diff --git a/libcxx/.clang-tidy b/libcxx/.clang-tidy
--- a/libcxx/.clang-tidy
+++ b/libcxx/.clang-tidy
@@ -1,2 +1,2 @@
 InheritParentConfig: true
-Checks: '-readability-identifier-naming,-llvm-header-guard,-misc-unconventional-assign-operator,-llvm-else-after-return'
+Checks: '-readability-identifier-naming,-llvm-header-guard,-misc-unconventional-assign-operator,-llvm-else-after-return,-llvm-qualified-auto'
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -924,7 +924,7 @@
   foreach (extra_site_define ${LIBCXX_EXTRA_SITE_DEFINES})
     # Allow defines such as DEFINE=VAL, transformed into "#define DEFINE VAL".
     string(REPLACE "=" " " extra_site_define "${extra_site_define}")
-    list(APPEND extra_site_defines "#define ${extra_site_define}")
+    list(APPEND extra_site_defines "#ifdef ${extra_site_define};#  undef ${extra_site_define};#endif;#define ${extra_site_define}")
   endforeach()
   string(REPLACE ";" "\n" extra_site_defines "${extra_site_defines}")
   config_define(${extra_site_defines} _LIBCPP_EXTRA_SITE_DEFINES)
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -56,7 +56,45 @@
    // ... add new file formats here ...
 #endif
 
+#if defined(__apple_build_version__)
+#  define _LIBCPP_COMPILER_CLANG_BASED
+#  define _LIBCPP_APPLE_CLANG_VER (__apple_build_version__ / 10000)
+#elif defined(__clang__)
+#  define _LIBCPP_COMPILER_CLANG_BASED
+#  define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
+#elif defined(__GNUC__)
+#  define _LIBCPP_COMPILER_GCC
+#elif defined(_MSC_VER)
+#  define _LIBCPP_COMPILER_MSVC
+#elif defined(__IBMCPP__)
+#  define _LIBCPP_COMPILER_IBM
+#endif
+
+#define _LIBCPP_TOSTRING2(x) #x
+#define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
+
+#ifdef _LIBCPP_COMPILER_CLANG_BASED
+#  define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("clang diagnostic push")
+#  define _LIBCPP_DIAGNOSTIC_POP _Pragma("clang diagnostic pop")
+#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(clang diagnostic ignored str))
+#  define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
+#elif defined(_LIBCPP_COMPILER_GCC)
+#  define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("GCC diagnostic push")
+#  define _LIBCPP_DIAGNOSTIC_POP _Pragma("GCC diagnostic pop")
+#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
+#  define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(GCC diagnostic ignored str))
+#else
+#  define _LIBCPP_DIAGNOSTIC_PUSH
+#  define _LIBCPP_DIAGNOSTIC_POP
+#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
+#  define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
+#endif
+
 #if _LIBCPP_ABI_VERSION >= 2
+// These macros are defined in some tests
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wmacro-redefined")
+
 // Change short string representation so that string data starts at offset 0,
 // improving its alignment in some cases.
 #  define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
@@ -114,6 +152,8 @@
 #  define _LIBCPP_ABI_DO_NOT_EXPORT_BASIC_STRING_COMMON
 // Remove vector base class
 #  define _LIBCPP_ABI_DO_NOT_EXPORT_VECTOR_BASE_COMMON
+
+_LIBCPP_DIAGNOSTIC_POP
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support
@@ -142,9 +182,6 @@
 #  define _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
 #endif
 
-#define _LIBCPP_TOSTRING2(x) #x
-#define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
-
 #if __cplusplus < 201103L
 #define _LIBCPP_CXX03_LANG
 #endif
@@ -185,20 +222,6 @@
 #define __has_include(...) 0
 #endif
 
-#if defined(__apple_build_version__)
-#  define _LIBCPP_COMPILER_CLANG_BASED
-#  define _LIBCPP_APPLE_CLANG_VER (__apple_build_version__ / 10000)
-#elif defined(__clang__)
-#  define _LIBCPP_COMPILER_CLANG_BASED
-#  define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
-#elif defined(__GNUC__)
-#  define _LIBCPP_COMPILER_GCC
-#elif defined(_MSC_VER)
-#  define _LIBCPP_COMPILER_MSVC
-#elif defined(__IBMCPP__)
-#  define _LIBCPP_COMPILER_IBM
-#endif
-
 #if defined(_LIBCPP_COMPILER_GCC) && __cplusplus < 201103L
 #error "libc++ does not support using GCC with C++03. Please enable C++11"
 #endif
@@ -1423,23 +1446,6 @@
    // the ABI inconsistent.
 #endif
 
