177  _Iterator(filter_view* __parent, _Vp_iter __current)
1178  : _M_current(std::move(__current)),
1179  _M_parent(__parent)
1180  { }
1181 
1182  constexpr _Vp_iter
1183  base() const &
1184  requires copyable<_Vp_iter>
1185  { return _M_current; }
1186 
1187  constexpr _Vp_iter
1188  base() &&
1189  { return std::move(_M_current); }
1190 
1191  constexpr range_reference_t<_Vp>
1192  operator*() const
1193  { return *_M_current; }
1194 
1195  constexpr _Vp_iter
1196  operator->() const
1197  requires __detail::__has_arrow<_Vp_iter>
1198  && copyable<_Vp_iter>
1199  { return _M_current; }
1200 
1201  constexpr _Iterator&
1202  operator++()
1203  {
1204  _M_current = __detail::find_if(std::move(++_M_current),
1205  ranges::end(_M_parent->_M_base),
1206  std::ref(*_M_parent->_M_pred));
1207  return *this;
1208  }
1209 
1210  constexpr void
1211  operator++(int)
1212  { ++*this; }
1213 
1214  constexpr _Iterator
1215  operator++(int) requires forward_range<_Vp>
1216  {
1217  auto __tmp = *this;
1218  ++*this;
1219  return __tmp;
1220  }
1221 
1222  constexpr _Iterator&
1223  operator--() requires bidirectional_range<_Vp>
1224  {
1225  do
1226  --_M_current;
1227  while (!std::__invoke(*_M_parent->_M_pred, *_M_current));
1228  return *this;
1229  }
1230 
1231  constexpr _Iterator
1232  operator--(int) requires bidirectional_range<_Vp>
1233  {
1234  auto __tmp = *this;
1235  --*this;
1236  return __tmp;
1237  }
1238 
1239  friend constexpr bool
1240  operator==(const _Iterator& __x, const _Iterator& __y)
1241  requires equality_comparable<_Vp_iter>
1242  { return __x._M_current == __y._M_current; }
1243 
1244  friend constexpr range_rvalue_reference_t<_Vp>
1245  iter_move(const _Iterator& __i)
1246  noexcept(noexcept(ranges::iter_move(__i._M_current)))
1247  { return ranges::iter_move(__i._M_current); }
1248 
1249  friend constexpr void
1250  iter_swap(const _Iterator& __x, const _Iterator& __y)
1251  noexcept(noexcept(ranges::iter_swap(__x._M_current, __y._M_current)))
1252  requires indirectly_swappable<_Vp_iter>
1253  { ranges::iter_swap(__x._M_current, __y._M_current); }
1254  };
1255 
1256  struct _Sentinel
1257  {
1258  private:
1259  sentinel_t<_Vp> _M_end = sentinel_t<_Vp>();
1260 
1261  constexpr bool
1262  __equal(const _Iterator& __i) const
1263  { return __i._M_current == _M_end; }
1264 
1265  public:
1266  _Sentinel() = default;
1267 
1268  constexpr explicit
1269  _Sentinel(filter_view* __parent)
1270  : _M_end(ranges::end(__parent->_M_base))
1271  { }
1272 
1273  constexpr sentinel_t<_Vp>
1274  base() const
1275  { return _M_end; }
1276 
1277  friend constexpr bool
1278  operator==(const _Iterator& __x, const _Sentinel& __y)
1279  { return __y.__equal(__x); }
1280  };
1281 
1282  [[no_unique_address]] __detail::__box<_Pred> _M_pred;
1283  [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
1284  _Vp _M_base = _Vp();
1285 
1286  public:
1287  filter_view() = default;
1288 
1289  constexpr
1290  filter_view(_Vp __base, _Pred __pred)
1291  : _M_pred(std::move(__pred)), _M_base(std::move(__base))
1292  { }
1293 
1294  constexpr _Vp
1295  base() const& requires copy_constructible<_Vp>
1296  { return _M_base; }
1297 
1298  constexpr _Vp
1299  base() &&
1300  { return std::move(_M_base); }
1301 
1302  constexpr const _Pred&
1303  pred() const
1304  { return *_M_pred; }
1305 
1306  constexpr _Iterator
1307  begin()
1308  {
1309  if (_M_cached_begin._M_has_value())
1310  return {this, _M_cached_begin._M_get(_M_base)};
1311 
1312  __glibcxx_assert(_M_pred.has_value());
1313  auto __it = __detail::find_if(ranges::begin(_M_base),
1314  ranges::end(_M_base),
1315  std::ref(*_M_pred));
1316  _M_cached_begin._M_set(_M_base, __it);
1317  return {this, std::move(__it)};
1318  }
1319 
1320  constexpr auto
1321  end()
1322  {
1323  if constexpr (common_range<_Vp>)
1324  return _Iterator{this, ranges::end(_M_base)};
1325  else
1326  return _Sentinel{this};
1327  }
1328  };
1329 
1330  template<typename _Range, typename _Pred>
1331  filter_view(_Range&&, _Pred) -> filter_view<views::all_t<_Range>, _Pred>;
1332 
1333  namespace views
1334  {
1335  namespace __detail
1336  {
1337  template<typename _Range, typename _Pred>
1338  concept __can_filter_view
1339  = requires { filter_view{std::declval<_Range>(), std::declval<_Pred>()}; };
1340  } // namespace __detail
1341 
1342  struct _Filter : __adaptor::_RangeAdaptor<_Filter>
1343  {
1344  template<viewable_range _Range, typename _Pred>
1345  requires __detail::__can_filter_view<_Range, _Pred>
1346  constexpr auto
1347  operator()(_Range&& __r, _Pred&& __p) const
1348  {
1349  return filter_view{std::forward<_Range>(__r), std::forward<_Pred>(__p)};
1350  }
1351 
1352  using _RangeAdaptor<_Filter>::operator();
1353  static constexpr int _S_arity = 2;
1354  };
1355 
1356  inline constexpr _Filter filter;
1357  } // namespace views
1358 
1359  template<input_range _Vp, copy_constructible _Fp>
1360  requires view<_Vp> && is_object_v<_Fp>
1361  && regular_invocable<_Fp&, range_reference_t<_Vp>>
1362  && std::__detail::__can_reference<invoke_result_t<_Fp&,
1363  range_reference_t<_Vp>>>
1364  class transform_view : public view_interface<transform_view<_Vp, _Fp>>
1365  {
1366  private:
1367  template<bool _Const>
1368  using _Base = __detail::__maybe_const_t<_Const, _Vp>;
1369 
1370  template<bool _Const>
1371  struct __iter_cat
1372  { };
1373 
1374  template<bool _Const>
1375  requires forward_range<_Base<_Const>>
1376  struct __iter_cat<_Const>
1377  {
1378  private:
1379  static auto
1380  _S_iter_cat()
1381  {
1382  using _Base = transform_view::_Base<_Const>;
1383  using _Res = invoke_result_t<_Fp&, range_reference_t<_Base>>;
1384  if constexpr (is_lvalue_reference_v<_Res>)
1385  {
1386  using _Cat
1387  = typename iterator_traits<iterator_t<_Base>>::iterator_category;
1388  if constexpr (derived_from<_Cat, contiguous_iterator_tag>)
1389  return random_access_iterator_tag{};
1390  else
1391  return _Cat{};
1392  }
1393  else
1394  return input_iterator_tag{};
1395  }
1396  public:
1397  using iterator_category = decltype(_S_iter_cat());
1398  };
1399 
1400  template<bool _Const>
1401  struct _Sentinel;
1402 
1403  template<bool _Const>
1404  struct _Iterator : __iter_cat<_Const>
1405  {
1406  private:
1407  using _Parent = __detail::__maybe_const_t<_Const, transform_view>;
1408  using _Base = transform_view::_Base<_Const>;
1409 
1410  static auto
1411  _S_iter_concept()
1412  {
1413  if constexpr (random_access_range<_Vp>)
1414  return random_access_iterator_tag{};
1415  else if constexpr (bidirectional_range<_Vp>)
1416  return bidirectional_iterator_tag{};
1417  else if constexpr (forward_range<_Vp>)
1418  return forward_iterator_tag{};
1419  else
1420  return input_iterator_tag{};
1421  }
1422 
1423  using _Base_iter = iterator_t<_Base>;
1424 
1425  _Base_iter _M_current = _Base_iter();
1426  _Parent* _M_parent = nullptr;
1427 
1428  public:
1429  using iterator_concept = decltype(_S_iter_concept());
1430  // iterator_category defined in __transform_view_iter_cat
1431  using value_type
1432  = remove_cvref_t<invoke_result_t<_Fp&, range_reference_t<_Base>>>;
1433  using difference_type = range_difference_t<_Base>;
1434 
1435  _Iterator() = default;
1436 
1437  constexpr
1438  _Iterator(_Parent* __parent, _Base_iter __current)
1439  : _M_current(std::move(__current)),
1440  _M_parent(__parent)
1441  { }
1442 
1443  constexpr
1444  _Iterator(_Iterator<!_Const> __i)
1445  requires _Const
1446  && convertible_to<iterator_t<_Vp>, _Base_iter>
1447  : _M_current(std::move(__i._M_current)), _M_parent(__i._M_parent)
1448  { }
1449 
1450  constexpr _Base_iter
1451  base() const &
1452  requires copyable<_Base_iter>
1453  { return _M_current; }
1454 
1455  constexpr _Base_iter
1456  base() &&
1457  { return std::move(_M_current); }
1458 
1459  constexpr decltype(auto)
1460  operator*() const
1461  noexcept(noexcept(std::__invoke(*_M_parent->_M_fun, *_M_current)))
1462  { return std::__invoke(*_M_parent->_M_fun, *_M_current); }
1463 
1464  constexpr _Iterator&
1465  operator++()
1466  {
1467  ++_M_current;
1468  return *this;
1469  }
1470 
1471  constexpr void
1472  operator++(int)
1473  { ++_M_current; }
1474 
1475  constexpr _Iterator
1476  operator++(int) requires forward_range<_Base>
1477  {
1478  auto __tmp = *this;
1479  ++*this;
1480  return __tmp;
1481  }
1482 
1483  constexpr _Iterator&
1484  operator--() requires bidirectional_range<_Base>
1485  {
1486  --_M_current;
1487  return *this;
1488  }
1489 
1490  constexpr _Iterator
1491  operator--(int) requires bidirectional_range<_Base>
1492  {
1493  auto __tmp = *this;
1494  --*this;
1495  return __tmp;
1496  }
1497 
1498  constexpr _Iterator&
