Index: libcxx/CREDITS.TXT
===================================================================
--- libcxx/CREDITS.TXT
+++ libcxx/CREDITS.TXT
@@ -41,6 +41,11 @@
 E: jbcoe@me.com
 D: Implementation of propagate_const.
 
+N: Christopher Di Bella
+E: cjdb@google.com
+E: cjdb.ns@gmail.com
+D: Library concepts.
+
 N: Glen Joseph Fernandes
 E: glenjofe@gmail.com
 D: Implementation of to_address.
Index: libcxx/include/CMakeLists.txt
===================================================================
--- libcxx/include/CMakeLists.txt
+++ libcxx/include/CMakeLists.txt
@@ -45,6 +45,7 @@
   compare
   complex
   complex.h
+  concepts
   condition_variable
   csetjmp
   csignal
Index: libcxx/include/concepts
===================================================================
--- /dev/null
+++ libcxx/include/concepts
@@ -0,0 +1,166 @@
+// -*- C++ -*-
+//===-------------------------- concepts ----------------------------------===//
+//
+// 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_CONCEPTS
+#define _LIBCPP_CONCEPTS
+
+/*
+    concepts synopsis
+namespace std {
+  // [concepts.lang], language-related concepts
+  // [concept.same], concept same_as
+  template<class T, class U>
+    concept same_as = see below;
+
+  // [concept.derived], concept derived_from
+  template<class Derived, class Base>
+    concept derived_from = see below;
+
+  // [concept.convertible], concept convertible_to
+  template<class From, class To>
+    concept convertible_to = see below;
+
+  // [concept.commonref], concept common_reference_with
+  template<class T, class U>
+    concept common_reference_with = see below;
+
+  // [concept.common], concept common_with
+  template<class T, class U>
+    concept common_with = see below;
+
+  // [concepts.arithmetic], arithmetic concepts
+  template<class T>
+    concept integral = see below;
+  template<class T>
+    concept signed_integral = see below;
+  template<class T>
+    concept unsigned_integral = see below;
+  template<class T>
+    concept floating_point = see below;
+
+  // [concept.assignable], concept assignable_from
+  template<class LHS, class RHS>
+    concept assignable_from = see below;
+
+  // [concept.swappable], concept swappable
+  namespace ranges {
+    inline namespace unspecified {
+      inline constexpr unspecified swap = unspecified;
+    }
+  }
+  template<class T>
+    concept swappable = see below;
+  template<class T, class U>
+    concept swappable_with = see below;
+
+  // [concept.destructible], concept destructible
+  template<class T>
+    concept destructible = see below;
+
+  // [concept.constructible], concept constructible_from
+  template<class T, class... Args>
+    concept constructible_from = see below;
+
+  // [concept.defaultconstructible], concept default_constructible
+  template<class T>
+    concept default_constructible = see below;
+
+  // [concept.moveconstructible], concept move_constructible
+  template<class T>
+    concept move_constructible = see below;
+
+  // [concept.copyconstructible], concept copy_constructible
+  template<class T>
+    concept copy_constructible = see below;
+
+  // [concepts.compare], comparison concepts
+  // [concept.boolean], concept boolean
+  template<class B>
+    concept boolean = see below;
+
+  // [concept.equalitycomparable], concept equality_comparable
+  template<class T>
+    concept equality_comparable = see below;
+  template<class T, class U>
+    concept equality_comparable_with = see below;
+
+  // [concept.totallyordered], concept totally_ordered
+  template<class T>
+    concept totally_ordered = see below;
+  template<class T, class U>
+    concept totally_ordered_with = see below;
+
+  // [concepts.object], object concepts
+  template<class T>
+    concept movable = see below;
+  template<class T>
+    concept copyable = see below;
+  template<class T>
+    concept semiregular = see below;
+  template<class T>
+    concept regular = see below;
+
+  // [concepts.callable], callable concepts
+  // [concept.invocable], concept invocable
+  template<class F, class... Args>
+    concept invocable = see below;
+
+  // [concept.regularinvocable], concept regular_invocable
+  template<class F, class... Args>
+    concept regular_invocable = see below;
+
+  // [concept.predicate], concept predicate
