diff --git a/libcxx/docs/Status/RangesAlgorithms.csv b/libcxx/docs/Status/RangesAlgorithms.csv
--- a/libcxx/docs/Status/RangesAlgorithms.csv
+++ b/libcxx/docs/Status/RangesAlgorithms.csv
@@ -60,11 +60,11 @@
 Write,unique_copy,Not assigned,n/a,Not started
 Write,partition_copy,Not assigned,n/a,Not started
 Write,partial_sort_copy,Not assigned,n/a,Not started
-Merge,merge,Not assigned,n/a,Not started
-Merge,set_difference,Not assigned,n/a,Not started
-Merge,set_intersection,Not assigned,n/a,Not started
-Merge,set_symmetric_difference,Not assigned,n/a,Not started
-Merge,set_union,Not assigned,n/a,Not started
+Merge,merge,Hui Xie,n/a,✅
+Merge,set_difference,Hui Xie,n/a,Not started
+Merge,set_intersection,Hui Xie,n/a,Not started
+Merge,set_symmetric_difference,Hui Xie,n/a,Not started
+Merge,set_union,Not assigned,Hui Xie,Not started
 Permutation,remove,Not assigned,n/a,Not started
 Permutation,remove_if,Not assigned,n/a,Not started
 Permutation,reverse,Nikolas Klauser,`D125752 <https://llvm.org/D125752>`_,✅
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -94,6 +94,7 @@
   __algorithm/ranges_lower_bound.h
   __algorithm/ranges_max.h
   __algorithm/ranges_max_element.h
+  __algorithm/ranges_merge.h
   __algorithm/ranges_min.h
   __algorithm/ranges_min_element.h
   __algorithm/ranges_minmax.h
diff --git a/libcxx/include/__algorithm/ranges_merge.h b/libcxx/include/__algorithm/ranges_merge.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/ranges_merge.h
@@ -0,0 +1,142 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_MERGE_H
+#define _LIBCPP___ALGORITHM_RANGES_MERGE_H
+
+#include "__algorithm/in_in_out_result.h"
+#include "__algorithm/ranges_copy.h"
+#include "__iterator/concepts.h"
+#include "__iterator/mergeable.h"
+#include "__functional/identity.h"
+#include "__functional/invoke.h"
+#include "__functional/ranges_operations.h"
+#include "__utility/move.h"
+#include "__ranges/access.h"
+#include "__ranges/concepts.h"
+#include "__ranges/dangling.h"
+#include "__type_traits/decay.h"
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+
+template <class _InIter1, class _InIter2, class _OutIter>
+using merge_result = in_in_out_result<_InIter1, _InIter2, _OutIter>;
+
+template <
+    class _InIter1,
+    class _Sent1,
+    class _InIter2,
+    class _Sent2,
+    class _OutIter,
+    class _Comp,
+    class _Proj1,
+    class _Proj2>
+_LIBCPP_HIDE_FROM_ABI constexpr ranges::merge_result<decay_t<_InIter1>, decay_t<_InIter2>, decay_t<_OutIter>>
+__merge_impl(
+    _InIter1&& __first1,
+    _Sent1&& __last1,
+    _InIter2&& __first2,
+    _Sent2&& __last2,
+    _OutIter&& __result,
+    _Comp&& __comp,
+    _Proj1&& __proj1,
+    _Proj2&& __proj2) {
+  for (; __first1 != __last1 && __first2 != __last2; ++__result) {
+    if (std::invoke(__comp, std::invoke(__proj2, *__first2), std::invoke(__proj1, *__first1))) {
+      *__result = *__first2;
+      ++__first2;
+    } else {
+      *__result = *__first1;
+      ++__first1;
+    }
+  }
+  auto __ret1 = ranges::copy(std::move(__first1), std::move(__last1), std::move(__result));
+  auto __ret2 = ranges::copy(std::move(__first2), std::move(__last2), std::move(__ret1.out));
+  return {std::move(__ret1.in), std::move(__ret2.in), std::move(__ret2.out)};
+}
+
+namespace __merge {
+
+  struct __fn {
+    template <
+        input_iterator _InIter1,
+        sentinel_for<_InIter1> _Sent1,
+        input_iterator _InIter2,
+        sentinel_for<_InIter2> _Sent2,
+        weakly_incrementable _OutIter,
+        class _Comp  = ranges::less,
+        class _Proj1 = identity,
+        class _Proj2 = identity >
+      requires std::mergeable<_InIter1, _InIter2, _OutIter, _Comp, _Proj1, _Proj2>
+    _LIBCPP_HIDE_FROM_ABI constexpr ranges::merge_result<_InIter1, _InIter2, _OutIter> operator()(
+        _InIter1 __first1,
+        _Sent1 __last1,
+        _InIter2 __first2,
+        _Sent2 __last2,
+        _OutIter __result,
+        _Comp __comp   = {},
+        _Proj1 __proj1 = {},
+        _Proj2 __proj2 = {}) const {
+      return ranges::__merge_impl(__first1, __last1, __first2, __last2, __result, __comp, __proj1, __proj2);
+    }
+
+    template <
+        ranges::input_range _Range1,
+        ranges::input_range _Range2,
+        std::weakly_incrementable _OutIter,
+        class _Comp  = ranges::less,
+        class _Proj1 = identity,
+        class _Proj2 = identity >
+      requires std::mergeable<
+          ranges::iterator_t<_Range1>,
+          ranges::iterator_t<_Range2>,
+          _OutIter,
+          _Comp,
+          _Proj1,
