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/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1437,11 +1437,11 @@
 #endif
 
 #if defined(_AIX) && !defined(_LIBCPP_COMPILER_GCC)
-#define _LIBCPP_PACKED_BYTE_FOR_AIX _Pragma("pack(1)")
-#define _LIBCPP_PACKED_BYTE_FOR_AIX_END _Pragma("pack(pop)")
+#  define _LIBCPP_PACKED_BYTE_FOR_AIX _Pragma("pack(1)")
+#  define _LIBCPP_PACKED_BYTE_FOR_AIX_END _Pragma("pack(pop)")
 #else
-#define _LIBCPP_PACKED_BYTE_FOR_AIX      /* empty */
-#define _LIBCPP_PACKED_BYTE_FOR_AIX_END  /* empty */
+#  define _LIBCPP_PACKED_BYTE_FOR_AIX      /* empty */
+#  define _LIBCPP_PACKED_BYTE_FOR_AIX_END  /* empty */
 #endif
 
 #endif // __cplusplus
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -33,7 +33,17 @@
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_FORMAT
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_RANGES
 
+// __USE_MINGW_ANSI_STDIO gets redefined on MinGW
+#ifdef __clang__
+#  pragma clang diagnostic push
+#  pragma clang diagnostic ignored "-Wmacro-redefined"
+#endif
+
 @_LIBCPP_ABI_DEFINES@
 @_LIBCPP_EXTRA_SITE_DEFINES@
 
+#ifdef __clang__
+#  pragma clang diagnostic pop
+#endif
+
 #endif // _LIBCPP_CONFIG_SITE
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) ; This is llvm.org/PR54114
     {  __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
@@ -21,15 +21,15 @@
 #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__)
 # include <__support/android/locale_bionic.h>
 #elif defined(__sun__)
-# include <xlocale.h>
 # include <__support/solaris/xlocale.h>
+# include <xlocale.h>
 #elif defined(_NEWLIB_VERSION)
 # include <__support/newlib/xlocale.h>
 #elif defined(__OpenBSD__)
diff --git a/libcxx/include/ext/__hash b/libcxx/include/ext/__hash
--- a/libcxx/include/ext/__hash
+++ b/libcxx/include/ext/__hash
@@ -12,6 +12,7 @@
 
 #  pragma GCC system_header
 
+#include <__config>
 #include <__string>
 #include <cstring>
 #include <string>
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 -DNOMINMAX'
+    '-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 -DNOMINMAX'
 ))
 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 -DNOMINMAX'
+    '-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 -DNOMINMAX'
 ))
 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/libcxx/clang_tidy.sh.cpp b/libcxx/test/libcxx/clang_tidy.sh.cpp
--- a/libcxx/test/libcxx/clang_tidy.sh.cpp
+++ b/libcxx/test/libcxx/clang_tidy.sh.cpp
@@ -230,8 +230,10 @@
 #endif // __cplusplus >= 201103L
 
 // extended headers
-#include <ext/hash_map>
-#include <ext/hash_set>
+#ifndef _LIBCXX_MODULES_BUILD
+#    include <ext/hash_map>
+#    include <ext/hash_set>
+#endif
 
