[libcxx] adds ranges::fold_left_with_iter and ranges::fold_left (#75259)

Notable things in this commit:

* refactors `__indirect_binary_left_foldable`, making it slightly
different (but equivalent) to _`indirect-binary-left-foldable`_, which
improves readability (a [patch to the Working Paper][patch] was made)
* omits `__cpo` namespace, since it is not required for implementing
niebloids (a cleanup should happen in 2024)
* puts tests ensuring invocable robustness and dangling correctness
inside the correctness testing to ensure that the algorithms' results
are still correct

[patch]: https://github.com/cplusplus/draft/pull/6734

18 files changed, 1062 insertions, 32 deletions

diff --git a/libcxx/docs/Status/RangesAlgorithms.csv b/libcxx/docs/Status/RangesAlgorithms.csv
index 17c8953..2fe530b 100644
--- a/libcxx/docs/Status/RangesAlgorithms.csv
+++ b/libcxx/docs/Status/RangesAlgorithms.csv
@@ -10,3 +10,9 @@ C++23,`shift_right <https://wg21.link/p2440r1>`_,Unassigned,No patch yet,Not sta
 C++23,`iota (algorithm) <https://wg21.link/p2440r1>`_,Unassigned,No patch yet,Not started
 C++23,`fold <https://wg21.link/p2322r5>`_,Unassigned,No patch yet,Not started
 C++23,`contains <https://wg21.link/p2302r2>`_,Zijun Zhao,No patch yet,In Progress
+C++23,`fold_left_with_iter <https://wg21.link/p2322r6>`_,Christopher Di Bella,N/A,Complete
+C++23,`fold_left <https://wg21.link/p2322r6>`_,Christopher Di Bella,N/A,Complete
+C++23,`fold_left_first_with_iter <https://wg21.link/p2322r6>`_,Christopher Di Bella,N/A,In progress
+C++23,`fold_left_first <https://wg21.link/p2322r6>`_,Christopher Di Bella,N/A,In progress
+C++23,`fold_right <https://wg21.link/p2322r6>`_,Christopher Di Bella,N/A,In progress
+C++23,`fold_right_last <https://wg21.link/p2322r6>`_,Christopher Di Bella,N/A,In progress
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index f1e5a24..746d581 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -23,6 +23,7 @@ set(files
   __algorithm/find_if.h
   __algorithm/find_if_not.h
   __algorithm/find_segment_if.h
+  __algorithm/fold.h
   __algorithm/for_each.h
   __algorithm/for_each_n.h
   __algorithm/for_each_segment.h
diff --git a/libcxx/include/__algorithm/fold.h b/libcxx/include/__algorithm/fold.h
new file mode 100644
index 0000000..88e6814
--- /dev/null
+++ b/libcxx/include/__algorithm/fold.h
@@ -0,0 +1,125 @@
+// -*- 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___ALGORITHM_FOLD_H
+#define _LIBCPP___ALGORITHM_FOLD_H
+
+#include <__concepts/assignable.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/invocable.h>
+#include <__concepts/movable.h>
+#include <__config>
+#include <__functional/invoke.h>
+#include <__functional/reference_wrapper.h>
+#include <__iterator/concepts.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__type_traits/decay.h>
+#include <__type_traits/invoke.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+namespace ranges {
+template <class _Ip, class _Tp>
+struct in_value_result {
+  _LIBCPP_NO_UNIQUE_ADDRESS _Ip in;
+  _LIBCPP_NO_UNIQUE_ADDRESS _Tp value;
+
+  template <class _I2, class _T2>
+    requires convertible_to<const _Ip&, _I2> && convertible_to<const _Tp&, _T2>
+  _LIBCPP_HIDE_FROM_ABI constexpr operator in_value_result<_I2, _T2>() const& {
+    return {in, value};
+  }
+
+  template <class _I2, class _T2>
+    requires convertible_to<_Ip, _I2> && convertible_to<_Tp, _T2>
+  _LIBCPP_HIDE_FROM_ABI constexpr operator in_value_result<_I2, _T2>() && {
+    return {std::move(in), std::move(value)};
+  }
+};
+
+template <class _Ip, class _Tp>
+using fold_left_with_iter_result = in_value_result<_Ip, _Tp>;
+
+template <class _Fp, class _Tp, class _Ip, class _Rp, class _Up = decay_t<_Rp>>
+concept __indirectly_binary_left_foldable_impl =
+    convertible_to<_Rp, _Up> &&                    //
+    movable<_Tp> &&                                //
+    movable<_Up> &&                                //
+    convertible_to<_Tp, _Up> &&                    //
+    invocable<_Fp&, _Up, iter_reference_t<_Ip>> && //
+    assignable_from<_Up&, invoke_result_t<_Fp&, _Up, iter_reference_t<_Ip>>>;
+
+template <class _Fp, class _Tp, class _Ip>
+concept __indirectly_binary_left_foldable =
+    copy_constructible<_Fp> &&                     //
+    invocable<_Fp&, _Tp, iter_reference_t<_Ip>> && //
+    __indirectly_binary_left_foldable_impl<_Fp, _Tp, _Ip, invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+
+struct __fold_left_with_iter {
+  template <input_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, __indirectly_binary_left_foldable<_Tp, _Ip> _Fp>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr auto
+  operator()(_Ip __first, _Sp __last, _Tp __init, _Fp __f) {
+    using _Up = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+
+    if (__first == __last) {
+      return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), _Up(std::move(__init))};
+    }
+
+    _Up __result = std::invoke(__f, std::move(__init), *__first);
+    for (++__first; __first != __last; ++__first) {
+      __result = std::invoke(__f, std::move(__result), *__first);
+    }
+
+    return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), std::move(__result)};
+  }
+
+  template <input_range _Rp, class _Tp, __indirectly_binary_left_foldable<_Tp, iterator_t<_Rp>> _Fp>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Rp&& __r, _Tp __init, _Fp __f) {
+    auto __result = operator()(ranges::begin(__r), ranges::end(__r), std::move(__init), std::ref(__f));
+
+    using _Up = decay_t<invoke_result_t<_Fp&, _Tp, range_reference_t<_Rp>>>;
+    return fold_left_with_iter_result<borrowed_iterator_t<_Rp>, _Up>{std::move(__result.in), std::move(__result.value)};
+  }
+};
+
+inline constexpr auto fold_left_with_iter = __fold_left_with_iter();
+
+struct __fold_left {
+  template <input_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, __indirectly_binary_left_foldable<_Tp, _Ip> _Fp>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr auto
+  operator()(_Ip __first, _Sp __last, _Tp __init, _Fp __f) {
+    return fold_left_with_iter(std::move(__first), std::move(__last), std::move(__init), std::ref(__f)).value;
+  }
+
+  template <input_range _Rp, class _Tp, __indirectly_binary_left_foldable<_Tp, iterator_t<_Rp>> _Fp>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Rp&& __r, _Tp __init, _Fp __f) {
+    return fold_left_with_iter(ranges::begin(__r), ranges::end(__r), std::move(__init), std::ref(__f)).value;
+  }
+};
+
+inline constexpr auto fold_left = __fold_left();
+} // namespace ranges
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ALGORITHM_FOLD_H
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 62dbec4..1176602 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -42,6 +42,9 @@ namespace ranges {
   template <class I>
     struct in_found_result;              // since C++20
 
