diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -97,8 +97,10 @@
   __bits
   __bsd_locale_defaults.h
   __bsd_locale_fallbacks.h
+  __compare/__synthesized.h  # NOT FOR REVIEW
   __compare/common_comparison_category.h
   __compare/ordering.h
+  __concepts/comparison.h  # NOT FOR REVIEW
   __config
   __debug
   __errc
diff --git a/libcxx/include/__compare/__synthesized.h b/libcxx/include/__compare/__synthesized.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__compare/__synthesized.h
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___COMPARE___SYNTHESIZED_H
+#define _LIBCPP___COMPARE___SYNTHESIZED_H
+
+#include <__config>
+#include <__compare/ordering.h>
+#include <__concepts/comparison.h>
+#include <__utility/declval.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)
+
+template <class T, class U>
+constexpr auto __synth_three_way(const T& __t, const U& __u)
+    requires requires {
+      { __t < __u } -> __boolean_testable;
+      { __u < __t } -> __boolean_testable;
+    }
+{
+  if constexpr (three_way_comparable_with<T, U>) {
+    return __t <=> __u;
+  } else {
+    if (__t < __u) return weak_ordering::less;
+    if (__u < __t) return weak_ordering::greater;
+    return weak_ordering::equivalent;
+  }
+}
+
+template <class T, class U = T>
+using __synth_three_way_result = decltype(__synth_three_way(declval<T&>(), declval<U&>()));
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___COMPARE___SYNTHESIZED_H
diff --git a/libcxx/include/__concepts/comparison.h b/libcxx/include/__concepts/comparison.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__concepts/comparison.h
@@ -0,0 +1,138 @@
+// THIS FILE NOT FOR REVIEW (see: https://reviews.llvm.org/D107584 )
+#ifndef INCLUDE___CONCEPTS_COMPARISON_H_
+#define INCLUDE___CONCEPTS_COMPARISON_H_
+
+#include <__config>
+#include <__compare/common_comparison_category.h>
+#include <__utility/forward.h>
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+// [concept.same]
+
+template<class _Tp, class _Up>
+concept __same_as_impl = _IsSame<_Tp, _Up>::value;
+
+template<class _Tp, class _Up>
+concept same_as = __same_as_impl<_Tp, _Up> && __same_as_impl<_Up, _Tp>;
+
+// [concept.convertible]
+template<class _From, class _To>
+concept convertible_to =
+  is_convertible_v<_From, _To> &&
+  requires(add_rvalue_reference_t<_From> (&__f)()) {
+    static_cast<_To>(__f());
+  };
+
+// [concept.commonref]
+template<class _Tp, class _Up>
+concept common_reference_with =
+  same_as<common_reference_t<_Tp, _Up>, common_reference_t<_Up, _Tp>> &&
+  convertible_to<_Tp, common_reference_t<_Tp, _Up>> &&
+  convertible_to<_Up, common_reference_t<_Tp, _Up>>;
+
+// [concept.booleantestable]
+template<class _Tp>
+concept __boolean_testable_impl = convertible_to<_Tp, bool>;
+
+template<class _Tp>
+concept __boolean_testable = __boolean_testable_impl<_Tp> && requires(_Tp&& __t) {
+  { !_VSTD::forward<_Tp>(__t) } -> __boolean_testable_impl;
+};
+
+// [concept.equalitycomparable]
+template<class _Tp, class _Up>
+concept __weakly_equality_comparable_with =
+  requires(__make_const_lvalue_ref<_Tp> __t, __make_const_lvalue_ref<_Up> __u) {
+    { __t == __u } -> __boolean_testable;
+    { __t != __u } -> __boolean_testable;
+    { __u == __t } -> __boolean_testable;
+    { __u != __t } -> __boolean_testable;
+  };
+
+template<class _Tp>
+concept equality_comparable = __weakly_equality_comparable_with<_Tp, _Tp>;
+
+template<class _Tp, class _Up>
+concept equality_comparable_with =
+  equality_comparable<_Tp> && equality_comparable<_Up> &&
+  common_reference_with<__make_const_lvalue_ref<_Tp>, __make_const_lvalue_ref<_Up>> &&
+  equality_comparable<
+    common_reference_t<
+      __make_const_lvalue_ref<_Tp>,
+      __make_const_lvalue_ref<_Up>>> &&
+  __weakly_equality_comparable_with<_Tp, _Up>;
+
+// [concept.totallyordered]
+
+template<class _Tp, class _Up>
+concept __partially_ordered_with =
+  requires(__make_const_lvalue_ref<_Tp> __t, __make_const_lvalue_ref<_Up> __u) {
+    { __t <  __u } -> __boolean_testable;
+    { __t >  __u } -> __boolean_testable;
+    { __t <= __u } -> __boolean_testable;
+    { __t >= __u } -> __boolean_testable;
+    { __u <  __t } -> __boolean_testable;
+    { __u >  __t } -> __boolean_testable;
+    { __u <= __t } -> __boolean_testable;
+    { __u >= __t } -> __boolean_testable;
+  };
+
+template<class _Tp>
+concept totally_ordered = equality_comparable<_Tp> && __partially_ordered_with<_Tp, _Tp>;
+
+template<class _Tp, class _Up>
+concept totally_ordered_with =
+  totally_ordered<_Tp> && totally_ordered<_Up> &&
+  equality_comparable_with<_Tp, _Up> &&
+  totally_ordered<
+    common_reference_t<
+      __make_const_lvalue_ref<_Tp>,
+      __make_const_lvalue_ref<_Up>>> &&
+  __partially_ordered_with<_Tp, _Up>;
+
+
+// NOT FOR REVIEW
+#if !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)
+
+template<class T, class Cat>
+concept __compares_as =
+  same_as<common_comparison_category_t<T, Cat>, Cat>;
+
+template<class T, class Cat = partial_ordering>
+concept three_way_comparable =
+  __weakly_equality_comparable_with<T, T> &&
+  __partially_ordered_with<T, T> &&
+  requires(const remove_reference_t<T>& a, const remove_reference_t<T>& b) {
+    { a <=> b } -> __compares_as<Cat>;
+  };
+
+template<class T, class U, class Cat = partial_ordering>
+concept three_way_comparable_with =
+  three_way_comparable<T, Cat> &&
+  three_way_comparable<U, Cat> &&
+  common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
+  three_way_comparable<
+    common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> &&
+  __weakly_equality_comparable_with<T, U> &&
+  __partially_ordered_with<T, U> &&
+  requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) {
+    { t <=> u } -> __compares_as<Cat>;
+    { u <=> t } -> __compares_as<Cat>;
+  };
+
+#endif // !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)
+// end NOT FOR REVIEW
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //INCLUDE___CONCEPTS_COMPARISON_H_
diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -10,6 +10,8 @@
 #define _LIBCPP___UTILITY_PAIR_H
 
