WIP: [libc++][ranges] Implement `ranges::stride_view`.

[hawkinsw]

Hello @ldionne and @philnik !! I am a *huge* fan of both of yours and really appreciate all the work that you devote to libc++. I saw a chance to contribute and wanted to try to help.

The code I have here is very rough and early, but I was hoping that you could provide some rough guidance! I have contributed small patches before but this is a new area for me and I have already learned a ton! I know that I need to complete the steps listed in the Pre-commit check list in docs/Contributing.rst and I am working on that now.

Thank you, again, for all the great work!

PS: I hope that you are the right ones to tag for review. I based the tag here on git blame output and I apologize if I am spamming you!

I hope that this contribution is the first of many (e.g., ranges::adjacent_view).
Will

[Mordante]

Thanks for working on this!

I'm not to familiar with ranges to I leave a real review to others. I mainly glossed over the patch.

Based on https://libcxx.llvm.org/Status/Ranges @H-G-Hristov is working on this too, @H-G-Hristov what is the status of your work?

If you want to work on a different view nobody is working on please let us know, we can put your name on it.

libcxx/include/__ranges/stride_view.h

187

This line is too long, please use clang-format to format the patch.

[hawkinsw]

In D156924#4555070, @Mordante wrote:

Thanks for working on this!

I'm not to familiar with ranges to I leave a real review to others. I mainly glossed over the patch.

Based on https://libcxx.llvm.org/Status/Ranges @H-G-Hristov is working on this too, @H-G-Hristov what is the status of your work?

If you want to work on a different view nobody is working on please let us know, we can put your name on it.

I am really eager to help however I can! I am a huge language geek and love open source. The past two days of coding on this little side project have been the most eye-opening C++ experience I have had since I started (mid 90s). If @H-G-Hristov wants to take over, that's great -- just assign me whatever needs to be done!

Will

libcxx/include/__ranges/stride_view.h

187

Will do!!

This is actually a meta question -- I ran clang-format (in the libcxx/ directory with the .clang-format file) and noticed that the result was nothing like what the other files looked like. I was worried that I had done something wrong.

[Mordante]

In D156924#4555100, @hawkinsw wrote:

In D156924#4555070, @Mordante wrote:

Thanks for working on this!

I'm not to familiar with ranges to I leave a real review to others. I mainly glossed over the patch.

Based on https://libcxx.llvm.org/Status/Ranges @H-G-Hristov is working on this too, @H-G-Hristov what is the status of your work?

If you want to work on a different view nobody is working on please let us know, we can put your name on it.

I am really eager to help however I can! I am a huge language geek and love open source. The past two days of coding on this little side project have been the most eye-opening C++ experience I have had since I started (mid 90s). If @H-G-Hristov wants to take over, that's great -- just assign me whatever needs to be done!

Will

I think nobody is working on the C++23 range based algorithms. I found some patches for other views. Unfortunately the Phabricator search is under heavy load so I can't search. Maybe visit our Discord channel and reach out to @varconst or @philnik they are most familiar in this area. If you don't know where our Discord is, the link is at the top of https://libcxx.llvm.org/Contributing.html

libcxx/include/__ranges/stride_view.h

187

Not all files are formatted. Currently we use clang-format-17. We like new files to be formatted.

[Mordante]

Pressed submit too fast ;-)

In D156924#4555100, @hawkinsw wrote:

I am really eager to help however I can! I am a huge language geek and love open source. The past two days of coding on this little side project have been the most eye-opening C++ experience I have had since I started (mid 90s).

Cool to hear! I too love to see the progress C++ has made over the years :-)

[hawkinsw]

Formatted patch using clang-format on @Mordante 's helpful suggestion.

[hawkinsw]

Thank you, @Mordante !

libcxx/include/__ranges/stride_view.h

53

Yes, I realize that this is crazy! It is dead code and I will eliminate it.

[H-G-Hristov]

@Mordante @hawkinsw My patch is about the same state like this one. I am still working on enumerate_view I need to add more tests before I submit it. I didn't have much time to work during the last month but I'd like to get it in Clang 18. Maybe its about time to submit a draft.

