diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -1,5 +1,6 @@
 set(files
   __algorithm/adjacent_find.h
+  __algorithm/algorithm_family.h
   __algorithm/all_of.h
   __algorithm/any_of.h
   __algorithm/binary_search.h
diff --git a/libcxx/include/__algorithm/algorithm_family.h b/libcxx/include/__algorithm/algorithm_family.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/algorithm_family.h
@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_ALGORITHM_FAMILY_H
+#define _LIBCPP___ALGORITHM_ALGORITHM_FAMILY_H
+
+#include <__algorithm/iterator_operations.h>
+#include <__algorithm/move.h>
+#include <__algorithm/ranges_move.h>
+#include <__config>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _AlgPolicy>
+struct _AlgoFamily;
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+template <>
+struct _AlgoFamily<_RangeAlgPolicy> {
+  static constexpr auto __move = ranges::move;
+};
+
+#endif
+
+template <>
+struct _AlgoFamily<_ClassicAlgPolicy> {
+
+  // move
+  template <class _InputIterator, class _OutputIterator>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 static _OutputIterator
+  __move(_InputIterator __first, _InputIterator __last, _OutputIterator __result) {
+    return std::move(
+        std::move(__first),
+        std::move(__last),
+        std::move(__result));
+  }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ALGORITHM_ALGORITHM_FAMILY_H
diff --git a/libcxx/include/__algorithm/inplace_merge.h b/libcxx/include/__algorithm/inplace_merge.h
--- a/libcxx/include/__algorithm/inplace_merge.h
+++ b/libcxx/include/__algorithm/inplace_merge.h
@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___ALGORITHM_INPLACE_MERGE_H
 #define _LIBCPP___ALGORITHM_INPLACE_MERGE_H
 
+#include <__algorithm/algorithm_family.h>
 #include <__algorithm/comp.h>
 #include <__algorithm/comp_ref_type.h>
 #include <__algorithm/iterator_operations.h>
@@ -38,204 +39,199 @@
 class __invert // invert the sense of a comparison
 {
 private:
-    _Predicate __p_;
+  _Predicate __p_;
+
 public:
-    _LIBCPP_INLINE_VISIBILITY __invert() {}
+  _LIBCPP_HIDE_FROM_ABI __invert() {}
 
-    _LIBCPP_INLINE_VISIBILITY
-    explicit __invert(_Predicate __p) : __p_(__p) {}
+  _LIBCPP_HIDE_FROM_ABI explicit __invert(_Predicate __p) : __p_(__p) {}
 
-    template <class _T1>
-    _LIBCPP_INLINE_VISIBILITY
-    bool operator()(const _T1& __x) {return !__p_(__x);}
+  template <class _T1>
+  _LIBCPP_HIDE_FROM_ABI bool operator()(const _T1& __x) {
+    return !__p_(__x);
+  }
 
-    template <class _T1, class _T2>
-    _LIBCPP_INLINE_VISIBILITY
-    bool operator()(const _T1& __x, const _T2& __y) {return __p_(__y, __x);}
+  template <class _T1, class _T2>
+  _LIBCPP_HIDE_FROM_ABI bool operator()(const _T1& __x, const _T2& __y) {
+    return __p_(__y, __x);
+  }
 };
 
-template <class _AlgPolicy, class _Compare, class _InputIterator1, class _InputIterator2,
-          class _OutputIterator>
-void __half_inplace_merge(_InputIterator1 __first1, _InputIterator1 __last1,
-                          _InputIterator2 __first2, _InputIterator2 __last2,
-                          _OutputIterator __result, _Compare __comp)
-{
-    for (; __first1 != __last1; ++__result)
-    {
-        if (__first2 == __last2)
-        {
-            // TODO(alg-policy): pass `_AlgPolicy` once it's supported by `move`.
-            _VSTD::move(__first1, __last1, __result);
-            return;
-        }
-
-        if (__comp(*__first2, *__first1))
-        {
-            *__result = _IterOps<_AlgPolicy>::__iter_move(__first2);
-            ++__first2;
-        }
-        else
-        {
-            *__result = _IterOps<_AlgPolicy>::__iter_move(__first1);
-            ++__first1;
-        }
+template <
+    class _AlgPolicy,
+    class _Compare,
+    class _InputIterator1,
+    class _Sent1,
+    class _InputIterator2,
+    class _Sent2,
+    class _OutputIterator>
+_LIBCPP_HIDE_FROM_ABI void __half_inplace_merge(
+    _InputIterator1 __first1,
+    _Sent1 __last1,
+    _InputIterator2 __first2,
+    _Sent2 __last2,
+    _OutputIterator __result,
+    _Compare&& __comp) {
+  for (; __first1 != __last1; ++__result) {
+    if (__first2 == __last2) {
+      _AlgoFamily<_AlgPolicy>::__move(__first1, __last1, __result);
+      return;
+    }
+
+    if (__comp(*__first2, *__first1)) {
+      *__result = _IterOps<_AlgPolicy>::__iter_move(__first2);
+      ++__first2;
+    } else {
+      *__result = _IterOps<_AlgPolicy>::__iter_move(__first1);
+      ++__first1;
     }
-    // __first2 through __last2 are already in the right spot.
+  }
+  // __first2 through __last2 are already in the right spot.
 }
 