 #include <__config>
+#include <__compare/__synthesized.h>
+#include <__compare/common_comparison_category.h>
 #include <__functional/unwrap_ref.h>
 #include <__tuple>
 #include <__utility/forward.h>
@@ -323,6 +325,22 @@
     return __x.first == __y.first && __x.second == __y.second;
 }
 
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)
+
+template <class _T1, class _T2>
+inline _LIBCPP_INLINE_VISIBILITY constexpr
+common_comparison_category_t<
+        __synth_three_way_result<_T1>,
+        __synth_three_way_result<_T2> >
+operator<=>(const pair<_T1,_T2>& __x, const pair<_T1,_T2>& __y)
+{
+    auto __result = __synth_three_way(__x.first, __y.first);
+    if (__result == 0) return __synth_three_way(__x.second, __y.second);
+    else return __result;
+}
+
+#else // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)
+
 template <class _T1, class _T2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 bool
@@ -363,6 +381,8 @@
     return !(__y < __x);
 }
 
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)
+
 template <class _T1, class _T2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename enable_if
diff --git a/libcxx/include/concepts b/libcxx/include/concepts
--- a/libcxx/include/concepts
+++ b/libcxx/include/concepts
@@ -7,6 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// CHANGES IN THIS FILE NOT FOR REVIEW
+
 #ifndef _LIBCPP_CONCEPTS
 #define _LIBCPP_CONCEPTS
 
@@ -130,6 +132,7 @@
 */
 
 #include <__config>
+#include <__concepts/comparison.h>
 #include <__functional/invoke.h>
 #include <__functional_base>
 #include <type_traits>
@@ -147,35 +150,12 @@
 