-#ifdef _LIBCPP_COMPILER_CLANG_BASED
-#  define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("clang diagnostic push")
-#  define _LIBCPP_DIAGNOSTIC_POP _Pragma("clang diagnostic pop")
-#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(clang diagnostic ignored str))
-#  define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
-#elif defined(_LIBCPP_COMPILER_GCC)
-#  define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("GCC diagnostic push")
-#  define _LIBCPP_DIAGNOSTIC_POP _Pragma("GCC diagnostic pop")
-#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
-#  define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(GCC diagnostic ignored str))
-#else
-#  define _LIBCPP_DIAGNOSTIC_PUSH
-#  define _LIBCPP_DIAGNOSTIC_POP
-#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
-#  define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
-#endif
-
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
diff --git a/libcxx/include/__iterator/iterator_traits.h b/libcxx/include/__iterator/iterator_traits.h
--- a/libcxx/include/__iterator/iterator_traits.h
+++ b/libcxx/include/__iterator/iterator_traits.h
@@ -198,7 +198,7 @@
     { __i +  __n } -> same_as<_Ip>;
     { __n +  __i } -> same_as<_Ip>;
     { __i -  __n } -> same_as<_Ip>;
-    { __i -  __i } -> same_as<decltype(__n)>;
+    { __i -  __i } -> same_as<decltype(__n)>; // NOLINT(misc-redundant-expression)
     {  __i[__n]  } -> convertible_to<iter_reference_t<_Ip>>;
   };
 } // namespace __iterator_traits_detail
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -21,8 +21,8 @@
 #include <utility>
 
 #if defined(_LIBCPP_MSVCRT_LIKE)
-# include <cstring>
 # include <__support/win32/locale_win32.h>
+# include <cstring>
 #elif defined(_AIX) || defined(__MVS__)
 # include <__support/ibm/xlocale.h>
 #elif defined(__ANDROID__)
diff --git a/libcxx/include/__ranges/reverse_view.h b/libcxx/include/__ranges/reverse_view.h
--- a/libcxx/include/__ranges/reverse_view.h
+++ b/libcxx/include/__ranges/reverse_view.h
@@ -111,22 +111,22 @@
   namespace views {
   namespace __reverse {
     template<class _Tp>
-    constexpr bool __is_reverse_view = false;
+    inline constexpr bool __is_reverse_view = false;
 
     template<class _Tp>
-    constexpr bool __is_reverse_view<reverse_view<_Tp>> = true;
+    inline constexpr bool __is_reverse_view<reverse_view<_Tp>> = true;
 
     template<class _Tp>
-    constexpr bool __is_sized_reverse_subrange = false;
+    inline constexpr bool __is_sized_reverse_subrange = false;
 
     template<class _Iter>
-    constexpr bool __is_sized_reverse_subrange<subrange<reverse_iterator<_Iter>, reverse_iterator<_Iter>, subrange_kind::sized>> = true;
+    inline constexpr bool __is_sized_reverse_subrange<subrange<reverse_iterator<_Iter>, reverse_iterator<_Iter>, subrange_kind::sized>> = true;
 
     template<class _Tp>
-    constexpr bool __is_unsized_reverse_subrange = false;
+    inline constexpr bool __is_unsized_reverse_subrange = false;
 
     template<class _Iter, subrange_kind _Kind>
-    constexpr bool __is_unsized_reverse_subrange<subrange<reverse_iterator<_Iter>, reverse_iterator<_Iter>, _Kind>> = _Kind == subrange_kind::unsized;
+    inline constexpr bool __is_unsized_reverse_subrange<subrange<reverse_iterator<_Iter>, reverse_iterator<_Iter>, _Kind>> = _Kind == subrange_kind::unsized;
 
     template<class _Tp>
     struct __unwrapped_reverse_subrange {
diff --git a/libcxx/include/ext/__hash b/libcxx/include/ext/__hash
--- a/libcxx/include/ext/__hash
+++ b/libcxx/include/ext/__hash
@@ -12,10 +12,15 @@
 