+          _Proj2> _LIBCPP_HIDE_FROM_ABI constexpr ranges::
+          merge_result<ranges::borrowed_iterator_t<_Range1>, ranges::borrowed_iterator_t<_Range2>, _OutIter>
+          operator()(
+              _Range1&& __range1,
+              _Range2&& __range2,
+              _OutIter __result,
+              _Comp __comp   = {},
+              _Proj1 __proj1 = {},
+              _Proj2 __proj2 = {}) const {
+      return ranges::__merge_impl(
+          ranges::begin(__range1),
+          ranges::end(__range1),
+          ranges::begin(__range2),
+          ranges::end(__range2),
+          std::move(__result),
+          std::move(__comp),
+          std::move(__proj1),
+          std::move(__proj2));
+    }
+  };
+
+} // namespace __merge
+
+inline namespace __cpo {
+  inline constexpr auto merge = __merge::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+#endif // _LIBCPP___ALGORITHM_RANGES_MERGE_H
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -470,6 +470,23 @@
     constexpr ranges::move_result<borrowed_iterator_t<R>, O>
       ranges::move(R&& r, O result);                                                                // since C++20
 
+  template<class I1, class I2, class O>
+    using merge_result = in_in_out_result<I1, I2, O>;                                               // since C++20
+
+  template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
+           weakly_incrementable O, class Comp = ranges::less, class Proj1 = identity,
+           class Proj2 = identity>
+    requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
+    constexpr merge_result<I1, I2, O>
+      merge(I1 first1, S1 last1, I2 first2, S2 last2, O result,
+            Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});                                    // since C++20
+
+  template<input_range R1, input_range R2, weakly_incrementable O, class Comp = ranges::less,
+           class Proj1 = identity, class Proj2 = identity>
+    requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
+    constexpr merge_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
+      merge(R1&& r1, R2&& r2, O result,
+            Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});                                    // since C++20
 
 }
 
@@ -1216,6 +1233,7 @@
 #include <__algorithm/ranges_minmax.h>
 #include <__algorithm/ranges_minmax_element.h>
 #include <__algorithm/ranges_mismatch.h>
+#include <__algorithm/ranges_merge.h>
 #include <__algorithm/ranges_move.h>
 #include <__algorithm/ranges_move_backward.h>
 #include <__algorithm/ranges_none_of.h>
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -338,6 +338,7 @@
       module ranges_minmax                   { private header "__algorithm/ranges_minmax.h" }
       module ranges_minmax_element           { private header "__algorithm/ranges_minmax_element.h" }
       module ranges_mismatch                 { private header "__algorithm/ranges_mismatch.h" }
+      module ranges_merge                    { private header "__algorithm/ranges_merge.h" }
       module ranges_move                     { private header "__algorithm/ranges_move.h" }
       module ranges_move_backward            { private header "__algorithm/ranges_move_backward.h" }
       module ranges_none_of                  { private header "__algorithm/ranges_none_of.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
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
@@ -79,7 +79,7 @@
 {
     void *a[10] = {};
     void *b[10] = {};
-    //void *half[5] = {};
+    void *half[5] = {};
     void **first = a;
     void **mid = a+5;
     void **last = a+10;
@@ -151,8 +151,8 @@
     (void)std::ranges::max(a, Less(&copies)); assert(copies == 0);
     (void)std::ranges::max_element(first, last, Less(&copies)); assert(copies == 0);
     (void)std::ranges::max_element(a, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::merge(first, mid, mid, last, first2, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::merge(half, half, b, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::merge(first, mid, mid, last, first2, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::merge(half, half, b, Less(&copies)); assert(copies == 0);
     (void)std::ranges::min(value, value, Less(&copies)); assert(copies == 0);
     (void)std::ranges::min({ value, value }, Less(&copies)); assert(copies == 0);
     (void)std::ranges::min(a, Less(&copies)); assert(copies == 0);
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
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
@@ -61,7 +61,7 @@
