[libc++] Optimize std::find for segmented iterators (#67224)

```
--------------------------------------------------------------------------
Benchmark                                              old             new
--------------------------------------------------------------------------
bm_find<std::deque<char>>/1                        6.06 ns         10.6 ns
bm_find<std::deque<char>>/2                        15.5 ns         10.6 ns
bm_find<std::deque<char>>/3                        19.0 ns         10.6 ns
bm_find<std::deque<char>>/4                        20.8 ns         10.6 ns
bm_find<std::deque<char>>/5                        22.0 ns         10.6 ns
bm_find<std::deque<char>>/6                        23.0 ns         10.5 ns
bm_find<std::deque<char>>/7                        24.8 ns         10.7 ns
bm_find<std::deque<char>>/8                        25.7 ns         10.6 ns
bm_find<std::deque<char>>/16                       28.3 ns         10.6 ns
bm_find<std::deque<char>>/64                       44.2 ns         27.0 ns
bm_find<std::deque<char>>/512                       133 ns         37.6 ns
bm_find<std::deque<char>>/4096                      867 ns         53.1 ns
bm_find<std::deque<char>>/32768                    6838 ns          160 ns
bm_find<std::deque<char>>/262144                  52897 ns         1495 ns
bm_find<std::deque<char>>/1048576                215621 ns         6077 ns
bm_find<std::deque<short>>/1                       6.03 ns         6.28 ns
bm_find<std::deque<short>>/2                       15.8 ns         15.8 ns
bm_find<std::deque<short>>/3                       20.5 ns         20.3 ns
bm_find<std::deque<short>>/4                       21.0 ns         21.0 ns
bm_find<std::deque<short>>/5                       23.0 ns         22.1 ns
bm_find<std::deque<short>>/6                       22.6 ns         23.0 ns
bm_find<std::deque<short>>/7                       23.4 ns         23.7 ns
bm_find<std::deque<short>>/8                       24.4 ns         24.9 ns
bm_find<std::deque<short>>/16                      26.6 ns         27.2 ns
bm_find<std::deque<short>>/64                      43.2 ns         40.9 ns
bm_find<std::deque<short>>/512                      124 ns         90.7 ns
bm_find<std::deque<short>>/4096                     845 ns          525 ns
bm_find<std::deque<short>>/32768                   7273 ns         3194 ns
bm_find<std::deque<short>>/262144                 53710 ns        24385 ns
bm_find<std::deque<short>>/1048576               216086 ns        96195 ns
bm_find<std::deque<int>>/1                         6.03 ns         10.3 ns
bm_find<std::deque<int>>/2                         15.6 ns         10.3 ns
bm_find<std::deque<int>>/3                         19.1 ns         10.3 ns
bm_find<std::deque<int>>/4                         22.3 ns         10.3 ns
bm_find<std::deque<int>>/5                         23.5 ns         10.4 ns
bm_find<std::deque<int>>/6                         23.1 ns         10.3 ns
bm_find<std::deque<int>>/7                         23.7 ns         10.2 ns
bm_find<std::deque<int>>/8                         24.5 ns         10.2 ns
bm_find<std::deque<int>>/16                        27.9 ns         26.6 ns
bm_find<std::deque<int>>/64                        42.6 ns         32.2 ns
bm_find<std::deque<int>>/512                        123 ns         43.0 ns
bm_find<std::deque<int>>/4096                       874 ns         93.5 ns
bm_find<std::deque<int>>/32768                     7031 ns          751 ns
bm_find<std::deque<int>>/262144                   57723 ns         6169 ns
bm_find<std::deque<int>>/1048576                 230867 ns        35851 ns
bm_ranges_find<std::deque<char>>/1                 5.97 ns         10.6 ns
bm_ranges_find<std::deque<char>>/2                 16.0 ns         10.5 ns
bm_ranges_find<std::deque<char>>/3                 19.5 ns         10.5 ns
bm_ranges_find<std::deque<char>>/4                 21.1 ns         10.6 ns
bm_ranges_find<std::deque<char>>/5                 22.8 ns         10.5 ns
bm_ranges_find<std::deque<char>>/6                 22.8 ns         10.6 ns
bm_ranges_find<std::deque<char>>/7                 23.4 ns         10.8 ns
bm_ranges_find<std::deque<char>>/8                 24.1 ns         10.5 ns
bm_ranges_find<std::deque<char>>/16                26.9 ns         10.6 ns
bm_ranges_find<std::deque<char>>/64                50.2 ns         27.2 ns
bm_ranges_find<std::deque<char>>/512                126 ns         38.3 ns
bm_ranges_find<std::deque<char>>/4096               868 ns         53.8 ns
bm_ranges_find<std::deque<char>>/32768             6695 ns          161 ns
bm_ranges_find<std::deque<char>>/262144           54411 ns         1497 ns
bm_ranges_find<std::deque<char>>/1048576         241699 ns         6042 ns
bm_ranges_find<std::deque<short>>/1                6.39 ns         6.31 ns
bm_ranges_find<std::deque<short>>/2                15.8 ns         15.9 ns
bm_ranges_find<std::deque<short>>/3                19.0 ns         19.8 ns
bm_ranges_find<std::deque<short>>/4                20.8 ns         20.9 ns
bm_ranges_find<std::deque<short>>/5                21.8 ns         22.1 ns
bm_ranges_find<std::deque<short>>/6                23.0 ns         23.0 ns
bm_ranges_find<std::deque<short>>/7                23.2 ns         23.9 ns
bm_ranges_find<std::deque<short>>/8                23.7 ns         24.4 ns
bm_ranges_find<std::deque<short>>/16               26.6 ns         26.8 ns
bm_ranges_find<std::deque<short>>/64               43.4 ns         39.7 ns
bm_ranges_find<std::deque<short>>/512               131 ns         90.5 ns
bm_ranges_find<std::deque<short>>/4096              851 ns          523 ns
bm_ranges_find<std::deque<short>>/32768            7370 ns         3166 ns
bm_ranges_find<std::deque<short>>/262144          60778 ns        24814 ns
bm_ranges_find<std::deque<short>>/1048576        229288 ns        99273 ns
bm_ranges_find<std::deque<int>>/1                  6.43 ns         10.2 ns
bm_ranges_find<std::deque<int>>/2                  16.6 ns         10.2 ns
bm_ranges_find<std::deque<int>>/3                  19.6 ns         10.2 ns
bm_ranges_find<std::deque<int>>/4                  21.0 ns         10.2 ns
bm_ranges_find<std::deque<int>>/5                  21.9 ns         10.4 ns
bm_ranges_find<std::deque<int>>/6                  22.7 ns         10.2 ns
bm_ranges_find<std::deque<int>>/7                  23.9 ns         10.2 ns
bm_ranges_find<std::deque<int>>/8                  23.8 ns         10.2 ns
bm_ranges_find<std::deque<int>>/16                 27.2 ns         27.1 ns
bm_ranges_find<std::deque<int>>/64                 42.4 ns         32.4 ns
bm_ranges_find<std::deque<int>>/512                 122 ns         43.0 ns
bm_ranges_find<std::deque<int>>/4096                895 ns         93.7 ns
bm_ranges_find<std::deque<int>>/32768              6890 ns          756 ns
bm_ranges_find<std::deque<int>>/262144            54025 ns         6102 ns
bm_ranges_find<std::deque<int>>/1048576          221558 ns        32783 ns
```