 // clang-format on
 
diff --git a/libcxx/test/libcxx/containers/gnu_cxx/hash_map.pass.cpp b/libcxx/test/libcxx/containers/gnu_cxx/hash_map.pass.cpp
--- a/libcxx/test/libcxx/containers/gnu_cxx/hash_map.pass.cpp
+++ b/libcxx/test/libcxx/containers/gnu_cxx/hash_map.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: modules-build
+
 // Prevent <ext/hash_map> from generating deprecated warnings for this test.
 #if defined(__DEPRECATED)
 #   undef __DEPRECATED
diff --git a/libcxx/test/libcxx/containers/gnu_cxx/hash_map_name_lookup.pass.cpp b/libcxx/test/libcxx/containers/gnu_cxx/hash_map_name_lookup.pass.cpp
--- a/libcxx/test/libcxx/containers/gnu_cxx/hash_map_name_lookup.pass.cpp
+++ b/libcxx/test/libcxx/containers/gnu_cxx/hash_map_name_lookup.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: modules-build
+
 // Poison the std:: names we might use inside __gnu_cxx to ensure they're
 // properly qualified.
 struct allocator;
diff --git a/libcxx/test/libcxx/containers/gnu_cxx/hash_set.pass.cpp b/libcxx/test/libcxx/containers/gnu_cxx/hash_set.pass.cpp
--- a/libcxx/test/libcxx/containers/gnu_cxx/hash_set.pass.cpp
+++ b/libcxx/test/libcxx/containers/gnu_cxx/hash_set.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: modules-build
+
 // Prevent <ext/hash_set> from generating deprecated warnings for this test.
 #if defined(__DEPRECATED)
 #   undef __DEPRECATED
diff --git a/libcxx/test/libcxx/containers/gnu_cxx/hash_set_name_lookup.pass.cpp b/libcxx/test/libcxx/containers/gnu_cxx/hash_set_name_lookup.pass.cpp
--- a/libcxx/test/libcxx/containers/gnu_cxx/hash_set_name_lookup.pass.cpp
+++ b/libcxx/test/libcxx/containers/gnu_cxx/hash_set_name_lookup.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: modules-build
+
 // Poison the std:: names we might use inside __gnu_cxx to ensure they're
 // properly qualified.
 struct allocator;
diff --git a/libcxx/test/libcxx/containers/sequences/deque/incomplete.pass.cpp b/libcxx/test/libcxx/containers/sequences/deque/incomplete.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/deque/incomplete.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/deque/incomplete.pass.cpp
@@ -11,7 +11,7 @@
 // deque()
 // deque::iterator()
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -D_LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
 
 #include <deque>
 #include <cassert>
diff --git a/libcxx/test/libcxx/double_include.sh.cpp b/libcxx/test/libcxx/double_include.sh.cpp
--- a/libcxx/test/libcxx/double_include.sh.cpp
+++ b/libcxx/test/libcxx/double_include.sh.cpp
@@ -231,8 +231,10 @@
 #endif // __cplusplus >= 201103L
 
 // extended headers
-#include <ext/hash_map>
-#include <ext/hash_set>
+#ifndef _LIBCXX_MODULES_BUILD
+#    include <ext/hash_map>
+#    include <ext/hash_set>
+#endif
 
 // clang-format on
 
diff --git a/libcxx/test/libcxx/extensions/hash/specializations.pass.cpp b/libcxx/test/libcxx/extensions/hash/specializations.pass.cpp
--- a/libcxx/test/libcxx/extensions/hash/specializations.pass.cpp
+++ b/libcxx/test/libcxx/extensions/hash/specializations.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: modules-build
+
 // NOTE: Undefined __DEPRECATED to prevent this test from failing with -Werror
 #undef __DEPRECATED
 #include <assert.h>
diff --git a/libcxx/test/libcxx/memory/trivial_abi/shared_ptr_arg.pass.cpp b/libcxx/test/libcxx/memory/trivial_abi/shared_ptr_arg.pass.cpp
--- a/libcxx/test/libcxx/memory/trivial_abi/shared_ptr_arg.pass.cpp
+++ b/libcxx/test/libcxx/memory/trivial_abi/shared_ptr_arg.pass.cpp
@@ -10,7 +10,7 @@
 
 // Test shared_ptr<T> with trivial_abi as parameter type.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -D_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI
 
 // XFAIL: gcc
 // UNSUPPORTED: c++03
diff --git a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_arg.pass.cpp b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_arg.pass.cpp
--- a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_arg.pass.cpp
+++ b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_arg.pass.cpp
@@ -10,7 +10,7 @@
 
 // Test unique_ptr<T> with trivial_abi as parameter type.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
 
 // XFAIL: gcc
 
diff --git a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_array.pass.cpp b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_array.pass.cpp
--- a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_array.pass.cpp
+++ b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_array.pass.cpp
@@ -10,7 +10,7 @@
 
 // Test unique_ptr<T[]> with trivial_abi as parameter type.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
 
 // XFAIL: gcc
 // UNSUPPORTED: c++03
diff --git a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_destruction_order.pass.cpp b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_destruction_order.pass.cpp
--- a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_destruction_order.pass.cpp
+++ b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_destruction_order.pass.cpp
@@ -10,7 +10,7 @@
 
 // Test arguments destruction order involving unique_ptr<T> with trivial_abi.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
 