1499  operator+=(difference_type __n) requires random_access_range<_Base>
1500  {
1501  _M_current += __n;
1502  return *this;
1503  }
1504 
1505  constexpr _Iterator&
1506  operator-=(difference_type __n) requires random_access_range<_Base>
1507  {
1508  _M_current -= __n;
1509  return *this;
1510  }
1511 
1512  constexpr decltype(auto)
1513  operator[](difference_type __n) const
1514  requires random_access_range<_Base>
1515  { return std::__invoke(*_M_parent->_M_fun, _M_current[__n]); }
1516 
1517  friend constexpr bool
1518  operator==(const _Iterator& __x, const _Iterator& __y)
1519  requires equality_comparable<_Base_iter>
1520  { return __x._M_current == __y._M_current; }
1521 
1522  friend constexpr bool
1523  operator<(const _Iterator& __x, const _Iterator& __y)
1524  requires random_access_range<_Base>
1525  { return __x._M_current < __y._M_current; }
1526 
1527  friend constexpr bool
1528  operator>(const _Iterator& __x, const _Iterator& __y)
1529  requires random_access_range<_Base>
1530  { return __y < __x; }
1531 
1532  friend constexpr bool
1533  operator<=(const _Iterator& __x, const _Iterator& __y)
1534  requires random_access_range<_Base>
1535  { return !(__y < __x); }
1536 
1537  friend constexpr bool
1538  operator>=(const _Iterator& __x, const _Iterator& __y)
1539  requires random_access_range<_Base>
1540  { return !(__x < __y); }
1541 
1542 #ifdef __cpp_lib_three_way_comparison
1543  friend constexpr auto
1544  operator<=>(const _Iterator& __x, const _Iterator& __y)
1545  requires random_access_range<_Base>
1546  && three_way_comparable<_Base_iter>
1547  { return __x._M_current <=> __y._M_current; }
1548 #endif
1549 
1550  friend constexpr _Iterator
1551  operator+(_Iterator __i, difference_type __n)
1552  requires random_access_range<_Base>
1553  { return {__i._M_parent, __i._M_current + __n}; }
1554 
1555  friend constexpr _Iterator
1556  operator+(difference_type __n, _Iterator __i)
1557  requires random_access_range<_Base>
1558  { return {__i._M_parent, __i._M_current + __n}; }
1559 
1560  friend constexpr _Iterator
1561  operator-(_Iterator __i, difference_type __n)
1562  requires random_access_range<_Base>
1563  { return {__i._M_parent, __i._M_current - __n}; }
1564 
1565  // _GLIBCXX_RESOLVE_LIB_DEFECTS
1566  // 3483. transform_view::iterator's difference is overconstrained
1567  friend constexpr difference_type
1568  operator-(const _Iterator& __x, const _Iterator& __y)
1569  requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
1570  { return __x._M_current - __y._M_current; }
1571 
1572  friend constexpr decltype(auto)
1573  iter_move(const _Iterator& __i) noexcept(noexcept(*__i))
1574  {
1575  if constexpr (is_lvalue_reference_v<decltype(*__i)>)
1576  return std::move(*__i);
1577  else
1578  return *__i;
1579  }
1580 
1581  friend _Iterator<!_Const>;
1582  template<bool> friend struct _Sentinel;
1583  };
1584 
1585  template<bool _Const>
1586  struct _Sentinel
1587  {
1588  private:
1589  using _Parent = __detail::__maybe_const_t<_Const, transform_view>;
1590  using _Base = transform_view::_Base<_Const>;
1591 
1592  template<bool _Const2>
1593  constexpr auto
1594  __distance_from(const _Iterator<_Const2>& __i) const
1595  { return _M_end - __i._M_current; }
1596 
1597  template<bool _Const2>
1598  constexpr bool
1599  __equal(const _Iterator<_Const2>& __i) const
1600  { return __i._M_current == _M_end; }
1601 
1602  sentinel_t<_Base> _M_end = sentinel_t<_Base>();
1603 
1604  public:
1605  _Sentinel() = default;
1606 
1607  constexpr explicit
1608  _Sentinel(sentinel_t<_Base> __end)
1609  : _M_end(__end)
1610  { }
1611 
1612  constexpr
1613  _Sentinel(_Sentinel<!_Const> __i)
1614  requires _Const
1615  && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
1616  : _M_end(std::move(__i._M_end))
1617  { }
1618 
1619  constexpr sentinel_t<_Base>
1620  base() const
1621  { return _M_end; }
1622 
1623  template<bool _Const2>
1624  requires sentinel_for<sentinel_t<_Base>,
1625  iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
1626  friend constexpr bool
1627  operator==(const _Iterator<_Const2>& __x, const _Sentinel& __y)
1628  { return __y.__equal(__x); }
1629 
1630  template<bool _Const2,
1631  typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
1632  requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
1633  friend constexpr range_difference_t<_Base2>
1634  operator-(const _Iterator<_Const2>& __x, const _Sentinel& __y)
1635  { return -__y.__distance_from(__x); }
1636 
1637  template<bool _Const2,
1638  typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
1639  requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
1640  friend constexpr range_difference_t<_Base2>
1641  operator-(const _Sentinel& __y, const _Iterator<_Const2>& __x)
1642  { return __y.__distance_from(__x); }
1643 
1644  friend _Sentinel<!_Const>;
1645  };
1646 
1647  [[no_unique_address]] __detail::__box<_Fp> _M_fun;
1648  _Vp _M_base = _Vp();
1649 
1650  public:
1651  transform_view() = default;
1652 
1653  constexpr
1654  transform_view(_Vp __base, _Fp __fun)
1655  : _M_fun(std::move(__fun)), _M_base(std::move(__base))
1656  { }
1657 
1658  constexpr _Vp
1659  base() const& requires copy_constructible<_Vp>
1660  { return _M_base ; }
1661 
1662  constexpr _Vp
1663  base() &&
1664  { return std::move(_M_base); }
1665 
1666  constexpr _Iterator<false>
1667  begin()
1668  { return _Iterator<false>{this, ranges::begin(_M_base)}; }
1669 
1670  constexpr _Iterator<true>
1671  begin() const
1672  requires range<const _Vp>
1673  && regular_invocable<const _Fp&, range_reference_t<const _Vp>>
1674  { return _Iterator<true>{this, ranges::begin(_M_base)}; }
1675 
1676  constexpr _Sentinel<false>
1677  end()
1678  { return _Sentinel<false>{ranges::end(_M_base)}; }
1679 
1680  constexpr _Iterator<false>
1681  end() requires common_range<_Vp>
1682  { return _Iterator<false>{this, ranges::end(_M_base)}; }
1683 
1684  constexpr _Sentinel<true>
1685  end() const
1686  requires range<const _Vp>
1687  && regular_invocable<const _Fp&, range_reference_t<const _Vp>>
1688  { return _Sentinel<true>{ranges::end(_M_base)}; }
1689 
1690  constexpr _Iterator<true>
1691  end() const
1692  requires common_range<const _Vp>
1693  && regular_invocable<const _Fp&, range_reference_t<const _Vp>>
1694  { return _Iterator<true>{this, ranges::end(_M_base)}; }
1695 
1696  constexpr auto
1697  size() requires sized_range<_Vp>
1698  { return ranges::size(_M_base); }
1699 
1700  constexpr auto
1701  size() const requires sized_range<const _Vp>
1702  { return ranges::size(_M_base); }
1703  };
1704 
1705  template<typename _Range, typename _Fp>
1706  transform_view(_Range&&, _Fp) -> transform_view<views::all_t<_Range>, _Fp>;
1707 
1708  namespace views
1709  {
1710  namespace __detail
1711  {
1712  template<typename _Range, typename _Fp>
1713  concept __can_transform_view
1714  = requires { transform_view{std::declval<_Range>(), std::declval<_Fp>()}; };
1715  } // namespace __detail
1716 
1717  struct _Transform : __adaptor::_RangeAdaptor<_Transform>
1718  {
1719  template<viewable_range _Range, typename _Fp>
1720  requires __detail::__can_transform_view<_Range, _Fp>
1721  constexpr auto
1722  operator()(_Range&& __r, _Fp&& __f) const
1723  {
1724  return transform_view{std::forward<_Range>(__r), std::forward<_Fp>(__f)};
1725  }
1726 
1727  using _RangeAdaptor<_Transform>::operator();
1728  static constexpr int _S_arity = 2;
1729  };
1730 
1731  inline constexpr _Transform transform;
1732  } // namespace views
1733 
1734  template<view _Vp>
1735  class take_view : public view_interface<take_view<_Vp>>
1736  {
1737  private:
1738  template<bool _Const>
1739  using _CI = counted_iterator<
1740  iterator_t<__detail::__maybe_const_t<_Const, _Vp>>>;
1741 
1742  template<bool _Const>
1743  struct _Sentinel
1744  {
1745  private:
1746  using _Base = __detail::__maybe_const_t<_Const, _Vp>;
1747  sentinel_t<_Base> _M_end = sentinel_t<_Base>();
1748 
1749  public:
1750  _Sentinel() = default;
1751 
1752  constexpr explicit
1753  _Sentinel(sentinel_t<_Base> __end)
1754  : _M_end(__end)
1755  { }
1756 
1757  constexpr
1758  _Sentinel(_Sentinel<!_Const> __s)
1759  requires _Const && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
1760  : _M_end(std::move(__s._M_end))
1761  { }
1762 
1763  constexpr sentinel_t<_Base>
1764  base() const
1765  { return _M_end; }
1766 
1767  friend constexpr bool
1768  operator==(const _CI<_Const>& __y, const _Sentinel& __x)
1769  { return __y.count() == 0 || __y.base() == __x._M_end; }
1770 
1771  template<bool _OtherConst = !_Const,
1772  typename _Base2 = __detail::__maybe_const_t<_OtherConst, _Vp>>
1773  requires sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