10 files changed, 253 insertions, 16 deletions

diff --git a/libcxx/benchmarks/algorithms/find.bench.cpp b/libcxx/benchmarks/algorithms/find.bench.cpp
index b87c575..6ff2d95 100644
--- a/libcxx/benchmarks/algorithms/find.bench.cpp
+++ b/libcxx/benchmarks/algorithms/find.bench.cpp
@@ -9,12 +9,15 @@
 #include <algorithm>
 #include <benchmark/benchmark.h>
 #include <cstring>
+#include <deque>
 #include <random>
 #include <vector>
 
-template <class T>
+template <class Container>
 static void bm_find(benchmark::State& state) {
-  std::vector<T> vec1(state.range(), '1');
+  using T = Container::value_type;
+
+  Container vec1(state.range(), '1');
   std::mt19937_64 rng(std::random_device{}());
 
   for (auto _ : state) {
@@ -25,13 +28,18 @@ static void bm_find(benchmark::State& state) {
     vec1[idx] = '1';
   }
 }
-BENCHMARK(bm_find<char>)->DenseRange(1, 8)->Range(16, 1 << 20);
-BENCHMARK(bm_find<short>)->DenseRange(1, 8)->Range(16, 1 << 20);
-BENCHMARK(bm_find<int>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_find<std::vector<char>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_find<std::vector<short>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_find<std::vector<int>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_find<std::deque<char>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_find<std::deque<short>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_find<std::deque<int>>)->DenseRange(1, 8)->Range(16, 1 << 20);
 
