[libc++][ranges] Implement ranges::mismatch

Implement `ranges::mismatch`

Reviewed By: Quuxplusone, ldionne, #libc

Spies: libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D117817

8 files changed, 411 insertions, 2 deletions

diff --git a/libcxx/docs/Status/RangesAlgorithms.csv b/libcxx/docs/Status/RangesAlgorithms.csv
index b8cc8baf9043..3f3cc3339262 100644
--- a/libcxx/docs/Status/RangesAlgorithms.csv
+++ b/libcxx/docs/Status/RangesAlgorithms.csv
@@ -7,7 +7,7 @@ Search,find_if,Christopher Di Bella,`D105792 <https://llvm.org/D105792>`_
 Search,find_if_not,Christopher Di Bella,`D105792 <https://llvm.org/D105792>`_
 Search,find_first_of,Not assigned,n/a,Not started
 Search,adjacent_find,Not assigned,n/a,Not started
-Search,mismatch,Not assigned,n/a,Not started
+Search,mismatch,Nikolas Klauser,`D117817 <https://llvm.org/D117817>`,Complete
 Search,equal,Not assigned,n/a,Not started
 Search,lexicographical_compare,Not assigned,n/a,Not started
 Search,partition_point,Christopher Di Bella,`D105794 <https://llvm.org/D105794>`_,Under review
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index df30c80997e8..cf7a992368d9 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -68,6 +68,7 @@ set(files
   __algorithm/push_heap.h
   __algorithm/ranges_max_element.h
   __algorithm/ranges_min_element.h
+  __algorithm/ranges_mismatch.h
   __algorithm/ranges_swap_ranges.h
   __algorithm/remove.h
   __algorithm/remove_copy.h
diff --git a/libcxx/include/__algorithm/ranges_mismatch.h b/libcxx/include/__algorithm/ranges_mismatch.h
new file mode 100644
index 000000000000..b886a106d914
--- /dev/null
+++ b/libcxx/include/__algorithm/ranges_mismatch.h
@@ -0,0 +1,85 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_RANGES_MISMATCH_H
+#define _LIBCPP___ALGORITHM_RANGES_MISMATCH_H
+
+#include <__algorithm/in_in_result.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/invoke.h>
+#include <__functional/ranges_operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/indirectly_comparable.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+namespace ranges {
+
+template <class _I1, class _I2>
+using mismatch_result = in_in_result<_I1, _I2>;
+
+namespace __mismatch {
+struct __fn {
+  template <class _I1, class _S1, class _I2, class _S2,
+            class _Pred, class _Proj1, class _Proj2>
+  static _LIBCPP_HIDE_FROM_ABI constexpr
+  mismatch_result<_I1, _I2>
+  __go(_I1 __first1, _S1 __last1, _I2 __first2, _S2 __last2,
+       _Pred& __pred, _Proj1& __proj1, _Proj2& __proj2) {
+    while (__first1 != __last1 && __first2 != __last2) {
+      if (!std::invoke(__pred, std::invoke(__proj1, *__first1), std::invoke(__proj2, *__first2)))
+        break;
+      ++__first1;
+      ++__first2;
+    }
+    return {std::move(__first1), std::move(__first2)};
+  }
+
+  template <input_iterator _I1, sentinel_for<_I1> _S1,
+            input_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>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  mismatch_result<_I1, _I2> operator()(_I1 __first1, _S1 __last1, _I2 __first2, _S2 __last2,
+                                       _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) const {
+    return __go(std::move(__first1), __last1, std::move(__first2), __last2, __pred, __proj1, __proj2);
+  }
+
+  template <input_range _R1, input_range _R2,
+            class _Pred = ranges::equal_to, class _Proj1 = identity, class _Proj2 = identity>
+    requires indirectly_comparable<iterator_t<_R1>, iterator_t<_R2>, _Pred, _Proj1, _Proj2>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  mismatch_result<borrowed_iterator_t<_R1>, borrowed_iterator_t<_R2>>
+  operator()(_R1&& __r1, _R2&& __r2, _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) const {