 {
     T a[10] = {};
     T b[10] = {};
-    //T half[5] = {};
+    T half[5] = {};
     T *first = a;
     T *mid = a+5;
     T *last = a+10;
@@ -134,8 +134,8 @@
     (void)std::ranges::max(a, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::max_element(first, last, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::max_element(a, Less(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::merge(first, mid, mid, last, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::merge(half, half, b, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::merge(first, mid, mid, last, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::merge(half, half, b, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::min(T(), T(), Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::min({ T(), T() }, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::min(a, Less(), Proj(&copies)); assert(copies == 0);
diff --git a/libcxx/test/libcxx/private_headers.verify.cpp b/libcxx/test/libcxx/private_headers.verify.cpp
--- a/libcxx/test/libcxx/private_headers.verify.cpp
+++ b/libcxx/test/libcxx/private_headers.verify.cpp
@@ -131,6 +131,7 @@
 #include <__algorithm/ranges_lower_bound.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_lower_bound.h'}}
 #include <__algorithm/ranges_max.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_max.h'}}
 #include <__algorithm/ranges_max_element.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_max_element.h'}}
+#include <__algorithm/ranges_merge.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_merge.h'}}
 #include <__algorithm/ranges_min.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_min.h'}}
 #include <__algorithm/ranges_min_element.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_min_element.h'}}
 #include <__algorithm/ranges_minmax.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_minmax.h'}}
diff --git a/libcxx/test/std/algorithms/alg.sorting/alg.merge/ranges_merge.pass.cpp b/libcxx/test/std/algorithms/alg.sorting/alg.merge/ranges_merge.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.sorting/alg.merge/ranges_merge.pass.cpp
@@ -0,0 +1,599 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// template<input_­iterator I1, sentinel_­for<I1> S1, input_­iterator I2, sentinel_­for<I2> S2,
+//          weakly_­incrementable O, class Comp = ranges::less, class Proj1 = identity,
+//          class Proj2 = identity>
+//   requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
+//   constexpr merge_result<I1, I2, O>
+//     merge(I1 first1, S1 last1, I2 first2, S2 last2, O result,
+//           Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});                                    // since C++20
+//
+// template<input_­range R1, input_­range R2, weakly_­incrementable O, class Comp = ranges::less,
+//          class Proj1 = identity, class Proj2 = identity>
+//   requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
+//   constexpr merge_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
+//     merge(R1&& r1, R2&& r2, O result,
+//           Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});                                    // since C++20
+
+#include <algorithm>
+#include <array>
+
+#include "almost_satisfies_types.h"
+#include "MoveOnly.h"
+#include "test_iterators.h"
+
+template <class... Args>
+concept HasMerge = requires(Args&&... args) { std::ranges::merge(std::forward<Args>(args)...); };
+
+// clang-format off
+
+using NotInputIter = InputIteratorNotDerivedFrom;
+using NotSentinel = SentinelForNotSemiregular;
+
+template <class It>
+using Sent = sentinel_wrapper<It>;
+
+// Test iterator overload's constraints:
+// =====================================
+
+// |     InIter1     |      Sent1        |    InIter2     |      Sent2      |    OutIter      |
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+static_assert( HasMerge<
+         int*,              int*,              int*,             int*,            int*       >);
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+// !std::input_iterator<I1>
+static_assert(!HasMerge<
+      NotInputIter,    Sent<NotInputIter>,     int*,             int*,            int*       >);
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+// !std::sentinel_for<S1, I1>
+static_assert(!HasMerge<
+        int*,            NotSentinel,          int*,             int*,            int*       >);
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+// !std::input_iterator<I2>
+static_assert(!HasMerge<
+        int*,               int*,          NotInputIter,   Sent<NotInputIter>,    int*       >);