 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
-// [concept.same]
-
-template<class _Tp, class _Up>
-concept __same_as_impl = _IsSame<_Tp, _Up>::value;
-
-template<class _Tp, class _Up>
-concept same_as = __same_as_impl<_Tp, _Up> && __same_as_impl<_Up, _Tp>;
-
 // [concept.derived]
 template<class _Dp, class _Bp>
 concept derived_from =
   is_base_of_v<_Bp, _Dp> &&
   is_convertible_v<const volatile _Dp*, const volatile _Bp*>;
 
-// [concept.convertible]
-template<class _From, class _To>
-concept convertible_to =
-  is_convertible_v<_From, _To> &&
-  requires(add_rvalue_reference_t<_From> (&__f)()) {
-    static_cast<_To>(__f());
-  };
-
-// [concept.commonref]
-template<class _Tp, class _Up>
-concept common_reference_with =
-  same_as<common_reference_t<_Tp, _Up>, common_reference_t<_Up, _Tp>> &&
-  convertible_to<_Tp, common_reference_t<_Tp, _Up>> &&
-  convertible_to<_Up, common_reference_t<_Tp, _Up>>;
-
 // [concept.common]
 template<class _Tp, class _Up>
 concept common_with =
@@ -332,66 +312,6 @@
     ranges::swap(_VSTD::forward<_Up>(__u), _VSTD::forward<_Tp>(__t));
   };
 
-// [concept.booleantestable]
-template<class _Tp>
-concept __boolean_testable_impl = convertible_to<_Tp, bool>;
-
-template<class _Tp>
-concept __boolean_testable = __boolean_testable_impl<_Tp> && requires(_Tp&& __t) {
-  { !std::forward<_Tp>(__t) } -> __boolean_testable_impl;
-};
-
-// [concept.equalitycomparable]
-template<class _Tp, class _Up>
-concept __weakly_equality_comparable_with =
-  requires(__make_const_lvalue_ref<_Tp> __t, __make_const_lvalue_ref<_Up> __u) {
-    { __t == __u } -> __boolean_testable;
-    { __t != __u } -> __boolean_testable;
-    { __u == __t } -> __boolean_testable;
-    { __u != __t } -> __boolean_testable;
-  };
-
-template<class _Tp>
-concept equality_comparable = __weakly_equality_comparable_with<_Tp, _Tp>;
-
-template<class _Tp, class _Up>
-concept equality_comparable_with =
-  equality_comparable<_Tp> && equality_comparable<_Up> &&
-  common_reference_with<__make_const_lvalue_ref<_Tp>, __make_const_lvalue_ref<_Up>> &&
-  equality_comparable<
-    common_reference_t<
-      __make_const_lvalue_ref<_Tp>,
-      __make_const_lvalue_ref<_Up>>> &&
-  __weakly_equality_comparable_with<_Tp, _Up>;
-
-// [concept.totallyordered]
-
-template<class _Tp, class _Up>
-concept __partially_ordered_with =
-  requires(__make_const_lvalue_ref<_Tp> __t, __make_const_lvalue_ref<_Up> __u) {
-    { __t <  __u } -> __boolean_testable;
-    { __t >  __u } -> __boolean_testable;
-    { __t <= __u } -> __boolean_testable;
-    { __t >= __u } -> __boolean_testable;
-    { __u <  __t } -> __boolean_testable;
-    { __u >  __t } -> __boolean_testable;
-    { __u <= __t } -> __boolean_testable;
-    { __u >= __t } -> __boolean_testable;
-  };
-
-template<class _Tp>
-concept totally_ordered = equality_comparable<_Tp> && __partially_ordered_with<_Tp, _Tp>;
-
-template<class _Tp, class _Up>
-concept totally_ordered_with =
-  totally_ordered<_Tp> && totally_ordered<_Up> &&
-  equality_comparable_with<_Tp, _Up> &&
-  totally_ordered<
-    common_reference_t<
-      __make_const_lvalue_ref<_Tp>,
-      __make_const_lvalue_ref<_Up>>> &&
-  __partially_ordered_with<_Tp, _Up>;
-
 // [concepts.object]
 template<class _Tp>
 concept movable =