 template <class _AlgPolicy, class _Compare, class _BidirectionalIterator>
-void
-__buffered_inplace_merge(_BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last,
-                _Compare __comp, typename iterator_traits<_BidirectionalIterator>::difference_type __len1,
-                                 typename iterator_traits<_BidirectionalIterator>::difference_type __len2,
-                typename iterator_traits<_BidirectionalIterator>::value_type* __buff)
-{
-    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
-    __destruct_n __d(0);
-    unique_ptr<value_type, __destruct_n&> __h2(__buff, __d);
-    if (__len1 <= __len2)
-    {
-        value_type* __p = __buff;
-        for (_BidirectionalIterator __i = __first; __i != __middle; __d.template __incr<value_type>(), (void) ++__i, (void) ++__p)
-            ::new ((void*)__p) value_type(_IterOps<_AlgPolicy>::__iter_move(__i));
-        std::__half_inplace_merge<_AlgPolicy, _Compare>(__buff, __p, __middle, __last, __first, __comp);
-    }
-    else
-    {
-        value_type* __p = __buff;
-        for (_BidirectionalIterator __i = __middle; __i != __last; __d.template __incr<value_type>(), (void) ++__i, (void) ++__p)
-            ::new ((void*)__p) value_type(_IterOps<_AlgPolicy>::__iter_move(__i));
-        typedef __unconstrained_reverse_iterator<_BidirectionalIterator> _RBi;
-        typedef __unconstrained_reverse_iterator<value_type*> _Rv;
-        typedef __invert<_Compare> _Inverted;
-        std::__half_inplace_merge<_AlgPolicy, _Inverted>(_Rv(__p), _Rv(__buff),
-                                    _RBi(__middle), _RBi(__first),
-                                    _RBi(__last), _Inverted(__comp));
-    }
+_LIBCPP_HIDE_FROM_ABI void __buffered_inplace_merge(
+    _BidirectionalIterator __first,
+    _BidirectionalIterator __middle,
+    _BidirectionalIterator __last,
+    _Compare&& __comp,
+    typename iterator_traits<_BidirectionalIterator>::difference_type __len1,
+    typename iterator_traits<_BidirectionalIterator>::difference_type __len2,
+    typename iterator_traits<_BidirectionalIterator>::value_type* __buff) {
+  typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
+  __destruct_n __d(0);
+  unique_ptr<value_type, __destruct_n&> __h2(__buff, __d);
+  if (__len1 <= __len2) {
+    value_type* __p = __buff;
+    for (_BidirectionalIterator __i = __first; __i != __middle;
+         __d.template __incr<value_type>(), (void)++__i, (void)++__p)
+      ::new ((void*)__p) value_type(_IterOps<_AlgPolicy>::__iter_move(__i));
+    std::__half_inplace_merge<_AlgPolicy>(__buff, __p, __middle, __last, __first, __comp);
+  } else {
+    value_type* __p = __buff;
+    for (_BidirectionalIterator __i = __middle; __i != __last;
+         __d.template __incr<value_type>(), (void)++__i, (void)++__p)
+      ::new ((void*)__p) value_type(_IterOps<_AlgPolicy>::__iter_move(__i));
+    typedef __unconstrained_reverse_iterator<_BidirectionalIterator> _RBi;
+    typedef __unconstrained_reverse_iterator<value_type*> _Rv;
+    typedef __invert<_Compare> _Inverted;
+    std::__half_inplace_merge<_AlgPolicy>(
+        _Rv(__p), _Rv(__buff), _RBi(__middle), _RBi(__first), _RBi(__last), _Inverted(__comp));
+  }
 }
 