+  template<class F, class... Args>
+    concept predicate = see below;
+
+  // [concept.relation], concept relation
+  template<class R, class T, class U>
+    concept relation = see below;
+
+  // [concept.equiv], concept equivalence_relation
+  template<class R, class T, class U>
+    concept equivalence_relation = see below;
+
+  // [concept.strictweakorder], concept strict_weak_order
+  template<class R, class T, class U>
+    concept strict_weak_order = see below;
+}
+
+*/
+
+#include <__config>
+#include <type_traits>
+#include <version>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && defined(__cpp_concepts) && __cpp_concepts >= 201811L
+
+// [concept.same]
+
+template<class _Tp, class _Up>
+concept __same_as_impl = _VSTD::_IsSame<_Tp, _Up>::value;
+
+template<class _Tp, class _Up>
+concept same_as = __same_as_impl<_Tp, _Up> && __same_as_impl<_Up, _Tp>;
+
+#endif //_LIBCPP_STD_VER > 17 && defined(__cpp_concepts) && __cpp_concepts >= 201811L
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP_CONCEPTS
Index: libcxx/include/module.modulemap
===================================================================
--- libcxx/include/module.modulemap
+++ libcxx/include/module.modulemap
@@ -267,6 +267,10 @@
     header "complex"
     export *
   }
+  module concepts {
+    header "concepts"
+    export *
+  }
   module condition_variable {
     header "condition_variable"
     export *
Index: libcxx/test/libcxx/double_include.sh.cpp
===================================================================
--- libcxx/test/libcxx/double_include.sh.cpp
+++ libcxx/test/libcxx/double_include.sh.cpp
@@ -49,6 +49,7 @@
 #include <compare>
 #include <complex>
 #include <complex.h>
+#include <concepts>
 #include <condition_variable>
 #include <csetjmp>
 #include <csignal>
Index: libcxx/test/std/concepts/lang/same_as.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/concepts/lang/same_as.pass.cpp
@@ -0,0 +1,295 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++98, c++03, c++11, c++14, c++17
+
+// template<class T, class U>
+// concept same_as;
+
+#include <concepts>
+#include <type_traits>
+
+struct S1 {};
+struct S2 {
+  int i;
+
+  int& f();
+  double g(int x) const;
+};
+struct S3 {
+  int& r;
+};
+struct S4 {
+  int&& r;
+};
+struct S5 {
+  int* p;
+};
+
+class C1 {};
+class C2 {
+  [[maybe_unused]] int i;
+};
+
+class C3 {
+public:
+  int i;
+};
+
+template <class T1, class T2 = T1>
+class C4 {
+  int t1;
+  int t2;
+};
+
+template <class T1, class T2 = T1>
+class C5 {
+  [[maybe_unused]] T1 t1;
+
+public:
+  T2 t2;
+};
+
+template <class T1, class T2 = T1>
+class C6 {
+public:
+  [[maybe_unused]] T1 t1;
+  [[maybe_unused]] T2 t2;
+};
+
+template <class T>
+struct identity {
+  using type = T;
+};
+
+template <template <typename> class Modifier = identity>
+void CheckSameAs() {
+  static_assert(
+      std::same_as<typename Modifier<int>::type, typename Modifier<int>::type>);
+  static_assert(
+      std::same_as<typename Modifier<S1>::type, typename Modifier<S1>::type>);
+  static_assert(
+      std::same_as<typename Modifier<S2>::type, typename Modifier<S2>::type>);
+  static_assert(
+      std::same_as<typename Modifier<S3>::type, typename Modifier<S3>::type>);
+  static_assert(
+      std::same_as<typename Modifier<S4>::type, typename Modifier<S4>::type>);
+  static_assert(
+      std::same_as<typename Modifier<S5>::type, typename Modifier<S5>::type>);
+  static_assert(
+      std::same_as<typename Modifier<C1>::type, typename Modifier<C1>::type>);
+  static_assert(
+      std::same_as<typename Modifier<C2>::type, typename Modifier<C2>::type>);
+  static_assert(
+      std::same_as<typename Modifier<C3>::type, typename Modifier<C3>::type>);
+  static_assert(std::same_as<typename Modifier<C4<int> >::type,
+                             typename Modifier<C4<int> >::type>);
+  static_assert(std::same_as<typename Modifier<C4<int&> >::type,
+                             typename Modifier<C4<int&> >::type>);
+  static_assert(std::same_as<typename Modifier<C4<int&&> >::type,
+                             typename Modifier<C4<int&&> >::type>);
+  static_assert(std::same_as<typename Modifier<C5<int> >::type,
+                             typename Modifier<C5<int> >::type>);
+  static_assert(std::same_as<typename Modifier<C5<int&> >::type,