+  template <class I, class T>
+    struct in_value_result;              // since C++23
+
   template<forward_iterator I, sentinel_for<I> S, class Proj = identity,
     indirect_strict_weak_order<projected<I, Proj>> Comp = ranges::less>                                   // since C++20
   constexpr I min_element(I first, S last, Comp comp = {}, Proj proj = {});
@@ -881,6 +884,23 @@ namespace ranges {
       ranges::search_n(R&& r, range_difference_t<R> count,
                        const T& value, Pred pred = {}, Proj proj = {});                             // since C++20
 
+  template<input_iterator I, sentinel_for<I> S, class T,
+           indirectly-binary-left-foldable<T, I> F>
+    constexpr auto ranges::fold_left(I first, S last, T init, F f);                                 // since C++23
+
+  template<input_range R, class T, indirectly-binary-left-foldable<T, iterator_t<R>> F>
+    constexpr auto fold_left(R&& r, T init, F f);                                                   // since C++23
+
+  template<class I, class T>
+    using fold_left_with_iter_result = in_value_result<I, T>;                                       // since C++23
+
+  template<input_iterator I, sentinel_for<I> S, class T,
+           indirectly-binary-left-foldable<T, I> F>
+    constexpr see below fold_left_with_iter(I first, S last, T init, F f);                          // since C++23
+
+  template<input_range R, class T, indirectly-binary-left-foldable<T, iterator_t<R>> F>
+    constexpr see below fold_left_with_iter(R&& r, T init, F f);                                    // since C++23
+
   template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, sentinel_for<I2> S2,
            class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
     requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
@@ -1786,6 +1806,7 @@ template <class BidirectionalIterator, class Compare>
 #include <__algorithm/find_first_of.h>
 #include <__algorithm/find_if.h>
 #include <__algorithm/find_if_not.h>
+#include <__algorithm/fold.h>
 #include <__algorithm/for_each.h>
 #include <__algorithm/for_each_n.h>
 #include <__algorithm/generate.h>
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 49d5e52..a37e962 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -666,6 +666,7 @@ module std_private_algorithm_find_first_of                               [system
 module std_private_algorithm_find_if                                     [system] { header "__algorithm/find_if.h" }
 module std_private_algorithm_find_if_not                                 [system] { header "__algorithm/find_if_not.h" }
 module std_private_algorithm_find_segment_if                             [system] { header "__algorithm/find_segment_if.h" }
+module std_private_algorithm_fold                                        [system] { header "__algorithm/fold.h" }
 module std_private_algorithm_for_each                                    [system] { header "__algorithm/for_each.h" }
 module std_private_algorithm_for_each_n                                  [system] { header "__algorithm/for_each_n.h" }
 module std_private_algorithm_for_each_segment                            [system] { header "__algorithm/for_each_segment.h" }
diff --git a/libcxx/modules/std/algorithm.inc b/libcxx/modules/std/algorithm.inc
index 246b55c..f6b35ef 100644
--- a/libcxx/modules/std/algorithm.inc
+++ b/libcxx/modules/std/algorithm.inc
@@ -16,7 +16,7 @@ export namespace std {
     using std::ranges::in_in_result;
     using std::ranges::in_out_out_result;
     using std::ranges::in_out_result;
-    // using std::ranges::in_value_result;
+    using std::ranges::in_value_result;
     using std::ranges::min_max_result;
     // using std::ranges::out_value_result;
   } // namespace ranges
@@ -157,15 +157,15 @@ export namespace std {
     // [alg.ends.with], ends with
     using std::ranges::ends_with;
 
-#  if 0
     // [alg.fold], fold
     using std::ranges::fold_left;
+    using std::ranges::fold_left_with_iter;
+    using std::ranges::fold_left_with_iter_result;
+#  if 0
     using std::ranges::fold_left_first;
     using std::ranges::fold_right;
     using std::ranges::fold_right_last;
     using std::ranges::fold_left_with_iter;
-    using std::ranges::fold_left_with_iter_result;
-    using std::ranges::fold_left_with_iter;
     using std::ranges::fold_left_first_with_iter;
     using std::ranges::fold_left_first_with_iter;
 #  endif
diff --git a/libcxx/test/libcxx/diagnostics/ranges.nodiscard_extensions.verify.cpp b/libcxx/test/libcxx/diagnostics/ranges.nodiscard_extensions.verify.cpp
index d11dcfc..f0a0e48 100644
--- a/libcxx/test/libcxx/diagnostics/ranges.nodiscard_extensions.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/ranges.nodiscard_extensions.verify.cpp
@@ -12,6 +12,8 @@
 