+    return __go(ranges::begin(__r1), ranges::end(__r1), ranges::begin(__r2), ranges::end(__r2),
+                __pred, __proj1, __proj2);
+  }
+};
+} // namespace __mismatch
+
+inline namespace __cpo {
+  constexpr inline auto mismatch = __mismatch::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ALGORITHM_RANGES_MISMATCH_H
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 6b5bc0cd9acb..72fdb29e50d6 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -44,6 +44,18 @@ namespace ranges {
   template<forward_range R, class Proj = identity,
     indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less> // since C++20
   constexpr borrowed_iterator_t<R> min_element(R&& r, Comp comp = {}, Proj proj = {});
+
+  template <input_iterator I1, sentinel_for<_I1> S1, input_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>
+  constexpr mismatch_result<_I1, _I2>
+  mismatch()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) // since C++20
+
+  template <input_range R1, input_range R2,
+          class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
+    requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
+  constexpr mismatch_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>>
+  mismatch(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {})                           // since C++20
 }
 
 template <class InputIterator, class Predicate>
@@ -765,6 +777,7 @@ template <class BidirectionalIterator, class Compare>
 #include <__algorithm/push_heap.h>
 #include <__algorithm/ranges_max_element.h>
 #include <__algorithm/ranges_min_element.h>
+#include <__algorithm/ranges_mismatch.h>
 #include <__algorithm/ranges_swap_ranges.h>
 #include <__algorithm/remove.h>
 #include <__algorithm/remove_copy.h>
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 1577c005dae4..a08b82b2d294 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -296,6 +296,7 @@ module std [system] {
       module push_heap                { private header "__algorithm/push_heap.h" }
       module ranges_max_element       { private header "__algorithm/ranges_max_element.h" }
       module ranges_min_element       { private header "__algorithm/ranges_min_element.h" }
+      module ranges_mismatch          { private header "__algorithm/ranges_mismatch.h" }
       module ranges_swap_ranges       { private header "__algorithm/ranges_swap_ranges.h" }
       module remove                   { private header "__algorithm/remove.h" }
       module remove_copy              { private header "__algorithm/remove_copy.h" }
diff --git a/libcxx/test/libcxx/diagnostics/detail.headers/algorithm/ranges_mismatch.module.verify.cpp b/libcxx/test/libcxx/diagnostics/detail.headers/algorithm/ranges_mismatch.module.verify.cpp
new file mode 100644
index 000000000000..da56aaaa0b31
--- /dev/null
+++ b/libcxx/test/libcxx/diagnostics/detail.headers/algorithm/ranges_mismatch.module.verify.cpp
@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_mismatch.h'}}
+#include <__algorithm/ranges_mismatch.h>
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/mismatch/ranges_mismatch.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/mismatch/ranges_mismatch.pass.cpp
new file mode 100644
index 000000000000..96c9e7b40588
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/mismatch/ranges_mismatch.pass.cpp
@@ -0,0 +1,294 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// template <input_iterator I1, sentinel_for<_I1> S1, input_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>
+// constexpr mismatch_result<_I1, _I2>
+// ranges::mismatch()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {})
+
+// template <input_range R1, input_range R2,
+//           class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
+//   requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
