[clang] [clang-tools-extra] [libcxx] [llvm] [libc++][ranges] P2542R8: Implement `views::concat` (PR #120920)
A. Jiang via cfe-commits
cfe-commits at lists.llvm.org
Sun Feb 16 05:41:51 PST 2025
================
@@ -0,0 +1,638 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___RANGES_CONCAT_VIEW_H
+#define _LIBCPP___RANGES_CONCAT_VIEW_H
+
+#include <__algorithm/ranges_find_if.h>
+#include <__assert>
+#include <__concepts/common_reference_with.h>
+#include <__concepts/constructible.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/copyable.h>
+#include <__concepts/derived_from.h>
+#include <__concepts/equality_comparable.h>
+#include <__concepts/swappable.h>
+#include <__config>
+#include <__functional/bind_back.h>
+#include <__functional/invoke.h>
+#include <__functional/reference_wrapper.h>
+#include <__iterator/concepts.h>
+#include <__iterator/default_sentinel.h>
+#include <__iterator/distance.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iter_move.h>
+#include <__iterator/iter_swap.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__memory/addressof.h>
+#include <__ranges/access.h>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/movable_box.h>
+#include <__ranges/non_propagating_cache.h>
+#include <__ranges/range_adaptor.h>
+#include <__ranges/size.h>
+#include <__ranges/view_interface.h>
+#include <__ranges/zip_view.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/decay.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_nothrow_convertible.h>
+#include <__type_traits/is_object.h>
+#include <__type_traits/make_unsigned.h>
+#include <__type_traits/maybe_const.h>
+#include <__utility/forward.h>
+#include <__utility/in_place.h>
+#include <__utility/move.h>
+#include <tuple>
+#include <variant>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+# pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+
+namespace ranges {
+
+# ifdef __cpp_pack_indexing
+template <class... _Tp>
+using __extract_last _LIBCPP_NODEBUG = _Tp...[sizeof...(_Tp) - 1];
+# else
+template <class _Tp, class... _Tail>
+struct __extract_last_impl : __extract_last_impl<_Tail...> {};
+template <class _Tp>
+struct __extract_last_impl<_Tp> {
+ using type _LIBCPP_NODEBUG = _Tp;
+};
+
+template <class... _Tp>
+using __extract_last _LIBCPP_NODEBUG = __extract_last_impl<_Tp...>::type;
+# endif
+
+template <class _Tp, class... _Tail>
+constexpr bool __derived_from_pack =
+ __derived_from_pack<_Tp, __extract_last<_Tail...>> && __derived_from_pack<_Tail...>;
+
+template <class _Tp, class _IterCategory>
+constexpr bool __derived_from_pack<_Tp, _IterCategory> = derived_from<_Tp, _IterCategory>;
+
+template <class _View, class... _Views>
+struct __last_view : __last_view<_Views...> {};
+
+template <class _View>
+struct __last_view<_View> {
+ using type = _View;
+};
+
+template <class _Ref, class _RRef, class _It>
+concept __concat_indirectly_readable_impl = requires(const _It __it) {
+ { *__it } -> convertible_to<_Ref>;
+ { ranges::iter_move(__it) } -> convertible_to<_RRef>;
+};
+
+template <class... _Rs>
+using __concat_reference_t _LIBCPP_NODEBUG = common_reference_t<range_reference_t<_Rs>...>;
+
+template <class... _Rs>
+using __concat_value_t _LIBCPP_NODEBUG = common_type_t<range_value_t<_Rs>...>;
+
+template <class... _Rs>
+using __concat_rvalue_reference_t _LIBCPP_NODEBUG = common_reference_t<range_rvalue_reference_t<_Rs>...>;
+
+template <class... _Rs>
+concept __concat_indirectly_readable =