@hawkinsw You can continue working on it if you like or maybe you can pick some of the other view which are more useful and interesting but I think also more difficult (join_with and join).
You can update the status here: https://libcxx.llvm.org/Status/Ranges.html

[hawkinsw]

In D156924#4558613, @H-G-Hristov wrote:

@Mordante @hawkinsw My patch is about the same state like this one. I am still working on enumerate_view I need to add more tests before I submit it. I didn't have much time to work during the last month but I'd like to get it in Clang 18. Maybe its about time to submit a draft.

@hawkinsw You can continue working on it if you like or maybe you can pick some of the other view which are more useful and interesting but I think also more difficult (join_with and join).
You can update the status here: https://libcxx.llvm.org/Status/Ranges.html

Hello @H-G-Hristov !! As a warmup to working on others, I will continue to hammer on this (my next goal is to write tests!) I don't want to waste your time (obviously!) but it's been fun to work on this and a great learning experience. The next goal I have is to write tests (I will start on that tonight!)

Does that sound okay?

[H-G-Hristov]

In D156924#4558648, @hawkinsw wrote:

In D156924#4558613, @H-G-Hristov wrote:

@Mordante @hawkinsw My patch is about the same state like this one. I am still working on enumerate_view I need to add more tests before I submit it. I didn't have much time to work during the last month but I'd like to get it in Clang 18. Maybe its about time to submit a draft.

@hawkinsw You can continue working on it if you like or maybe you can pick some of the other view which are more useful and interesting but I think also more difficult (join_with and join).
You can update the status here: https://libcxx.llvm.org/Status/Ranges.html

Hello @H-G-Hristov !! As a warmup to working on others, I will continue to hammer on this (my next goal is to write tests!) I don't want to waste your time (obviously!) but it's been fun to work on this and a great learning experience. The next goal I have is to write tests (I will start on that tonight!)

Does that sound okay?

Sure, please continue.

[hawkinsw]

Marked div_ceil as ABI invisible and removed caching.

[hawkinsw]

Fixed the commit against which this diff should be applied.

[hawkinsw]

Remove inclusion of spurious change to libcxx-lit.

[hawkinsw]

clang-tidy'd the code.

[hawkinsw]

Added a first, basic test and also did some more clang-tidy'ing.

[H-G-Hristov]

@hawkinsw You can look at my WIP for stuff you can easily copy: https://reviews.llvm.org/D157192

[hawkinsw]

In D156924#4563133, @H-G-Hristov wrote:

@hawkinsw You can look at my WIP for stuff you can easily copy: https://reviews.llvm.org/D157192

Thank you -- very kind of you! Two heads (especially yours -- which is smarter than mine!) are better than one!

[hawkinsw]

Fix some ADL warnings, compilation errors and implementation details.

[hawkinsw]

Bring in the awesome tests that @H-G-Hristov already wrote!

[hawkinsw]

More cargo crating from @H-G-Hristov's excellent work.

[hawkinsw]

Check in the modified feature test macro code.

[hawkinsw]

Adding some missing include files.

[hawkinsw]

Make div-ceil system only by adding __ prefix.

[hawkinsw]

Add initial tests for ::size

[hawkinsw]

(maybe) last clang-tidy/format and add base tests.

[hawkinsw]

Clean up the skeleton test file (remove references to std::ranges::enumerate).

[hawkinsw]

@H-G-Hristov Thank you for all the contributions -- your code was immensely helpful. I think that I am making some progress! If you have any thoughts on the way that things are going with the implementation, please let me know! Thanks again!

[H-G-Hristov]

In D156924#4573070, @hawkinsw wrote:

@H-G-Hristov Thank you for all the contributions -- your code was immensely helpful. I think that I am making some progress! If you have any thoughts on the way that things are going with the implementation, please let me know! Thanks again!

I'm also a rookie at this, so treat anything with a pinch of salt. This is may WIP patch for enumerate_view: https://reviews.llvm.org/D157193 I think it has some similarities to stride_view. It's missing a few converting constructor tests but after rebase it should get green. I do expect that it will get a lot of scrutiny and feedback in the review but I think you can check it for what tests you need to write. I've based my tests on already existing tests.

I am also going to update the status page with my patch soon: https://libcxx.llvm.org/Status/Ranges.html If you like I could update it for you too. Or you can do it yourself.

N.B. You should update the synopses. The implemented papers/issues, etc.