1774  friend constexpr bool
1775  operator==(const _CI<_OtherConst>& __y, const _Sentinel& __x)
1776  { return __y.count() == 0 || __y.base() == __x._M_end; }
1777 
1778  friend _Sentinel<!_Const>;
1779  };
1780 
1781  range_difference_t<_Vp> _M_count = 0;
1782  _Vp _M_base = _Vp();
1783 
1784  public:
1785  take_view() = default;
1786 
1787  constexpr
1788  take_view(_Vp base, range_difference_t<_Vp> __count)
1789  : _M_count(std::move(__count)), _M_base(std::move(base))
1790  { }
1791 
1792  constexpr _Vp
1793  base() const& requires copy_constructible<_Vp>
1794  { return _M_base; }
1795 
1796  constexpr _Vp
1797  base() &&
1798  { return std::move(_M_base); }
1799 
1800  constexpr auto
1801  begin() requires (!__detail::__simple_view<_Vp>)
1802  {
1803  if constexpr (sized_range<_Vp>)
1804  {
1805  if constexpr (random_access_range<_Vp>)
1806  return ranges::begin(_M_base);
1807  else
1808  {
1809  auto __sz = size();
1810  return counted_iterator{ranges::begin(_M_base), __sz};
1811  }
1812  }
1813  else
1814  return counted_iterator{ranges::begin(_M_base), _M_count};
1815  }
1816 
1817  constexpr auto
1818  begin() const requires range<const _Vp>
1819  {
1820  if constexpr (sized_range<const _Vp>)
1821  {
1822  if constexpr (random_access_range<const _Vp>)
1823  return ranges::begin(_M_base);
1824  else
1825  {
1826  auto __sz = size();
1827  return counted_iterator{ranges::begin(_M_base), __sz};
1828  }
1829  }
1830  else
1831  return counted_iterator{ranges::begin(_M_base), _M_count};
1832  }
1833 
1834  constexpr auto
1835  end() requires (!__detail::__simple_view<_Vp>)
1836  {
1837  if constexpr (sized_range<_Vp>)
1838  {
1839  if constexpr (random_access_range<_Vp>)
1840  return ranges::begin(_M_base) + size();
1841  else
1842  return default_sentinel;
1843  }
1844  else
1845  return _Sentinel<false>{ranges::end(_M_base)};
1846  }
1847 
1848  constexpr auto
1849  end() const requires range<const _Vp>
1850  {
1851  if constexpr (sized_range<const _Vp>)
1852  {
1853  if constexpr (random_access_range<const _Vp>)
1854  return ranges::begin(_M_base) + size();
1855  else
1856  return default_sentinel;
1857  }
1858  else
1859  return _Sentinel<true>{ranges::end(_M_base)};
1860  }
1861 
1862  constexpr auto
1863  size() requires sized_range<_Vp>
1864  {
1865  auto __n = ranges::size(_M_base);
1866  return std::min(__n, static_cast<decltype(__n)>(_M_count));
1867  }
1868 
1869  constexpr auto
1870  size() const requires sized_range<const _Vp>
1871  {
1872  auto __n = ranges::size(_M_base);
1873  return std::min(__n, static_cast<decltype(__n)>(_M_count));
1874  }
1875  };
1876 
1877  // _GLIBCXX_RESOLVE_LIB_DEFECTS
1878  // 3447. Deduction guides for take_view and drop_view have different
1879  // constraints
1880  template<typename _Range>
1881  take_view(_Range&&, range_difference_t<_Range>)
1882  -> take_view<views::all_t<_Range>>;
1883 
1884  template<typename _Tp>
1885  inline constexpr bool enable_borrowed_range<take_view<_Tp>>
1886  = enable_borrowed_range<_Tp>;
1887 
1888  namespace views
1889  {
1890  namespace __detail
1891  {
1892  template<typename _Range, typename _Tp>
1893  concept __can_take_view
1894  = requires { take_view{std::declval<_Range>(), std::declval<_Tp>()}; };
1895  } // namespace __detail
1896 
1897  struct _Take : __adaptor::_RangeAdaptor<_Take>
1898  {
1899  template<viewable_range _Range, typename _Tp>
1900  requires __detail::__can_take_view<_Range, _Tp>
1901  constexpr auto
1902  operator()(_Range&& __r, _Tp&& __n) const
1903  {
1904  return take_view{std::forward<_Range>(__r), std::forward<_Tp>(__n)};
1905  }
1906 
1907  using _RangeAdaptor<_Take>::operator();
1908  static constexpr int _S_arity = 2;
1909  };
1910 
1911  inline constexpr _Take take;
1912  } // namespace views
1913 
1914  template<view _Vp, typename _Pred>
1915  requires input_range<_Vp> && is_object_v<_Pred>
1916  && indirect_unary_predicate<const _Pred, iterator_t<_Vp>>
1917  class take_while_view : public view_interface<take_while_view<_Vp, _Pred>>
1918  {
1919  template<bool _Const>
1920  struct _Sentinel
1921  {
1922  private:
1923  using _Base = __detail::__maybe_const_t<_Const, _Vp>;
1924 
1925  sentinel_t<_Base> _M_end = sentinel_t<_Base>();
1926  const _Pred* _M_pred = nullptr;
1927 
1928  public:
1929  _Sentinel() = default;
1930 
1931  constexpr explicit
1932  _Sentinel(sentinel_t<_Base> __end, const _Pred* __pred)
1933  : _M_end(__end), _M_pred(__pred)
1934  { }
1935 
1936  constexpr
1937  _Sentinel(_Sentinel<!_Const> __s)
1938  requires _Const && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
1939  : _M_end(__s._M_end), _M_pred(__s._M_pred)
1940  { }
1941 
1942  constexpr sentinel_t<_Base>
1943  base() const { return _M_end; }
1944 
1945  friend constexpr bool
1946  operator==(const iterator_t<_Base>& __x, const _Sentinel& __y)
1947  { return __y._M_end == __x || !std::__invoke(*__y._M_pred, *__x); }
1948 
1949  template<bool _OtherConst = !_Const,
1950  typename _Base2 = __detail::__maybe_const_t<_OtherConst, _Vp>>
1951  requires sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
1952  friend constexpr bool
1953  operator==(const iterator_t<_Base2>& __x, const _Sentinel& __y)
1954  { return __y._M_end == __x || !std::__invoke(*__y._M_pred, *__x); }
1955 
1956  friend _Sentinel<!_Const>;
1957  };
1958 
1959  [[no_unique_address]] __detail::__box<_Pred> _M_pred;
1960  _Vp _M_base = _Vp();
1961 
1962  public:
1963  take_while_view() = default;
1964 
1965  constexpr
1966  take_while_view(_Vp base, _Pred __pred)
1967  : _M_pred(std::move(__pred)), _M_base(std::move(base))
1968  { }
1969 
1970  constexpr _Vp
1971  base() const& requires copy_constructible<_Vp>
1972  { return _M_base; }
1973 
1974  constexpr _Vp
1975  base() &&
1976  { return std::move(_M_base); }
1977 
1978  constexpr const _Pred&
1979  pred() const
1980  { return *_M_pred; }
1981 
1982  constexpr auto
1983  begin() requires (!__detail::__simple_view<_Vp>)
1984  { return ranges::begin(_M_base); }
1985 
1986  constexpr auto
1987  begin() const requires range<const _Vp>
1988  && indirect_unary_predicate<const _Pred, iterator_t<const _Vp>>
1989  { return ranges::begin(_M_base); }
1990 
1991  constexpr auto
1992  end() requires (!__detail::__simple_view<_Vp>)
1993  { return _Sentinel<false>(ranges::end(_M_base),
1994  std::__addressof(*_M_pred)); }
1995 
1996  constexpr auto
1997  end() const requires range<const _Vp>
1998  && indirect_unary_predicate<const _Pred, iterator_t<const _Vp>>
1999  { return _Sentinel<true>(ranges::end(_M_base),
2000  std::__addressof(*_M_pred)); }
2001  };
2002 
2003  template<typename _Range, typename _Pred>
2004  take_while_view(_Range&&, _Pred)
2005  -> take_while_view<views::all_t<_Range>, _Pred>;
2006 
2007  namespace views
2008  {
2009  namespace __detail
2010  {
2011  template<typename _Range, typename _Pred>
2012  concept __can_take_while_view
2013  = requires { take_while_view{std::declval<_Range>(), std::declval<_Pred>()}; };
2014  } // namespace __detail
2015 
2016  struct _TakeWhile : __adaptor::_RangeAdaptor<_TakeWhile>
2017  {
2018  template<viewable_range _Range, typename _Pred>
2019  requires __detail::__can_take_while_view<_Range, _Pred>
2020  constexpr auto
2021  operator()(_Range&& __r, _Pred&& __p) const
2022  {
2023  return take_while_view{std::forward<_Range>(__r), std::forward<_Pred>(__p)};
2024  }
2025 
2026  using _RangeAdaptor<_TakeWhile>::operator();
2027  static constexpr int _S_arity = 2;
2028  };
2029 
2030  inline constexpr _TakeWhile take_while;
2031  } // namespace views
2032 
2033  template<view _Vp>
2034  class drop_view : public view_interface<drop_view<_Vp>>
2035  {
2036  private:
2037  range_difference_t<_Vp> _M_count = 0;
2038  _Vp _M_base = _Vp();
2039 
2040  // ranges::next(begin(base), count, end(base)) is O(1) if _Vp satisfies
2041  // both random_access_range and sized_range. Otherwise, cache its result.
2042  static constexpr bool _S_needs_cached_begin
2043  = !(random_access_range<const _Vp> && sized_range<const _Vp>);
2044  [[no_unique_address]]
2045  __detail::__maybe_present_t<_S_needs_cached_begin,
2046  __detail::_CachedPosition<_Vp>>
2047  _M_cached_begin;
2048 
2049  public:
2050  drop_view() = default;
2051 
2052  constexpr
2053  drop_view(_Vp __base, range_difference_t<_Vp> __count)
2054  : _M_count(__count), _M_base(std::move(__base))
2055  { __glibcxx_assert(__count >= 0); }
2056 
2057  constexpr _Vp
2058  base() const& requires copy_constructible<_Vp>
2059  { return _M_base; }
2060 
2061  constexpr _Vp
2062  base() &&
2063  { return std::move(_M_base); }
2064 
2065  // This overload is disabled for simple views with constant-time begin().