 #include <algorithm>
 
+#include "test_macros.h"
+
 void test() {
   int range[1];
   int* iter = range;
@@ -87,4 +89,15 @@ void test() {
   std::ranges::unique(iter, iter); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   std::ranges::upper_bound(range, 1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   std::ranges::upper_bound(iter, iter, 1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+#if TEST_STD_VER >= 23
+  std::ranges::fold_left(range, 0, std::plus());
+  // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::ranges::fold_left(iter, iter, 0, std::plus());
+  // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::ranges::fold_left_with_iter(range, 0, std::plus());
+  // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::ranges::fold_left_with_iter(iter, iter, 0, std::plus());
+  // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
 }
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
new file mode 100644
index 0000000..cf089b2
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
@@ -0,0 +1,320 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// template<input_iterator I, sentinel_for<I> S, class T,
+//          indirectly-binary-left-foldable<T, I> F>
+//   constexpr see below ranges::fold_left_with_iter(I first, S last, T init, F f);
+//
+// template<input_range R, class T, indirectly-binary-left-foldable<T, iterator_t<R>> F>
+//   constexpr see below ranges::fold_left_with_iter(R&& r, T init, F f);
+
+// template<input_iterator I, sentinel_for<I> S, class T,
+//          indirectly-binary-left-foldable<T, I> F>
+//   constexpr see below ranges::fold_left(I first, S last, T init, F f);
+//
+// template<input_range R, class T, indirectly-binary-left-foldable<T, iterator_t<R>> F>
+//   constexpr see below ranges::fold_left(R&& r, T init, F f);
+
+#include <algorithm>
+#include <cassert>
+#include <concepts>
+#include <deque>
+#include <forward_list>
+#include <functional>
+#include <iterator>
+#include <list>
+#include <ranges>
+#include <string_view>
+#include <string>
+#include <vector>
+
+#include "test_macros.h"
+#include "test_range.h"
+#include "invocable_with_telemetry.h"
+#include "maths.h"
+
+#if !defined(TEST_HAS_NO_LOCALIZATION)
+#  include <sstream>
+#endif
+
+using std::ranges::fold_left;
+using std::ranges::fold_left_with_iter;
+
+template <class Result, class Range, class T>
+concept is_in_value_result =
+    std::same_as<Result, std::ranges::fold_left_with_iter_result<std::ranges::iterator_t<Range>, T>>;
+
+template <class Result, class T>
+concept is_dangling_with = std::same_as<Result, std::ranges::fold_left_with_iter_result<std::ranges::dangling, T>>;
+
+struct Long {
+  int value;
+
+  constexpr Long(int const x) : value(x) {}
+
+  constexpr Long plus(int const x) const { return Long{value + x}; }
+
+  friend constexpr bool operator==(Long const& x, Long const& y) = default;
+};
+
+template <std::ranges::input_range R, class T, class F, std::equality_comparable Expected>
+  requires std::copyable<R>
+constexpr void check_iterator(R& r, T const& init, F f, Expected const& expected) {
+  {
+    is_in_value_result<R, Expected> decltype(auto) result = fold_left_with_iter(r.begin(), r.end(), init, f);
+    assert(result.in == r.end());
+    assert(result.value == expected);
+  }
+  {
+    auto telemetry                                        = invocable_telemetry();
+    auto f2                                               = invocable_with_telemetry(f, telemetry);
+    is_in_value_result<R, Expected> decltype(auto) result = fold_left_with_iter(r.begin(), r.end(), init, f2);
+    assert(result.in == r.end());
+    assert(result.value == expected);
+    assert(telemetry.invocations == std::ranges::distance(r));
+    assert(telemetry.moves == 0);
+    assert(telemetry.copies == 1);
+  }
+
+  {
+    std::same_as<Expected> decltype(auto) result = fold_left(r.begin(), r.end(), init, f);
+    assert(result == expected);
+  }
+  {
+    auto telemetry                               = invocable_telemetry();
+    auto f2                                      = invocable_with_telemetry(f, telemetry);
+    std::same_as<Expected> decltype(auto) result = fold_left(r.begin(), r.end(), init, f2);
+    assert(result == expected);
+    assert(telemetry.invocations == std::ranges::distance(r));
+    assert(telemetry.moves == 0);
+    assert(telemetry.copies == 1);
+  }
+}
+
+template <std::ranges::input_range R, class T, class F, std::equality_comparable Expected>
+  requires std::copyable<R>
+constexpr void check_lvalue_range(R& r, T const& init, F f, Expected const& expected) {
+  {
+    is_in_value_result<R, Expected> decltype(auto) result = fold_left_with_iter(r, init, f);
+    assert(result.in == r.end());
+    assert(result.value == expected);
+  }
+  {
+    auto telemetry                               = invocable_telemetry();
+    auto f2                                      = invocable_with_telemetry(f, telemetry);
+    std::same_as<Expected> decltype(auto) result = fold_left(r, init, f2);
+    assert(result == expected);
+    assert(telemetry.invocations == std::ranges::distance(r));
+    assert(telemetry.moves == 0);
+    assert(telemetry.copies == 1);
+  }
+
+  {
+    std::same_as<Expected> decltype(auto) result = fold_left(r, init, f);
+    assert(result == expected);
+  }
+  {
+    auto telemetry                               = invocable_telemetry();
+    auto f2                                      = invocable_with_telemetry(f, telemetry);
+    std::same_as<Expected> decltype(auto) result = fold_left(r, init, f2);
+    assert(result == expected);
+    assert(telemetry.invocations == std::ranges::distance(r));
+    assert(telemetry.moves == 0);
+    assert(telemetry.copies == 1);
+  }
+}
+
+template <std::ranges::input_range R, class T, class F, std::equality_comparable Expected>
+  requires std::copyable<R>
+constexpr void check_rvalue_range(R& r, T const& init, F f, Expected const& expected) {
+  {
+    auto r2                                          = r;
+    is_dangling_with<Expected> decltype(auto) result = fold_left_with_iter(std::move(r2), init, f);
+    assert(result.value == expected);
+  }
+  {
+    auto telemetry                                   = invocable_telemetry();
+    auto f2                                          = invocable_with_telemetry(f, telemetry);
+    auto r2                                          = r;
+    is_dangling_with<Expected> decltype(auto) result = fold_left_with_iter(std::move(r2), init, f2);
+    assert(result.value == expected);
+    assert(telemetry.invocations == std::ranges::distance(r));
+    assert(telemetry.moves == 0);
+    assert(telemetry.copies == 1);
+  }
+
+  {
+    auto r2                                      = r;