+// constexpr mismatch_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>>
+// ranges::mismatch(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {})
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <ranges>
+
+#include "test_iterators.h"
+
+template <class Iter1, class Iter2>
+constexpr void test_iterators(Iter1 begin1, Iter1 end1, Iter2 begin2, Iter2 end2, int* expected1, int* expected2) {
+  using Expected = std::ranges::mismatch_result<Iter1, Iter2>;
+  std::same_as<Expected> auto ret = std::ranges::mismatch(std::move(begin1), sentinel_wrapper<Iter1>(std::move(end1)),
+                                                          std::move(begin2), sentinel_wrapper<Iter2>(std::move(end2)));
+  assert(base(ret.in1) == expected1);
+  assert(base(ret.in2) == expected2);
+}
+
+template <class Iter1, class Iter2>
+constexpr void test_iters() {
+  int a[] = {1, 2, 3, 4, 5};
+  int b[] = {1, 2, 3, 5, 4};
+
+  test_iterators(Iter1(a), Iter1(a + 5), Iter2(b), Iter2(b + 5), a + 3, b + 3);
+}
+
+constexpr bool test() {
+  test_iters<cpp17_input_iterator<int*>, cpp17_input_iterator<int*>>();
+  test_iters<cpp17_input_iterator<int*>, cpp20_input_iterator<int*>>();
+  test_iters<cpp17_input_iterator<int*>, forward_iterator<int*>>();
+  test_iters<cpp17_input_iterator<int*>, bidirectional_iterator<int*>>();
+  test_iters<cpp17_input_iterator<int*>, random_access_iterator<int*>>();
+  test_iters<cpp17_input_iterator<int*>, contiguous_iterator<int*>>();
+  test_iters<cpp17_input_iterator<int*>, int*>();
+
+  test_iters<cpp20_input_iterator<int*>, cpp17_input_iterator<int*>>();
+  test_iters<cpp20_input_iterator<int*>, cpp20_input_iterator<int*>>();
+  test_iters<cpp20_input_iterator<int*>, forward_iterator<int*>>();
+  test_iters<cpp20_input_iterator<int*>, bidirectional_iterator<int*>>();
+  test_iters<cpp20_input_iterator<int*>, random_access_iterator<int*>>();
+  test_iters<cpp20_input_iterator<int*>, contiguous_iterator<int*>>();
+  test_iters<cpp20_input_iterator<int*>, int*>();
+
+  test_iters<forward_iterator<int*>, cpp17_input_iterator<int*>>();
+  test_iters<forward_iterator<int*>, cpp20_input_iterator<int*>>();
+  test_iters<forward_iterator<int*>, forward_iterator<int*>>();
+  test_iters<forward_iterator<int*>, bidirectional_iterator<int*>>();
+  test_iters<forward_iterator<int*>, random_access_iterator<int*>>();
+  test_iters<forward_iterator<int*>, contiguous_iterator<int*>>();
+  test_iters<forward_iterator<int*>, int*>();
+
+  test_iters<bidirectional_iterator<int*>, cpp17_input_iterator<int*>>();
+  test_iters<bidirectional_iterator<int*>, cpp20_input_iterator<int*>>();
+  test_iters<bidirectional_iterator<int*>, forward_iterator<int*>>();
+  test_iters<bidirectional_iterator<int*>, bidirectional_iterator<int*>>();
+  test_iters<bidirectional_iterator<int*>, random_access_iterator<int*>>();
+  test_iters<bidirectional_iterator<int*>, contiguous_iterator<int*>>();
+  test_iters<bidirectional_iterator<int*>, int*>();
+
+  test_iters<random_access_iterator<int*>, cpp17_input_iterator<int*>>();
+  test_iters<random_access_iterator<int*>, cpp20_input_iterator<int*>>();
+  test_iters<random_access_iterator<int*>, forward_iterator<int*>>();
+  test_iters<random_access_iterator<int*>, bidirectional_iterator<int*>>();
+  test_iters<random_access_iterator<int*>, random_access_iterator<int*>>();
+  test_iters<random_access_iterator<int*>, contiguous_iterator<int*>>();
+  test_iters<random_access_iterator<int*>, int*>();
+
+  test_iters<contiguous_iterator<int*>, cpp17_input_iterator<int*>>();
+  test_iters<contiguous_iterator<int*>, cpp20_input_iterator<int*>>();
+  test_iters<contiguous_iterator<int*>, forward_iterator<int*>>();
+  test_iters<contiguous_iterator<int*>, bidirectional_iterator<int*>>();