[hawkinsw]

Prevent __to_unsigned_like from using ADL.

[hawkinsw]

Add iterator operator== tests.

[hawkinsw]

Adding some tests (generally making progress).

[hawkinsw]

Add tests for enable_borrowed_range.

[var-const]

@hawkinsw First of all, thanks a ton for working on this! LLVM is currently in the process of moving away from Phabricator to GitHub reviews -- see https://discourse.llvm.org/t/logistics-of-the-transition-to-github-prs-for-libc-libc-abi-libunwind/73130. Since the review for this patch hasn't really started yet, I think it might be easier to open a new review on GitHub. Would that work for you? If you have a strong preference to keep the patch on Phabricator for now, that's ok, I can start the review here.

Btw, can you give a very brief outline on the current status of the patch? I notice it's still marked as WIP so I'm just curious which parts are still TODO. Would it make sense to start reviewing it now, or should I wait a bit? Let me know, and once again, thanks a lot for working on this!

[hawkinsw]

In D156924#4635672, @var-const wrote:

@hawkinsw First of all, thanks a ton for working on this! LLVM is currently in the process of moving away from Phabricator to GitHub reviews -- see https://discourse.llvm.org/t/logistics-of-the-transition-to-github-prs-for-libc-libc-abi-libunwind/73130. Since the review for this patch hasn't really started yet, I think it might be easier to open a new review on GitHub. Would that work for you? If you have a strong preference to keep the patch on Phabricator for now, that's ok, I can start the review here.

You got it!! I want to make sure that I do what the community wants! I will move it there and then comment back here and let you know where it lands!

Btw, can you give a very brief outline on the current status of the patch? I notice it's still marked as WIP so I'm just curious which parts are still TODO. Would it make sense to start reviewing it now, or should I wait a bit? Let me know, and once again, thanks a lot for working on this!

Though I am a long-time C++ developer, I have only submitted minor optimizations to STL implementations to date. So, I _think_ that what is here is something that is roughly complete. What I am adding now are tests and it's taking a little longer than I had hoped. My day job as a teacher just ramped up again.

That is to say, I would love to start getting feedback on the patch and start refining it! I have had so much fun doing the implementation so far and learned a ton. I know that your review will help me learn even more!

Thank you for being so encouraging.

[hawkinsw]

In light of the transition to GitHub PRs, I am closing this revision in favor of https://github.com/llvm/llvm-project/pull/65200.

libcxx/include/CMakeLists.txt

set(files
   __ranges/rbegin.h
   __ranges/ref_view.h
   __ranges/rend.h
   __ranges/repeat_view.h
   __ranges/reverse_view.h
   __ranges/single_view.h
   __ranges/size.h
   __ranges/split_view.h
+  __ranges/stride_view.h
   __ranges/subrange.h
   __ranges/take_view.h
   __ranges/take_while_view.h
   __ranges/to.h
   __ranges/transform_view.h
   __ranges/view_interface.h
   __ranges/views.h
   __ranges/zip_view.h