+    std::same_as<Expected> decltype(auto) result = fold_left(std::move(r2), init, f);
+    assert(result == expected);
+  }
+  {
+    auto telemetry                               = invocable_telemetry();
+    auto f2                                      = invocable_with_telemetry(f, telemetry);
+    auto r2                                      = r;
+    std::same_as<Expected> decltype(auto) result = fold_left(std::move(r2), init, f2);
+    assert(result == expected);
+    assert(telemetry.invocations == std::ranges::distance(r));
+    assert(telemetry.moves == 0);
+    assert(telemetry.copies == 1);
+  }
+}
+
+template <std::ranges::input_range R, class T, class F, std::equality_comparable Expected>
+  requires std::copyable<R>
+constexpr void check(R r, T const& init, F f, Expected const& expected) {
+  check_iterator(r, init, f, expected);
+  check_lvalue_range(r, init, f, expected);
+  check_rvalue_range(r, init, f, expected);
+}
+
+constexpr void empty_range_test_case() {
+  auto const data = std::vector<int>{};
+  check(data, 100, std::plus(), 100);
+  check(data, -100, std::multiplies(), -100);
+
+  check(data | std::views::take_while([](auto) { return false; }), 1.23, std::plus(), 1.23);
+  check(data, Long(52), &Long::plus, Long(52));
+}
+
+constexpr void common_range_test_case() {
+  auto const data = std::vector<int>{1, 2, 3, 4};
+  check(data, 0, std::plus(), triangular_sum(data));
+  check(data, 1, std::multiplies(), factorial(data.back()));
+
+  auto multiply_with_prev = [n = 1](auto const x, auto const y) mutable {
+    auto const result = x * y * n;
+    n                 = y;
+    return static_cast<std::size_t>(result);
+  };
+  check(data, 1, multiply_with_prev, factorial(data.size()) * factorial(data.size() - 1));
+
+  auto fib = [n = 1](auto x, auto) mutable {
+    auto old_x = x;
+    x += n;
+    n = old_x;
+    return x;
+  };
+  check(data, 0, fib, fibonacci(data.back()));
+
+  check(data, Long(0), &Long::plus, Long(triangular_sum(data)));
+}
+
+constexpr void non_common_range_test_case() {
+  auto parse = [](std::string_view const s) {
+    return s == "zero"  ? 0.0
+         : s == "one"   ? 1.0
+         : s == "two"   ? 2.0
+         : s == "three" ? 3.0
+         : s == "four"  ? 4.0
+         : s == "five"  ? 5.0
+         : s == "six"   ? 6.0
+         : s == "seven" ? 7.0
+         : s == "eight" ? 8.0
+         : s == "nine"  ? 9.0
+                        : (assert(false), 10.0); // the number here is arbitrary
+  };
+
+  {
+    auto data  = std::vector<std::string>{"five", "three", "two", "six", "one", "four"};
+    auto range = data | std::views::transform(parse);
+    check(range, 0, std::plus(), triangular_sum(range));
+  }
+
+  {
+    auto data           = std::string("five three two six one four");
+    auto to_string_view = [](auto&& r) {
+      auto const n = std::ranges::distance(r);
+      return std::string_view(&*r.begin(), n);
+    };
+    auto range =
+        std::views::lazy_split(data, ' ') | std::views::transform(to_string_view) | std::views::transform(parse);
+    check(range, 0, std::plus(), triangular_sum(range));
+  }
+}
+
+constexpr bool test_case() {
+  empty_range_test_case();
+  common_range_test_case();
+  non_common_range_test_case();
+  return true;
+}
+
+// Most containers aren't constexpr
+void runtime_only_test_case() {
+#if !defined(TEST_HAS_NO_LOCALIZATION)
+  { // istream_view is a genuine input range and needs specific handling.
+    constexpr auto raw_data = "Shells Orange Syrup Baratie Cocoyashi Loguetown";
+    constexpr auto expected = "WindmillShellsOrangeSyrupBaratieCocoyashiLoguetown";
+    auto const init         = std::string("Windmill");
+
+    {
+      auto input = std::istringstream(raw_data);
+      auto data  = std::views::istream<std::string>(input);
+      is_in_value_result<std::ranges::basic_istream_view<std::string, char>, std::string> decltype(auto) result =
+          fold_left_with_iter(data.begin(), data.end(), init, std::plus());
+
+      assert(result.in == data.end());
+      assert(result.value == expected);
+    }
+    {
+      auto input = std::istringstream(raw_data);
+      auto data  = std::views::istream<std::string>(input);
+      is_in_value_result<std::ranges::basic_istream_view<std::string, char>, std::string> decltype(auto) result =
+          fold_left_with_iter(data, init, std::plus());
+      assert(result.in == data.end());
+      assert(result.value == expected);
+    }
+    {
+      auto input = std::istringstream(raw_data);
+      auto data  = std::views::istream<std::string>(input);
+      assert(fold_left(data.begin(), data.end(), init, std::plus()) == expected);
+    }
+    {
+      auto input = std::istringstream(raw_data);
+      auto data  = std::views::istream<std::string>(input);
+      assert(fold_left(data, init, std::plus()) == expected);
+    }
+  }
+#endif
+  {
+    auto const data     = std::forward_list<int>{1, 3, 5, 7, 9};
+    auto const n        = std::ranges::distance(data);
+    auto const expected = static_cast<float>(n * n); // sum of n consecutive odd numbers = n^2
+    check(data, 0.0f, std::plus(), expected);
+  }
+
+  {
+    auto const data     = std::list<int>{2, 4, 6, 8, 10, 12};
+    auto const expected = triangular_sum(data);
+    check(data, 0, std::plus<long>(), static_cast<long>(expected));
+  }
+
+  {
+    auto const data     = std::deque<double>{-1.1, -2.2, -3.3, -4.4, -5.5, -6.6};
+    auto plus           = [](int const x, double const y) { return x + y; };
+    auto const expected = -21.6; // int(  0.0) + -1.1 =   0 + -1.1 =  -1.1
+                                 // int(- 1.1) + -2.2 = - 1 + -2.2 =  -3.2
+                                 // int(- 3.2) + -3.3 = - 3 + -3.3 =  -6.3
+                                 // int(- 6.3) + -4.4 = - 6 + -4.4 = -10.4
+                                 // int(-10.4) + -5.5 = -10 + -5.5 = -15.5
+                                 // int(-15.5) + -6.6 = -15 + -6.6 = -21.6.
+    check(data, 0.0, plus, expected);
+  }
+}
+
+int main(int, char**) {
+  test_case();
+  static_assert(test_case());
+  runtime_only_test_case();
+  return 0;
+}
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/requirements.compile.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/requirements.compile.pass.cpp
new file mode 100644
index 0000000..cad96d4
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/requirements.compile.pass.cpp
@@ -0,0 +1,259 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// Checks that `std::ranges::fold_left_with_iter`'s requirements reject parameters that don't meet
+// the overloads' constraints.
+
+#include <algorithm>
+#include <concepts>
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <ranges>