+                             typename Modifier<C5<int&> >::type>);
+  static_assert(std::same_as<typename Modifier<C5<int&&> >::type,
+                             typename Modifier<C5<int&&> >::type>);
+  static_assert(std::same_as<typename Modifier<C6<int> >::type,
+                             typename Modifier<C6<int> >::type>);
+  static_assert(std::same_as<typename Modifier<C6<int&> >::type,
+                             typename Modifier<C6<int&> >::type>);
+  static_assert(std::same_as<typename Modifier<C6<int&&> >::type,
+                             typename Modifier<C6<int&&> >::type>);
+
+  static_assert(std::same_as<typename Modifier<void>::type,
+                             typename Modifier<void>::type>);
+}
+
+template <template <typename> class Modifier1,
+          template <typename> class Modifier2>
+void CheckNotSameAs() {
+  static_assert(!std::same_as<typename Modifier1<int>::type,
+                              typename Modifier2<int>::type>);
+  static_assert(!std::same_as<typename Modifier1<S1>::type,
+                              typename Modifier2<S1>::type>);
+  static_assert(!std::same_as<typename Modifier1<S2>::type,
+                              typename Modifier2<S2>::type>);
+  static_assert(!std::same_as<typename Modifier1<S3>::type,
+                              typename Modifier2<S3>::type>);
+  static_assert(!std::same_as<typename Modifier1<S4>::type,
+                              typename Modifier2<S4>::type>);
+  static_assert(!std::same_as<typename Modifier1<S5>::type,
+                              typename Modifier2<S5>::type>);
+  static_assert(!std::same_as<typename Modifier1<C1>::type,
+                              typename Modifier2<C1>::type>);
+  static_assert(!std::same_as<typename Modifier1<C2>::type,
+                              typename Modifier2<C2>::type>);
+  static_assert(!std::same_as<typename Modifier1<C3>::type,
+                              typename Modifier2<C3>::type>);
+  static_assert(!std::same_as<typename Modifier1<C4<int> >::type,
+                              typename Modifier2<C4<int> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C4<int&> >::type,
+                              typename Modifier2<C4<int&> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C4<int&&> >::type,
+                              typename Modifier2<C4<int&&> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C5<int> >::type,
+                              typename Modifier2<C5<int> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C5<int&> >::type,
+                              typename Modifier2<C5<int&> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C5<int&&> >::type,
+                              typename Modifier2<C5<int&&> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C6<int> >::type,
+                              typename Modifier2<C6<int> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C6<int&> >::type,
+                              typename Modifier2<C6<int&> >::type>);
+  static_assert(!std::same_as<typename Modifier1<C6<int&&> >::type,
+                              typename Modifier2<C6<int&&> >::type>);
+}
+
+// Checks subsumption works as intended
+template <class T, class U>
+requires std::same_as<T, U> void SubsumptionTest();
+
+// clang-format off
+template <class T, class U>
+requires std::same_as<U, T> && true // NOLINT(readability-simplify-boolean-expr)
+int SubsumptionTest();
+// clang-format on
+
+static_assert(std::same_as<int, decltype(SubsumptionTest<int, int>())>);
+static_assert(std::same_as<int, decltype(SubsumptionTest<void, void>())>);
+static_assert(
+    std::same_as<int, decltype(SubsumptionTest<int (*)(), int (*)()>())>);
+static_assert(
+    std::same_as<
+        int, decltype(SubsumptionTest<double (&)(int), double (&)(int)>())>);
+static_assert(
+    std::same_as<int, decltype(SubsumptionTest<int S2::*, int S2::*>())>);
+static_assert(
+    std::same_as<int,
+                 decltype(SubsumptionTest<int& (S2::*)(), int& (S2::*)()>())>);
+
+int main(int, char**) {
+  { // Checks std::same_as<T, T> is true
+    CheckSameAs();
+
+    // Checks std::same_as<T&, T&> is true
+    CheckSameAs<std::add_lvalue_reference>();
+
+    // Checks std::same_as<T&&, T&&> is true
+    CheckSameAs<std::add_rvalue_reference>();
+