2066  constexpr auto
2067  begin()
2068  requires (!(__detail::__simple_view<_Vp>
2069  && random_access_range<const _Vp>
2070  && sized_range<const _Vp>))
2071  {
2072  if constexpr (_S_needs_cached_begin)
2073  if (_M_cached_begin._M_has_value())
2074  return _M_cached_begin._M_get(_M_base);
2075 
2076  auto __it = ranges::next(ranges::begin(_M_base),
2077  _M_count, ranges::end(_M_base));
2078  if constexpr (_S_needs_cached_begin)
2079  _M_cached_begin._M_set(_M_base, __it);
2080  return __it;
2081  }
2082 
2083  // _GLIBCXX_RESOLVE_LIB_DEFECTS
2084  // 3482. drop_view's const begin should additionally require sized_range
2085  constexpr auto
2086  begin() const
2087  requires random_access_range<const _Vp> && sized_range<const _Vp>
2088  {
2089  return ranges::next(ranges::begin(_M_base), _M_count,
2090  ranges::end(_M_base));
2091  }
2092 
2093  constexpr auto
2094  end() requires (!__detail::__simple_view<_Vp>)
2095  { return ranges::end(_M_base); }
2096 
2097  constexpr auto
2098  end() const requires range<const _Vp>
2099  { return ranges::end(_M_base); }
2100 
2101  constexpr auto
2102  size() requires sized_range<_Vp>
2103  {
2104  const auto __s = ranges::size(_M_base);
2105  const auto __c = static_cast<decltype(__s)>(_M_count);
2106  return __s < __c ? 0 : __s - __c;
2107  }
2108 
2109  constexpr auto
2110  size() const requires sized_range<const _Vp>
2111  {
2112  const auto __s = ranges::size(_M_base);
2113  const auto __c = static_cast<decltype(__s)>(_M_count);
2114  return __s < __c ? 0 : __s - __c;
2115  }
2116  };
2117 
2118  template<typename _Range>
2119  drop_view(_Range&&, range_difference_t<_Range>)
2120  -> drop_view<views::all_t<_Range>>;
2121 
2122  template<typename _Tp>
2123  inline constexpr bool enable_borrowed_range<drop_view<_Tp>>
2124  = enable_borrowed_range<_Tp>;
2125 
2126  namespace views
2127  {
2128  namespace __detail
2129  {
2130  template<typename _Range, typename _Tp>
2131  concept __can_drop_view
2132  = requires { drop_view{std::declval<_Range>(), std::declval<_Tp>()}; };
2133  } // namespace __detail
2134 
2135  struct _Drop : __adaptor::_RangeAdaptor<_Drop>
2136  {
2137  template<viewable_range _Range, typename _Tp>
2138  requires __detail::__can_drop_view<_Range, _Tp>
2139  constexpr auto
2140  operator()(_Range&& __r, _Tp&& __n) const
2141  {
2142  return drop_view{std::forward<_Range>(__r), std::forward<_Tp>(__n)};
2143  }
2144 
2145  using _RangeAdaptor<_Drop>::operator();
2146  static constexpr int _S_arity = 2;
2147  };
2148 
2149  inline constexpr _Drop drop;
2150  } // namespace views
2151 
2152  template<view _Vp, typename _Pred>
2153  requires input_range<_Vp> && is_object_v<_Pred>
2154  && indirect_unary_predicate<const _Pred, iterator_t<_Vp>>
2155  class drop_while_view : public view_interface<drop_while_view<_Vp, _Pred>>
2156  {
2157  private:
2158  [[no_unique_address]] __detail::__box<_Pred> _M_pred;
2159  [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
2160  _Vp _M_base = _Vp();
2161 
2162  public:
2163  drop_while_view() = default;
2164 
2165  constexpr
2166  drop_while_view(_Vp __base, _Pred __pred)
2167  : _M_pred(std::move(__pred)), _M_base(std::move(__base))
2168  { }
2169 
2170  constexpr _Vp
2171  base() const& requires copy_constructible<_Vp>
2172  { return _M_base; }
2173 
2174  constexpr _Vp
2175  base() &&
2176  { return std::move(_M_base); }
2177 
2178  constexpr const _Pred&
2179  pred() const
2180  { return *_M_pred; }
2181 
2182  constexpr auto
2183  begin()
2184  {
2185  if (_M_cached_begin._M_has_value())
2186  return _M_cached_begin._M_get(_M_base);
2187 
2188  auto __it = __detail::find_if_not(ranges::begin(_M_base),
2189  ranges::end(_M_base),
2190  std::cref(*_M_pred));
2191  _M_cached_begin._M_set(_M_base, __it);
2192  return __it;
2193  }
2194 
2195  constexpr auto
2196  end()
2197  { return ranges::end(_M_base); }
2198  };
2199 
2200  template<typename _Range, typename _Pred>
2201  drop_while_view(_Range&&, _Pred)
2202  -> drop_while_view<views::all_t<_Range>, _Pred>;
2203 
2204  template<typename _Tp, typename _Pred>
2205  inline constexpr bool enable_borrowed_range<drop_while_view<_Tp, _Pred>>
2206  = enable_borrowed_range<_Tp>;
2207 
2208  namespace views
2209  {
2210  namespace __detail
2211  {
2212  template<typename _Range, typename _Pred>
2213  concept __can_drop_while_view
2214  = requires { drop_while_view{std::declval<_Range>(), std::declval<_Pred>()}; };
2215  } // namespace __detail
2216 
2217  struct _DropWhile : __adaptor::_RangeAdaptor<_DropWhile>
2218  {
2219  template<viewable_range _Range, typename _Pred>
2220  requires __detail::__can_drop_while_view<_Range, _Pred>
2221  constexpr auto
2222  operator()(_Range&& __r, _Pred&& __p) const
2223  {
2224  return drop_while_view{std::forward<_Range>(__r),
2225  std::forward<_Pred>(__p)};
2226  }
2227 
2228  using _RangeAdaptor<_DropWhile>::operator();
2229  static constexpr int _S_arity = 2;
2230  };
2231 
2232  inline constexpr _DropWhile drop_while;
2233  } // namespace views
2234 
2235  template<input_range _Vp>
2236  requires view<_Vp> && input_range<range_reference_t<_Vp>>
2237  && (is_reference_v<range_reference_t<_Vp>>
2238  || view<range_value_t<_Vp>>)
2239  class join_view : public view_interface<join_view<_Vp>>
2240  {
2241  private:
2242  using _InnerRange = range_reference_t<_Vp>;
2243 
2244  template<bool _Const>
2245  using _Base = __detail::__maybe_const_t<_Const, _Vp>;
2246 
2247  template<bool _Const>
2248  using _Outer_iter = iterator_t<_Base<_Const>>;
2249 
2250  template<bool _Const>
2251  using _Inner_iter = iterator_t<range_reference_t<_Base<_Const>>>;
2252 
2253  template<bool _Const>
2254  static constexpr bool _S_ref_is_glvalue
2255  = is_reference_v<range_reference_t<_Base<_Const>>>;
2256 
2257  template<bool _Const>
2258  struct __iter_cat
2259  { };
2260 
2261  template<bool _Const>
2262  requires _S_ref_is_glvalue<_Const>
2263  && forward_range<_Base<_Const>>
2264  && forward_range<range_reference_t<_Base<_Const>>>
2265  struct __iter_cat<_Const>
2266  {
2267  private:
2268  static constexpr auto
2269  _S_iter_cat()
2270  {
2271  using _Outer_iter = join_view::_Outer_iter<_Const>;
2272  using _Inner_iter = join_view::_Inner_iter<_Const>;
2273  using _OuterCat = typename iterator_traits<_Outer_iter>::iterator_category;
2274  using _InnerCat = typename iterator_traits<_Inner_iter>::iterator_category;
2275  if constexpr (derived_from<_OuterCat, bidirectional_iterator_tag>
2276  && derived_from<_InnerCat, bidirectional_iterator_tag>)
2277  return bidirectional_iterator_tag{};
2278  else if constexpr (derived_from<_OuterCat, forward_iterator_tag>
2279  && derived_from<_InnerCat, forward_iterator_tag>)
2280  return forward_iterator_tag{};
2281  else
2282  return input_iterator_tag{};
2283  }
2284  public:
2285  using iterator_category = decltype(_S_iter_cat());
2286  };
2287 
2288  template<bool _Const>
2289  struct _Sentinel;
2290 
2291  template<bool _Const>
2292  struct _Iterator : __iter_cat<_Const>
2293  {
2294  private:
2295  using _Parent = __detail::__maybe_const_t<_Const, join_view>;
2296  using _Base = join_view::_Base<_Const>;
2297 
2298  static constexpr bool _S_ref_is_glvalue
2299  = join_view::_S_ref_is_glvalue<_Const>;
2300 
2301  constexpr void
2302  _M_satisfy()
2303  {
2304  auto __update_inner = [this] (range_reference_t<_Base> __x) -> auto&
2305  {
2306  if constexpr (_S_ref_is_glvalue)
2307  return __x;
2308  else
2309  return (_M_parent->_M_inner = views::all(std::move(__x)));
2310  };
2311 
2312  for (; _M_outer != ranges::end(_M_parent->_M_base); ++_M_outer)
2313  {
2314  auto& __inner = __update_inner(*_M_outer);
2315  _M_inner = ranges::begin(__inner);
2316  if (_M_inner != ranges::end(__inner))
2317  return;
2318  }
2319 
2320  if constexpr (_S_ref_is_glvalue)
2321  _M_inner = _Inner_iter();
2322  }
2323 
2324  static constexpr auto
2325  _S_iter_concept()
2326  {
2327  if constexpr (_S_ref_is_glvalue
2328  && bidirectional_range<_Base>
2329  && bidirectional_range<range_reference_t<_Base>>)
2330  return bidirectional_iterator_tag{};
2331  else if constexpr (_S_ref_is_glvalue
2332  && forward_range<_Base>
2333  && forward_range<range_reference_t<_Base>>)
2334  return forward_iterator_tag{};
2335  else
2336  return input_iterator_tag{};
2337  }
2338 
2339  using _Outer_iter = join_view::_Outer_iter<_Const>;
2340  using _Inner_iter = join_view::_Inner_iter<_Const>;
2341 
2342  _Outer_iter _M_outer = _Outer_iter();
2343  _Inner_iter _M_inner = _Inner_iter();
2344  _Parent* _M_parent = nullptr;
2345 
2346  public:
2347  using iterator_concept = decltype(_S_iter_concept());
2348  // iterator_category defined in __join_view_iter_cat
2349  using value_type = range_value_t<range_reference_t<_Base>>;
2350  using difference_type
2351  = common_type_t<range_difference_t<_Base>,
2352  range_difference_t<range_reference_t<_Base>>>;
2353 
2354  _Iterator() = default;
2355 
2356  constexpr
2357  _Iterator(_Parent* __parent, _Outer_iter __outer)
2358  : _M_outer(std::move(__outer)),
2359  _M_parent(__parent)