-template <class T>
+template <class Container>
 static void bm_ranges_find(benchmark::State& state) {
-  std::vector<T> vec1(state.range(), '1');
+  using T = Container::value_type;
+
+  Container vec1(state.range(), '1');
   std::mt19937_64 rng(std::random_device{}());
 
   for (auto _ : state) {
@@ -42,9 +50,12 @@ static void bm_ranges_find(benchmark::State& state) {
     vec1[idx] = '1';
   }
 }
-BENCHMARK(bm_ranges_find<char>)->DenseRange(1, 8)->Range(16, 1 << 20);
-BENCHMARK(bm_ranges_find<short>)->DenseRange(1, 8)->Range(16, 1 << 20);
-BENCHMARK(bm_ranges_find<int>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_ranges_find<std::vector<char>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_ranges_find<std::vector<short>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_ranges_find<std::vector<int>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_ranges_find<std::deque<char>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_ranges_find<std::deque<short>>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_ranges_find<std::deque<int>>)->DenseRange(1, 8)->Range(16, 1 << 20);
 
 static void bm_vector_bool_find(benchmark::State& state) {
   std::vector<bool> vec1(state.range(), false);
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 533b7ac..7d0defa 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -22,6 +22,7 @@ set(files
   __algorithm/find_first_of.h
   __algorithm/find_if.h
   __algorithm/find_if_not.h
+  __algorithm/find_segment_if.h
   __algorithm/for_each.h
   __algorithm/for_each_n.h
   __algorithm/for_each_segment.h
diff --git a/libcxx/include/__algorithm/find.h b/libcxx/include/__algorithm/find.h
index 0118489..754e597 100644
--- a/libcxx/include/__algorithm/find.h
+++ b/libcxx/include/__algorithm/find.h
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP___ALGORITHM_FIND_H
 #define _LIBCPP___ALGORITHM_FIND_H
 
+#include <__algorithm/find_segment_if.h>
 #include <__algorithm/min.h>
 #include <__algorithm/unwrap_iter.h>
 #include <__bit/countr.h>
@@ -18,8 +19,10 @@
 #include <__functional/identity.h>
 #include <__functional/invoke.h>
 #include <__fwd/bit_reference.h>
+#include <__iterator/segmented_iterator.h>
 #include <__string/constexpr_c_functions.h>
 #include <__type_traits/is_same.h>
+#include <__utility/move.h>
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 #  include <cwchar>
@@ -118,6 +121,34 @@ __find_impl(__bit_iterator<_Cp, _IsConst> __first, __bit_iterator<_Cp, _IsConst>
   return std::__find_bool<false>(__first, static_cast<typename _Cp::size_type>(__last - __first));
 }
 
+// segmented iterator implementation
+
+template <class>
+struct __find_segment;
+
+template <class _SegmentedIterator,
+          class _Tp,
+          class _Proj,
+          __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _SegmentedIterator
+__find_impl(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value, _Proj& __proj) {
+  return std::__find_segment_if(std::move(__first), std::move(__last), __find_segment<_Tp>(__value), __proj);
+}
+
+template <class _Tp>
+struct __find_segment {
+  const _Tp& __value_;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __find_segment(const _Tp& __value) : __value_(__value) {}
+
+  template <class _InputIterator, class _Proj>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _InputIterator
+  operator()(_InputIterator __first, _InputIterator __last, _Proj& __proj) const {
+    return std::__find_impl(__first, __last, __value_, __proj);
+  }
+};
+
+// public API
 template <class _InputIterator, class _Tp>
 _LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _InputIterator
 find(_InputIterator __first, _InputIterator __last, const _Tp& __value) {
diff --git a/libcxx/include/__algorithm/find_segment_if.h b/libcxx/include/__algorithm/find_segment_if.h
new file mode 100644
index 0000000..9d6064f
--- /dev/null
+++ b/libcxx/include/__algorithm/find_segment_if.h
@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_FIND_SEGMENT_IF_H
+#define _LIBCPP___ALGORITHM_FIND_SEGMENT_IF_H
+
+#include <__config>
+#include <__iterator/segmented_iterator.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// __find_segment_if is a utility function for optimizing iteration over segmented iterators linearly.
+// [__first, __last) has to be a segmented range. __pred is expected to take a range of local iterators and the __proj.
+// It returns an iterator to the first element that satisfies the predicate, or a one-past-the-end iterator if there was
+// no match. __proj may be anything that should be passed to __pred, but is expected to be a projection to support
+// ranges algorithms, or __identity for classic algorithms.
+
+template <class _SegmentedIterator, class _Pred, class _Proj>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _SegmentedIterator
+__find_segment_if(_SegmentedIterator __first, _SegmentedIterator __last, _Pred __pred, _Proj& __proj) {
+  using _Traits = __segmented_iterator_traits<_SegmentedIterator>;
+
+  auto __sfirst = _Traits::__segment(__first);
+  auto __slast  = _Traits::__segment(__last);
+
+  // We are in a single segment, so we might not be at the beginning or end
+  if (__sfirst == __slast)
+    return _Traits::__compose(__sfirst, __pred(_Traits::__local(__first), _Traits::__local(__last), __proj));
+
+  { // We have more than one segment. Iterate over the first segment, since we might not start at the beginning
+    auto __llast = _Traits::__end(__sfirst);
+    auto __liter = __pred(_Traits::__local(__first), __llast, __proj);
+    if (__liter != __llast)
+      return _Traits::__compose(__sfirst, __liter);
+  }
+  ++__sfirst;
+
+  // Iterate over the segments which are guaranteed to be completely in the range
+  while (__sfirst != __slast) {
+    auto __llast = _Traits::__end(__sfirst);
+    auto __liter = __pred(_Traits::__begin(__sfirst), _Traits::__end(__sfirst), __proj);
+    if (__liter != __llast)
+      return _Traits::__compose(__sfirst, __liter);
+    ++__sfirst;
+  }
+
+  // Iterate over the last segment
+  return _Traits::__compose(__sfirst, __pred(_Traits::__begin(__sfirst), _Traits::__local(__last), __proj));
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ALGORITHM_FIND_SEGMENT_IF_H
diff --git a/libcxx/include/deque b/libcxx/include/deque
index d45793c..eabcc7b 100644
--- a/libcxx/include/deque
+++ b/libcxx/include/deque
@@ -440,7 +440,7 @@ public:
   }
 