+
+#include "test_iterators.h"
+
+// FIXME(cjdb): deduplicate
+struct bad_iterator_category {
+  using value_type        = int;
+  using difference_type   = std::ptrdiff_t;
+  using iterator_category = void;
+
+  value_type operator*() const;
+
+  bad_iterator_category& operator++();
+  void operator++(int);
+};
+
+// Covers indirectly_readable<I> too
+template <std::input_or_output_iterator T>
+  requires(!std::input_iterator<T>)
+void requires_input_iterator() {
+  struct bad_range {
+    T begin();
+    std::unreachable_sentinel_t end();
+  };
+
+  static_assert(!requires(bad_range r) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), std::unreachable_sentinel, 0, std::plus());
+  });
+  static_assert(!requires(bad_range r) { std::ranges::fold_left_with_iter(r, 0, std::plus()); });
+
+  static_assert(!requires(bad_range r) {
+    std::ranges::fold_left(r.begin(), r.end(), std::unreachable_sentinel, 0, std::plus());
+  });
+
+  static_assert(!requires(bad_range r) { std::ranges::fold_left(r, 0, std::plus()); });
+}
+
+template <std::equality_comparable S>
+  requires(!std::sentinel_for<int*, S>)
+void requires_sentinel() {
+  static_assert(!requires(S first, S last) { std::ranges::fold_left_with_iter(first, last, 0, std::plus()); });
+  static_assert(!requires(S first, S last) { std::ranges::fold_left(first, last, 0, std::plus()); });
+}
+
+struct non_copy_constructible_callable {
+  non_copy_constructible_callable(non_copy_constructible_callable&&)      = default;
+  non_copy_constructible_callable(non_copy_constructible_callable const&) = delete;
+
+  int operator()(int, int) const;
+};
+
+template <class F>
+  requires(!std::copy_constructible<F>)
+void requires_copy_constructible_F() {
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), 0, std::move(f));
+  });
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) {
+    std::ranges::fold_left_with_iter(r, 0, std::move(f));
+  });
+
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) {
+    std::ranges::fold_left(r.begin(), r.end(), 0, std::move(f));
+  });
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) { std::ranges::fold_left(r, 0, std::move(f)); });
+}
+
+struct not_invocable_with_lvalue_rhs {
+  int operator()(int, int&&);
+};
+
+template <class F>
+  requires(!std::invocable<F&, int, std::iter_reference_t<int*>>)
+void requires_raw_invocable() {
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), 0, f);
+  });
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) { std::ranges::fold_left_with_iter(r, 0, f); });
+
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) {
+    std::ranges::fold_left(r.begin(), r.end(), 0, f);
+  });
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, F f) { std::ranges::fold_left(r, 0, f); });
+}
+
+struct S {};
+
+struct non_decayable_result {
+  S volatile& operator()(S, S) const;
+};
+
+template <std::invocable<S, std::iter_reference_t<S*>> F>
+  requires(!std::convertible_to<std::invoke_result_t<F&, S, std::iter_reference_t<S*>>,
+                                std::decay_t<std::invoke_result_t<F&, S, std::iter_reference_t<S*>>>>)
+void requires_decaying_invoke_result() {
+  static_assert(!requires(std::ranges::subrange<S*, S*> r, S init, F f) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), init, f);
+  });
+  static_assert(!requires(std::ranges::subrange<S*, S*> r, S init, F f) {
+    std::ranges::fold_left_with_iter(r, init, f);
+  });
+
+  static_assert(!requires(std::ranges::subrange<S*, S*> r, S init, F f) {
+    std::ranges::fold_left(r.begin(), r.end(), init, f);
+  });
+  static_assert(!requires(std::ranges::subrange<S*, S*> r, S init, F f) { std::ranges::fold_left(r, init, f); });
+}
+
+struct non_movable {
+  non_movable(int);
+  non_movable(non_movable&&) = delete;
+
+  int apply(non_movable const&) const;
+};
+
+template <class T>
+  requires(!std::movable<T>)
+void requires_movable_init() {
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), init, &T::apply);
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) {
+    std::ranges::fold_left_with_iter(r, init, &T::apply);
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) {
+    std::ranges::fold_left(r.begin(), r.end(), init, &T::apply);
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) { std::ranges::fold_left(r, init, &T::apply); });
+}
+
+struct result_not_movable_after_decay {
+  result_not_movable_after_decay(int);
+  result_not_movable_after_decay(result_not_movable_after_decay&&) = delete;
+  result_not_movable_after_decay(result_not_movable_after_decay const&);
+
+  friend result_not_movable_after_decay const& operator+(int, result_not_movable_after_decay const&);
+  friend result_not_movable_after_decay const& operator+(result_not_movable_after_decay const&, int);
+  friend result_not_movable_after_decay const&
+  operator+(result_not_movable_after_decay const&, result_not_movable_after_decay const&);
+};
+
+template <class T>
+  requires(!std::movable<T>)
+void requires_movable_decayed() {
+  static_assert(!requires(std::ranges::subrange<T*, T*> r) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), 0, std::plus());
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r) { std::ranges::fold_left_with_iter(r, 0, std::plus()); });
+
+  static_assert(!requires(std::ranges::subrange<T*, T*> r) {
+    std::ranges::fold_left(r.begin(), r.end(), 0, T::apply);
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r) { std::ranges::fold_left(r, 0, std::plus()); });
+}
+
+struct not_convertible_to_int {
+  friend int operator+(not_convertible_to_int, not_convertible_to_int);
+  friend int operator+(not_convertible_to_int, int);
+  friend int operator+(int, not_convertible_to_int);
+};
+
+template <class T>
+  requires(!std::convertible_to<T, int>)
+void requires_init_is_convertible_to_decayed() {
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, T init) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), init, std::plus());
+  });
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, T init) {
+    std::ranges::fold_left_with_iter(r, init, std::plus());
+  });
+
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, T init) {
+    std::ranges::fold_left(r.begin(), r.end(), init, std::plus());
+  });
+  static_assert(!requires(std::ranges::subrange<int*, int*> r, T init) {
+    std::ranges::fold_left(r, init, std::plus());
+  });
+}
+
+struct not_invocable_with_decayed {
+  not_invocable_with_decayed(int);
+  friend not_invocable_with_decayed& operator+(int, not_invocable_with_decayed&);
+  friend not_invocable_with_decayed& operator+(not_invocable_with_decayed&, int);
+  friend not_invocable_with_decayed& operator+(not_invocable_with_decayed volatile&, not_invocable_with_decayed&);
