diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -106,6 +106,7 @@
   __concepts/class_or_enum.h
   __concepts/common_reference_with.h
   __concepts/common_with.h
+  __concepts/complete_object_type.h
   __concepts/constructible.h
   __concepts/convertible_to.h
   __concepts/copyable.h
diff --git a/libcxx/include/__concepts/complete_object_type.h b/libcxx/include/__concepts/complete_object_type.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__concepts/complete_object_type.h
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_COMPLETE_OBJECT_TYPE_H
+#define _LIBCPP___CONCEPTS_COMPLETE_OBJECT_TYPE_H
+
+#include <__config>
+
+#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)
+
+template <class _Tp>
+concept __complete_object_type = requires {
+  sizeof(_Tp);
+};
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CONCEPTS_COMPLETE_OBJECT_TYPE_H
diff --git a/libcxx/include/__iterator/concepts.h b/libcxx/include/__iterator/concepts.h
--- a/libcxx/include/__iterator/concepts.h
+++ b/libcxx/include/__iterator/concepts.h
@@ -11,6 +11,7 @@
 #define _LIBCPP___ITERATOR_CONCEPTS_H
 
 #include <__config>
+#include <__concepts/complete_object_type.h>
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/iter_move.h>
 #include <__iterator/iterator_traits.h>
@@ -174,7 +175,7 @@
   derived_from<_ITER_CONCEPT<_Ip>, contiguous_iterator_tag> &&
   is_lvalue_reference_v<iter_reference_t<_Ip>> &&
   same_as<iter_value_t<_Ip>, remove_cvref_t<iter_reference_t<_Ip>>> &&
-  (is_pointer_v<_Ip> || requires { sizeof(__pointer_traits_element_type<_Ip>); }) &&
+  (is_pointer_v<_Ip> || __complete_object_type<__pointer_traits_element_type<_Ip>>) &&
   requires(const _Ip& __i) {
     { _VSTD::to_address(__i) } -> same_as<add_pointer_t<iter_reference_t<_Ip>>>;
   };
diff --git a/libcxx/include/__ranges/access.h b/libcxx/include/__ranges/access.h
--- a/libcxx/include/__ranges/access.h
+++ b/libcxx/include/__ranges/access.h
@@ -10,6 +10,7 @@
 #define _LIBCPP___RANGES_ACCESS_H
 
 #include <__config>
+#include <__concepts/complete_object_type.h>
 #include <__iterator/concepts.h>
 #include <__iterator/readable_traits.h>
 #include <__ranges/enable_borrowed_range.h>
@@ -36,9 +37,6 @@
   template <class _Tp>
   concept __can_borrow =
       is_lvalue_reference_v<_Tp> || enable_borrowed_range<remove_cvref_t<_Tp> >;
-
-  template<class _Tp>
-  concept __is_complete = requires { sizeof(_Tp); };
 } // namespace ranges
 
 // [range.access.begin]
@@ -66,7 +64,7 @@
     template <class _Tp>
     requires is_array_v<remove_cv_t<_Tp>>
     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Tp& __t) const noexcept {
-      constexpr bool __complete = __is_complete<iter_value_t<_Tp> >;
+      constexpr bool __complete = __complete_object_type<iter_value_t<_Tp> >;
       if constexpr (__complete) { // used to disable cryptic diagnostic
         return __t + 0;
       }
@@ -131,7 +129,7 @@
   public:
     template <class _Tp, size_t _Np>
     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Tp (&__t)[_Np]) const noexcept {
-      constexpr bool __complete = __is_complete<remove_cv_t<_Tp> >;
+      constexpr bool __complete = __complete_object_type<remove_cv_t<_Tp> >;
       if constexpr (__complete) { // used to disable cryptic diagnostic
         return __t + _Np;
       }
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -380,6 +380,7 @@
       module class_or_enum              { private header "__concepts/class_or_enum.h"             }
       module common_reference_with      { private header "__concepts/common_reference_with.h"     }
       module common_with                { private header "__concepts/common_with.h"               }
+      module complete                   { private header "__concepts/complete.h"                  }
       module constructible              { private header "__concepts/constructible.h"             }
       module convertible_to             { private header "__concepts/convertible_to.h"            }
       module copyable                   { private header "__concepts/copyable.h"                  }
diff --git a/libcxx/test/libcxx/diagnostics/detail.headers/concepts/complete.module.verify.cpp b/libcxx/test/libcxx/diagnostics/detail.headers/concepts/complete.module.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/diagnostics/detail.headers/concepts/complete.module.verify.cpp
@@ -0,0 +1,16 @@
+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '___concepts/complete.h'}}
+#include <___concepts/complete.h>