libcxx/include/__ranges/stride_view.h

+// -*- 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_STRIDE_VIEW_H
+#define _LIBCPP___RANGES_STRIDE_VIEW_H
+
+#include "__compare/three_way_comparable.h"
+#include "__iterator/default_sentinel.h"
+#include <__concepts/convertible_to.h>
+#include <__concepts/constructible.h>
+#include <__config>
+#include <__functional/bind_back.h>
+#include <__iterator/concepts.h>
+#include <__iterator/next.h>
+#include <__iterator/reverse_iterator.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/enable_borrowed_range.h>
+#include <__ranges/non_propagating_cache.h>
+#include <__ranges/range_adaptor.h>
+#include <__ranges/size.h>
+#include <__ranges/subrange.h>
+#include <__ranges/view_interface.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+#include <iterator>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20
+
+namespace ranges {
+  template<input_range _View>
+    requires view<_View>
+  class stride_view : public view_interface<stride_view<_View>> {
+    // We cache begin() whenever ranges::next is not guaranteed O(1) to provide an
+    // amortized O(1) begin() method.
+    using difference_type = range_difference_t<_View>;
+    static constexpr bool _UseCache = !random_access_range<_View> && !common_range<_View>;
+    using _Cache = _If<_UseCache, __non_propagating_cache<reverse_iterator<iterator_t<_View>>>, __empty_cache>;
+
+
+    _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
+    _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
+    difference_type __stride_ = 0;
+
+    template <bool _Const>
+    class __iterator;
+    template <bool _Const>
+    class __sentinel;
+
+  public:
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr explicit stride_view(_View __base, difference_type __stride) : __base_(std::move(__base)), __stride_(__stride) {}
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr _View base() const& requires copy_constructible<_View> { return __base_; }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr _View base() && { return std::move(__base_); }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto stride() { return __stride_; }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto begin() requires (!__simple_view<_View>) {
+      return __iterator<false>{*this, ranges::begin(__base_)};
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto begin() const requires ranges::range<const _View> {
+      return __iterator<true>{*this, ranges::begin(__base_)};
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto end() requires (!__simple_view<_View> && 
+      common_range<_View> &&
+      sized_range<_View> &&
+      forward_range<_View>
+    )
+    {
+      auto __missing = (__stride_ - ranges::distance(__base_) % __stride_) % __stride_;
+      return __iterator<false>{*this, ranges::end(__base_), __missing};
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto end() requires (!__simple_view<_View> && 
+      common_range<_View> &&
+      !bidirectional_range<_View>
+    )
+    {
+      return __iterator<false>{*this, ranges::end(__base_)};
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto end() requires (!__simple_view<_View>) {
+      return std::default_sentinel;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto end() const requires (range<const _View> &&
+      common_range<const _View> &&
+      sized_range<const _View> &&
+      forward_range<const _View>
+    )
+    {
+      auto __missing = (__stride_ - ranges::distance(__base_) % __stride_) % __stride_;
+      return __iterator<true>{*this, ranges::end(__base_), __missing};
+
+    }
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto end() const requires (range<const _View> &&
+      common_range<_View> &&
+      !bidirectional_range<_View>
+    )
+    {
+      return __iterator<true>{*this, ranges::end(__base_)};
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto end() const requires (range<const _View>) {
+      return std::default_sentinel;
+    }
+  }; // class stride_view 
+
+  template<class _View>
+  struct __stride_iterator_category { };
+
+  template<forward_range _View>
+  struct __stride_iterator_category<_View> {
+    using _Cat = typename iterator_traits<iterator_t<_View>>::iterator_category;
+    using iterator_category =
+      _If<derived_from<_Cat, random_access_iterator_tag>, random_access_iterator_tag,
+      /* else */                                          _Cat
+    >;
+  };
+
+  template<input_range _View>
+    requires view<_View>
+  template<bool _Const>
+  class stride_view<_View>::__iterator : public __stride_iterator_category<_View> {
+
+    template <class I>
+    constexpr static I div_ceil(I __left, I __right) {
+      I __r = __left / __right;
+      if (__left % __right) {
+        __r++;
+      }
+      return __r;
+    }
+
+    using _Parent = __maybe_const<_Const, stride_view<_View>>;
+    using _Base = __maybe_const<_Const, _View>;
+
+  public:
+    using difference_type = range_difference_t<_Base>;
+    using value_type = range_value_t<_Base>;
+    using iterator_concept =
+      _If<random_access_range<_Base>, random_access_iterator_tag,
+      _If<bidirectional_range<_Base>, bidirectional_iterator_tag,
+      _If<forward_range<_Base>,       forward_iterator_tag,
+      /* else */                      input_iterator_tag
+    >>>;
+
+    _LIBCPP_NO_UNIQUE_ADDRESS iterator_t<_Base> __current_ = iterator_t<_Base>();
+    _LIBCPP_NO_UNIQUE_ADDRESS ranges::sentinel_t<_Base> __end_ = ranges::sentinel_t<_Base>();
+    difference_type __stride_ = 0;
+    difference_type __missing_ = 0;
+
+    // using iterator_category = inherited;
+    _LIBCPP_HIDE_FROM_ABI
+    __iterator() requires default_initializable<iterator_t<_Base>> = default;
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator(__iterator<!_Const> __i) requires _Const && std::convertible_to<ranges::iterator_t<_View>, ranges::iterator_t<_Base>> && std::convertible_to<ranges::sentinel_t<_View>, ranges::sentinel_t<_Base>>
+      : __current_(std::move(__i.__current_)), __end_(std::move(__i.__end_)), __stride_(__i.__stride_), __missing_(__i.__missing_)
+    { }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator(_Parent &__parent, ranges::iterator_t<_Base> __current, difference_type __missing = 0)
+      : __current_(std::move(__current)), __end_(ranges::end(__parent.__base_)), __stride_(__parent.__stride_), __missing_(__missing)
+    { }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr iterator_t<_View> const& base() const& noexcept { return __current_; }
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr iterator_t<_View> base() && { return std::move(__current_); }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr decltype(auto) operator*() const { return *__current_; }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator& operator++() {
+      __missing_ = ranges::advance(__current_, __stride_, __end_);
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr void operator++(int) { ++*this; }
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator operator++(int) requires forward_range<_Base> {