+};
+
+template <class T>
+  requires(!std::invocable<std::plus<>&, T, T&>)
+void requires_invocable_with_decayed() {
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, int init) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), init, std::plus());
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, int init) {
+    std::ranges::fold_left_with_iter(r, init, std::plus());
+  });
+
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, int init) {
+    std::ranges::fold_left(r.begin(), r.end(), init, std::plus());
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, int init) { std::ranges::fold_left(r, init, std::plus()); });
+}
+
+struct not_assignable_to_decayed {
+  not_assignable_to_decayed();
+  not_assignable_to_decayed(not_assignable_to_decayed&);
+  not_assignable_to_decayed(not_assignable_to_decayed const&);
+  not_assignable_to_decayed(not_assignable_to_decayed volatile&);
+  not_assignable_to_decayed(not_assignable_to_decayed const volatile&);
+  friend not_assignable_to_decayed volatile& operator+(not_assignable_to_decayed, not_assignable_to_decayed);
+};
+
+template <class T>
+  requires(!std::assignable_from<T&, T volatile&>)
+void requires_assignable_from_invoke_result() {
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) {
+    std::ranges::fold_left_with_iter(r.begin(), r.end(), init, std::plus());
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) {
+    std::ranges::fold_left_with_iter(r, init, std::plus());
+  });
+
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) {
+    std::ranges::fold_left(r.begin(), r.end(), init, std::plus());
+  });
+  static_assert(!requires(std::ranges::subrange<T*, T*> r, T init) { std::ranges::fold_left(r, init, std::plus()); });
+}
+
+void test() {
+  requires_input_iterator<bad_iterator_category>();
+  requires_sentinel<cpp17_input_iterator<int*>>();
+  requires_copy_constructible_F<non_copy_constructible_callable>();
+  requires_raw_invocable<not_invocable_with_lvalue_rhs>();
+  requires_decaying_invoke_result<non_decayable_result>();
+  requires_movable_init<non_movable>();
+  requires_movable_decayed<result_not_movable_after_decay>();
+  requires_init_is_convertible_to_decayed<not_convertible_to_int>();
+  requires_invocable_with_decayed<not_invocable_with_decayed>();
+  requires_assignable_from_invoke_result<not_assignable_to_decayed>();
+}
diff --git a/libcxx/test/std/algorithms/algorithms.results/in_value_result.pass.cpp b/libcxx/test/std/algorithms/algorithms.results/in_value_result.pass.cpp
new file mode 100644
index 0000000..f4023b8
--- /dev/null
+++ b/libcxx/test/std/algorithms/algorithms.results/in_value_result.pass.cpp
@@ -0,0 +1,104 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// template <class I, class T>
+// struct in_value_result;
+
+#include <algorithm>
+#include <cassert>
+#include <type_traits>
+#include <utility>
+
+#include "MoveOnly.h"
+
+struct A {
+  explicit A(int);
+};
+// no implicit conversion
+static_assert(!std::is_constructible_v<std::ranges::in_value_result<A, A>, std::ranges::in_value_result<int, int>>);
+
+struct B {
+  B(int);
+};
+// implicit conversion
+static_assert(std::is_constructible_v<std::ranges::in_value_result<B, B>, std::ranges::in_value_result<int, int>>);
+static_assert(std::is_constructible_v<std::ranges::in_value_result<B, B>, std::ranges::in_value_result<int, int>&>);
+static_assert(
+    std::is_constructible_v<std::ranges::in_value_result<B, B>, const std::ranges::in_value_result<int, int>>);
+static_assert(
+    std::is_constructible_v<std::ranges::in_value_result<B, B>, const std::ranges::in_value_result<int, int>&>);
+
+struct C {
+  C(int&);
+};
+static_assert(!std::is_constructible_v<std::ranges::in_value_result<C, C>, std::ranges::in_value_result<int, int>&>);
+
+// has to be convertible via const&
+static_assert(std::is_convertible_v<std::ranges::in_value_result<int, int>&, std::ranges::in_value_result<long, long>>);
+static_assert(
+    std::is_convertible_v<const std::ranges::in_value_result<int, int>&, std::ranges::in_value_result<long, long>>);
+static_assert(
+    std::is_convertible_v<std::ranges::in_value_result<int, int>&&, std::ranges::in_value_result<long, long>>);
+static_assert(
+    std::is_convertible_v<const std::ranges::in_value_result<int, int>&&, std::ranges::in_value_result<long, long>>);
+
+// should be move constructible
+static_assert(std::is_move_constructible_v<std::ranges::in_value_result<MoveOnly, int>>);
+static_assert(std::is_move_constructible_v<std::ranges::in_value_result<int, MoveOnly>>);
+
+// should not copy constructible with move-only type
+static_assert(!std::is_copy_constructible_v<std::ranges::in_value_result<MoveOnly, int>>);
+static_assert(!std::is_copy_constructible_v<std::ranges::in_value_result<int, MoveOnly>>);
+
+struct NotConvertible {};
+// conversions should not work if there is no conversion
+static_assert(
+    !std::is_convertible_v<std::ranges::in_value_result<NotConvertible, int>, std::ranges::in_value_result<int, int>>);
+static_assert(
+    !std::is_convertible_v<std::ranges::in_value_result<int, NotConvertible>, std::ranges::in_value_result<int, int>>);
+
+template <class T>
+struct ConvertibleFrom {
+  constexpr ConvertibleFrom(T c) : content{c} {}
+  T content;
+};
+
+constexpr bool test() {
+  {
+    std::ranges::in_value_result<int, double> res{10, 0.};
+    assert(res.in == 10);
+    assert(res.value == 0.);
+    std::ranges::in_value_result<ConvertibleFrom<int>, ConvertibleFrom<double>> res2 = res;
+    assert(res2.in.content == 10);
+    assert(res2.value.content == 0.);
+  }
+  {
+    std::ranges::in_value_result<MoveOnly, int> res{MoveOnly{}, 2};
+    assert(res.in.get() == 1);
+    assert(res.value == 2);
+    auto res2 = static_cast<std::ranges::in_value_result<MoveOnly, int>>(std::move(res));
+    assert(res.in.get() == 0);
+    assert(res2.in.get() == 1);
+    assert(res2.value == 2);
+  }
+  {
+    auto [in, value] = std::ranges::in_value_result<int, int>{1, 2};
+    assert(in == 1);
+    assert(value == 2);
+  }
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/algorithms/algorithms.results/no_unique_address.compile.pass.cpp b/libcxx/test/std/algorithms/algorithms.results/no_unique_address.compile.pass.cpp
index 8e0a819..34dbd64 100644
--- a/libcxx/test/std/algorithms/algorithms.results/no_unique_address.compile.pass.cpp
+++ b/libcxx/test/std/algorithms/algorithms.results/no_unique_address.compile.pass.cpp
@@ -53,6 +53,14 @@ static_assert(sizeof(std::ranges::in_out_out_result<Empty, Empty, char>) == 2);
 static_assert(sizeof(std::ranges::in_out_out_result<int, Empty, Empty2>) == sizeof(int));
 static_assert(sizeof(std::ranges::in_out_out_result<Empty, Empty, Empty>) == 3);
 