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+// !std::sentinel_for<S2, I2>
+static_assert(!HasMerge<
+        int*,               int*,              int*,          NotSentinel,        int*       >);
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+// !std::weakly_incrementable<O>
+static_assert(!HasMerge<
+        int*,               int*,              int*,             int*,     WeaklyIncrementableNotMovable>);
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+// !std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
+static_assert(!HasMerge<
+      MoveOnly*,          MoveOnly*,         MoveOnly*,        MoveOnly*,        MoveOnly*    >);
+// |-----------------|-------------------|----------------|-----------------|-----------------|  
+
+// Test range overload's constraints:
+// =====================================
+
+template <class It, class St = sentinel_wrapper<It>>
+using Range = UncheckedRange<It, St>;
+
+
+// |         R1             |             R2           |          OutIter         |
+// |------------------------|--------------------------|--------------------------| 
+static_assert(HasMerge<
+      Range<int*, int*>,           Range<int*, int*>,            int*            >);
+// |------------------------|--------------------------|--------------------------| 
+// !std::input_range<R1>
+static_assert(!HasMerge<
+      Range<NotInputIter>,         Range<int*>,                  int*            >);
+// |------------------------|--------------------------|--------------------------| 
+// !std::input_range<R2>
+static_assert(!HasMerge<
+      Range<int*>,                 Range<NotInputIter>,          int*            >);
+// |------------------------|--------------------------|--------------------------| 
+// !std::weakly_incrementable<O>
+static_assert(!HasMerge<
+       Range<int*>,                Range<int*>,     WeaklyIncrementableNotMovable >);
+// |------------------------|--------------------------|--------------------------| 
+// !mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
+static_assert(!HasMerge<
+      Range<MoveOnly*>,            Range<MoveOnly*>,           MoveOnly*         >);
+// |------------------------|--------------------------|--------------------------| 
+
+// clang-format on
+
+// test the range is correctly merged for different kinds of input ranges and output iterator
+template <class...>
+struct type_list {};
+
+template <class... In1, class... In2, class... Out, std::size_t... Is>
+constexpr auto cartesian_product_impl(
+    std::tuple<In1...>, std::tuple<In2...>, std::tuple<Out...>, std::index_sequence<Is...>)
+    -> type_list< type_list<
+        std::tuple_element_t<Is / (sizeof...(In2) * sizeof...(Out)), std::tuple<In1...>>,
+        std::tuple_element_t<Is % (sizeof...(In2) * sizeof...(Out)) / sizeof...(Out), std::tuple<In2...>>,
+        std::tuple_element_t<Is % sizeof...(Out), std::tuple<Out...>> >... >;
+
+template <class... In1, class... In2, class... Out>
+constexpr auto
+cartesian_product(type_list<In1...>, type_list<In2...>, type_list<Out...>) -> decltype(cartesian_product_impl(
+    std::declval<std::tuple<In1...>>(),
+    std::declval<std::tuple<In2...>>(),
+    std::declval<std::tuple<Out...>>(),
+    std::make_index_sequence<sizeof...(In1) * sizeof...(In2) * sizeof...(Out)>())) {
+  return {};
+}
+
+using In1Iters = type_list<
+    cpp20_input_iterator<int*>,
+    forward_iterator<int*>,
+    bidirectional_iterator<int*>,
+    random_access_iterator<int*>,
+    contiguous_iterator<int*> >;
+
+using In2Iters = In1Iters;
+
+using OutIters = type_list<
+    cpp17_output_iterator<int*>,
+    cpp20_output_iterator<int*>,
+    cpp17_input_iterator<int*>,
+    cpp20_input_iterator<int*>,
+    forward_iterator<int*>,
+    bidirectional_iterator<int*>,
+    random_access_iterator<int*>,
+    contiguous_iterator<int*>>;
+
+template <class Test, class... Sigs>
+constexpr void runAllPermutations(Test test, type_list<Sigs...>) {
+  (test(Sigs{}), ...);
+}
+
+constexpr void runAllPermutations(auto test) {
+  runAllPermutations(test, cartesian_product(In1Iters{}, In2Iters{}, OutIters{}));
+}
+
+template <class In1, class In2, class Out, std::size_t N1, std::size_t N2>
+constexpr void testMergeImpl(std::array<int, N1>& in1, std::array<int, N2>& in2, const auto& expected) {
+  // TODO: std::ranges::merge calls std::ranges::copy
+  // std::ranges::copy(contiguous_iterator<int*>, sentinel_wrapper<contiguous_iterator<int*>>, contiguous_iterator<int*>) doesn't seem to work.