+      auto __tmp = *this;
+      ++*this;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator& operator--() requires bidirectional_range<_Base> {
+      ranges::advance(__current_, __missing_ - __stride_);
+      __missing_ = 0;
+      return *this;
+    }
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator operator--(int) requires bidirectional_range<_Base> {
+      auto __tmp = *this;
+      --*this;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator& operator+=(difference_type __s) requires random_access_range<_Base> {
+      if (__s > 0) { 
+        ranges::advance(__current_, __stride_ * (__s - 1));
+        __missing_ = ranges::advance(__current_, __stride_, __end_);
+      } else if (__s < 0) {
+        ranges::advance(__current_, __stride_ * __s + __missing_);
+        __missing_ = 0;
+      }
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator& operator-=(difference_type __s) requires random_access_range<_Base> {
+      return *this += -__s;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __iterator& operator[](difference_type __s) const requires random_access_range<_Base> {
+      return *(*this + __s);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr bool operator==(__iterator const& __x, default_sentinel_t) {
+      return __x.__current_ == __x.__end_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr bool operator==(__iterator const& __x, __iterator const& __y)
+      requires equality_comparable<iterator_t<_Base>>
+    {
+      return __x.__current_ == __y.__current_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr bool operator<(__iterator const& __x, __iterator const& __y)
+      requires random_access_range<_Base>
+    {
+      return __x.__current_ < __y.__current_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr bool operator>(__iterator const& __x, __iterator const& __y)
+      requires random_access_range<_Base>
+    {
+      return __y < __x;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr bool operator<=(__iterator const& __x, __iterator const& __y)
+      requires random_access_range<_Base>
+    {
+      return !(__y < __x);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr bool operator>=(__iterator const& __x, __iterator const& __y)
+      requires random_access_range<_Base>
+    {
+      return !(__x < __y);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr bool operator<=>(__iterator const& __x, __iterator const& __y)
+      requires random_access_range<_Base> && three_way_comparable<iterator_t<_Base>>
+    {
+      return __x.__current_ <=> __y.__current_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr __iterator operator+(__iterator const& __i, difference_type __s)
+      requires random_access_range<_Base>
+    {
+      auto __r = __i;
+      __r += __s;
+      return __r;
+    }
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr __iterator operator+(difference_type __s, __iterator const &__i)
+      requires random_access_range<_Base>
+    {
+      auto __r = __i;
+      __r += __s;
+      return __r;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr __iterator operator-(__iterator const& __i, difference_type __s)
+      requires random_access_range<_Base>
+    {
+      auto __r = __i;
+      __r -= __s;
+      return __r;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr difference_type operator-( __iterator const &__x, __iterator const &__y)
+      requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>> && forward_range<_Base>
+    {
+      auto __n = __x.__current_ - __y.__current_;
+      return (__n + __x.__missing_ - __y.__missing_) / __x.__stride_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr difference_type operator-( __iterator const &__x, __iterator const &__y)
+      requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
+    {
+      auto __n = __x.__current_ - __y.__current_;
+      if (__n < 0) {
+        return -div_ceil(-__n, __x.__stride_);
+      }
+      return div_ceil(__n, __x.__stride_);
+    }
+
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr difference_type operator-(default_sentinel_t, __iterator const &__x)
+      requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base>>
+    {
+      return div_ceil(__x.__end_ - __x.__current_, __x.__stride_);
+    }
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr difference_type operator-( __iterator const &__x, default_sentinel_t __y)
+      requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base>>
+    {
+      return -(__y - __x);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr range_rvalue_reference_t<_Base> iter_move(__iterator const& __it)
+      noexcept(noexcept(ranges::iter_move(__it.__current_)))
+    {
+      return ranges::iter_move(__it.__current_);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    friend constexpr void iter_swap(__iterator const& __x, __iterator const& __y)
+      noexcept(noexcept(ranges::iter_swap(__x.__current_, __y.__current_)))
+      requires indirectly_swappable<iterator_t<_Base>>
+    {
+      return ranges::iter_swap(__x.__current_, __y.__current_);
+    }
+  }; // class stride_view::__iterator
+
+  template<input_range _View>
+    requires view<_View>
+  template<bool _Const>
+  class stride_view<_View>::__sentinel {
+  public:
+    sentinel_t<_View> __end_ = sentinel_t<_View>();
+
+    _LIBCPP_HIDE_FROM_ABI
+    __sentinel() = default;
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr explicit __sentinel(stride_view<_View>& __parent)
+      : __end_(ranges::end(__parent.__base_))
+    { }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr sentinel_t<_View> base() const { return __end_; }
+
+    _LIBCPP_HIDE_FROM_ABI friend constexpr bool
+    operator==(__iterator<true> const& __x, __sentinel const& __y) {
+      return __x.__current_ == __y.__end_;
+    }
+  }; // class stride_view::__sentinel
+
+  template<class _Tp>
+  inline constexpr bool enable_borrowed_range<stride_view<_Tp>> = enable_borrowed_range<_Tp>;
+
+  namespace views {
+  namespace __stride {
+    // removed this.
+    //noexcept(noexcept(reverse_view{std::forward<_Range>(__range)}))
+    struct __fn {
+      template<viewable_range _Range>
+      [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
+      constexpr auto operator()(_Range&& __range, range_difference_t<_Range> __n) const
+        -> decltype(      stride_view{std::forward<_Range>(__range), __n})
+        { return          stride_view{std::forward<_Range>(__range), __n}; }
+
+      template<class _Np>
+      [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
+      constexpr auto operator()(_Np &&__n) const
+        { return __range_adaptor_closure_t(std::__bind_back(*this, std::forward<_Np>(__n))); }
+    };
+  } // namespace __stride
+
+  inline namespace __cpo {
+    inline constexpr auto stride = __stride::__fn{};
+  } // namespace __cpo
+  } // namespace views
+} // namespace ranges
+
+#endif // _LIBCPP_STD_VER >= 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___RANGES_STRIDE_VIEW_H