+#if TEST_STD_VER >= 23
+
+static_assert(sizeof(std::ranges::in_value_result<Empty, int>) == sizeof(int));
+static_assert(sizeof(std::ranges::in_value_result<int, Empty>) == sizeof(int));
+static_assert(sizeof(std::ranges::in_value_result<Empty, Empty2>) == sizeof(Empty2));
+
+#endif // TEST_STD_VER
+
 // In min_max_result both elements have the same type, so they can't have the same address.
 // So the only way to test that [[no_unique_address]] is used is to have it in another struct
 struct MinMaxNoUniqueAddress {
diff --git a/libcxx/test/std/algorithms/ranges_result_alias_declarations.compile.pass.cpp b/libcxx/test/std/algorithms/ranges_result_alias_declarations.compile.pass.cpp
index a72c3a3..6940b23 100644
--- a/libcxx/test/std/algorithms/ranges_result_alias_declarations.compile.pass.cpp
+++ b/libcxx/test/std/algorithms/ranges_result_alias_declarations.compile.pass.cpp
@@ -16,6 +16,8 @@
 #include <memory>
 #include <type_traits>
 
+#include "test_macros.h"
+
 using namespace std::ranges;
 
 static_assert(std::is_same_v<in_fun_result<int, long>, for_each_result<int, long>>);
@@ -59,4 +61,10 @@ static_assert(std::is_same_v<min_max_result<int>, minmax_element_result<int>>);
 static_assert(std::is_same_v<in_found_result<int>, next_permutation_result<int>>);
 static_assert(std::is_same_v<in_found_result<int>, prev_permutation_result<int>>);
 
+#if TEST_STD_VER >= 23
+
+static_assert(std::is_same_v<in_value_result<int, long>, fold_left_with_iter_result<int, long>>);
+
 // static_assert(std::is_same_v<out_value_result<int>, iota_result<int>>);
+
+#endif // TEST_STD_VER
diff --git a/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.input/input_iterator.compile.pass.cpp b/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.input/input_iterator.compile.pass.cpp
index 217c68f..0c72c70 100644
--- a/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.input/input_iterator.compile.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.input/input_iterator.compile.pass.cpp
@@ -81,6 +81,7 @@ struct not_indirectly_readable {
 };
 static_assert(!std::indirectly_readable<not_indirectly_readable> && !std::input_iterator<not_indirectly_readable>);
 
+// FIXME(cjdb): deduplicate
 struct bad_iterator_category {
   using value_type = int;
   using difference_type = std::ptrdiff_t;
diff --git a/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp b/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp
index fa005d1..494e9fd 100644
--- a/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp
+++ b/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp
@@ -86,6 +86,10 @@ static_assert(test(std::ranges::find_end, a, a));
 static_assert(test(std::ranges::find_first_of, a, a));
 static_assert(test(std::ranges::find_if, a, odd));
 static_assert(test(std::ranges::find_if_not, a, odd));
+#if TEST_STD_VER >= 23
+static_assert(test(std::ranges::fold_left, a, 0, std::plus()));
+static_assert(test(std::ranges::fold_left_with_iter, a, 0, std::plus()));
+#endif
 static_assert(test(std::ranges::for_each, a, odd));
 static_assert(test(std::ranges::for_each_n, a, 10, odd));
 static_assert(test(std::ranges::generate, a, gen));
diff --git a/libcxx/test/std/ranges/range.adaptors/range.all/range.ref.view/borrowing.compile.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.all/range.ref.view/borrowing.compile.pass.cpp
index 4405742..2887ef1 100644
--- a/libcxx/test/std/ranges/range.adaptors/range.all/range.ref.view/borrowing.compile.pass.cpp
+++ b/libcxx/test/std/ranges/range.adaptors/range.all/range.ref.view/borrowing.compile.pass.cpp
@@ -15,6 +15,6 @@
 
 #include "test_range.h"
 