+    // Checks std::same_as<const T, const T> is true
+    CheckSameAs<std::add_const>();
+
+    // Checks std::same_as<volatile T, volatile T> is true
+    CheckSameAs<std::add_volatile>();
+
+    // Checks std::same_as<const volatile T, const volatile T> is true
+    CheckSameAs<std::add_cv>();
+
+    // Checks std::same_as<T*, T*> is true
+    CheckSameAs<std::add_pointer>();
+
+    // Checks concrete types are identical
+    static_assert(std::same_as<void, void>);
+
+    using Void = void;
+    static_assert(std::same_as<void, Void>);
+
+    static_assert(std::same_as<int[1], int[1]>);
+    static_assert(std::same_as<int[2], int[2]>);
+
+    static_assert(std::same_as<int (*)(), int (*)()>);
+    static_assert(std::same_as<void (&)(), void (&)()>);
+    static_assert(std::same_as<S1& (*)(S1), S1& (*)(S1)>);
+    static_assert(std::same_as<C1& (&)(S1, int), C1& (&)(S1, int)>);
+
+    static_assert(std::same_as<int S2::*, int S2::*>);
+    static_assert(std::same_as<double S2::*, double S2::*>);
+
+    static_assert(std::same_as<int& (S2::*)(), int& (S2::*)()>);
+    static_assert(std::same_as<double& (S2::*)(int), double& (S2::*)(int)>);
+  }
+
+  { // Checks that `T` and `T&` are distinct types
+    CheckNotSameAs<identity, std::add_lvalue_reference>();
+    CheckNotSameAs<std::add_lvalue_reference, identity>();
+
+    // Checks that `T` and `T&&` are distinct types
+    CheckNotSameAs<identity, std::add_rvalue_reference>();
+    CheckNotSameAs<std::add_rvalue_reference, identity>();
+
+    // Checks that `T` and `const T` are distinct types
+    CheckNotSameAs<identity, std::add_const>();
+    CheckNotSameAs<std::add_const, identity>();
+
+    // Checks that `T` and `volatile T` are distinct types
+    CheckNotSameAs<identity, std::add_volatile>();
+    CheckNotSameAs<std::add_volatile, identity>();
+
+    // Checks that `T` and `const volatile T` are distinct types
+    CheckNotSameAs<identity, std::add_cv>();
+    CheckNotSameAs<std::add_cv, identity>();
+
+    // Checks that `const T` and `volatile T` are distinct types
+    CheckNotSameAs<std::add_const, std::add_volatile>();
+    CheckNotSameAs<std::add_volatile, std::add_const>();
+
+    // Checks that `const T` and `const volatile T` are distinct types
+    CheckNotSameAs<std::add_const, std::add_cv>();
+    CheckNotSameAs<std::add_cv, std::add_const>();
+
+    // Checks that `volatile T` and `const volatile T` are distinct types
+    CheckNotSameAs<std::add_volatile, std::add_cv>();
+    CheckNotSameAs<std::add_cv, std::add_volatile>();
+
+    // Checks `T&` and `T&&` are distinct types
+    CheckNotSameAs<std::add_lvalue_reference, std::add_rvalue_reference>();
+    CheckNotSameAs<std::add_rvalue_reference, std::add_lvalue_reference>();
+  }
+
+  { // Checks different type names are distinct types
+    static_assert(!std::same_as<S1, C1>);
+    static_assert(!std::same_as<C4<int>, C5<int> >);
+    static_assert(!std::same_as<C4<int>, C5<int> >);
+    static_assert(!std::same_as<C5<int, double>, C5<double, int> >);
+
+    static_assert(!std::same_as<int&, const int&>);
+    static_assert(!std::same_as<int&, volatile int&>);
+    static_assert(!std::same_as<int&, const volatile int&>);
+
+    static_assert(!std::same_as<int&&, const int&>);
+    static_assert(!std::same_as<int&&, volatile int&>);
+    static_assert(!std::same_as<int&&, const volatile int&>);
+
+    static_assert(!std::same_as<int&, const int&&>);
+    static_assert(!std::same_as<int&, volatile int&&>);
+    static_assert(!std::same_as<int&, const volatile int&&>);
+
+    static_assert(!std::same_as<int&&, const int&&>);
+    static_assert(!std::same_as<int&&, volatile int&&>);
+    static_assert(!std::same_as<int&&, const volatile int&&>);
+
+    static_assert(!std::same_as<void, int>);
+
+    static_assert(!std::same_as<int[1], int[2]>);
+    static_assert(!std::same_as<double[1], int[2]>);
+
+    static_assert(!std::same_as<int* (*)(), const int* (*)()>);
+    static_assert(!std::same_as<void (&)(), void (&)(S1)>);
+    static_assert(!std::same_as<S1 (*)(S1), S1& (*)(S1)>);
+    static_assert(!std::same_as<C3 (&)(int), C1& (&)(S1, int)>);
+
+    static_assert(!std::same_as<int S2::*, double S2::*>);
+
+    static_assert(!std::same_as<int& (S2::*)(), double& (S2::*)(int)>);
+  }
+
+  return 0;
+}