+  // It seems that std::ranges::copy calls std::copy, which unwraps contiguous_iterator<int*> into int*, and then it failed because there is no == between int* and sentinel_wrapper<contiguous_iterator<int*>>
+  using Sent1 = std::conditional_t<std::contiguous_iterator<In1>, In1, sentinel_wrapper<In1>>;
+  using Sent2 = std::conditional_t<std::contiguous_iterator<In2>, In2, sentinel_wrapper<In2>>;
+
+  // iterator overload
+  {
+    std::array<int, N1 + N2> out;
+    std::same_as<std::ranges::merge_result<In1, In2, Out>> decltype(auto) result = std::ranges::merge(
+        In1{in1.data()},
+        Sent1{In1{in1.data() + in1.size()}},
+        In2{in2.data()},
+        Sent2{In2{in2.data() + in2.size()}},
+        Out{out.data()});
+    assert(std::ranges::equal(out, expected));
+
+    assert(base(result.in1) == in1.data() + in1.size());
+    assert(base(result.in2) == in2.data() + in2.size());
+    assert(base(result.out) == out.data() + out.size());
+  }
+
+  // range overload
+  {
+    std::array<int, N1 + N2> out;
+    std::ranges::subrange r1{In1{in1.data()}, Sent1{In1{in1.data() + in1.size()}}};
+    std::ranges::subrange r2{In2{in2.data()}, Sent2{In2{in2.data() + in2.size()}}};
+    std::same_as<std::ranges::merge_result<In1, In2, Out>> decltype(auto) result =
+        std::ranges::merge(r1, r2, Out{out.data()});
+    assert(std::ranges::equal(out, expected));
+
+    assert(base(result.in1) == in1.data() + in1.size());
+    assert(base(result.in2) == in2.data() + in2.size());
+    assert(base(result.out) == out.data() + out.size());
+  }
+}
+
+constexpr bool test() {
+  // Test merge produces correct result for different kinds of iterators
+  {
+    constexpr auto testMerge = []<class In1, class In2, class Out>(type_list<In1, In2, Out>) {
+      {
+        std::array in1{0, 1, 5, 6, 9, 10};
+        std::array in2{3, 6, 7, 9, 13, 15, 100};
+        std::array expected{0, 1, 3, 5, 6, 6, 7, 9, 9, 10, 13, 15, 100};
+        testMergeImpl<In1, In2, Out>(in1, in2, expected);
+      }
+
+      {
+        std::array in1{0, 1, 2};
+        std::array in2{0, 1, 2};
+        std::array expected{0, 0, 1, 1, 2, 2};
+        testMergeImpl<In1, In2, Out>(in1, in2, expected);
+      }
+
+      {
+        std::array in1{0};
+        std::array in2{0, 0, 0, 1};
+        std::array expected{0, 0, 0, 0, 1};
+        testMergeImpl<In1, In2, Out>(in1, in2, expected);
+      }
+    };
+    runAllPermutations(testMerge);
+  }
+
+  // check that ranges::dangling is returned for non-borrowed_range and iterator_t is returned for borrowed_range
+  {
+    constexpr auto testDangling = []<class In1, class In2, class Out>(type_list<In1, In2, Out>) {
+      struct NonBorrowRange {
+        int* data_;
+        size_t size_;
+
+        // TODO: std::ranges::merge calls std::ranges::copy
+        // std::ranges::copy(contiguous_iterator<int*>, sentinel_wrapper<contiguous_iterator<int*>>, contiguous_iterator<int*>) doesn't seem to work.
