diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -70,9 +70,14 @@
   __algorithm/pop_heap.h
   __algorithm/prev_permutation.h
   __algorithm/pstl_any_all_none_of.h
+  __algorithm/pstl_backend.h
+  __algorithm/pstl_backends/cpu_backend.h
+  __algorithm/pstl_backends/cpu_backends/for_each.h
+  __algorithm/pstl_backends/cpu_backends/serial.h
   __algorithm/pstl_fill.h
   __algorithm/pstl_find.h
   __algorithm/pstl_for_each.h
+  __algorithm/pstl_frontend_dispatch.h
   __algorithm/push_heap.h
   __algorithm/ranges_adjacent_find.h
   __algorithm/ranges_all_of.h
diff --git a/libcxx/include/__algorithm/pstl_backend.h b/libcxx/include/__algorithm/pstl_backend.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backend.h
@@ -0,0 +1,84 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_PSTL_BACKEND_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKEND_H
+
+#include <__algorithm/pstl_backends/cpu_backend.h>
+#include <__config>
+#include <execution>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+/*
+TODO: Documentation of how backends work
+
+A PSTL parallel backend is a tag type to which the following functions are associated, at minimum:
+
+  template <class _ExecutionPolicy, class _Iterator, class _Func>
+  void __pstl_for_each(_Backend, _ExecutionPolicy&&, _Iterator __first, _Iterator __last, _Func __f);
+
+// TODO: Complete this list
+
+The following functions are optional but can be provided. If provided, they are used by the corresponding
+algorithms, otherwise they are implemented in terms of other algorithms. If none of the optional algorithms are
+implemented, all the algorithms will eventually forward to the basis algorithms listed above:
+
+  template <class _ExecutionPolicy, class _Iterator, class _Size, class _Func>
+  void __pstl_for_each_n(_Backend, _ExecutionPolicy&&, _Iterator __first, _Size __n, _Func __f);
+
+// TODO: Complete this list
+
+*/
+
+template <class>
+void __pstl_for_each_n();
+
+template <class _ExecutionPolicy>
+struct __select_backend;
+
+template <>
+struct __select_backend<std::execution::sequenced_policy> {
+  using type = __cpu_backend_tag;
+};
+
+template <>
+struct __select_backend<std::execution::unsequenced_policy> {
+  using type = __cpu_backend_tag;
+};
+
+#  if defined(_PSTL_CPU_BACKEND_SERIAL)
+#    include <__algorithm/pstl_backends/cpu_backend.h>
+template <>
+struct __select_backend<std::execution::parallel_policy> {
+  using type = __cpu_backend_tag;
+};
+
+template <>
+struct __select_backend<std::execution::parallel_unsequenced_policy> {
+  using type = __cpu_backend_tag;
+};
+
+#  else
+
+// ...New vendors can add parallel backends here...
+
+#    error "Invalid choice of a PSTL parallel backend"
+#  endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKEND_H
diff --git a/libcxx/include/__algorithm/pstl_backends/cpu_backend.h b/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_PSTL_BACKENDS_CPU_BACKEND_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKEND_H
+
+#include <__config>
+
+/*
+
+  // _Functor takes a subrange for [__first, __last) that should be executed in serial
+  template <class _RandomAccessIterator, class _Functor>
+  void __parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Functor __func);
+
+  TODO: Document the parallel backend
+*/
+
+#include <__algorithm/pstl_backends/cpu_backends/for_each.h>
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKEND_H
diff --git a/libcxx/include/__algorithm/pstl_backends/cpu_backends/backend.h b/libcxx/include/__algorithm/pstl_backends/cpu_backends/backend.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backends/cpu_backends/backend.h
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_PSTL_BACKENDS_CPU_BACKEND_BACKEND_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKEND_BACKEND_H
+
+#include <__config>
+
+#if defined _LIBCPP_HAS_NO_THREADS || defined _PSTL_CPU_BACKEND_SERIAL
+#  include <__algorithm/pstl_backends/cpu_backends/serial.h>
+#else
+#  error "Invalid CPU backend choice"
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct __cpu_backend_tag {};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKEND_BACKEND_H
diff --git a/libcxx/include/__algorithm/pstl_backends/cpu_backends/for_each.h b/libcxx/include/__algorithm/pstl_backends/cpu_backends/for_each.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backends/cpu_backends/for_each.h
@@ -0,0 +1,55 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_PSTL_BACKENDS_CPU_BACKNEDS_FOR_EACH_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKNEDS_FOR_EACH_H
+
+#include <__algorithm/for_each.h>
+#include <__algorithm/pstl_backends/cpu_backends/backend.h>
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__type_traits/is_execution_policy.h>
+#include <__utility/terminate_on_exception.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Iterator, class _DifferenceType, class _Function>
+_LIBCPP_HIDE_FROM_ABI _Iterator __simd_walk_1(_Iterator __first, _DifferenceType __n, _Function __f) noexcept {
+  _PSTL_PRAGMA_SIMD
+  for (_DifferenceType __i = 0; __i < __n; ++__i)
+    __f(__first[__i]);
+
+  return __first + __n;
+}
+
+template <class _ExecutionPolicy, class _ForwardIterator, class _Functor>
+void __pstl_for_each(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __last, _Functor __func) {
+  if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
+                __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
+    std::__terminate_on_exception([&] {
+      std::__par_backend::__parallel_for(
+          __first, __last, [__func](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
+            std::__pstl_for_each<__remove_parallel_policy_t<_ExecutionPolicy>>(
+                __cpu_backend_tag{}, __brick_first, __brick_last, __func);
+          });
+    });
+  } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
+                       __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
+    std::__simd_walk_1(__first, __last - __first, __func);
+  } else {
+    std::for_each(__first, __last, __func);
+  }
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKNEDS_FOR_EACH_H
diff --git a/libcxx/include/__algorithm/pstl_backends/cpu_backends/serial.h b/libcxx/include/__algorithm/pstl_backends/cpu_backends/serial.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backends/cpu_backends/serial.h
@@ -0,0 +1,38 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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_PSTL_BACKENDS_CPU_BACKENDS_SERIAL_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_SERIAL_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace __par_backend::inline __serial_cpu_backend {
+
+template <class _RandomAccessIterator, class _Fp>
+_LIBCPP_HIDE_FROM_ABI void __parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Fp __f) {
+  __f(__first, __last);
+}
+
+// TODO: Complete this list
+
+} // namespace __par_backend::inline __serial_cpu_backend
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_SERIAL_H
diff --git a/libcxx/include/__algorithm/pstl_for_each.h b/libcxx/include/__algorithm/pstl_for_each.h
--- a/libcxx/include/__algorithm/pstl_for_each.h
+++ b/libcxx/include/__algorithm/pstl_for_each.h
@@ -11,12 +11,16 @@
 