 template <class _AlgPolicy, class _Compare, class _BidirectionalIterator>
-void
-__inplace_merge(_BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last,
-                _Compare __comp, typename iterator_traits<_BidirectionalIterator>::difference_type __len1,
-                                 typename iterator_traits<_BidirectionalIterator>::difference_type __len2,
-                typename iterator_traits<_BidirectionalIterator>::value_type* __buff, ptrdiff_t __buff_size)
-{
-    using _Ops = _IterOps<_AlgPolicy>;
-
-    typedef typename iterator_traits<_BidirectionalIterator>::difference_type difference_type;
-    while (true)
-    {
-        // if __middle == __last, we're done
-        if (__len2 == 0)
-            return;
-        if (__len1 <= __buff_size || __len2 <= __buff_size)
-            return std::__buffered_inplace_merge<_AlgPolicy, _Compare>
-                   (__first, __middle, __last, __comp, __len1, __len2, __buff);
-        // shrink [__first, __middle) as much as possible (with no moves), returning if it shrinks to 0
-        for (; true; ++__first, (void) --__len1)
-        {
-            if (__len1 == 0)
-                return;
-            if (__comp(*__middle, *__first))
-                break;
-        }
-        // __first < __middle < __last
-        // *__first > *__middle
-        // partition [__first, __m1) [__m1, __middle) [__middle, __m2) [__m2, __last) such that
-        //     all elements in:
-        //         [__first, __m1)  <= [__middle, __m2)
-        //         [__middle, __m2) <  [__m1, __middle)
-        //         [__m1, __middle) <= [__m2, __last)
-        //     and __m1 or __m2 is in the middle of its range
-        _BidirectionalIterator __m1;  // "median" of [__first, __middle)
-        _BidirectionalIterator __m2;  // "median" of [__middle, __last)
-        difference_type __len11;      // distance(__first, __m1)
-        difference_type __len21;      // distance(__middle, __m2)
-        // binary search smaller range
-        if (__len1 < __len2)
-        {   // __len >= 1, __len2 >= 2
-            __len21 = __len2 / 2;
-            __m2 = __middle;
-            _Ops::advance(__m2, __len21);
-            // TODO: replace _ClassicAlgPolicy and __identity with _AlgPolicy and projection
-            __m1 = std::__upper_bound<_ClassicAlgPolicy>(__first, __middle, *__m2, __comp, std::__identity());
-            __len11 = _Ops::distance(__first, __m1);
-        }
-        else
-        {
-            if (__len1 == 1)
-            {   // __len1 >= __len2 && __len2 > 0, therefore __len2 == 1
-                // It is known *__first > *__middle
-              _Ops::iter_swap(__first, __middle);
-                return;
-            }
-            // __len1 >= 2, __len2 >= 1
-            __len11 = __len1 / 2;
-            __m1 = __first;
-            _Ops::advance(__m1, __len11);
-            __m2 = std::lower_bound(__middle, __last, *__m1, __comp);
-            __len21 = _Ops::distance(__middle, __m2);
-        }
-        difference_type __len12 = __len1 - __len11;  // distance(__m1, __middle)
-        difference_type __len22 = __len2 - __len21;  // distance(__m2, __last)
-        // [__first, __m1) [__m1, __middle) [__middle, __m2) [__m2, __last)
-        // swap middle two partitions
-        // TODO(alg-policy): pass `_AlgPolicy` once it's supported by `rotate`.
-        __middle = _VSTD::rotate(__m1, __middle, __m2);
-        // __len12 and __len21 now have swapped meanings
-        // merge smaller range with recursive call and larger with tail recursion elimination
-        if (__len11 + __len21 < __len12 + __len22)
-        {
-            std::__inplace_merge<_AlgPolicy, _Compare>(
-                __first, __m1, __middle, __comp, __len11, __len21, __buff, __buff_size);
-//          _VSTD::__inplace_merge<_Compare>(__middle, __m2, __last, __comp, __len12, __len22, __buff, __buff_size);
-            __first = __middle;
-            __middle = __m2;
-            __len1 = __len12;
-            __len2 = __len22;
-        }
-        else
-        {
-            std::__inplace_merge<_AlgPolicy, _Compare>(
-                __middle, __m2, __last, __comp, __len12, __len22, __buff, __buff_size);
-//          _VSTD::__inplace_merge<_Compare>(__first, __m1, __middle, __comp, __len11, __len21, __buff, __buff_size);
-            __last = __middle;
-            __middle = __m1;
-            __len1 = __len11;
-            __len2 = __len21;
-        }
+ void __inplace_merge(
+    _BidirectionalIterator __first,
+    _BidirectionalIterator __middle,
+    _BidirectionalIterator __last,
+    _Compare&& __comp,
+    typename iterator_traits<_BidirectionalIterator>::difference_type __len1,
+    typename iterator_traits<_BidirectionalIterator>::difference_type __len2,
+    typename iterator_traits<_BidirectionalIterator>::value_type* __buff,
+    ptrdiff_t __buff_size) {
+  using _Ops = _IterOps<_AlgPolicy>;
+
+  typedef typename iterator_traits<_BidirectionalIterator>::difference_type difference_type;
+  while (true) {
+    // if __middle == __last, we're done
+    if (__len2 == 0)
+      return;
+    if (__len1 <= __buff_size || __len2 <= __buff_size)
+      return std::__buffered_inplace_merge<_AlgPolicy>(__first, __middle, __last, __comp, __len1, __len2, __buff);
+    // shrink [__first, __middle) as much as possible (with no moves), returning if it shrinks to 0
+    for (; true; ++__first, (void)--__len1) {
+      if (__len1 == 0)
+        return;
+      if (__comp(*__middle, *__first))
+        break;
+    }
+    // __first < __middle < __last
+    // *__first > *__middle
+    // partition [__first, __m1) [__m1, __middle) [__middle, __m2) [__m2, __last) such that
+    //     all elements in:
+    //         [__first, __m1)  <= [__middle, __m2)
+    //         [__middle, __m2) <  [__m1, __middle)
+    //         [__m1, __middle) <= [__m2, __last)
+    //     and __m1 or __m2 is in the middle of its range
+    _BidirectionalIterator __m1; // "median" of [__first, __middle)
+    _BidirectionalIterator __m2; // "median" of [__middle, __last)
+    difference_type __len11;     // distance(__first, __m1)
+    difference_type __len21;     // distance(__middle, __m2)
+    // binary search smaller range
+    if (__len1 < __len2) { // __len >= 1, __len2 >= 2
+      __len21 = __len2 / 2;
+      __m2    = __middle;
+      _Ops::advance(__m2, __len21);
+      // TODO: replace _ClassicAlgPolicy and __identity with _AlgPolicy and projection
+      __m1    = std::__upper_bound<_ClassicAlgPolicy>(__first, __middle, *__m2, __comp, std::__identity());
+      __len11 = _Ops::distance(__first, __m1);
+    } else {
+      if (__len1 == 1) { // __len1 >= __len2 && __len2 > 0, therefore __len2 == 1
+        // It is known *__first > *__middle
+        _Ops::iter_swap(__first, __middle);
+        return;
+      }
+      // __len1 >= 2, __len2 >= 1
+      __len11 = __len1 / 2;
+      __m1    = __first;
+      _Ops::advance(__m1, __len11);
+      __m2    = std::lower_bound(__middle, __last, *__m1, __comp);
+      __len21 = _Ops::distance(__middle, __m2);
     }
+    difference_type __len12 = __len1 - __len11; // distance(__m1, __middle)
+    difference_type __len22 = __len2 - __len21; // distance(__m2, __last)
+    // [__first, __m1) [__m1, __middle) [__middle, __m2) [__m2, __last)
+    // swap middle two partitions
+    // TODO(alg-policy): pass `_AlgPolicy` once it's supported by `rotate`.
+    __middle = _VSTD::rotate(__m1, __middle, __m2);
+    // __len12 and __len21 now have swapped meanings
+    // merge smaller range with recursive call and larger with tail recursion elimination
+    if (__len11 + __len21 < __len12 + __len22) {
+      std::__inplace_merge<_AlgPolicy>(__first, __m1, __middle, __comp, __len11, __len21, __buff, __buff_size);
+      __first  = __middle;
+      __middle = __m2;
+      __len1   = __len12;
+      __len2   = __len22;
+    } else {
+      std::__inplace_merge<_AlgPolicy>(__middle, __m2, __last, __comp, __len12, __len22, __buff, __buff_size);
+      __last   = __middle;
+      __middle = __m1;
+      __len1   = __len11;
+      __len2   = __len21;
+    }
+  }
+}
+
+template <class _AlgPolicy, class _BidirectionalIterator, class _Compare>
+_LIBCPP_HIDE_FROM_ABI void __inplace_merge(
+    _BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last, _Compare&& __comp) {
+  typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
+  typedef typename iterator_traits<_BidirectionalIterator>::difference_type difference_type;
+  difference_type __len1     = _IterOps<_AlgPolicy>::distance(__first, __middle);
+  difference_type __len2     = _IterOps<_AlgPolicy>::distance(__middle, __last);
+  difference_type __buf_size = _VSTD::min(__len1, __len2);
+  // TODO: Remove the use of std::get_temporary_buffer
+  _LIBCPP_SUPPRESS_DEPRECATED_PUSH
+  pair<value_type*, ptrdiff_t> __buf = _VSTD::get_temporary_buffer<value_type>(__buf_size);
+  _LIBCPP_SUPPRESS_DEPRECATED_POP
+  unique_ptr<value_type, __return_temporary_buffer> __h(__buf.first);
+  std::__inplace_merge<_AlgPolicy>(
+      std::move(__first), std::move(__middle), std::move(__last), __comp, __len1, __len2, __buf.first, __buf.second);
 }
 