 // XFAIL: gcc
 
diff --git a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_ret.pass.cpp b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_ret.pass.cpp
--- a/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_ret.pass.cpp
+++ b/libcxx/test/libcxx/memory/trivial_abi/unique_ptr_ret.pass.cpp
@@ -10,7 +10,7 @@
 
 // Test unique_ptr<T> with trivial_abi as return-type.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -D_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
 
 // XFAIL: gcc
 
diff --git a/libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp b/libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp
--- a/libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp
+++ b/libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp
@@ -10,7 +10,7 @@
 
 // Test weak_ptr<T> with trivial_abi as return-type.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -D_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI
 
 // XFAIL: gcc
 
diff --git a/libcxx/test/libcxx/min_max_macros.compile.pass.cpp b/libcxx/test/libcxx/min_max_macros.compile.pass.cpp
--- a/libcxx/test/libcxx/min_max_macros.compile.pass.cpp
+++ b/libcxx/test/libcxx/min_max_macros.compile.pass.cpp
@@ -366,10 +366,12 @@
 #endif // __cplusplus >= 201103L
 
 // extended headers
-#include <ext/hash_map>
+#ifndef _LIBCXX_MODULES_BUILD
+#    include <ext/hash_map>
 TEST_MACROS();
-#include <ext/hash_set>
+#    include <ext/hash_set>
 TEST_MACROS();
+#endif
 
 // clang-format on
 
diff --git a/libcxx/test/libcxx/nasty_macros.compile.pass.cpp b/libcxx/test/libcxx/nasty_macros.compile.pass.cpp
--- a/libcxx/test/libcxx/nasty_macros.compile.pass.cpp
+++ b/libcxx/test/libcxx/nasty_macros.compile.pass.cpp
@@ -341,8 +341,10 @@
 #endif // __cplusplus >= 201103L
 
 // extended headers
-#include <ext/hash_map>
-#include <ext/hash_set>
+#ifndef _LIBCXX_MODULES_BUILD
+#    include <ext/hash_map>
+#    include <ext/hash_set>
+#endif
 
 // clang-format on
 
diff --git a/libcxx/test/libcxx/no_assert_include.compile.pass.cpp b/libcxx/test/libcxx/no_assert_include.compile.pass.cpp
--- a/libcxx/test/libcxx/no_assert_include.compile.pass.cpp
+++ b/libcxx/test/libcxx/no_assert_include.compile.pass.cpp
@@ -226,8 +226,10 @@
 #endif // __cplusplus >= 201103L
 
 // extended headers
-#include <ext/hash_map>
-#include <ext/hash_set>
+#ifndef _LIBCXX_MODULES_BUILD
+#    include <ext/hash_map>
+#    include <ext/hash_set>
+#endif
 
 // clang-format on
 
diff --git a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.geo/bad_engine.verify.cpp b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.geo/bad_engine.verify.cpp
--- a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.geo/bad_engine.verify.cpp
+++ b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.geo/bad_engine.verify.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
+// ADDITIONAL_COMPILE_FLAGS: -Wno-sign-compare -Wno-shift-count-negative
 
 // <random>
 
diff --git a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.negbin/bad_engine.verify.cpp b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.negbin/bad_engine.verify.cpp
--- a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.negbin/bad_engine.verify.cpp
+++ b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.negbin/bad_engine.verify.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
+// ADDITIONAL_COMPILE_FLAGS: -Wno-shift-count-negative
 
 // <random>
 
diff --git a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.norm/rand.dist.norm.f/bad_engine.verify.cpp b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.norm/rand.dist.norm.f/bad_engine.verify.cpp
--- a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.norm/rand.dist.norm.f/bad_engine.verify.cpp
+++ b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.norm/rand.dist.norm.f/bad_engine.verify.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
+// ADDITIONAL_COMPILE_FLAGS: -Wno-sign-compare
 
 // <random>
 
diff --git a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.pois/rand.dist.pois.poisson/bad_engine.verify.cpp b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.pois/rand.dist.pois.poisson/bad_engine.verify.cpp
--- a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.pois/rand.dist.pois.poisson/bad_engine.verify.cpp
+++ b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.pois/rand.dist.pois.poisson/bad_engine.verify.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
+// ADDITIONAL_COMPILE_FLAGS: -Wno-shift-count-negative
 