+ common_reference_with<__concat_reference_t<_Rs...>&&, __concat_value_t<_Rs...>&> &&
+ common_reference_with<__concat_reference_t<_Rs...>&&, __concat_rvalue_reference_t<_Rs...>&&> &&
+ common_reference_with<__concat_rvalue_reference_t<_Rs...>&&, __concat_value_t<_Rs...> const&> &&
+ (__concat_indirectly_readable_impl<__concat_reference_t<_Rs...>,
+ __concat_rvalue_reference_t<_Rs...>,
+ iterator_t<_Rs>> &&
+ ...);
+
+template <class... _Rs>
+concept __concatable = requires {
+ typename __concat_reference_t<_Rs...>;
+ typename __concat_value_t<_Rs...>;
+ typename __concat_rvalue_reference_t<_Rs...>;
+} && __concat_indirectly_readable<_Rs...>;
+
+template <bool _Const, class... _Rs>
+concept __concat_is_random_access =
+ (random_access_range<__maybe_const<_Const, _Rs>> && ...) && (sized_range<__maybe_const<_Const, _Rs>> && ...);
+
+template <bool _Const, class... _Rs>
+concept __concat_is_bidirectional =
+ ((bidirectional_range<__maybe_const<_Const, _Rs>> && ...) && (common_range<__maybe_const<_Const, _Rs>> && ...));
+
+template <bool _Const, class... _Views>
+concept __all_forward = (forward_range<__maybe_const<_Const, _Views>> && ...);
+
+template <bool _Const, class... _Tp>
+struct __apply_drop_first;
+
+template <bool _Const, class _Head, class... _Tail>
+struct __apply_drop_first<_Const, _Head, _Tail...> {
+ static constexpr bool value = (sized_range<__maybe_const<_Const, _Tail>> && ...);
+};
+
+template <input_range... _Views>
+ requires(view<_Views> && ...) && (sizeof...(_Views) > 0) && __concatable<_Views...>
+class concat_view : public view_interface<concat_view<_Views...>> {
+ tuple<_Views...> __views_;
+
+ template <bool _Const>
+ class __iterator;
+
+public:
+ _LIBCPP_HIDE_FROM_ABI constexpr concat_view() = default;
+
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit concat_view(_Views... __views) : __views_(std::move(__views)...) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator<false> begin()
+ requires(!(__simple_view<_Views> && ...))
+ {
+ __iterator<false> __it(this, in_place_index<0>, ranges::begin(std::get<0>(__views_)));
+ __it.template __satisfy<0>();
+ return __it;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator<true> begin() const
+ requires((range<const _Views> && ...) && __concatable<const _Views...>)
+ {
+ __iterator<true> __it(this, in_place_index<0>, ranges::begin(std::get<0>(__views_)));
+ __it.template __satisfy<0>();
+ return __it;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+ requires(!(__simple_view<_Views> && ...))
+ {
+ if constexpr (common_range<typename __last_view<_Views...>::type>) {
+ constexpr auto __n = sizeof...(_Views);
+ return __iterator<false>(this, in_place_index<__n - 1>, ranges::end(std::get<__n - 1>(__views_)));
+ } else {
+ return default_sentinel;
+ }
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+ requires(range<const _Views> && ...)
+ {
+ if constexpr (common_range<typename __last_view<_Views...>::type>) {
+ constexpr auto __n = sizeof...(_Views);
+ return __iterator<true>(this, in_place_index<__n - 1>, ranges::end(std::get<__n - 1>(__views_)));
+ } else {
+ return default_sentinel;
+ }
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+ requires(sized_range<_Views> && ...)
+ {
+ return std::apply(
+ [](auto... __sizes) { return (make_unsigned_t<common_type_t<decltype(__sizes)...>>(__sizes) + ...); },
+ ranges::__tuple_transform(ranges::size, __views_));
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+ requires(sized_range<const _Views> && ...)