 template <class _BidirectionalIterator, class _Compare>
-inline _LIBCPP_INLINE_VISIBILITY
-void
-inplace_merge(_BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last,
-              _Compare __comp)
-{
-    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
-    typedef typename iterator_traits<_BidirectionalIterator>::difference_type difference_type;
-    difference_type __len1 = _VSTD::distance(__first, __middle);
-    difference_type __len2 = _VSTD::distance(__middle, __last);
-    difference_type __buf_size = _VSTD::min(__len1, __len2);
-// TODO: Remove the use of std::get_temporary_buffer
-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
-    pair<value_type*, ptrdiff_t> __buf = _VSTD::get_temporary_buffer<value_type>(__buf_size);
-_LIBCPP_SUPPRESS_DEPRECATED_POP
-    unique_ptr<value_type, __return_temporary_buffer> __h(__buf.first);
-    typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
-    return _VSTD::__inplace_merge<_ClassicAlgPolicy, _Comp_ref>(__first, __middle, __last, __comp, __len1, __len2,
-                                            __buf.first, __buf.second);
+inline _LIBCPP_HIDE_FROM_ABI void inplace_merge(
+    _BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last, _Compare __comp) {
+  typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
+  std::__inplace_merge<_ClassicAlgPolicy>(
+      std::move(__first), std::move(__middle), std::move(__last), static_cast<_Comp_ref>(__comp));
 }
 