2360  { _M_satisfy(); }
2361 
2362  constexpr
2363  _Iterator(_Iterator<!_Const> __i)
2364  requires _Const
2365  && convertible_to<iterator_t<_Vp>, _Outer_iter>
2366  && convertible_to<iterator_t<_InnerRange>, _Inner_iter>
2367  : _M_outer(std::move(__i._M_outer)), _M_inner(__i._M_inner),
2368  _M_parent(__i._M_parent)
2369  { }
2370 
2371  constexpr decltype(auto)
2372  operator*() const
2373  { return *_M_inner; }
2374 
2375  // _GLIBCXX_RESOLVE_LIB_DEFECTS
2376  // 3500. join_view::iterator::operator->() is bogus
2377  constexpr _Inner_iter
2378  operator->() const
2379  requires __detail::__has_arrow<_Inner_iter>
2380  && copyable<_Inner_iter>
2381  { return _M_inner; }
2382 
2383  constexpr _Iterator&
2384  operator++()
2385  {
2386  auto&& __inner_range = [this] () -> auto&& {
2387  if constexpr (_S_ref_is_glvalue)
2388  return *_M_outer;
2389  else
2390  return _M_parent->_M_inner;
2391  }();
2392  if (++_M_inner == ranges::end(__inner_range))
2393  {
2394  ++_M_outer;
2395  _M_satisfy();
2396  }
2397  return *this;
2398  }
2399 
2400  constexpr void
2401  operator++(int)
2402  { ++*this; }
2403 
2404  constexpr _Iterator
2405  operator++(int)
2406  requires _S_ref_is_glvalue && forward_range<_Base>
2407  && forward_range<range_reference_t<_Base>>
2408  {
2409  auto __tmp = *this;
2410  ++*this;
2411  return __tmp;
2412  }
2413 
2414  constexpr _Iterator&
2415  operator--()
2416  requires _S_ref_is_glvalue && bidirectional_range<_Base>
2417  && bidirectional_range<range_reference_t<_Base>>
2418  && common_range<range_reference_t<_Base>>
2419  {
2420  if (_M_outer == ranges::end(_M_parent->_M_base))
2421  _M_inner = ranges::end(*--_M_outer);
2422  while (_M_inner == ranges::begin(*_M_outer))
2423  _M_inner = ranges::end(*--_M_outer);
2424  --_M_inner;
2425  return *this;
2426  }
2427 
2428  constexpr _Iterator
2429  operator--(int)
2430  requires _S_ref_is_glvalue && bidirectional_range<_Base>
2431  && bidirectional_range<range_reference_t<_Base>>
2432  && common_range<range_reference_t<_Base>>
2433  {
2434  auto __tmp = *this;
2435  --*this;
2436  return __tmp;
2437  }
2438 
2439  friend constexpr bool
2440  operator==(const _Iterator& __x, const _Iterator& __y)
2441  requires _S_ref_is_glvalue
2442  && equality_comparable<_Outer_iter>
2443  && equality_comparable<_Inner_iter>
2444  {
2445  return (__x._M_outer == __y._M_outer
2446  && __x._M_inner == __y._M_inner);
2447  }
2448 
2449  friend constexpr decltype(auto)
2450  iter_move(const _Iterator& __i)
2451  noexcept(noexcept(ranges::iter_move(__i._M_inner)))
2452  { return ranges::iter_move(__i._M_inner); }
2453 
2454  friend constexpr void
2455  iter_swap(const _Iterator& __x, const _Iterator& __y)
2456  noexcept(noexcept(ranges::iter_swap(__x._M_inner, __y._M_inner)))
2457  requires indirectly_swappable<_Inner_iter>
2458  { return ranges::iter_swap(__x._M_inner, __y._M_inner); }
2459 
2460  friend _Iterator<!_Const>;
2461  template<bool> friend struct _Sentinel;
2462  };
2463 
2464  template<bool _Const>
2465  struct _Sentinel
2466  {
2467  private:
2468  using _Parent = __detail::__maybe_const_t<_Const, join_view>;
2469  using _Base = join_view::_Base<_Const>;
2470 
2471  template<bool _Const2>
2472  constexpr bool
2473  __equal(const _Iterator<_Const2>& __i) const
2474  { return __i._M_outer == _M_end; }
2475 
2476  sentinel_t<_Base> _M_end = sentinel_t<_Base>();
2477 
2478  public:
2479  _Sentinel() = default;
2480 
2481  constexpr explicit
2482  _Sentinel(_Parent* __parent)
2483  : _M_end(ranges::end(__parent->_M_base))
2484  { }
2485 
2486  constexpr
2487  _Sentinel(_Sentinel<!_Const> __s)
2488  requires _Const && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
2489  : _M_end(std::move(__s._M_end))
2490  { }
2491 
2492  template<bool _Const2>
2493  requires sentinel_for<sentinel_t<_Base>,
2494  iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
2495  friend constexpr bool
2496  operator==(const _Iterator<_Const2>& __x, const _Sentinel& __y)
2497  { return __y.__equal(__x); }
2498 
2499  friend _Sentinel<!_Const>;
2500  };
2501 
2502  // XXX: _M_inner is "present only when !is_reference_v<_InnerRange>"
2503  [[no_unique_address]]
2504  __detail::__maybe_present_t<!is_reference_v<_InnerRange>,
2505  views::all_t<_InnerRange>> _M_inner;
2506  _Vp _M_base = _Vp();
2507 
2508  public:
2509  join_view() = default;
2510 
2511  constexpr explicit
2512  join_view(_Vp __base)
2513  : _M_base(std::move(__base))
2514  { }
2515 
2516  constexpr _Vp
2517  base() const& requires copy_constructible<_Vp>
2518  { return _M_base; }
2519 
2520  constexpr _Vp
2521  base() &&
2522  { return std::move(_M_base); }
2523 
2524  constexpr auto
2525  begin()
2526  {
2527  constexpr bool __use_const
2528  = (__detail::__simple_view<_Vp>
2529  && is_reference_v<range_reference_t<_Vp>>);
2530  return _Iterator<__use_const>{this, ranges::begin(_M_base)};
2531  }
2532 
2533  constexpr auto
2534  begin() const
2535  requires input_range<const _Vp>
2536  && is_reference_v<range_reference_t<const _Vp>>
2537  {
2538  return _Iterator<true>{this, ranges::begin(_M_base)};
2539  }
2540 
2541  constexpr auto
2542  end()
2543  {
2544  if constexpr (forward_range<_Vp> && is_reference_v<_InnerRange>
2545  && forward_range<_InnerRange>
2546  && common_range<_Vp> && common_range<_InnerRange>)
2547  return _Iterator<__detail::__simple_view<_Vp>>{this,
2548  ranges::end(_M_base)};
2549  else
2550  return _Sentinel<__detail::__simple_view<_Vp>>{this};
2551  }
2552 
2553  constexpr auto
2554  end() const
2555  requires input_range<const _Vp>
2556  && is_reference_v<range_reference_t<const _Vp>>
2557  {
2558  if constexpr (forward_range<const _Vp>
2559  && is_reference_v<range_reference_t<const _Vp>>
2560  && forward_range<range_reference_t<const _Vp>>
2561  && common_range<const _Vp>
2562  && common_range<range_reference_t<const _Vp>>)
2563  return _Iterator<true>{this, ranges::end(_M_base)};
2564  else
2565  return _Sentinel<true>{this};
2566  }
2567  };
2568 
2569  template<typename _Range>
2570  explicit join_view(_Range&&) -> join_view<views::all_t<_Range>>;
2571 
2572  namespace views
2573  {
2574  namespace __detail
2575  {
2576  template<typename _Range>
2577  concept __can_join_view
2578  = requires { join_view<all_t<_Range>>{std::declval<_Range>()}; };
2579  } // namespace __detail
2580 
2581  struct _Join : __adaptor::_RangeAdaptorClosure
2582  {
2583  template<viewable_range _Range>
2584  requires __detail::__can_join_view<_Range>
2585  constexpr auto
2586  operator()(_Range&& __r) const
2587  {
2588  // _GLIBCXX_RESOLVE_LIB_DEFECTS
2589  // 3474. Nesting join_views is broken because of CTAD
2590  return join_view<all_t<_Range>>{std::forward<_Range>(__r)};
2591  }
2592  };
2593 
2594  inline constexpr _Join join;
2595  } // namespace views
2596 
2597  namespace __detail
2598  {
2599  template<auto>
2600  struct __require_constant;
2601 
2602  template<typename _Range>
2603  concept __tiny_range = sized_range<_Range>
2604  && requires
2605  { typename __require_constant<remove_reference_t<_Range>::size()>; }
2606  && (remove_reference_t<_Range>::size() <= 1);
2607 
2608  template<typename _Base>
2609  struct __split_view_outer_iter_cat
2610  { };
2611 
2612  template<forward_range _Base>
2613  struct __split_view_outer_iter_cat<_Base>
2614  { using iterator_category = input_iterator_tag; };
2615 
2616  template<typename _Base>
2617  struct __split_view_inner_iter_cat
2618  { };
2619 
2620  template<forward_range _Base>
2621  struct __split_view_inner_iter_cat<_Base>
2622  {
2623  private:
2624  static constexpr auto
2625  _S_iter_cat()
2626  {
2627  using _Cat = typename iterator_traits<iterator_t<_Base>>::iterator_category;
2628  if constexpr (derived_from<_Cat, forward_iterator_tag>)
2629  return forward_iterator_tag{};
2630  else
2631  return _Cat{};
2632  }
2633  public:
2634  using iterator_category = decltype(_S_iter_cat());
2635  };
2636  }
2637 
2638  template<input_range _Vp, forward_range _Pattern>
2639  requires view<_Vp> && view<_Pattern>
2640  && indirectly_comparable<iterator_t<_Vp>, iterator_t<_Pattern>,
2641  ranges::equal_to>
2642  && (forward_range<_Vp> || __detail::__tiny_range<_Pattern>)
2643  class split_view : public view_interface<split_view<_Vp, _Pattern>>
2644  {
2645  private:
2646  template<bool _Const>
2647  using _Base = __detail::__maybe_const_t<_Const, _Vp>;
2648 
2649  template<bool _Const>
2650  struct _InnerIter;
2651 
2652  template<bool _Const>
2653  struct _OuterIter
2654  : __detail::__split_view_outer_iter_cat<_Base<_Const>>
2655  {
2656  private:
2657  using _Parent = __detail::__maybe_const_t<_Const, split_view>;
2658  using _Base = split_view::_Base<_Const>;
2659 
2660  constexpr bool
2661  __at_end() const
2662  { return __current() == ranges::end(_M_parent->_M_base); }
2663 
2664  // [range.split.outer] p1
2665  // Many of the following specifications refer to the notional member
2666  // current of outer-iterator. current is equivalent to current_ if
2667  // V models forward_range, and parent_->current_ otherwise.