 // <random>
 
diff --git a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.uni/rand.dist.uni.int/bad_engine.verify.cpp b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.uni/rand.dist.uni.int/bad_engine.verify.cpp
--- a/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.uni/rand.dist.uni.int/bad_engine.verify.cpp
+++ b/libcxx/test/libcxx/numerics/rand/rand.dis/rand.dist.uni/rand.dist.uni.int/bad_engine.verify.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
+// ADDITIONAL_COMPILE_FLAGS: -Wno-sign-compare
 
 // <random>
 
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/generate_header_tests.py b/libcxx/utils/generate_header_tests.py
--- a/libcxx/utils/generate_header_tests.py
+++ b/libcxx/utils/generate_header_tests.py
@@ -93,7 +93,9 @@
 #endif // __cplusplus >= 201103L
 
 // extended headers
+#ifndef _LIBCXX_MODULES_BUILD
 {extended_headers}
+#endif
 """
 
 
@@ -166,7 +168,7 @@
 
 
 def produce_extended_headers(post_include=None, exclusions=None):
-    return produce_headers([include_path, 'ext'], 0, post_include=post_include, exclusions=exclusions)
+    return produce_headers([include_path, 'ext'], 1, post_include=post_include, exclusions=exclusions)
 
 
 def replace_generated_headers(test_path, test_str):
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
@@ -312,8 +312,8 @@
         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')
diff --git a/libcxx/utils/libcxx/test/params.py b/libcxx/utils/libcxx/test/params.py
--- a/libcxx/utils/libcxx/test/params.py
+++ b/libcxx/utils/libcxx/test/params.py
@@ -77,6 +77,7 @@
               AddFeature('modules-build'),
               AddCompileFlag('-fmodules'),
               AddCompileFlag('-fcxx-modules'), # AppleClang disregards -fmodules entirely when compiling C++. This enables modules for C++.
+              AddCompileFlag('-D_LIBCXX_MODULES_BUILD') # Needed to exclude headers that are not supported in a modules build
             ] if modules else []),
 
   Parameter(name='enable_exceptions', choices=[True, False], type=bool, default=True,
diff --git a/libcxxabi/test/configs/apple-libc++abi-backdeployment.cfg.in b/libcxxabi/test/configs/apple-libc++abi-backdeployment.cfg.in
--- a/libcxxabi/test/configs/apple-libc++abi-backdeployment.cfg.in
+++ b/libcxxabi/test/configs/apple-libc++abi-backdeployment.cfg.in
@@ -45,7 +45,7 @@
     '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
 ))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{install}/include/c++/v1 -DLIBCXXABI_NO_TIMER -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS ' +
+    '-nostdinc++ -I %{install}/include/c++/v1 -DLIBCXXABI_NO_TIMER -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS ' +
     '-I %{libcxx}/test/support -I %{libcxx}/src'
 ))
 config.substitutions.append(('%{link_flags}',
diff --git a/libcxxabi/test/configs/apple-libc++abi-shared.cfg.in b/libcxxabi/test/configs/apple-libc++abi-shared.cfg.in
--- a/libcxxabi/test/configs/apple-libc++abi-shared.cfg.in
+++ b/libcxxabi/test/configs/apple-libc++abi-shared.cfg.in
@@ -6,7 +6,7 @@
     '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
 ))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{install}/include/c++/v1 -DLIBCXXABI_NO_TIMER -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS ' +
+    '-nostdinc++ -I %{install}/include/c++/v1 -DLIBCXXABI_NO_TIMER -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS ' +
     '-I %{libcxx}/test/support -I %{libcxx}/src'
 ))
 config.substitutions.append(('%{link_flags}',
diff --git a/libcxxabi/test/configs/ibm-libc++abi-shared.cfg.in b/libcxxabi/test/configs/ibm-libc++abi-shared.cfg.in
--- a/libcxxabi/test/configs/ibm-libc++abi-shared.cfg.in
+++ b/libcxxabi/test/configs/ibm-libc++abi-shared.cfg.in
@@ -4,7 +4,7 @@
 
 config.substitutions.append(('%{flags}',''))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -isystem %{install}/include/c++/v1 ' + 
+    '-nostdinc++ -I %{install}/include/c++/v1 ' +
     '-D__LIBC_NO_CPP_MATH_OVERLOADS__ -DLIBCXXABI_NO_TIMER ' +
     '-D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS ' +
     '-I %{libcxx}/test/support -I %{libcxx}/src'