+  test_iters<contiguous_iterator<int*>, random_access_iterator<int*>>();
+  test_iters<contiguous_iterator<int*>, contiguous_iterator<int*>>();
+  test_iters<contiguous_iterator<int*>, int*>();
+
+  test_iters<int*, cpp17_input_iterator<int*>>();
+  test_iters<int*, cpp20_input_iterator<int*>>();
+  test_iters<int*, forward_iterator<int*>>();
+  test_iters<int*, bidirectional_iterator<int*>>();
+  test_iters<int*, random_access_iterator<int*>>();
+  test_iters<int*, contiguous_iterator<int*>>();
+  test_iters<int*, int*>();
+
+  { // test with a range
+    std::array<int, 5> a = {1, 2, 3, 4, 5};
+    std::array<int, 5> b = {1, 2, 3, 5, 4};
+    using Expected = std::ranges::mismatch_result<int*, int*>;
+    std::same_as<Expected> auto ret = std::ranges::mismatch(a, b);
+    assert(ret.in1 == a.begin() + 3);
+    assert(ret.in2 == b.begin() + 3);
+  }
+
+  { // test with non-iterator sentinel
+    int a[] = {1, 2, 3, 4, 5};
+    int b[] = {1, 2, 3, 5, 4};
+
+    using Iter = int*;
+    using Sentinel = sentinel_wrapper<Iter>;
+    using Expected = std::ranges::mismatch_result<Iter, Iter>;
+
+    std::same_as<Expected> auto r = std::ranges::mismatch(Iter(a), Sentinel(a + 5), Iter(b), Sentinel(b + 5));
+    assert(r.in1 == a + 3);
+    assert(r.in2 == b + 3);
+  }
+
+  { // test with different array sizes
+    {
+      int a[] = {1, 2, 3};
+      int b[] = {1, 2};
+      test_iterators(a, a + 3, b, b + 2, a + 2, b + 2);
+      using Expected = std::ranges::mismatch_result<int*, int*>;
+      std::same_as<Expected> auto ret = std::ranges::mismatch(a, b);
+      assert(ret.in1 == a + 2);
+      assert(ret.in2 == b + 2);
+    }
+    {
+      int a[] = {1, 2};
+      int b[] = {1, 2, 3};
+      test_iterators(a, a + 2, b, b + 3, a + 2, b + 2);
+      using Expected = std::ranges::mismatch_result<int*, int*>;
+      std::same_as<Expected> auto ret = std::ranges::mismatch(a, b);
+      assert(ret.in1 == a + 2);
+      assert(ret.in2 == b + 2);
+    }
+  }
+
+  { // test with borrowed ranges
+    int r1[] = {1, 2, 3, 4, 5};
+    int r2[] = {1, 2, 3, 5, 4};
+
+    using Expected = std::ranges::mismatch_result<int*, int*>;
+    {
+      std::same_as<Expected> auto ret = std::ranges::mismatch(r1, std::views::all(r2));
+      assert(ret.in1 == r1 + 3);
+      assert(ret.in2 == r2 + 3);
+    }
+    {
+      std::same_as<Expected> auto ret = std::ranges::mismatch(std::views::all(r1), r2);
+      assert(ret.in1 == r1 + 3);
+      assert(ret.in2 == r2 + 3);
+    }
+    {
+      std::same_as<Expected> auto ret = std::ranges::mismatch(std::views::all(r1), std::views::all(r2));
+      assert(ret.in1 == r1 + 3);
+      assert(ret.in2 == r2 + 3);
+    }
+  }
+
+  { // test structured bindings
+    int a[] = {1, 2, 3, 4};
+    int b[] = {1, 2, 4, 8, 16};
+    auto [ai, bi] = std::ranges::mismatch(a, b);
+    assert(ai == a + 2);
+    assert(bi == b + 2);
+    auto [aj, bj] = std::ranges::mismatch(a, a+4, b, b+5);
+    assert(aj == a + 2);
+    assert(bj == b + 2);
+  }
+
+  { // test predicate
+    {
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      int b[] = {6, 5, 8, 2, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, a + 8, b, b + 8, std::ranges::greater{});
+      assert(ret.in1 == a + 4);
+      assert(ret.in2 == b + 4);
+      assert(*ret.in1 == 5);
+      assert(*ret.in2 == 5);
+    }
+
+    {
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      int b[] = {6, 5, 8, 2, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, b, std::ranges::greater{});
+      assert(ret.in1 == a + 4);
+      assert(ret.in2 == b + 4);
+      assert(*ret.in1 == 5);
+      assert(*ret.in2 == 5);
+    }
+  }
+
+  { // test projection
+    {
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      int b[] = {6, 5, 8, 2, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, b,