2668  constexpr auto&
2669  __current() noexcept
2670  {
2671  if constexpr (forward_range<_Vp>)
2672  return _M_current;
2673  else
2674  return _M_parent->_M_current;
2675  }
2676 
2677  constexpr auto&
2678  __current() const noexcept
2679  {
2680  if constexpr (forward_range<_Vp>)
2681  return _M_current;
2682  else
2683  return _M_parent->_M_current;
2684  }
2685 
2686  _Parent* _M_parent = nullptr;
2687 
2688  // XXX: _M_current is present only if "V models forward_range"
2689  [[no_unique_address]]
2690  __detail::__maybe_present_t<forward_range<_Vp>,
2691  iterator_t<_Base>> _M_current;
2692 
2693  public:
2694  using iterator_concept = conditional_t<forward_range<_Base>,
2695  forward_iterator_tag,
2696  input_iterator_tag>;
2697  // iterator_category defined in __split_view_outer_iter_cat
2698  using difference_type = range_difference_t<_Base>;
2699 
2700  struct value_type : view_interface<value_type>
2701  {
2702  private:
2703  _OuterIter _M_i = _OuterIter();
2704 
2705  public:
2706  value_type() = default;
2707 
2708  constexpr explicit
2709  value_type(_OuterIter __i)
2710  : _M_i(std::move(__i))
2711  { }
2712 
2713  constexpr _InnerIter<_Const>
2714  begin() const
2715  requires copyable<_OuterIter>
2716  { return _InnerIter<_Const>{_M_i}; }
2717 
2718  constexpr _InnerIter<_Const>
2719  begin()
2720  requires (!copyable<_OuterIter>)
2721  { return _InnerIter<_Const>{std::move(_M_i)}; }
2722 
2723  constexpr default_sentinel_t
2724  end() const
2725  { return default_sentinel; }
2726  };
2727 
2728  _OuterIter() = default;
2729 
2730  constexpr explicit
2731  _OuterIter(_Parent* __parent) requires (!forward_range<_Base>)
2732  : _M_parent(__parent)
2733  { }
2734 
2735  constexpr
2736  _OuterIter(_Parent* __parent, iterator_t<_Base> __current)
2737  requires forward_range<_Base>
2738  : _M_parent(__parent),
2739  _M_current(std::move(__current))
2740  { }
2741 
2742  constexpr
2743  _OuterIter(_OuterIter<!_Const> __i)
2744  requires _Const
2745  && convertible_to<iterator_t<_Vp>, iterator_t<_Base>>
2746  : _M_parent(__i._M_parent), _M_current(std::move(__i._M_current))
2747  { }
2748 
2749  constexpr value_type
2750  operator*() const
2751  { return value_type{*this}; }
2752 
2753  constexpr _OuterIter&
2754  operator++()
2755  {
2756  // _GLIBCXX_RESOLVE_LIB_DEFECTS
2757  // 3505. split_view::outer-iterator::operator++ misspecified
2758  const auto __end = ranges::end(_M_parent->_M_base);
2759  if (__current() == __end)
2760  return *this;
2761  const auto [__pbegin, __pend] = subrange{_M_parent->_M_pattern};
2762  if (__pbegin == __pend)
2763  ++__current();
2764  else if constexpr (__detail::__tiny_range<_Pattern>)
2765  {
2766  __current() = __detail::find(std::move(__current()), __end,
2767  *__pbegin);
2768  if (__current() != __end)
2769  ++__current();
2770  }
2771  else
2772  do
2773  {
2774  auto [__b, __p]
2775  = __detail::mismatch(__current(), __end, __pbegin, __pend);
2776  if (__p == __pend)
2777  {
2778  __current() = __b;
2779  break;
2780  }
2781  } while (++__current() != __end);
2782  return *this;
2783  }
2784 
2785  constexpr decltype(auto)
2786  operator++(int)
2787  {
2788  if constexpr (forward_range<_Base>)
2789  {
2790  auto __tmp = *this;
2791  ++*this;
2792  return __tmp;
2793  }
2794  else
2795  ++*this;
2796  }
2797 
2798  friend constexpr bool
2799  operator==(const _OuterIter& __x, const _OuterIter& __y)
2800  requires forward_range<_Base>
2801  { return __x._M_current == __y._M_current; }
2802 
2803  friend constexpr bool
2804  operator==(const _OuterIter& __x, default_sentinel_t)
2805  { return __x.__at_end(); };
2806 
2807  friend _OuterIter<!_Const>;
2808  friend _InnerIter<_Const>;
2809  };
2810 
2811  template<bool _Const>
2812  struct _InnerIter
2813  : __detail::__split_view_inner_iter_cat<_Base<_Const>>
2814  {
2815  private:
2816  using _Base = split_view::_Base<_Const>;
2817 
2818  constexpr bool
2819  __at_end() const
2820  {
2821  auto [__pcur, __pend] = subrange{_M_i._M_parent->_M_pattern};
2822  auto __end = ranges::end(_M_i._M_parent->_M_base);
2823  if constexpr (__detail::__tiny_range<_Pattern>)
2824  {
2825  const auto& __cur = _M_i_current();
2826  if (__cur == __end)
2827  return true;
2828  if (__pcur == __pend)
2829  return _M_incremented;
2830  return *__cur == *__pcur;
2831  }
2832  else
2833  {
2834  auto __cur = _M_i_current();
2835  if (__cur == __end)
2836  return true;
2837  if (__pcur == __pend)
2838  return _M_incremented;
2839  do
2840  {
2841  if (*__cur != *__pcur)
2842  return false;
2843  if (++__pcur == __pend)
2844  return true;
2845  } while (++__cur != __end);
2846  return false;
2847  }
2848  }
2849 
2850  constexpr auto&
2851  _M_i_current() noexcept
2852  { return _M_i.__current(); }
2853 
2854  constexpr auto&
2855  _M_i_current() const noexcept
2856  { return _M_i.__current(); }
2857 
2858  _OuterIter<_Const> _M_i = _OuterIter<_Const>();
2859  bool _M_incremented = false;
2860 
2861  public:
2862  using iterator_concept
2863  = typename _OuterIter<_Const>::iterator_concept;
2864  // iterator_category defined in __split_view_inner_iter_cat
2865  using value_type = range_value_t<_Base>;
2866  using difference_type = range_difference_t<_Base>;
2867 
2868  _InnerIter() = default;
2869 
2870  constexpr explicit
2871  _InnerIter(_OuterIter<_Const> __i)
2872  : _M_i(std::move(__i))
2873  { }
2874 
2875  constexpr decltype(auto)
2876  operator*() const
2877  { return *_M_i_current(); }
2878 
2879  constexpr _InnerIter&
2880  operator++()
2881  {
2882  _M_incremented = true;
2883  if constexpr (!forward_range<_Base>)
2884  if constexpr (_Pattern::size() == 0)
2885  return *this;
2886  ++_M_i_current();
2887  return *this;
2888  }
2889 
2890  constexpr decltype(auto)
2891  operator++(int)
2892  {
2893  if constexpr (forward_range<_Base>)
2894  {
2895  auto __tmp = *this;
2896  ++*this;
2897  return __tmp;
2898  }
2899  else
2900  ++*this;
2901  }
2902 
2903  friend constexpr bool
2904  operator==(const _InnerIter& __x, const _InnerIter& __y)
2905  requires forward_range<_Base>
2906  { return __x._M_i == __y._M_i; }
2907 
2908  friend constexpr bool
2909  operator==(const _InnerIter& __x, default_sentinel_t)
2910  { return __x.__at_end(); }
2911 
2912  friend constexpr decltype(auto)
2913  iter_move(const _InnerIter& __i)
2914  noexcept(noexcept(ranges::iter_move(__i._M_i_current())))
2915  { return ranges::iter_move(__i._M_i_current()); }
2916 
2917  friend constexpr void
2918  iter_swap(const _InnerIter& __x, const _InnerIter& __y)
2919  noexcept(noexcept(ranges::iter_swap(__x._M_i_current(),
2920  __y._M_i_current())))
2921  requires indirectly_swappable<iterator_t<_Base>>
2922  { ranges::iter_swap(__x._M_i_current(), __y._M_i_current()); }
2923  };
2924 
2925  _Pattern _M_pattern = _Pattern();
2926  // XXX: _M_current is "present only if !forward_range<V>"
2927  [[no_unique_address]]
2928  __detail::__maybe_present_t<!forward_range<_Vp>,
2929  iterator_t<_Vp>> _M_current;
2930  _Vp _M_base = _Vp();
2931 
2932 
2933  public:
2934  split_view() = default;
2935 
2936  constexpr
2937  split_view(_Vp __base, _Pattern __pattern)
2938  : _M_pattern(std::move(__pattern)), _M_base(std::move(__base))
2939  { }
2940 
2941  template<input_range _Range>
2942  requires constructible_from<_Vp, views::all_t<_Range>>
2943  && constructible_from<_Pattern, single_view<range_value_t<_Range>>>
2944  constexpr
2945  split_view(_Range&& __r, range_value_t<_Range> __e)
2946  : _M_pattern(std::move(__e)),
2947  _M_base(views::all(std::forward<_Range>(__r)))
2948  { }
2949 
2950  constexpr _Vp
2951  base() const& requires copy_constructible<_Vp>
2952  { return _M_base; }
2953 
2954  constexpr _Vp
2955  base() &&
2956  { return std::move(_M_base); }
2957 
2958  constexpr auto
2959  begin()
2960  {
2961  if constexpr (forward_range<_Vp>)
2962  return _OuterIter<__detail::__simple_view<_Vp>>{
2963  this, ranges::begin(_M_base)};
2964  else
2965  {
2966  _M_current = ranges::begin(_M_base);
2967  return _OuterIter<false>{this};
2968  }
2969  }
2970 
2971  constexpr auto
2972  begin() const requires forward_range<_Vp> && forward_range<const _Vp>
2973  {
2974  return _OuterIter<true>{this, ranges::begin(_M_base)};
2975  }
2976 
2977  constexpr auto
2978  end() requires forward_range<_Vp> && common_range<_Vp>
2979  {
2980  return _OuterIter<__detail::__simple_view<_Vp>>{
2981  this, ranges::end(_M_base)};
2982  }
2983 
2984  constexpr auto
2985  end() const
2986  {
2987  if constexpr (forward_range<_Vp>
2988  && forward_range<const _Vp>
2989  && common_range<const _Vp>)
2990  return _OuterIter<true>{this, ranges::end(_M_base)};
2991  else
2992  return default_sentinel;
2993  }
2994  };
2995 
2996  template<typename _Range, typename _Pattern>
2997  split_view(_Range&&, _Pattern&&)
2998  -> split_view<views::all_t<_Range>, views::all_t<_Pattern>>;
2999 
3000  template<input_range _Range>
3001  split_view(_Range&&, range_value_t<_Range>)
3002  -> split_view<views::all_t<_Range>, single_view<range_value_t<_Range>>>;
3003 
3004  namespace views
3005  {
3006  namespace __detail
3007  {
3008  template<typename _Range, typename _Pattern>
3009  concept __can_split_view
3010  = requires { split_view{std::declval<_Range>(), std::declval<_Pattern>()}; };
3011  } // namespace __detail
3012 
3013  struct _Split : __adaptor::_RangeAdaptor<_Split>
3014  {
3015  template<viewable_range _Range, typename _Pattern>
3016  requires __detail::__can_split_view<_Range, _Pattern>
3017  constexpr auto
3018  operator()(_Range&& __r, _Pattern&& __f) const
3019  {
3020  return split_view{std::forward<_Range>(__r), std::forward<_Pattern>(__f)};
3021  }
3022 
3023  using _RangeAdaptor<_Split>::operator();
3024  static constexpr int _S_arity = 2;
3025  };
3026 
3027  inline constexpr _Split split;
3028  } // namespace views
3029 
3030  namespace views
3031  {
3032  struct _Counted
3033  {
3034  template<input_or_output_iterator _Iter>
3035  constexpr auto
3036  operator()(_Iter __i, iter_difference_t<_Iter> __n) const
3037  {
3038  if constexpr (random_access_iterator<_Iter>)
3039  return subrange{__i, __i + __n};
3040  else
3041  return subrange{counted_iterator{std::move(__i), __n},
3042  default_sentinel};
3043  }
3044  };
3045 
3046  inline constexpr _Counted counted{};
3047  } // namespace views
3048 
3049  template<view _Vp>
3050  requires (!common_range<_Vp>) && copyable<iterator_t<_Vp>>
3051  class common_view : public view_interface<common_view<_Vp>>
3052  {
3053  private:
3054  _Vp _M_base = _Vp();
3055 
3056  public:
3057  common_view() = default;
3058 
3059  constexpr explicit
3060  common_view(_Vp __r)
3061  : _M_base(std::move(__r))
3062  { }
3063 
3064  /* XXX: LWG 3280 didn't remove this constructor, but I think it should?