 #include <__algorithm/for_each.h>
 #include <__algorithm/for_each_n.h>
+#include <__algorithm/pstl_backend.h>
+#include <__algorithm/pstl_frontend_dispatch.h>
 #include <__config>
 #include <__iterator/iterator_traits.h>
 #include <__pstl/internal/parallel_backend.h>
 #include <__pstl/internal/unseq_backend_simd.h>
 #include <__type_traits/is_execution_policy.h>
+#include <__type_traits/is_valid.h>
 #include <__type_traits/remove_cvref.h>
+#include <__type_traits/void_t.h>
 #include <__utility/terminate_on_exception.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -30,41 +34,34 @@
 template <class _ExecutionPolicy,
           class _ForwardIterator,
           class _Function,
-          enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
+          class _RawPolicy                                    = __remove_cvref_t<_ExecutionPolicy>,
+          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI void
 for_each(_ExecutionPolicy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Function __func) {
-  if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
-    std::__terminate_on_exception([&] {
-      __pstl::__par_backend::__parallel_for(
-          {},
-          __policy,
-          __first,
-          __last,
-          [&__policy, __func](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
-            std::for_each(std::__remove_parallel_policy(__policy), __brick_first, __brick_last, __func);
-          });
-    });
-  } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
-    __pstl::__unseq_backend::__simd_walk_1(__first, __last - __first, __func);
-  } else {
-    std::for_each(__first, __last, __func);
-  }
+  using _Backend = typename __select_backend<_RawPolicy>::type;
+  std::__pstl_for_each<_RawPolicy>(_Backend{}, std::move(__first), std::move(__last), std::move(__func));
 }
 