+ {
+ return std::apply(
+ [](auto... __sizes) { return (make_unsigned_t<common_type_t<decltype(__sizes)...>>(__sizes) + ...); },
+ ranges::__tuple_transform(ranges::size, __views_));
+ }
+};
+
+template <class... _Views>
+concat_view(_Views&&...) -> concat_view<views::all_t<_Views>...>;
+
+template <input_range... _Views>
+ requires(view<_Views> && ...) && (sizeof...(_Views) > 0) && __concatable<_Views...>
+template <bool _Const>
+class concat_view<_Views...>::__iterator {
+public:
+ constexpr static bool derive_pack_random_iterator =
+ __derived_from_pack<typename iterator_traits<iterator_t<__maybe_const<_Const, _Views>>>::iterator_category...,
+ random_access_iterator_tag>;
+ constexpr static bool derive_pack_bidirectional_iterator =
+ __derived_from_pack<typename iterator_traits<iterator_t<__maybe_const<_Const, _Views>>>::iterator_category...,
+ bidirectional_iterator_tag>;
+ constexpr static bool derive_pack_forward_iterator =
+ __derived_from_pack<typename iterator_traits< iterator_t<__maybe_const<_Const, _Views>>>::iterator_category...,
+ forward_iterator_tag>;
+ using iterator_category =
+ _If<!is_reference_v<__concat_reference_t<__maybe_const<_Const, _Views>...>>,
+ input_iterator_tag,
+ _If<derive_pack_random_iterator,
+ random_access_iterator_tag,
+ _If<derive_pack_bidirectional_iterator,
+ bidirectional_iterator_tag,
+ _If<derive_pack_forward_iterator, forward_iterator_tag, input_iterator_tag > > > >;
+ using iterator_concept =
+ _If<__concat_is_random_access<_Const, _Views...>,
+ random_access_iterator_tag,
+ _If<__concat_is_bidirectional<_Const, _Views...>,
+ bidirectional_iterator_tag,
+ _If< __all_forward<_Const, _Views...>, forward_iterator_tag, input_iterator_tag > > >;
+ using value_type = __concat_value_t<__maybe_const<_Const, _Views>...>;
+ using difference_type = common_type_t<range_difference_t<__maybe_const<_Const, _Views>>...>;
+ using __base_iter _LIBCPP_NODEBUG = variant<iterator_t<__maybe_const<_Const, _Views>>...>;
+
+ __base_iter __it_;
+ __maybe_const<_Const, concat_view>* __parent_ = nullptr;
+
+ template <std::size_t _Idx>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __satisfy() {
+ if constexpr (_Idx < (sizeof...(_Views) - 1)) {
+ if (std::get<_Idx>(__it_) == ranges::end(std::get<_Idx>(__parent_->__views_))) {
+ __it_.template emplace<_Idx + 1>(ranges::begin(std::get<_Idx + 1>(__parent_->__views_)));
+ __satisfy<_Idx + 1>();
+ }
+ }
+ }
+
+ template <std::size_t _Idx>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __prev() {
+ if constexpr (_Idx == 0) {
+ --std::get<0>(__it_);
+ } else {
+ if (std::get<_Idx>(__it_) == ranges::begin(std::get<_Idx>(__parent_->__views_))) {
+ using __prev_view = __maybe_const<_Const, tuple_element_t<_Idx - 1, tuple<_Views...>>>;
+ if constexpr (common_range<__prev_view>) {
+ __it_.template emplace<_Idx - 1>(ranges::end(std::get<_Idx - 1>(__parent_->__views_)));
+ } else {
+ __it_.template emplace<_Idx - 1>(ranges::__next(ranges::begin(std::get<_Idx - 1>(__parent_->__views_)),
+ ranges::size(std::get<_Idx - 1>(__parent_->__views_))));
+ }
+ __prev<_Idx - 1>();
+ } else {
+ --std::get<_Idx>(__it_);
+ }
+ }
+ }
+
+ template <std::size_t _Idx>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __advance_fwd(difference_type __offset, difference_type __steps) {
+ using __underlying_diff_type = iter_difference_t<variant_alternative_t<_Idx, __base_iter>>;
+ if constexpr (_Idx == sizeof...(_Views) - 1) {
+ std::get<_Idx>(__it_) += static_cast<__underlying_diff_type>(__steps);
+ } else {
+ difference_type __n_size = ranges::size(std::get<_Idx>(__parent_->__views_));
+ if (__offset + __steps < __n_size) {
+ std::get<_Idx>(__it_) += static_cast<__underlying_diff_type>(__steps);
+ } else {
+ __it_.template emplace<_Idx + 1>(ranges::begin(std::get<_Idx + 1>(__parent_->__views_)));