3065  template<viewable_range _Range>
3066  requires (!common_range<_Range>)
3067  && constructible_from<_Vp, views::all_t<_Range>>
3068  constexpr explicit
3069  common_view(_Range&& __r)
3070  : _M_base(views::all(std::forward<_Range>(__r)))
3071  { }
3072  */
3073 
3074  constexpr _Vp
3075  base() const& requires copy_constructible<_Vp>
3076  { return _M_base; }
3077 
3078  constexpr _Vp
3079  base() &&
3080  { return std::move(_M_base); }
3081 
3082  constexpr auto
3083  begin()
3084  {
3085  if constexpr (random_access_range<_Vp> && sized_range<_Vp>)
3086  return ranges::begin(_M_base);
3087  else
3088  return common_iterator<iterator_t<_Vp>, sentinel_t<_Vp>>
3089  (ranges::begin(_M_base));
3090  }
3091 
3092  constexpr auto
3093  begin() const requires range<const _Vp>
3094  {
3095  if constexpr (random_access_range<const _Vp> && sized_range<const _Vp>)
3096  return ranges::begin(_M_base);
3097  else
3098  return common_iterator<iterator_t<const _Vp>, sentinel_t<const _Vp>>
3099  (ranges::begin(_M_base));
3100  }
3101 
3102  constexpr auto
3103  end()
3104  {
3105  if constexpr (random_access_range<_Vp> && sized_range<_Vp>)
3106  return ranges::begin(_M_base) + ranges::size(_M_base);
3107  else
3108  return common_iterator<iterator_t<_Vp>, sentinel_t<_Vp>>
3109  (ranges::end(_M_base));
3110  }
3111 
3112  constexpr auto
3113  end() const requires range<const _Vp>
3114  {
3115  if constexpr (random_access_range<const _Vp> && sized_range<const _Vp>)
3116  return ranges::begin(_M_base) + ranges::size(_M_base);
3117  else
3118  return common_iterator<iterator_t<const _Vp>, sentinel_t<const _Vp>>
3119  (ranges::end(_M_base));
3120  }
3121 
3122  constexpr auto
3123  size() requires sized_range<_Vp>
3124  { return ranges::size(_M_base); }
3125 
3126  constexpr auto
3127  size() const requires sized_range<const _Vp>
3128  { return ranges::size(_M_base); }
3129  };
3130 
3131  template<typename _Range>
3132  common_view(_Range&&) -> common_view<views::all_t<_Range>>;
3133 
3134  template<typename _Tp>
3135  inline constexpr bool enable_borrowed_range<common_view<_Tp>>
3136  = enable_borrowed_range<_Tp>;
3137 
3138  namespace views
3139  {
3140  namespace __detail
3141  {
3142  template<typename _Range>
3143  concept __already_common = common_range<_Range>
3144  && requires { views::all(std::declval<_Range>()); };
3145 
3146  template<typename _Range>
3147  concept __can_common_view
3148  = requires { common_view{std::declval<_Range>()}; };
3149  } // namespace __detail
3150 
3151  struct _Common : __adaptor::_RangeAdaptorClosure
3152  {
3153  template<viewable_range _Range>
3154  requires __detail::__already_common<_Range>
3155  || __detail::__can_common_view<_Range>
3156  constexpr auto
3157  operator()(_Range&& __r) const
3158  {
3159  if constexpr (__detail::__already_common<_Range>)
3160  return views::all(std::forward<_Range>(__r));
3161  else
3162  return common_view{std::forward<_Range>(__r)};
3163  }
3164  };
3165 
3166  inline constexpr _Common common;
3167  } // namespace views
3168 
3169  template<view _Vp>
3170  requires bidirectional_range<_Vp>
3171  class reverse_view : public view_interface<reverse_view<_Vp>>
3172  {
3173  private:
3174  static constexpr bool _S_needs_cached_begin
3175  = !common_range<_Vp> && !random_access_range<_Vp>;
3176 
3177  [[no_unique_address]]
3178  __detail::__maybe_present_t<_S_needs_cached_begin,
3179  __detail::_CachedPosition<_Vp>>
3180  _M_cached_begin;
3181  _Vp _M_base = _Vp();
3182 
3183  public:
3184  reverse_view() = default;
3185 
3186  constexpr explicit
3187  reverse_view(_Vp __r)
3188  : _M_base(std::move(__r))
3189  { }
3190 
3191  constexpr _Vp
3192  base() const& requires copy_constructible<_Vp>
3193  { return _M_base; }
3194 
3195  constexpr _Vp
3196  base() &&
3197  { return std::move(_M_base); }
3198 
3199  constexpr reverse_iterator<iterator_t<_Vp>>
3200  begin()
3201  {
3202  if constexpr (_S_needs_cached_begin)
3203  if (_M_cached_begin._M_has_value())
3204  return std::make_reverse_iterator(_M_cached_begin._M_get(_M_base));
3205 
3206  auto __it = ranges::next(ranges::begin(_M_base), ranges::end(_M_base));
3207  if constexpr (_S_needs_cached_begin)
3208  _M_cached_begin._M_set(_M_base, __it);
3209  return std::make_reverse_iterator(std::move(__it));
3210  }
3211 
3212  constexpr auto
3213  begin() requires common_range<_Vp>
3214  { return std::make_reverse_iterator(ranges::end(_M_base)); }
3215 
3216  constexpr auto
3217  begin() const requires common_range<const _Vp>
3218  { return std::make_reverse_iterator(ranges::end(_M_base)); }
3219 
3220  constexpr reverse_iterator<iterator_t<_Vp>>
3221  end()
3222  { return std::make_reverse_iterator(ranges::begin(_M_base)); }
3223 
3224  constexpr auto
3225  end() const requires common_range<const _Vp>
3226  { return std::make_reverse_iterator(ranges::begin(_M_base)); }
3227 
3228  constexpr auto
3229  size() requires sized_range<_Vp>
3230  { return ranges::size(_M_base); }
3231 
3232  constexpr auto
3233  size() const requires sized_range<const _Vp>
3234  { return ranges::size(_M_base); }
3235  };
3236 
3237  template<typename _Range>
3238  reverse_view(_Range&&) -> reverse_view<views::all_t<_Range>>;
3239 
3240  template<typename _Tp>
3241  inline constexpr bool enable_borrowed_range<reverse_view<_Tp>>
3242  = enable_borrowed_range<_Tp>;
3243 
3244  namespace views
3245  {
3246  namespace __detail
3247  {
3248  template<typename>
3249  inline constexpr bool __is_reversible_subrange = false;
3250 
3251  template<typename _Iter, subrange_kind _Kind>
3252  inline constexpr bool
3253  __is_reversible_subrange<subrange<reverse_iterator<_Iter>,
3254  reverse_iterator<_Iter>,
3255  _Kind>> = true;
3256 
3257  template<typename>
3258  inline constexpr bool __is_reverse_view = false;
3259 
3260  template<typename _Vp>
3261  inline constexpr bool __is_reverse_view<reverse_view<_Vp>> = true;
3262 
3263  template<typename _Range>
3264  concept __can_reverse_view
3265  = requires { reverse_view{std::declval<_Range>()}; };
3266  } // namespace __detail
3267 
3268  struct _Reverse : __adaptor::_RangeAdaptorClosure
3269  {
3270  template<viewable_range _Range>
3271  requires __detail::__is_reverse_view<remove_cvref_t<_Range>>
3272  || __detail::__is_reversible_subrange<remove_cvref_t<_Range>>
3273  || __detail::__can_reverse_view<_Range>
3274  constexpr auto
3275  operator()(_Range&& __r) const
3276  {
3277  using _Tp = remove_cvref_t<_Range>;
3278  if constexpr (__detail::__is_reverse_view<_Tp>)
3279  return std::forward<_Range>(__r).base();
3280  else if constexpr (__detail::__is_reversible_subrange<_Tp>)
3281  {
3282  using _Iter = decltype(ranges::begin(__r).base());
3283  if constexpr (sized_range<_Tp>)
3284  return subrange<_Iter, _Iter, subrange_kind::sized>
3285  {__r.end().base(), __r.begin().base(), __r.size()};
3286  else
3287  return subrange<_Iter, _Iter, subrange_kind::unsized>
3288  {__r.end().base(), __r.begin().base()};
3289  }
3290  else
3291  return reverse_view{std::forward<_Range>(__r)};
3292  }
3293  };
3294 
3295  inline constexpr _Reverse reverse;
3296  } // namespace views
3297 
3298  namespace __detail
3299  {
3300  template<typename _Tp, size_t _Nm>
3301  concept __has_tuple_element = requires(_Tp __t)
3302  {
3303  typename tuple_size<_Tp>::type;
3304  requires _Nm < tuple_size_v<_Tp>;
3305  typename tuple_element_t<_Nm, _Tp>;
3306  { std::get<_Nm>(__t) }
3307  -> convertible_to<const tuple_element_t<_Nm, _Tp>&>;
3308  };
3309 
3310  template<typename _Tp, size_t _Nm>
3311  concept __returnable_element
3312  = is_reference_v<_Tp> || move_constructible<tuple_element_t<_Nm, _Tp>>;
3313  }
3314 
3315  template<input_range _Vp, size_t _Nm>
3316  requires view<_Vp>
3317  && __detail::__has_tuple_element<range_value_t<_Vp>, _Nm>
3318  && __detail::__has_tuple_element<remove_reference_t<range_reference_t<_Vp>>,
3319  _Nm>
3320  && __detail::__returnable_element<range_reference_t<_Vp>, _Nm>
3321  class elements_view : public view_interface<elements_view<_Vp, _Nm>>
3322  {
3323  public:
3324  elements_view() = default;
3325 
3326  constexpr explicit
3327  elements_view(_Vp base)
3328  : _M_base(std::move(base))
3329  { }
3330 
3331  constexpr _Vp
3332  base() const& requires copy_constructible<_Vp>
3333  { return _M_base; }
3334 
3335  constexpr _Vp
3336  base() &&
3337  { return std::move(_M_base); }
3338 
3339  constexpr auto
3340  begin() requires (!__detail::__simple_view<_Vp>)
3341  { return _Iterator<false>(ranges::begin(_M_base)); }
3342 
3343  constexpr auto
3344  begin() const requires range<const _Vp>
3345  { return _Iterator<true>(ranges::begin(_M_base)); }
3346 
3347  constexpr auto
3348  end() requires (!__detail::__simple_view<_Vp> && !common_range<_Vp>)
3349  { return _Sentinel<false>{ranges::end(_M_base)}; }
3350 
3351  constexpr auto
3352  end() requires (!__detail::__simple_view<_Vp> && common_range<_Vp>)
3353  { return _Iterator<false>{ranges::end(_M_base)}; }
3354 
3355  constexpr auto
3356  end() const requires range<const _Vp>
3357  { return _Sentinel<true>{ranges::end(_M_base)}; }
3358 
3359  constexpr auto
3360  end() const requires common_range<const _Vp>
3361  { return _Iterator<true>{ranges::end(_M_base)}; }
3362 
3363  constexpr auto
3364  size() requires sized_range<_Vp>
3365  { return ranges::size(_M_base); }
3366 
3367  constexpr auto
3368  size() const requires sized_range<const _Vp>
3369  { return ranges::size(_M_base); }