-static_assert(std::ranges::borrowed_range<std::ranges::ref_view<BorrowedRange>>);
+static_assert(std::ranges::borrowed_range<std::ranges::ref_view<BorrowedRange<>>>);
 static_assert(std::ranges::borrowed_range<std::ranges::ref_view<BorrowedView>>);
 static_assert(std::ranges::borrowed_range<std::ranges::ref_view<NonBorrowedView>>);
diff --git a/libcxx/test/support/invocable_with_telemetry.h b/libcxx/test/support/invocable_with_telemetry.h
new file mode 100644
index 0000000..612bbec
--- /dev/null
+++ b/libcxx/test/support/invocable_with_telemetry.h
@@ -0,0 +1,89 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 TEST_SUPPORT_INVOCABLE_WITH_TELEMETRY_H
+#define TEST_SUPPORT_INVOCABLE_WITH_TELEMETRY_H
+
+#include <cassert>
+#include <concepts>
+#include <functional>
+#include <utility>
+
+#if TEST_STD_VER < 20
+#  error invocable_with_telemetry requires C++20
+#else
+struct invocable_telemetry {
+  int invocations;
+  int moves;
+  int copies;
+};
+
+template <class F>
+class invocable_with_telemetry {
+public:
+  constexpr invocable_with_telemetry(F f, invocable_telemetry& telemetry) : f_(f), telemetry_(&telemetry) {}
+
+  constexpr invocable_with_telemetry(invocable_with_telemetry&& other)
+    requires std::move_constructible<F>
+      : f_(std::move(other.f_)),
+        telemetry_(assert(other.telemetry_ != nullptr), std::exchange(other.telemetry_, nullptr)) {
+    ++telemetry_->moves;
+  }
+
+  constexpr invocable_with_telemetry(invocable_with_telemetry const& other)
+    requires std::copy_constructible<F>
+      : f_(other.f_), telemetry_((assert(other.telemetry_ != nullptr), other.telemetry_)) {
+    ++telemetry_->copies;
+  }
+
+  constexpr invocable_with_telemetry& operator=(invocable_with_telemetry&& other)
+    requires std::movable<F>
+  {
+    // Not using move-and-swap idiom to ensure that copies and moves remain accurate.
+    assert(&other != this);
+    assert(other.telemetry_ != nullptr);
+
+    f_         = std::move(other.f_);
+    telemetry_ = std::exchange(other.telemetry_, nullptr);
+
+    ++telemetry_->moves;
+    return *this;
+  }
+
+  constexpr invocable_with_telemetry& operator=(invocable_with_telemetry const& other)
+    requires std::copyable<F>
+  {
+    // Not using copy-and-swap idiom to ensure that copies and moves remain accurate.
+    assert(&other != this);
+    assert(other.telemetry_ != nullptr);
+
+    f_         = other.f_;
+    telemetry_ = other.telemetry_;
+
+    ++telemetry_->copies;
+    return *this;
+  }
+
+  template <class... Args>
+    requires std::invocable<F&, Args...>
+  constexpr decltype(auto) operator()(Args&&... args) noexcept(std::is_nothrow_invocable_v<F&, Args...>) {
+    assert(telemetry_);
+    ++telemetry_->invocations;
+    return std::invoke(f_, std::forward<Args>(args)...);
+  }
+
+private:
+  F f_                            = F();
+  invocable_telemetry* telemetry_ = nullptr;
+};
+
+template <class F>
+invocable_with_telemetry(F f, int& invocations, int& moves, int& copies) -> invocable_with_telemetry<F>;
+
+#endif // TEST_STD_VER < 20
+#endif // TEST_SUPPORT_INVOCABLE_WITH_TELEMETRY_H
diff --git a/libcxx/test/support/maths.h b/libcxx/test/support/maths.h
new file mode 100644
index 0000000..11c507b
--- /dev/null
+++ b/libcxx/test/support/maths.h
@@ -0,0 +1,68 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Implementations of well-known functions in mathematics that are useful for
+// testing algorithms.
+
+#ifndef LIBCXX_TEST_MATHS_H
+#define LIBCXX_TEST_MATHS_H
+
+#include <algorithm>
+#include <cassert>
+#include <concepts>
+#include <ranges>
+#include <vector>
+
+template <std::ranges::forward_range R>
+constexpr std::ranges::range_value_t<R> triangular_sum(R& input) {
+  assert(not std::ranges::empty(input));
+  auto [min, max] = std::ranges::minmax_element(input);
+  return static_cast<std::ranges::range_value_t<R>>(
+      (static_cast<double>(std::ranges::distance(input)) / 2) * (*min + *max));
+}
+
+template <std::integral I>
+constexpr I factorial(I const n) {
+  assert(n >= 0);
+  auto result = I(1);
+  for (auto i = I(1); i <= n; ++i) {
+    result *= i;
+  }
+
+  return result;
+}
+static_assert(factorial(0) == 1);
+static_assert(factorial(1) == 1);
+static_assert(factorial(2) == 2);
+static_assert(factorial(3) == 6);
+static_assert(factorial(4) == 24);
+static_assert(factorial(5) == 120);
+
+template <std::integral I>
+constexpr I fibonacci(I const n) {
+  assert(n >= 0);
+
+  auto result = I(0);
+  auto prev   = I(1);
+  for (auto i = I(0); i < n; ++i) {
+    result += std::exchange(prev, result);
+  }
+  return result;
+}
+static_assert(fibonacci(0) == 0);
+static_assert(fibonacci(1) == 1);
+static_assert(fibonacci(2) == 1);
+static_assert(fibonacci(3) == 2);
+static_assert(fibonacci(4) == 3);
+static_assert(fibonacci(5) == 5);
+static_assert(fibonacci(6) == 8);
+static_assert(fibonacci(7) == 13);
+static_assert(fibonacci(8) == 21);
+static_assert(fibonacci(9) == 34);
+
+#endif // LIBCXX_TEST_MATHS_H
diff --git a/libcxx/test/support/test_range.h b/libcxx/test/support/test_range.h
index eea8ce1..3f03b4d 100644
--- a/libcxx/test/support/test_range.h
+++ b/libcxx/test/support/test_range.h
@@ -15,62 +15,64 @@
 #include "test_iterators.h"
 
 #if TEST_STD_VER < 17
-#error "test/support/test_range.h" can only be included in builds supporting ranges
+#  error "test/support/test_range.h" can only be included in builds supporting ranges
 #endif
 
 struct sentinel {
   bool operator==(std::input_or_output_iterator auto const&) const;
 };
 
-template <template <class...> class I>
-requires std::input_or_output_iterator<I<int*> >
+template <template <class...> class I, class T = int>
+  requires std::input_or_output_iterator<I<T*> >
 struct test_range {
-  I<int*> begin();
-  I<int const*> begin() const;
+  I<T*> begin();
+  I<T const*> begin() const;
   sentinel end();
   sentinel end() const;
 };
 
-template <template <class...> class I>
-requires std::input_or_output_iterator<I<int*> >
+template <template <class...> class I, class T = int>
+  requires std::input_or_output_iterator<I<T*> >
 struct test_non_const_range {
-  I<int*> begin();
+  I<T*> begin();
   sentinel end();
 };
 
-template <template <class...> class I>
-requires std::input_or_output_iterator<I<int*> >
+template <template <class...> class I, class T = int>
+  requires std::input_or_output_iterator<I<T*> >
 struct test_common_range {
-  I<int*> begin();
-  I<int const*> begin() const;
-  I<int*> end();
-  I<int const*> end() const;
+  I<T*> begin();
+  I<T const*> begin() const;
+  I<T*> end();
+  I<T const*> end() const;
 };
 
-template <template <class...> class I>
-requires std::input_or_output_iterator<I<int*> >
+template <template <class...> class I, class T = int>
+  requires std::input_or_output_iterator<I<T*> >
 struct test_non_const_common_range {
-  I<int*> begin();
-  I<int*> end();
+  I<T*> begin();
+  I<T*> end();
 };
 
-template <template <class...> class I>
-requires std::input_or_output_iterator<I<int*> >
+template <template <class...> class I, class T = int>
+  requires std::input_or_output_iterator<I<T*> >
 struct test_view : std::ranges::view_base {
-  I<int*> begin();
-  I<int const*> begin() const;
+  I<T*> begin();
+  I<T const*> begin() const;
   sentinel end();
   sentinel end() const;
 };
 
+template <class T = int>
 struct BorrowedRange {
-  int *begin() const;
-  int *end() const;
+  T* begin() const;
+  T* end() const;
   BorrowedRange(BorrowedRange&&) = delete;
 };
-template<> inline constexpr bool std::ranges::enable_borrowed_range<BorrowedRange> = true;
-static_assert(!std::ranges::view<BorrowedRange>);
-static_assert(std::ranges::borrowed_range<BorrowedRange>);
+template <class T>
+inline constexpr bool std::ranges::enable_borrowed_range<BorrowedRange<T>> = true;
+static_assert(!std::ranges::view<BorrowedRange<>>);
+static_assert(std::ranges::borrowed_range<BorrowedRange<>>);
 
 using BorrowedView = std::ranges::empty_view<int>;
 static_assert(std::ranges::view<BorrowedView>);