libcxx/include/module.modulemap.in

 module std_private_ranges_repeat_view                [system] { header "__ranges/repeat_view.h" }
 module std_private_ranges_reverse_view               [system] { header "__ranges/reverse_view.h" }
 module std_private_ranges_single_view                [system] { header "__ranges/single_view.h" }
 module std_private_ranges_size                       [system] {
   header "__ranges/size.h"
   export std_private_type_traits_make_unsigned
 }
 module std_private_ranges_split_view                 [system] { header "__ranges/split_view.h" }
+module std_private_ranges_stride_view                 [system] { header "__ranges/stride_view.h" }
 module std_private_ranges_subrange                   [system] {
   header "__ranges/subrange.h"
   export std_private_ranges_subrange_fwd
 }
 module std_private_ranges_subrange_fwd               [system] {
   header "__fwd/subrange.h"
   export std_private_iterator_concepts
 }

libcxx/include/ranges

 #include <__ranges/rbegin.h>
 #include <__ranges/ref_view.h>
 #include <__ranges/rend.h>
 #include <__ranges/repeat_view.h>
 #include <__ranges/reverse_view.h>
 #include <__ranges/single_view.h>
 #include <__ranges/size.h>
 #include <__ranges/split_view.h>
+#include <__ranges/stride_view.h>
 #include <__ranges/subrange.h>
 #include <__ranges/take_view.h>
 #include <__ranges/take_while_view.h>
 #include <__ranges/to.h>
 #include <__ranges/transform_view.h>
 #include <__ranges/view_interface.h>
 #include <__ranges/views.h>
 #include <__ranges/zip_view.h>