3370 
3371  private:
3372  template<bool _Const>
3373  using _Base = __detail::__maybe_const_t<_Const, _Vp>;
3374 
3375  template<bool _Const>
3376  struct __iter_cat
3377  { };
3378 
3379  template<bool _Const>
3380  requires forward_range<_Base<_Const>>
3381  struct __iter_cat<_Const>
3382  {
3383  private:
3384  static auto _S_iter_cat()
3385  {
3386  using _Base = elements_view::_Base<_Const>;
3387  using _Cat = iterator_traits<iterator_t<_Base>>::iterator_category;
3388  using _Res = decltype((std::get<_Nm>(*std::declval<iterator_t<_Base>>())));
3389  if constexpr (!is_lvalue_reference_v<_Res>)
3390  return input_iterator_tag{};
3391  else if constexpr (derived_from<_Cat, random_access_iterator_tag>)
3392  return random_access_iterator_tag{};
3393  else
3394  return _Cat{};
3395  }
3396  public:
3397  using iterator_category = decltype(_S_iter_cat());
3398  };
3399 
3400  template<bool _Const>
3401  struct _Sentinel;
3402 
3403  template<bool _Const>
3404  struct _Iterator : __iter_cat<_Const>
3405  {
3406  private:
3407  using _Base = elements_view::_Base<_Const>;
3408 
3409  iterator_t<_Base> _M_current = iterator_t<_Base>();
3410 
3411  static constexpr decltype(auto)
3412  _S_get_element(const iterator_t<_Base>& __i)
3413  {
3414  if constexpr (is_reference_v<range_reference_t<_Base>>)
3415  return std::get<_Nm>(*__i);
3416  else
3417  {
3418  using _Et = remove_cv_t<tuple_element_t<_Nm, range_reference_t<_Base>>>;
3419  return static_cast<_Et>(std::get<_Nm>(*__i));
3420  }
3421  }
3422 
3423  static auto
3424  _S_iter_concept()
3425  {
3426  if constexpr (random_access_range<_Vp>)
3427  return random_access_iterator_tag{};
3428  else if constexpr (bidirectional_range<_Vp>)
3429  return bidirectional_iterator_tag{};
3430  else if constexpr (forward_range<_Vp>)
3431  return forward_iterator_tag{};
3432  else
3433  return input_iterator_tag{};
3434  }
3435 
3436  friend _Iterator<!_Const>;
3437 
3438  public:
3439  using iterator_concept = decltype(_S_iter_concept());
3440  // iterator_category defined in elements_view::__iter_cat
3441  using value_type
3442  = remove_cvref_t<tuple_element_t<_Nm, range_value_t<_Base>>>;
3443  using difference_type = range_difference_t<_Base>;
3444 
3445  _Iterator() = default;
3446 
3447  constexpr explicit
3448  _Iterator(iterator_t<_Base> current)
3449  : _M_current(std::move(current))
3450  { }
3451 
3452  constexpr
3453  _Iterator(_Iterator<!_Const> i)
3454  requires _Const && convertible_to<iterator_t<_Vp>, iterator_t<_Base>>
3455  : _M_current(std::move(i._M_current))
3456  { }
3457 
3458  constexpr iterator_t<_Base>
3459  base() const&
3460  requires copyable<iterator_t<_Base>>
3461  { return _M_current; }
3462 
3463  constexpr iterator_t<_Base>
3464  base() &&
3465  { return std::move(_M_current); }
3466 
3467  constexpr decltype(auto)
3468  operator*() const
3469  { return _S_get_element(_M_current); }
3470 
3471  constexpr _Iterator&
3472  operator++()
3473  {
3474  ++_M_current;
3475  return *this;
3476  }
3477 
3478  constexpr void
3479  operator++(int)
3480  { ++_M_current; }
3481 
3482  constexpr _Iterator
3483  operator++(int) requires forward_range<_Base>
3484  {
3485  auto __tmp = *this;
3486  ++_M_current;
3487  return __tmp;
3488  }
3489 
3490  constexpr _Iterator&
3491  operator--() requires bidirectional_range<_Base>
3492  {
3493  --_M_current;
3494  return *this;
3495  }
3496 
3497  constexpr _Iterator
3498  operator--(int) requires bidirectional_range<_Base>
3499  {
3500  auto __tmp = *this;
3501  --_M_current;
3502  return __tmp;
3503  }
3504 
3505  constexpr _Iterator&
3506  operator+=(difference_type __n)
3507  requires random_access_range<_Base>
3508  {
3509  _M_current += __n;
3510  return *this;
3511  }
3512 
3513  constexpr _Iterator&
3514  operator-=(difference_type __n)
3515  requires random_access_range<_Base>
3516  {
3517  _M_current -= __n;
3518  return *this;
3519  }
3520 
3521  constexpr decltype(auto)
3522  operator[](difference_type __n) const
3523  requires random_access_range<_Base>
3524  { return _S_get_element(_M_current + __n); }
3525 
3526  friend constexpr bool
3527  operator==(const _Iterator& __x, const _Iterator& __y)
3528  requires equality_comparable<iterator_t<_Base>>
3529  { return __x._M_current == __y._M_current; }
3530 
3531  friend constexpr bool
3532  operator<(const _Iterator& __x, const _Iterator& __y)
3533  requires random_access_range<_Base>
3534  { return __x._M_current < __y._M_current; }
3535 
3536  friend constexpr bool
3537  operator>(const _Iterator& __x, const _Iterator& __y)
3538  requires random_access_range<_Base>
3539  { return __y._M_current < __x._M_current; }
3540 
3541  friend constexpr bool
3542  operator<=(const _Iterator& __x, const _Iterator& __y)
3543  requires random_access_range<_Base>
3544  { return !(__y._M_current > __x._M_current); }
3545 
3546  friend constexpr bool
3547  operator>=(const _Iterator& __x, const _Iterator& __y)
3548  requires random_access_range<_Base>
3549  { return !(__x._M_current > __y._M_current); }
3550 
3551 #ifdef __cpp_lib_three_way_comparison
3552  friend constexpr auto
3553  operator<=>(const _Iterator& __x, const _Iterator& __y)
3554  requires random_access_range<_Base>
3555  && three_way_comparable<iterator_t<_Base>>
3556  { return __x._M_current <=> __y._M_current; }
3557 #endif
3558 
3559  friend constexpr _Iterator
3560  operator+(const _Iterator& __x, difference_type __y)
3561  requires random_access_range<_Base>
3562  { return _Iterator{__x} += __y; }
3563 
3564  friend constexpr _Iterator
3565  operator+(difference_type __x, const _Iterator& __y)
3566  requires random_access_range<_Base>
3567  { return __y + __x; }
3568 
3569  friend constexpr _Iterator
3570  operator-(const _Iterator& __x, difference_type __y)
3571  requires random_access_range<_Base>
3572  { return _Iterator{__x} -= __y; }
3573 
3574  // _GLIBCXX_RESOLVE_LIB_DEFECTS
3575  // 3483. transform_view::iterator's difference is overconstrained
3576  friend constexpr difference_type
3577  operator-(const _Iterator& __x, const _Iterator& __y)
3578  requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
3579  { return __x._M_current - __y._M_current; }
3580 
3581  friend _Sentinel<_Const>;
3582  };
3583 
3584  template<bool _Const>
3585  struct _Sentinel
3586  {
3587  private:
3588  constexpr bool
3589  _M_equal(const _Iterator<_Const>& __x) const
3590  { return __x._M_current == _M_end; }
3591 
3592  using _Base = elements_view::_Base<_Const>;
3593  sentinel_t<_Base> _M_end = sentinel_t<_Base>();
3594 
3595  public:
3596  _Sentinel() = default;
3597 
3598  constexpr explicit
3599  _Sentinel(sentinel_t<_Base> __end)
3600  : _M_end(std::move(__end))
3601  { }
3602 
3603  constexpr
3604  _Sentinel(_Sentinel<!_Const> __other)
3605  requires _Const
3606  && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
3607  : _M_end(std::move(__other._M_end))
3608  { }
3609 
3610  constexpr sentinel_t<_Base>
3611  base() const
3612  { return _M_end; }
3613 
3614  template<bool _Const2>
3615  requires sentinel_for<sentinel_t<_Base>,
3616  iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
3617  friend constexpr bool
3618  operator==(const _Iterator<_Const2>& __x, const _Sentinel& __y)
3619  { return __y._M_equal(__x); }
3620 
3621  template<bool _Const2,
3622  typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
3623  requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
3624  friend constexpr range_difference_t<_Base2>
3625  operator-(const _Iterator<_Const2>& __x, const _Sentinel& __y)
3626  { return __x._M_current - __y._M_end; }
3627 
3628  template<bool _Const2,
3629  typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
3630  requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
3631  friend constexpr range_difference_t<_Base>
3632  operator-(const _Sentinel& __x, const _Iterator<_Const2>& __y)
3633  { return __x._M_end - __y._M_current; }
3634 
3635  friend _Sentinel<!_Const>;
3636  };
3637 
3638  _Vp _M_base = _Vp();
3639  };
3640 
3641  template<typename _Tp, size_t _Nm>
3642  inline constexpr bool enable_borrowed_range<elements_view<_Tp, _Nm>>
3643  = enable_borrowed_range<_Tp>;
3644 
3645  template<typename _Range>
3646  using keys_view = elements_view<views::all_t<_Range>, 0>;
3647 
3648  template<typename _Range>
3649  using values_view = elements_view<views::all_t<_Range>, 1>;
3650 
3651  namespace views
3652  {
3653  namespace __detail
3654  {
3655  template<size_t _Nm, typename _Range>
3656  concept __can_elements_view
3657  = requires { elements_view<all_t<_Range>, _Nm>{std::declval<_Range>()}; };
3658  } // namespace __detail
3659 
3660  template<size_t _Nm>
3661  struct _Elements : __adaptor::_RangeAdaptorClosure
3662  {
3663  template<viewable_range _Range>
3664  requires __detail::__can_elements_view<_Nm, _Range>
3665  constexpr auto
3666  operator()(_Range&& __r) const
3667  {
3668  return elements_view<all_t<_Range>, _Nm>{std::forward<_Range>(__r)};
3669  }
3670  };
3671 
3672  template<size_t _Nm>
3673  inline constexpr _Elements<_Nm> elements;
3674  inline constexpr auto keys = elements<0>;
3675  inline constexpr auto values = elements<1>;
3676  } // namespace views
3677 
3678 } // namespace ranges
3679 
3680  namespace views = ranges::views;
3681 
3682 _GLIBCXX_END_NAMESPACE_VERSION
3683 } // namespace
3684 #endif // library concepts
3685 #endif // C++2a
3686 #endif /* _GLIBCXX_RANGES */