 template <class _BidirectionalIterator>
-inline _LIBCPP_INLINE_VISIBILITY
-void
-inplace_merge(_BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last)
-{
-    _VSTD::inplace_merge(__first, __middle, __last,
-                        __less<typename iterator_traits<_BidirectionalIterator>::value_type>());
+inline _LIBCPP_HIDE_FROM_ABI void
+inplace_merge(_BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last) {
+  std::inplace_merge(__first, __middle, __last, __less<typename iterator_traits<_BidirectionalIterator>::value_type>());
 }
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__algorithm/ranges_inplace_merge.h b/libcxx/include/__algorithm/ranges_inplace_merge.h
--- a/libcxx/include/__algorithm/ranges_inplace_merge.h
+++ b/libcxx/include/__algorithm/ranges_inplace_merge.h
@@ -10,6 +10,7 @@
 #define _LIBCPP___ALGORITHM_RANGES_INPLACE_MERGE_H
 
 #include <__algorithm/inplace_merge.h>
+#include <__algorithm/iterator_operations.h>
 #include <__algorithm/make_projected.h>
 #include <__config>
 #include <__functional/identity.h>
@@ -17,6 +18,7 @@
 #include <__functional/ranges_operations.h>
 #include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
 #include <__iterator/projected.h>
 #include <__iterator/sortable.h>
 #include <__ranges/access.h>
@@ -36,28 +38,38 @@
 namespace ranges {
 namespace __inplace_merge {
 