+                                       std::ranges::greater{},
+                                       [](int i) { return i == 5 ? +100 : i; },
+                                       [](int i) { return i == 5 ? -100 : i; });
+      assert(ret.in1 == a + 5);
+      assert(ret.in2 == b + 5);
+      assert(*ret.in1 == 1);
+      assert(*ret.in2 == 1);
+    }
+    {
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, a,
+                                       std::less<double>{},
+                                       [](int i) { return i * 1.01; },
+                                       [c = 0](int i) mutable { return c++ < 5 ? i * 1.02 : i; });
+      assert(ret.in1 == a + 5);
+      assert(ret.in2 == a + 5);
+      assert(*ret.in1 == 1);
+      assert(*ret.in2 == 1);
+    }
+    {
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      int b[] = {6, 5, 8, 2, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, a + 8, b, b + 8,
+                                       std::ranges::greater{},
+                                       [](int i) { return i == 5 ? +100 : i; },
+                                       [](int i) { return i == 5 ? -100 : i; });
+      assert(ret.in1 == a + 5);
+      assert(ret.in2 == b + 5);
+      assert(*ret.in1 == 1);
+      assert(*ret.in2 == 1);
+    }
+    {
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, a + 8, a, a + 8,
+                                       std::less<double>{},
+                                       [](int i) { return i * 1.01; },
+                                       [c = 0](int i) mutable { return c++ < 5 ? i * 1.02 : i; });
+      assert(ret.in1 == a + 5);
+      assert(ret.in2 == a + 5);
+      assert(*ret.in1 == 1);
+      assert(*ret.in2 == 1);
+    }
+  }
+
+  { // test predicate and projection call count
+    {
+      int pred_count = 0;
+      int proj1_count = 0;
+      int proj2_count = 0;
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, a,
+                                       [&](int lhs, int rhs) { ++pred_count; return lhs == rhs; },
+                                       [&](int i) { ++proj1_count; return i; },
+                                       [&](int i) { ++proj2_count; return i; });
+      assert(ret.in1 == a + 8);
+      assert(ret.in2 == a + 8);
+      assert(pred_count == 8);
+      assert(proj1_count == 8);
+      assert(proj2_count == 8);
+    }
+    {
+      int pred_count = 0;
+      int proj1_count = 0;
+      int proj2_count = 0;
+      int a[] = {7, 6, 9, 3, 5, 1, 2, 4};
+      auto ret = std::ranges::mismatch(a, a + 8, a, a + 8,
+                                       [&](int lhs, int rhs) { ++pred_count; return lhs == rhs; },
+                                       [&](int i) { ++proj1_count; return i; },
+                                       [&](int i) { ++proj2_count; return i; });
+      assert(ret.in1 == a + 8);
+      assert(ret.in2 == a + 8);
+      assert(pred_count == 8);
+      assert(proj1_count == 8);
+      assert(proj2_count == 8);
+    }
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
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 d2a08ae600f6..08370ac07558 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
@@ -104,7 +104,7 @@ static_assert(test(std::ranges::max_element, a));
 static_assert(test(std::ranges::min_element, a));
 //static_assert(test(std::ranges::minmax, a));
 //static_assert(test(std::ranges::minmax_element, a));
-//static_assert(test(std::ranges::mismatch, a, a));
+static_assert(test(std::ranges::mismatch, a, a));
 //static_assert(test(std::ranges::move, a, a));
 //static_assert(test(std::ranges::move_backward, a, a));
 //static_assert(test(std::ranges::next_permutation, a));