   static _LIBCPP_HIDE_FROM_ABI _Iterator __compose(__segment_iterator __segment, __local_iterator __local) {
-        if (__local == __end(__segment)) {
+        if (__segment && __local == __end(__segment)) {
             ++__segment;
             return _Iterator(__segment, *__segment);
         }
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 7e93e0a..5f57a8a 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -665,6 +665,7 @@ module std_private_algorithm_find_end                                    [system
 module std_private_algorithm_find_first_of                               [system] { header "__algorithm/find_first_of.h" }
 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_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/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
index eece983..1a6c0d1 100644
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
@@ -15,6 +15,7 @@
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
+#include <deque>
 
 #include "test_macros.h"
 
diff --git a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
index afbbc22..2e15e5c 100644
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
@@ -15,6 +15,7 @@
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
+#include <deque>
 
 #include "test_macros.h"
 
@@ -224,10 +225,19 @@ constexpr bool all_the_algorithms()
     return true;
 }
 
-int main(int, char**)
-{
-    all_the_algorithms();
-    static_assert(all_the_algorithms());
+void test_deque() {
+  std::deque<T> d;
+  int copies  = 0;
+  void* value = nullptr;
+
+  (void)std::ranges::find(d, value, Proj(&copies));
+  assert(copies == 0);
+}
+
+int main(int, char**) {
+  test_deque();
+  all_the_algorithms();
+  static_assert(all_the_algorithms());
 
-    return 0;
+  return 0;
 }
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp
index 3141f14..0afc573 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp
@@ -19,6 +19,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <deque>
 #include <vector>
 #include <type_traits>
 
@@ -115,6 +116,52 @@ struct TestTypes {
   }
 };
 
+void test_deque() {
+  { // empty deque
+    std::deque<int> data;
+    assert(std::find(data.begin(), data.end(), 4) == data.end());
+  }
+
+  { // single element - match
+    std::deque<int> data;
+    data.push_back(4);
+    assert(std::find(data.begin(), data.end(), 4) == data.begin());
+  }
+
+  { // single element - no match
+    std::deque<int> data;
+    data.push_back(3);
+    assert(std::find(data.begin(), data.end(), 4) == data.end());
+  }
+
+  // many elements
+  int sizes[] = {2, 3, 1023, 1024, 1025, 2047, 2048, 2049};
+  for (auto size : sizes) {
+    { // last element match
+      std::deque<int> data;
+      data.resize(size);
+      std::fill(data.begin(), data.end(), 3);
+      data[size - 1] = 4;
+      assert(std::find(data.begin(), data.end(), 4) == data.end() - 1);
+    }
+
+    { // second-last element match
+      std::deque<int> data;
+      data.resize(size);
+      std::fill(data.begin(), data.end(), 3);
+      data[size - 2] = 4;
+      assert(std::find(data.begin(), data.end(), 4) == data.end() - 2);
+    }
+
+    { // no match
+      std::deque<int> data;
+      data.resize(size);
+      std::fill(data.begin(), data.end(), 3);
+      assert(std::find(data.begin(), data.end(), 4) == data.end());
+    }
+  }
+}
+
 TEST_CONSTEXPR_CXX20 bool test() {
   types::for_each(types::integer_types(), TestTypes<char>());
   types::for_each(types::integer_types(), TestTypes<int>());
@@ -124,10 +171,21 @@ TEST_CONSTEXPR_CXX20 bool test() {
   Test<TriviallyComparable<wchar_t>, TriviallyComparable<wchar_t>>().operator()<TriviallyComparable<wchar_t>*>();
 #endif
 