+        // It seems that std::ranges::copy calls std::copy, which unwraps contiguous_iterator<int*> into int*, and then it failed because there is no == between int* and sentinel_wrapper<contiguous_iterator<int*>>
+        using Sent = std::conditional_t<std::contiguous_iterator<In2>, In2, sentinel_wrapper<In2>>;
+
+        constexpr NonBorrowRange(int* d, size_t s) : data_{d}, size_{s} {}
+
+        constexpr In2 begin() const { return In2{data_}; };
+        constexpr Sent end() const { return Sent{In2{data_ + size_}}; };
+      };
+
+      std::array r1{3, 6, 7, 9};
+      std::array r2{2, 3, 4};
+      std::array<int, 7> out;
+      std::same_as<std::ranges::merge_result<std::array<int, 4>::iterator, std::ranges::dangling, int*>> decltype(auto)
+          result = std::ranges::merge(r1, NonBorrowRange{r2.data(), r2.size()}, out.data());
+      assert(base(result.in1) == r1.end());
+      assert(base(result.out) == out.data() + out.size());
+      assert(std::ranges::equal(out, std::array{2, 3, 3, 4, 6, 7, 9}));
+    };
+    runAllPermutations(testDangling);
+  }
+
+  // check that every element is copied exactly once
+  struct TracedCopy {
+    int copy_assign = 0;
+    int data        = 0;
+
+    constexpr TracedCopy() = default;
+    constexpr TracedCopy(int i) : data(i) {}
+    constexpr TracedCopy(const TracedCopy& other) : copy_assign(other.copy_assign), data(other.data) {}
+
+    constexpr TracedCopy(TracedCopy&& other)            = delete;
+    constexpr TracedCopy& operator=(TracedCopy&& other) = delete;
+
+    constexpr TracedCopy& operator=(const TracedCopy& other) {
+      copy_assign = other.copy_assign + 1;
+      data        = other.data;
+      return *this;
+    }
+
+    constexpr bool operator==(const TracedCopy& o) const { return data == o.data; }
+    constexpr auto operator<=>(const TracedCopy& o) const { return data <=> o.data; }
+  };
+  {
+    std::array<TracedCopy, 3> r1{3, 5, 8};
+    std::array<TracedCopy, 3> r2{1, 3, 8};
+    using Iter = std::array<TracedCopy, 3>::iterator;
+
+    // iterator overload
+    {
+      std::array<TracedCopy, 6> out;
+      std::same_as<std::ranges::merge_result<Iter, Iter, TracedCopy*>> decltype(auto) result =
+          std::ranges::merge(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data());
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+      assert(std::ranges::equal(out, std::array<TracedCopy, 6>{1, 3, 3, 5, 8, 8}));
+
+      std::ranges::all_of(out, [](const TracedCopy& e) { return e.copy_assign == 1; });
+    }
+
+    // range overload
+    {
+      std::array<TracedCopy, 6> out;
+      std::same_as<std::ranges::merge_result<Iter, Iter, TracedCopy*>> decltype(auto) result =
+          std::ranges::merge(r1, r2, out.data());
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+      assert(std::ranges::equal(out, std::array<TracedCopy, 6>{1, 3, 3, 5, 8, 8}));
+
+      std::ranges::all_of(out, [](const TracedCopy& e) { return e.copy_assign == 1; });
+    }
+  }
+
+  struct IntAndID {
+    int data;
+    int id;
+
+    constexpr auto operator==(const IntAndID& o) const { return data == o.data; }
+    constexpr auto operator<=>(const IntAndID& o) const { return data <=> o.data; }
+  };
+
+  // Algorithm is stable: equal elements should merged in the original order
+  {
+    std::array<IntAndID, 3> r1{{{0, 0}, {0, 1}, {0, 2}}};
+    std::array<IntAndID, 3> r2{{{1, 0}, {1, 1}, {1, 2}}};
+
+    // iterator overload
+    {
+      std::array<IntAndID, 6> out;
+      std::ranges::merge(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data());
+
+      assert(std::ranges::equal(out, std::array{0, 0, 0, 1, 1, 1}, std::ranges::equal_to{}, &IntAndID::data));
+      // ID should be in their original order
+      assert(std::ranges::equal(out, std::array{0, 1, 2, 0, 1, 2}, std::ranges::equal_to{}, &IntAndID::id));
+    }
+
+    // range overload
+    {
+      std::array<IntAndID, 6> out;
+      std::ranges::merge(r1, r2, out.data());
+
+      assert(std::ranges::equal(out, std::array{0, 0, 0, 1, 1, 1}, std::ranges::equal_to{}, &IntAndID::data));
+      // ID should be in their original order