 #  pragma GCC system_header
 
-#include <__string>
+#include <__config>
 #include <cstring>
 #include <string>
 
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wprivate-header")
+#include <__string>
+_LIBCPP_DIAGNOSTIC_POP
+
 namespace __gnu_cxx {
 
 template <typename _Tp> struct _LIBCPP_TEMPLATE_VIS hash { };
diff --git a/libcxx/include/ext/hash_map b/libcxx/include/ext/hash_map
--- a/libcxx/include/ext/hash_map
+++ b/libcxx/include/ext/hash_map
@@ -201,7 +201,6 @@
 
 */
 
-#include <__algorithm/is_permutation.h>
 #include <__config>
 #include <__hash_table>
 #include <ext/__hash>
@@ -209,6 +208,11 @@
 #include <stdexcept>
 #include <type_traits>
 
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wprivate-header")
+#include <__algorithm/is_permutation.h>
+_LIBCPP_DIAGNOSTIC_POP
+
 #if defined(__DEPRECATED) && __DEPRECATED
 #if defined(_LIBCPP_WARNING)
     _LIBCPP_WARNING("Use of the header <ext/hash_map> is deprecated.  Migrate to <unordered_map>")
diff --git a/libcxx/include/ext/hash_set b/libcxx/include/ext/hash_set
--- a/libcxx/include/ext/hash_set
+++ b/libcxx/include/ext/hash_set
@@ -192,12 +192,16 @@
 
 */
 
-#include <__algorithm/is_permutation.h>
 #include <__config>
 #include <__hash_table>
 #include <ext/__hash>
 #include <functional>
 
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wprivate-header")
+#include <__algorithm/is_permutation.h>
+_LIBCPP_DIAGNOSTIC_POP
+
 #if defined(__DEPRECATED) && __DEPRECATED
 #if defined(_LIBCPP_WARNING)
     _LIBCPP_WARNING("Use of the header <ext/hash_set> is deprecated.  Migrate to <unordered_set>")
diff --git a/libcxx/test/configs/apple-libc++-shared.cfg.in b/libcxx/test/configs/apple-libc++-shared.cfg.in
--- a/libcxx/test/configs/apple-libc++-shared.cfg.in
+++ b/libcxx/test/configs/apple-libc++-shared.cfg.in
@@ -13,7 +13,7 @@
     '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
 ))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{install}/include/c++/v1 -I %{libcxx}/test/support'
+    '-nostdinc++ -I %{install}/include/c++/v1 -I %{libcxx}/test/support'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib++ -L %{install}/lib -lc++'
diff --git a/libcxx/test/configs/ibm-libc++-shared.cfg.in b/libcxx/test/configs/ibm-libc++-shared.cfg.in
--- a/libcxx/test/configs/ibm-libc++-shared.cfg.in
+++ b/libcxx/test/configs/ibm-libc++-shared.cfg.in
@@ -6,7 +6,7 @@
 
 config.substitutions.append(('%{flags}', ''))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -D__LIBC_NO_CPP_MATH_OVERLOADS__ -isystem %{install}/include/c++/v1 -I %{libcxx}/test/support'
+    '-nostdinc++ -D__LIBC_NO_CPP_MATH_OVERLOADS__ -I %{install}/include/c++/v1 -I %{libcxx}/test/support'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib++ -L %{install}/lib -lc++ -lc++abi -latomic -Wl,-bbigtoc'
diff --git a/libcxx/test/configs/llvm-libc++-mingw.cfg.in b/libcxx/test/configs/llvm-libc++-mingw.cfg.in
--- a/libcxx/test/configs/llvm-libc++-mingw.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-mingw.cfg.in
@@ -5,7 +5,7 @@
 
 config.substitutions.append(('%{flags}', ''))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{include} -isystem %{target-include} -I %{libcxx}/test/support'
+    '-nostdinc++ -I %{include} -I %{target-include} -I %{libcxx}/test/support'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib++ -L %{lib} -lc++'
diff --git a/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in b/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
--- a/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
@@ -5,7 +5,7 @@
 