+ __advance_fwd<_Idx + 1>(0, __offset + __steps - __n_size);
+ }
+ }
+ }
+
+ template <std::size_t _Idx>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __advance_bwd(difference_type __offset, difference_type __steps) {
+ using __underlying_diff_type = iter_difference_t<variant_alternative_t<_Idx, __base_iter>>;
+ if constexpr (_Idx == 0) {
+ std::get<_Idx>(__it_) -= static_cast<__underlying_diff_type>(__steps);
+ } else {
+ if (__offset >= __steps) {
+ std::get<_Idx>(__it_) -= static_cast<__underlying_diff_type>(__steps);
+ } else {
+ auto __prev_size = ranges::__distance(std::get<_Idx - 1>(__parent_->__views_));
+ __it_.template emplace<_Idx - 1>(ranges::begin(std::get<_Idx - 1>(__parent_->__views_)) + __prev_size);
+ __advance_bwd<_Idx - 1>(__prev_size, __steps - __offset);
+ }
+ }
+ }
+
+ template <size_t... _Is, typename _Func>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __apply_at_index(size_t __index, _Func&& __func, index_sequence<_Is...>) {
+ ((__index == _Is ? (__func(integral_constant<size_t, _Is>{}), 0) : 0), ...);
+ }
+
+ template <size_t _Idx, typename _Func>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __apply_at_index(size_t __index, _Func&& __func) {
+ __apply_at_index(__index, std::forward<_Func>(__func), make_index_sequence<_Idx>{});
+ }
+
+ template <class... _Args>
+ _LIBCPP_HIDE_FROM_ABI explicit constexpr __iterator(__maybe_const<_Const, concat_view>* __parent, _Args&&... __args)
+ requires constructible_from<__base_iter, _Args&&...>
+ : __it_(std::forward<_Args>(__args)...), __parent_(__parent) {}
+
+public:
+ _LIBCPP_HIDE_FROM_ABI __iterator() = default;
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator(__iterator<!_Const> __i)
+ requires _Const && (convertible_to<iterator_t<_Views>, iterator_t<const _Views>> && ...)
+ : __it_([&__src = __i.it_]<size_t... _Indices>(size_t __idx, index_sequence<_Indices...>) -> __base_iter {
+ using __src_lref = decltype((__src));
+ using __construction_fptr = __base_iter (*)(__src_lref);
+ static constexpr __construction_fptr __vtable[]{[](__src_lref __src_var) -> __base_iter {
+ return __base_iter(in_place_index<_Indices>, std::get<_Indices>(std::move(__src_var)));
+ }...};
+ return __vtable[__idx](__src);
+ }(__i.index(), make_index_sequence<variant_size_v<__base_iter>>{})),
+ __parent_(__i.parent_) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator*() const {
+ return std::visit(
+ [](auto&& __it) -> __concat_reference_t<__maybe_const<_Const, _Views>...> { return *__it; }, __it_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator++() {
+ size_t __active_index = __it_.index();
+ __apply_at_index<std::variant_size_v<decltype(__it_)>>(__active_index, [&](auto __index_constant) {
+ constexpr size_t __i = __index_constant.value;
+ ++std::get<__i>(__it_);
+ __satisfy<__i>();
+ });
+ return *this;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr void operator++(int) { ++*this; }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator operator++(int)
+ requires(forward_range<__maybe_const<_Const, _Views>> && ...)
+ {
+ auto __tmp = *this;
+ ++*this;
+ return __tmp;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator--()
+ requires __concat_is_bidirectional<_Const, _Views...>
+ {
+ size_t __active_index = __it_.index();
+ __apply_at_index<std::variant_size_v<decltype(__it_)>>(__active_index, [&](auto __index_constant) {
+ constexpr size_t __i = __index_constant.value;
+ __prev<__i>();
+ });
+ return *this;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator operator--(int)
+ requires __concat_is_bidirectional<_Const, _Views...>
+ {
+ auto __tmp = *this;
+ --*this;
+ return __tmp;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator& __x, const __iterator& __y)
+ requires(equality_comparable<iterator_t<__maybe_const<_Const, _Views>>> && ...)