+      assert(std::ranges::equal(out, std::array{0, 1, 2, 0, 1, 2}, std::ranges::equal_to{}, &IntAndID::id));
+    }
+  }
+
+  // Equal elements in R1 should be merged before equal elements in R2
+  {
+    std::array<IntAndID, 3> r1{{{0, 1}, {1, 1}, {2, 1}}};
+    std::array<IntAndID, 3> r2{{{0, 2}, {1, 2}, {2, 2}}};
+
+    // iterator overload
+    {
+      std::array<IntAndID, 6> out;
+      std::ranges::merge(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data());
+
+      assert(std::ranges::equal(out, std::array{0, 0, 1, 1, 2, 2}, std::ranges::equal_to{}, &IntAndID::data));
+      // ID 1 (from R1) should be in front of ID 2 (from R2)
+      assert(std::ranges::equal(out, std::array{1, 2, 1, 2, 1, 2}, std::ranges::equal_to{}, &IntAndID::id));
+    }
+
+    // range overload
+    {
+      std::array<IntAndID, 6> out;
+      std::ranges::merge(r1, r2, out.data());
+
+      assert(std::ranges::equal(out, std::array{0, 0, 1, 1, 2, 2}, std::ranges::equal_to{}, &IntAndID::data));
+      // ID 1 (from R1) should be in front of ID 2 (from R2)
+      assert(std::ranges::equal(out, std::array{1, 2, 1, 2, 1, 2}, std::ranges::equal_to{}, &IntAndID::id));
+    }
+  }
+
+  struct Data {
+    int data;
+
+    constexpr bool smallerThan(const Data& o) const { return data < o.data; }
+  };
+  // Test custom comparator
+  {
+    std::array r1{Data{4}, Data{8}, Data{12}};
+    std::array r2{Data{5}, Data{9}};
+    using Iter1 = std::array<Data, 3>::iterator;
+    using Iter2 = std::array<Data, 2>::iterator;
+
+    // iterator overload
+    {
+      std::array<Data, 5> out;
+      std::same_as<std::ranges::merge_result<Iter1, Iter2, Data*>> decltype(auto) result =
+          std::ranges::merge(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data(), [](const Data& x, const Data& y) {
+            return x.data < y.data;
+          });
+
+      assert(std::ranges::equal(out, std::array{4, 5, 8, 9, 12}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+
+    // range overload
+    {
+      std::array<Data, 5> out;
+      std::same_as<std::ranges::merge_result<Iter1, Iter2, Data*>> decltype(auto) result =
+          std::ranges::merge(r1, r2, out.data(), [](const Data& x, const Data& y) { return x.data < y.data; });
+
+      assert(std::ranges::equal(out, std::array{4, 5, 8, 9, 12}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+  }
+
+  // Test Projection
+  {
+    std::array r1{Data{4}, Data{8}, Data{12}};
+    std::array r2{Data{5}, Data{9}};
+    using Iter1 = std::array<Data, 3>::iterator;
+    using Iter2 = std::array<Data, 2>::iterator;
+
+    const auto proj = [](const Data& d) { return d.data; };
+
+    // iterator overload
+    {
+      std::array<Data, 5> out;
+      std::same_as<std::ranges::merge_result<Iter1, Iter2, Data*>> decltype(auto) result =
+          std::ranges::merge(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data(), std::ranges::less{}, proj, proj);
+
+      assert(std::ranges::equal(out, std::array{4, 5, 8, 9, 12}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+
+    // range overload
+    {
+      std::array<Data, 5> out;
+      std::same_as<std::ranges::merge_result<Iter1, Iter2, Data*>> decltype(auto) result =
+          std::ranges::merge(r1, r2, out.data(), std::ranges::less{}, proj, proj);
+
+      assert(std::ranges::equal(out, std::array{4, 5, 8, 9, 12}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+
+    // member pointer Comparator
+    {
+      std::array<Data, 5> out;
+      std::same_as<std::ranges::merge_result<Iter1, Iter2, Data*>> decltype(auto) result =
+          std::ranges::merge(r1, r2, out.data(), &Data::smallerThan);
+
+      assert(std::ranges::equal(out, std::array{4, 5, 8, 9, 12}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+
+    // member pointer Projection
+    {
+      std::array<Data, 5> out;
+      std::same_as<std::ranges::merge_result<Iter1, Iter2, Data*>> decltype(auto) result =
+          std::ranges::merge(r1, r2, out.data(), std::ranges::less{}, &Data::data, &Data::data);
+
+      assert(std::ranges::equal(out, std::array{4, 5, 8, 9, 12}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+  }
+
+  // Complexity: at most N - 1 comparisons and applications of each projection.