 template <class _ExecutionPolicy,
           class _ForwardIterator,
           class _Size,
           class _Function,
-          enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
+          class _RawPolicy                                    = __remove_cvref_t<_ExecutionPolicy>,
+          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI void
 for_each_n(_ExecutionPolicy&& __policy, _ForwardIterator __first, _Size __size, _Function __func) {
-  if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
-    std::for_each(__policy, __first, __first + __size, __func);
-  } else {
-    std::for_each_n(__first, __size, __func);
-  }
+  return std::__pstl_frontend_dispatch(
+      _LIBCPP_PSTL_CUSTOMIZATION_POINT(__pstl_for_each_n),
+      [&](_ForwardIterator __first, _Size __size, _Function __func) {
+        if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
+          std::for_each(__policy, std::move(__first), __first + __size, std::move(__func));
+        } else {
+          std::for_each_n(std::move(__first), __size, std::move(__func));
+        }
+      },
+      std::move(__first),
+      __size,
+      std::move(__func));
 }
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__algorithm/pstl_frontend_dispatch.h b/libcxx/include/__algorithm/pstl_frontend_dispatch.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_frontend_dispatch.h
@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_PSTL_FRONTEND_DISPATCH
+#define _LIBCPP___ALGORITHM_PSTL_FRONTEND_DISPATCH
+
+#include <__config>
+#include <__type_traits/is_callable.h>
+#include <__utility/forward.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#  define _LIBCPP_PSTL_CUSTOMIZATION_POINT(name)                                                                       \
+    [](auto&&... __args) -> decltype(std::name<_RawPolicy>(typename __select_backend<_RawPolicy>::type{},              \
+                                                           std::forward<decltype(__args)>(__args)...)) {               \
+      return std::name<_RawPolicy>(                                                                                    \
+          typename __select_backend<_RawPolicy>::type{}, std::forward<decltype(__args)>(__args)...);                   \
+    }
+
+template <class _SpecializedImpl, class _GenericImpl, class... _Args>
+_LIBCPP_HIDE_FROM_ABI decltype(auto)
+__pstl_frontend_dispatch(_SpecializedImpl __specialized, _GenericImpl __generic, _Args&&... __args) {
+  if constexpr (__is_callable<_SpecializedImpl, _Args...>::value) {
+    return __specialized(std::forward<_Args>(__args)...);
+  } else {
+    return __generic(std::forward<_Args>(__args)...);
+  }
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_FRONTEND_DISPATCH
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1273,6 +1273,7 @@
 
 // TODO: Make this a proper configuration option
 #define _PSTL_PAR_BACKEND_SERIAL
+#define _PSTL_CPU_BACKEND_SERIAL
 
 #define _PSTL_PRAGMA(x) _Pragma(#    x)
 
diff --git a/libcxx/include/__type_traits/is_execution_policy.h b/libcxx/include/__type_traits/is_execution_policy.h
--- a/libcxx/include/__type_traits/is_execution_policy.h
+++ b/libcxx/include/__type_traits/is_execution_policy.h
@@ -36,10 +36,20 @@
 template <class _Tp>
 inline constexpr bool __is_parallel_execution_policy_v = __is_parallel_execution_policy_impl<__remove_cvref_t<_Tp>>;
 