-struct __fn {
+  struct __fn {
+    template <class _Iter, class _Sent, class _Comp, class _Proj>
+    _LIBCPP_HIDE_FROM_ABI static constexpr auto
+    __inplace_merge_impl(_Iter __first, _Iter __middle, _Sent __last, _Comp&& __comp, _Proj&& __proj) {
+      auto __last_iter = ranges::next(__middle, __last);
+      std::__inplace_merge<_RangeAlgPolicy>(
+          std::move(__first), std::move(__middle), __last_iter, ranges::__make_projected_comp(__comp, __proj));
+      return __last_iter;
+    }
 
-  template <bidirectional_iterator _Iter, sentinel_for<_Iter> _Sent, class _Comp = ranges::less, class _Proj = identity>
-  requires sortable<_Iter, _Comp, _Proj>
-  _LIBCPP_HIDE_FROM_ABI
-  _Iter operator()(_Iter __first, _Iter __middle, _Sent __last, _Comp __comp = {}, _Proj __proj = {}) const {
-    // TODO: implement
-    (void)__first; (void)__middle; (void)__last; (void)__comp; (void)__proj;
-    return {};
-  }
+    template <
+        bidirectional_iterator _Iter,
+        sentinel_for<_Iter> _Sent,
+        class _Comp = ranges::less,
+        class _Proj = identity>
+      requires sortable<_Iter, _Comp, _Proj>
+    _LIBCPP_HIDE_FROM_ABI _Iter
+    operator()(_Iter __first, _Iter __middle, _Sent __last, _Comp __comp = {}, _Proj __proj = {}) const {
+      return __inplace_merge_impl(
+          std::move(__first), std::move(__middle), std::move(__last), std::move(__comp), std::move(__proj));
+    }
 
-  template <bidirectional_range _Range, class _Comp = ranges::less, class _Proj = identity>
-  requires sortable<iterator_t<_Range>, _Comp, _Proj>
-  _LIBCPP_HIDE_FROM_ABI
-  borrowed_iterator_t<_Range> operator()(_Range&& __range, iterator_t<_Range> __middle,
-                                            _Comp __comp = {}, _Proj __proj = {}) const {
-    // TODO: implement
-    (void)__range; (void)__middle; (void)__comp; (void)__proj;
-    return {};
-  }
-
-};
+    template <bidirectional_range _Range, class _Comp = ranges::less, class _Proj = identity>
+      requires sortable<
+          iterator_t<_Range>,
+          _Comp,
+          _Proj> _LIBCPP_HIDE_FROM_ABI borrowed_iterator_t<_Range>
+      operator()(_Range&& __range, iterator_t<_Range> __middle, _Comp __comp = {}, _Proj __proj = {}) const {
+      return __inplace_merge_impl(
+          ranges::begin(__range), std::move(__middle), ranges::end(__range), std::move(__comp), std::move(__proj));
+    }
+  };
 
 } // namespace __inplace_merge
 
diff --git a/libcxx/include/__algorithm/stable_sort.h b/libcxx/include/__algorithm/stable_sort.h
--- a/libcxx/include/__algorithm/stable_sort.h
+++ b/libcxx/include/__algorithm/stable_sort.h
@@ -203,7 +203,7 @@
     }
     std::__stable_sort<_AlgPolicy, _Compare>(__first, __m, __comp, __l2, __buff, __buff_size);
     std::__stable_sort<_AlgPolicy, _Compare>(__m, __last, __comp, __len - __l2, __buff, __buff_size);
-    std::__inplace_merge<_AlgPolicy, _Compare>(__first, __m, __last, __comp, __l2, __len - __l2, __buff, __buff_size);
+    std::__inplace_merge<_AlgPolicy>(__first, __m, __last, __comp, __l2, __len - __l2, __buff, __buff_size);
 }
 
 template <class _AlgPolicy, class _RandomAccessIterator, class _Compare>
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -774,7 +774,7 @@
       set_symmetric_difference(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>
@@ -787,13 +787,13 @@
            indirect_strict_weak_order<const T*, projected<I, Proj>> Comp = ranges::less>
     constexpr subrange<I>
       equal_range(I first, S last, const T& value, Comp comp = {}, Proj proj = {});                 // since C++20
-  
+
   template<forward_range R, class T, class Proj = identity,
            indirect_strict_weak_order<const T*, projected<iterator_t<R>, Proj>> Comp =
              ranges::less>
     constexpr borrowed_subrange_t<R>
       equal_range(R&& r, const T& value, Comp comp = {}, Proj proj = {});                           // since C++20
-  
+
   template<class I1, class I2, class O>
     using set_union_result = in_in_out_result<I1, I2, O>;                                           // since C++20
 