 config.substitutions.append(('%{flags}', '--driver-mode=g++'))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{include} -isystem %{target-include} -I %{libcxx}/test/support -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_STDIO_ISO_WIDE_SPECIFIERS'
+    '-nostdinc++ -I %{include} -I %{target-include} -I %{libcxx}/test/support -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_STDIO_ISO_WIDE_SPECIFIERS'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib -L %{lib} -lc++ -lmsvcrt -lmsvcprt -loldnames'
diff --git a/libcxx/test/configs/llvm-libc++-shared-gcc.cfg.in b/libcxx/test/configs/llvm-libc++-shared-gcc.cfg.in
--- a/libcxx/test/configs/llvm-libc++-shared-gcc.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-shared-gcc.cfg.in
@@ -6,7 +6,7 @@
 
 config.substitutions.append(('%{flags}', ''))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{include} -isystem %{target-include} -I %{libcxx}/test/support'
+    '-nostdinc++ -I %{include} -I %{target-include} -I %{libcxx}/test/support'
 ))
 config.substitutions.append(('%{link_flags}',
     '-L %{lib} -Wl,-rpath,%{lib} -nodefaultlibs -lc++ -lm -lgcc_s -lgcc -lpthread -lc -lgcc_s -lgcc -latomic'
diff --git a/libcxx/test/configs/llvm-libc++-shared.cfg.in b/libcxx/test/configs/llvm-libc++-shared.cfg.in
--- a/libcxx/test/configs/llvm-libc++-shared.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-shared.cfg.in
@@ -7,7 +7,7 @@
     '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
 ))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{include} -isystem %{target-include} -I %{libcxx}/test/support'
+    '-nostdinc++ -I %{include} -I %{target-include} -I %{libcxx}/test/support'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib++ -L %{lib} -Wl,-rpath,%{lib} -lc++ -pthread'
diff --git a/libcxx/test/configs/llvm-libc++-static-clangcl.cfg.in b/libcxx/test/configs/llvm-libc++-static-clangcl.cfg.in
--- a/libcxx/test/configs/llvm-libc++-static-clangcl.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-static-clangcl.cfg.in
@@ -5,7 +5,7 @@
 
 config.substitutions.append(('%{flags}', '--driver-mode=g++'))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{include} -isystem %{target-include} -I %{libcxx}/test/support -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_STDIO_ISO_WIDE_SPECIFIERS'
+    '-nostdinc++ -I %{include} -I %{target-include} -I %{libcxx}/test/support -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_STDIO_ISO_WIDE_SPECIFIERS'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib -L %{lib} -llibc++ -lmsvcrt -lmsvcprt -loldnames'
diff --git a/libcxx/test/configs/llvm-libc++-static.cfg.in b/libcxx/test/configs/llvm-libc++-static.cfg.in
--- a/libcxx/test/configs/llvm-libc++-static.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-static.cfg.in
@@ -7,7 +7,7 @@
     '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
 ))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{include} -isystem %{target-include} -I %{libcxx}/test/support'
+    '-nostdinc++ -I %{include} -I %{target-include} -I %{libcxx}/test/support'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib++ -L %{lib} -lc++ -lc++abi -pthread'
diff --git a/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp b/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp
--- a/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp
+++ b/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp
@@ -13,6 +13,11 @@
 
 // Test the size and alignment matches that of an array of a given type.
 
+// Ignore error about requesting a large alignment not being ABI compatible with older AIX systems.
+#if defined(_AIX)
+# pragma clang diagnostic ignored "-Waix-compat"
+#endif
+
 #include <array>
 #include <iterator>
 #include <type_traits>
@@ -20,7 +25,6 @@
 
 #include "test_macros.h"
 
-
 template <class T, size_t Size>
 struct MyArray {
   T elems[Size];
diff --git a/libcxx/utils/libcxx/test/config.py b/libcxx/utils/libcxx/test/config.py
--- a/libcxx/utils/libcxx/test/config.py
+++ b/libcxx/utils/libcxx/test/config.py
@@ -308,8 +308,8 @@
         if triple is not None:
             cxx_target_headers = os.path.join(path, triple, cxx, version)
             if os.path.isdir(cxx_target_headers):
-                self.cxx.compile_flags += ['-isystem', cxx_target_headers]
-        self.cxx.compile_flags += ['-isystem', cxx_headers]
+                self.cxx.compile_flags += ['-I', cxx_target_headers]
+        self.cxx.compile_flags += ['-I', cxx_headers]
         if self.libcxx_obj_root is not None:
             cxxabi_headers = os.path.join(self.libcxx_obj_root, 'include',
                                           'c++build')