+ {
+ return __x.__it_ == __y.__it_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator[](difference_type __n) const
+ requires __concat_is_random_access<_Const, _Views...>
+ {
+ return *((*this) + __n);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(const __iterator& __it, difference_type __n)
+ requires __concat_is_random_access<_Const, _Views...>
+ {
+ auto __temp = __it;
+ __temp += __n;
+ return __temp;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(difference_type __n, const __iterator& __it)
+ requires __concat_is_random_access<_Const, _Views...>
+ {
+ return __it + __n;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator+=(difference_type __n)
+ requires __concat_is_random_access<_Const, _Views...>
+ {
+ size_t __active_index = __it_.index();
+ if (__n > 0) {
+ std::visit(
+ [&](auto& __active_it) {
+ __apply_at_index<std::tuple_size_v<decltype(__parent_->__views_)>>(
+ __active_index, [&](auto __index_constant) {
+ constexpr size_t __i = __index_constant.value;
+ auto& __active_view = std::get<__i>(__parent_->__views_);
+ difference_type __idx = __active_it - ranges::begin(__active_view);
+ __advance_fwd<__i>(__idx, __n);
+ });
+ },
+ __it_);
+ }
+
+ else if (__n < 0) {
+ std::visit(
+ [&](auto& __active_it) {
+ __apply_at_index<std::tuple_size_v<decltype(__parent_->__views_)>>(
+ __active_index, [&](auto __index_constant) {
+ constexpr size_t __i = __index_constant.value;
+ auto& __active_view = std::get<__i>(__parent_->__views_);
+ difference_type __idx = __active_it - ranges::begin(__active_view);
+ __advance_bwd<__i>(__idx, -__n);
+ });
+ },
+ __it_);
+ }
+
+ return *this;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator-=(difference_type __n)
+ requires __concat_is_random_access<_Const, _Views...>
+ {
+ *this += -__n;
+ return *this;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator& __it, default_sentinel_t) {
+ constexpr auto __last_idx = sizeof...(_Views) - 1;
+ return __it.__it_.index() == __last_idx &&
+ std::get<__last_idx>(__it.__it_) == ranges::end(std::get<__last_idx>(__it.__parent_->__views_));
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator<(const __iterator& __x, const __iterator& __y)
+ requires(random_access_range<__maybe_const<_Const, _Views>> && ...)
+ {
+ return __x.__it_ < __y.__it_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator>(const __iterator& __x, const __iterator& __y)
+ requires(random_access_range<__maybe_const<_Const, _Views>> && ...)
+ {
+ return __x.__it_ > __y.__it_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator<=(const __iterator& __x, const __iterator& __y)
+ requires(random_access_range<__maybe_const<_Const, _Views>> && ...)
+ {
+ return __x.__it_ <= __y.__it_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator>=(const __iterator& __x, const __iterator& __y)
+ requires(random_access_range<__maybe_const<_Const, _Views>> && ...)
+ {
+ return __x.__it_ >= __y.__it_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr auto operator<=>(const __iterator& __x, const __iterator& __y)
+ requires((random_access_range<__maybe_const<_Const, _Views>> && ...) &&
+ (three_way_comparable<__maybe_const<_Const, _Views>> && ...))
+ {
+ return __x.__it_ <=> __y.__it_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr decltype(auto) iter_move(const __iterator& __it) noexcept(
+
+ ((is_nothrow_invocable_v< decltype(ranges::iter_move), const iterator_t<__maybe_const<_Const, _Views>>& >) &&
+ ...) &&
+ ((is_nothrow_convertible_v< range_rvalue_reference_t<__maybe_const<_Const, _Views>>,
+ __concat_rvalue_reference_t<__maybe_const<_Const, _Views>...> >) &&
+ ...))
+
+ {
+ return std::visit(
+ [](const auto& __i) -> __concat_rvalue_reference_t<__maybe_const<_Const, _Views>...> {
+ return ranges::iter_move(__i);
+ },
+ __it.__it_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr void iter_swap(const __iterator& __x, const __iterator& __y)
+
+ noexcept((noexcept(ranges::swap(*__x, *__y))) &&
+ (noexcept(ranges::iter_swap(std::declval<const iterator_t<__maybe_const<_Const, _Views>>>(),
+ std::declval<const iterator_t<__maybe_const<_Const, _Views>>>())) &&
+ ...))