+namespace execution {
+struct __disable_user_instantiations_tag {
+  explicit __disable_user_instantiations_tag() = default;
+};
+} // namespace execution
+
+template <class _ExecutionPolicy>
+_LIBCPP_HIDE_FROM_ABI auto
+__remove_parallel_policy(const _ExecutionPolicy& = _ExecutionPolicy{execution::__disable_user_instantiations_tag{}});
+
 // Removes the "parallel" part of an execution policy.
 // For example, turns par_unseq into unseq, and par into seq.
 template <class _ExecutionPolicy>
-_LIBCPP_HIDE_FROM_ABI const auto& __remove_parallel_policy(_ExecutionPolicy&&);
+using __remove_parallel_policy_t = decltype(std::__remove_parallel_policy<_ExecutionPolicy>());
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/execution b/libcxx/include/execution
--- a/libcxx/include/execution
+++ b/libcxx/include/execution
@@ -48,10 +48,6 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace execution {
-struct __disable_user_instantiations_tag {
-  explicit __disable_user_instantiations_tag() = default;
-};
-
 struct sequenced_policy {
   _LIBCPP_HIDE_FROM_ABI constexpr explicit sequenced_policy(__disable_user_instantiations_tag) {}
   sequenced_policy(const sequenced_policy&)            = delete;
@@ -135,12 +131,11 @@
 struct is_execution_policy : bool_constant<is_execution_policy_v<_Tp>> {};
 
 template <class _ExecutionPolicy>
-_LIBCPP_HIDE_FROM_ABI const auto& __remove_parallel_policy(_ExecutionPolicy&&) {
-  using _ExecPol = __remove_cvref_t<_ExecutionPolicy>;
-  if constexpr (is_same_v<_ExecPol, execution::parallel_policy>) {
-    return execution::seq;
-  } else if constexpr (is_same_v<_ExecPol, execution::parallel_unsequenced_policy>) {
-    return execution::__unseq;
+_LIBCPP_HIDE_FROM_ABI auto __remove_parallel_policy(const _ExecutionPolicy&) {
+  if constexpr (is_same_v<_ExecutionPolicy, execution::parallel_policy>) {
+    return execution::sequenced_policy(execution::__disable_user_instantiations_tag{});
+  } else if constexpr (is_same_v<_ExecutionPolicy, execution::parallel_unsequenced_policy>) {
+    return execution::__unsequenced_policy{execution::__disable_user_instantiations_tag{}};
   }
 }
 
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
@@ -114,6 +114,10 @@
 #include <__algorithm/partition_point.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/partition_point.h'}}
 #include <__algorithm/pop_heap.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pop_heap.h'}}
 #include <__algorithm/prev_permutation.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/prev_permutation.h'}}
+#include <__algorithm/pstl_backends/cpu_backend.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backend.h'}}
+#include <__algorithm/pstl_backends/cpu_backends/backend.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backends/backend.h'}}
+#include <__algorithm/pstl_backends/cpu_backends/for_each.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backends/for_each.h'}}
+#include <__algorithm/pstl_backends/cpu_backends/serial.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backends/serial.h'}}
 #include <__algorithm/push_heap.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/push_heap.h'}}
 #include <__algorithm/ranges_adjacent_find.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_adjacent_find.h'}}
 #include <__algorithm/ranges_all_of.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_all_of.h'}}
@@ -740,6 +744,7 @@
 #include <__type_traits/is_union.h> // expected-error@*:* {{use of private header from outside its module: '__type_traits/is_union.h'}}
 #include <__type_traits/is_unsigned.h> // expected-error@*:* {{use of private header from outside its module: '__type_traits/is_unsigned.h'}}
 #include <__type_traits/is_unsigned_integer.h> // expected-error@*:* {{use of private header from outside its module: '__type_traits/is_unsigned_integer.h'}}
+#include <__type_traits/is_valid.h> // expected-error@*:* {{use of private header from outside its module: '__type_traits/is_valid.h'}}
 #include <__type_traits/is_valid_expansion.h> // expected-error@*:* {{use of private header from outside its module: '__type_traits/is_valid_expansion.h'}}
 #include <__type_traits/is_void.h> // expected-error@*:* {{use of private header from outside its module: '__type_traits/is_void.h'}}
 #include <__type_traits/is_volatile.h> // expected-error@*:* {{use of private header from outside its module: '__type_traits/is_volatile.h'}}