+  {
+    Data r1[] = {{0}, {1}, {2}, {3}, {4}, {5}, {6}, {7}, {8}, {9}};
+    Data r2[] = {{0}, {1}, {2}, {3}, {4}, {5}, {6}, {7}, {8}, {9}};
+    std::array<Data, 20> out;
+    std::size_t numberOfComp  = 0;
+    std::size_t numberOfProj1 = 0;
+    std::size_t numberOfProj2 = 0;
+
+    const auto comp = [&numberOfComp](int x, int y) {
+      ++numberOfComp;
+      return x < y;
+    };
+
+    const auto proj1 = [&numberOfProj1](const Data& d) {
+      ++numberOfProj1;
+      return d.data;
+    };
+
+    const auto proj2 = [&numberOfProj2](const Data& d) {
+      ++numberOfProj2;
+      return d.data;
+    };
+
+    std::ranges::merge(r1, r2, out.data(), comp, proj1, proj2);
+    assert(std::ranges::equal(
+        out,
+        std::array{0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9},
+        std::ranges::equal_to{},
+        &Data::data));
+    assert(numberOfComp < out.size());
+    assert(numberOfProj1 < out.size());
+    assert(numberOfProj2 < out.size());
+  }
+
+  // Comparator convertible to bool
+  {
+    struct ConvertibleToBool {
+      bool b;
+      constexpr operator bool() const { return b; }
+    };
+    Data r1[] = {{2}, {4}};
+    Data r2[] = {{3}, {4}, {5}};
+    std::array<Data, 5> out;
+
+    const auto comp = [](const Data& x, const Data& y) { return ConvertibleToBool{x.data < y.data}; };
+
+    std::ranges::merge(r1, r2, out.data(), comp);
+    assert(std::ranges::equal(out, std::array{2, 3, 4, 4, 5}, std::ranges::equal_to{}, &Data::data));
+  }
+
+  // One range is empty
+  {
+    auto r1                = std::views::empty<Data>;
+    std::array<Data, 3> r2 = {{{3}, {4}, {5}}};
+
+    // iterator overload
+    {
+      std::array<Data, 3> out;
+      std::same_as<std::ranges::merge_result<Data*, Data*, Data*>> decltype(auto) result =
+          std::ranges::merge(r1.begin(), r1.end(), r2.begin(), r2.begin() + r2.size(), out.data(), &Data::smallerThan);
+
+      assert(std::ranges::equal(out, std::array{3, 4, 5}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+
+    // range overload
+    {
+      std::array<Data, 3> out;
+      std::same_as<std::ranges::merge_result<Data*, std::array<Data, 3>::iterator, Data*>> decltype(auto) result =
+          std::ranges::merge(r1, r2, out.data(), &Data::smallerThan);
+
+      assert(std::ranges::equal(out, std::array{3, 4, 5}, std::ranges::equal_to{}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+  }
+
+  // Both ranges are empty
+  {
+    auto r1 = std::views::empty<TracedCopy>;
+    auto r2 = std::views::empty<TracedCopy>;
+
+    // iterator overload
+    {
+      std::array<TracedCopy, 1> out{};
+      std::same_as<std::ranges::merge_result<TracedCopy*, TracedCopy*, TracedCopy*>> decltype(auto) result =
+          std::ranges::merge(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data());
+
+      assert(out[0].copy_assign == 0);
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.begin());
+    }
+
+    // range overload
+    {
+      std::array<TracedCopy, 1> out{};
+      std::same_as<std::ranges::merge_result<TracedCopy*, TracedCopy*, TracedCopy*>> decltype(auto) result =
+          std::ranges::merge(r1, r2, out.data());
+
+      assert(out[0].copy_assign == 0);
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.begin());
+    }
+  }
+
+  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
--- 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
@@ -98,7 +98,7 @@
 //static_assert(test(std::ranges::make_heap, a));
 static_assert(test(std::ranges::max, a));
 static_assert(test(std::ranges::max_element, a));
-//static_assert(test(std::ranges::merge, a, a, a));
+static_assert(test(std::ranges::merge, a, a, a));
 static_assert(test(std::ranges::min, a));
 static_assert(test(std::ranges::min_element, a));
 static_assert(test(std::ranges::minmax, a));