+
+ requires swappable_with<iter_reference_t<__iterator>, iter_reference_t<__iterator>> &&
+ (... && indirectly_swappable<iterator_t<__maybe_const<_Const, _Views>>>)
+ {
+ std::visit(
+ [&](const auto& __it1, const auto& __it2) {
+ if constexpr (is_same_v<decltype(__it1), decltype(__it2)>) {
+ ranges::iter_swap(__it1, __it2);
+ } else {
+ ranges::swap(*__x, *__y);
+ }
+ },
+ __x.__it_,
+ __y.__it_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(const __iterator& __x, const __iterator& __y)
+ requires __concat_is_random_access<_Const, _Views...>
+ {
+ size_t __ix = __x.__it_.index();
+ size_t __iy = __y.__it_.index();
+
+ if (__ix > __iy) {
+ std::visit(
+ [&](auto& __it_x, auto& __it_y) {
+ __it_x.template __apply_at_index<tuple_size_v<decltype(__x.__parent_->__views_)>>(
+ __ix, [&](auto __index_constant_x) {
+ constexpr size_t __index_x = __index_constant_x.value;
+ auto __dx = ranges::__distance(ranges::begin(std::get<__index_x>(__x.__parent_->__views_)), __it_x);
+
+ __it_y.template __apply_at_index<tuple_size_v<decltype(__y.__parent_->__views_)>>(
+ __iy, [&](auto __index_constant_y) {
+ constexpr size_t __index_y = __index_constant_y.value;
+ auto __dy =
+ ranges::__distance(ranges::begin(std::get<__index_y>(__y.__parent_->__views_)), __it_y);
+ difference_type __s = 0;
+ for (size_t __idx = __index_y + 1; __idx < __index_x; __idx++) {
+ __s += ranges::size(std::get<__idx>(__x.__parent_->__views_));
+ }
+ return __dy + __s + __dx;
+ });
+ });
+ },
+ __x.__it_,
+ __y.__it_);
+ } else if (__ix < __iy) {
+ return -(__y - __x);
+ } else {
+ std::visit([&](const auto& __it1, const auto& __it2) { return __it1 - __it2; }, __x.__it_, __y.__it_);
+ }
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator-(const __iterator& __it, difference_type __n)
+ requires __concat_is_random_access<_Const, _Views...>
+ {
+ auto __temp = __it;
+ __temp -= __n;
+ return __temp;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(const __iterator& __x, default_sentinel_t)
+ requires(sized_sentinel_for<sentinel_t<__maybe_const<_Const, _Views>>, iterator_t<__maybe_const<_Const, _Views>>> &&
+ ...) &&
+ (__apply_drop_first<_Const, _Views...>::value)
+ {
+ size_t __ix = __x.__it_.index();
+ std::visit(
+ [&](auto& __it_x) {
+ __it_x.template __apply_at_index<tuple_size_v<decltype(__x.__parent_->__views_)>>(
+ __ix, [&](auto __index_constant) {
+ constexpr size_t __index_x = __index_constant.value;
+ auto __dx = ranges::__distance(ranges::begin(std::get<__index_x>(__x.__parent_->__views_)), __it_x);
+
+ difference_type __s = 0;
+ for (size_t __idx = 0; __idx < __index_x; __idx++) {
+ __s += ranges::size(std::get<__idx>(__x.__parent_->__views_));
+ }
+
+ return -(__dx + __s);
+ });
+ },
+ __x.__it_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(default_sentinel_t, const __iterator& __x)
+ requires(sized_sentinel_for<sentinel_t<__maybe_const<_Const, _Views>>, iterator_t<__maybe_const<_Const, _Views>>> &&
+ ...) &&
+ (__apply_drop_first<_Const, _Views...>::value)
+ {
+ -(__x - default_sentinel);
+ }
+};
+
+namespace views {
+namespace __concat {
+struct __fn {
+ template <class... _Views>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Views... __views) const
+ noexcept(noexcept(concat_view(std::forward<_Views>(__views)...)))
+ -> decltype(concat_view(std::forward<_Views>(__views)...)) {
+ return concat_view(std::forward<_Views>(__views)...);
+ }
+};
----------------
frederick-vs-ja wrote:
It seems that the `views::all` case in [[range.concat.overview]](https://eel.is/c++draft/range.concat.overview) was completely ignored.
IMO it would be helpful to write a helper `constexpr` variable template to record both strategy (__all, __concat, or __none) and noexcept status.
https://github.com/llvm/llvm-project/pull/120920
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