@@ -818,13 +818,24 @@
              ranges::less>
     constexpr bool includes(I1 first1, S1 last1, I2 first2, S2 last2, Comp comp = {},
                             Proj1 proj1 = {}, Proj2 proj2 = {});                                   // Since C++20
-  
+
   template<input_range R1, input_range R2, class Proj1 = identity,
            class Proj2 = identity,
            indirect_strict_weak_order<projected<iterator_t<R1>, Proj1>,
                                       projected<iterator_t<R2>, Proj2>> Comp = ranges::less>
     constexpr bool includes(R1&& r1, R2&& r2, Comp comp = {},
                             Proj1 proj1 = {}, Proj2 proj2 = {});                                   // Since C++20
+
+  template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less,
+           class Proj = identity>
+    requires sortable<I, Comp, Proj>
+    I inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {});                    // Since C++20
+
+  template<bidirectional_range R, class Comp = ranges::less, class Proj = identity>
+    requires sortable<iterator_t<R>, Comp, Proj>
+    borrowed_iterator_t<R>
+      inplace_merge(R&& r, iterator_t<R> middle, Comp comp = {},
+                    Proj proj = {});                                                               // Since C++20
 }
 
     constexpr bool     // constexpr in C++20
@@ -1575,6 +1586,7 @@
 #include <__algorithm/ranges_find_if_not.h>
 #include <__algorithm/ranges_for_each.h>
 #include <__algorithm/ranges_for_each_n.h>
+#include <__algorithm/ranges_inplace_merge.h>
 #include <__algorithm/ranges_includes.h>
 #include <__algorithm/ranges_is_partitioned.h>
 #include <__algorithm/ranges_is_sorted.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
@@ -239,6 +239,7 @@
 
     module __algorithm {
       module adjacent_find                   { private header "__algorithm/adjacent_find.h" }
+      module algorithm_family                { private header "__algorithm/algorithm_family.h" }
       module all_of                          { private header "__algorithm/all_of.h" }
       module any_of                          { private header "__algorithm/any_of.h" }
       module binary_search                   { private header "__algorithm/binary_search.h" }
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
@@ -37,6 +37,7 @@
 // DO NOT MANUALLY EDIT ANYTHING BETWEEN THE MARKERS BELOW
 // GENERATED-MARKER
 #include <__algorithm/adjacent_find.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/adjacent_find.h'}}
+#include <__algorithm/algorithm_family.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/algorithm_family.h'}}
 #include <__algorithm/all_of.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/all_of.h'}}
 #include <__algorithm/any_of.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/any_of.h'}}
 #include <__algorithm/binary_search.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/binary_search.h'}}
diff --git a/libcxx/test/std/algorithms/alg.sorting/alg.merge/ranges_inplace_merge.pass.cpp b/libcxx/test/std/algorithms/alg.sorting/alg.merge/ranges_inplace_merge.pass.cpp
--- a/libcxx/test/std/algorithms/alg.sorting/alg.merge/ranges_inplace_merge.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.sorting/alg.merge/ranges_inplace_merge.pass.cpp
@@ -31,7 +31,23 @@
 #include "almost_satisfies_types.h"
 #include "test_iterators.h"
 
-// TODO: SFINAE tests.
+template <
+    class Iter   = int*,
+    class Middle = int*,
+    class Sent   = int*,
+    class Comp   = std::ranges::less,
+    class Proj   = std::identity>
+concept HasInplaceMergeIter =
+    requires(Iter&& iter, Middle&& mid, Sent&& sent, Comp&& comp, Proj&& proj) {
+      std::ranges::inplace_merge(
+          std::forward<Iter>(iter),
+          std::forward<Middle>(mid),
+          std::forward<Sent>(sent),
+          std::forward<Comp>(comp),
+          std::forward<Proj>(proj));
+    };
+
+static_assert(HasInplaceMergeIter<int*, int*, int*>);
 
 constexpr bool test() {
   // TODO: main tests.
diff --git a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
--- a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
+++ b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
@@ -69,6 +69,7 @@
   copy("copy_headers") {
     sources = [
       "__algorithm/adjacent_find.h",
+      "__algorithm/algorithm_family.h",
       "__algorithm/all_of.h",
       "__algorithm/any_of.h",
       "__algorithm/binary_search.h",