+  // TODO: Remove the `_LIBCPP_ENABLE_EXPERIMENTAL` check once we have the FTM guarded or views::join isn't
+  // experimental anymore
+#if TEST_STD_VER >= 20 && (!defined(_LIBCPP_VERSION) || defined(_LIBCPP_ENABLE_EXPERIMENTAL))
+  {
+    std::vector<std::vector<int>> vec = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
+    auto view                         = vec | std::views::join;
+    assert(std::find(view.begin(), view.end(), 4) == std::next(view.begin(), 3));
+  }
+#endif
+
   return true;
 }
 
 int main(int, char**) {
+  test_deque();
   test();
 #if TEST_STD_VER >= 20
   static_assert(test());
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp
index dbb5874..4ae049c 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp
@@ -26,6 +26,7 @@
 #include <algorithm>
 #include <array>
 #include <cassert>
+#include <deque>
 #include <ranges>
 #include <vector>
 
@@ -130,6 +131,17 @@ constexpr bool test() {
                     });
                   });
 
+  // TODO: Remove the `_LIBCPP_ENABLE_EXPERIMENTAL` check once we have the FTM guarded or views::join isn't
+  // experimental anymore
+#if TEST_STD_VER >= 20 && (!defined(_LIBCPP_VERSION) || defined(_LIBCPP_ENABLE_EXPERIMENTAL))
+  {
+    std::vector<std::vector<int>> vec = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
+    auto view                         = vec | std::views::join;
+    assert(std::ranges::find(view.begin(), view.end(), 4) == std::next(view.begin(), 3));
+    assert(std::ranges::find(view, 4) == std::next(view.begin(), 3));
+  }
+#endif
+
   { // check that the first element is returned
     {
       struct S {
@@ -205,7 +217,57 @@ public:
   friend bool operator==(const Comparable& lhs, long long rhs) { return comparable_data[lhs.index_] == rhs; }
 };
 
+void test_deque() {
+  { // empty deque
+    std::deque<int> data;
+    assert(std::ranges::find(data, 4) == data.end());
+    assert(std::ranges::find(data.begin(), data.end(), 4) == data.end());
+  }
+
+  { // single element - match
+    std::deque<int> data = {4};
+    assert(std::ranges::find(data, 4) == data.begin());
+    assert(std::ranges::find(data.begin(), data.end(), 4) == data.begin());
+  }
+
+  { // single element - no match
+    std::deque<int> data = {3};
+    assert(std::ranges::find(data, 4) == data.end());
+    assert(std::ranges::find(data.begin(), data.end(), 4) == data.end());
+  }
+
+  // many elements
+  for (auto size : {2, 3, 1023, 1024, 1025, 2047, 2048, 2049}) {
+    { // last element match
+      std::deque<int> data;
+      data.resize(size);
+      std::fill(data.begin(), data.end(), 3);
+      data[size - 1] = 4;
+      assert(std::ranges::find(data, 4) == data.end() - 1);
+      assert(std::ranges::find(data.begin(), data.end(), 4) == data.end() - 1);
+    }
+
+    { // second-last element match
+      std::deque<int> data;
+      data.resize(size);
+      std::fill(data.begin(), data.end(), 3);
+      data[size - 2] = 4;
+      assert(std::ranges::find(data, 4) == data.end() - 2);
+      assert(std::ranges::find(data.begin(), data.end(), 4) == data.end() - 2);
+    }
+
+    { // no match
+      std::deque<int> data;
+      data.resize(size);
+      std::fill(data.begin(), data.end(), 3);
+      assert(std::ranges::find(data, 4) == data.end());
+      assert(std::ranges::find(data.begin(), data.end(), 4) == data.end());
+    }
+  }
+}
+
 int main(int, char**) {
+  test_deque();
   test();
   static_assert(test());