diff --git a/libcxx/docs/Contributing.rst b/libcxx/docs/Contributing.rst
index b812b958e6a0a9..9806f9b445c472 100644
--- a/libcxx/docs/Contributing.rst
+++ b/libcxx/docs/Contributing.rst
@@ -37,7 +37,7 @@ sure you don't forget anything:
 - Did you mark all functions and type declarations with the :ref:`proper visibility macro <visibility-macros>`?
 - If you added a header:
 
-  - Did you add it to ``include/module.modulemap``?
+  - Did you add it to ``include/module.modulemap.in``?
   - Did you add it to ``include/CMakeLists.txt``?
   - If it's a public header, did you add a test under ``test/libcxx`` that the new header defines ``_LIBCPP_VERSION``? See ``test/libcxx/algorithms/version.pass.cpp`` for an example. NOTE: This should be automated.
   - If it's a public header, did you update ``utils/generate_header_inclusion_tests.py``?
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 8f2a66f86f8cfd..3f89fee658cbbe 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -618,7 +618,6 @@ set(files
   map
   math.h
   memory
-  module.modulemap
   mutex
   new
   numbers
@@ -669,9 +668,17 @@ set(files
   wctype.h
   )
 
+foreach(feature LIBCXX_ENABLE_FILESYSTEM LIBCXX_ENABLE_LOCALIZATION LIBCXX_ENABLE_THREADS LIBCXX_ENABLE_WIDE_CHARACTERS)
+  if (NOT ${${feature}})
+    set(requires_${feature} "requires LIBCXX_CONFIGURED_WITHOUT_SUPPORT_FOR_THIS_HEADER")
+  endif()
+endforeach()
+
 configure_file("__config_site.in" "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site" @ONLY)
+configure_file("module.modulemap.in" "${LIBCXX_GENERATED_INCLUDE_DIR}/module.modulemap" @ONLY)
 
-set(_all_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site")
+set(_all_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site"
+                  "${LIBCXX_GENERATED_INCLUDE_DIR}/module.modulemap")
 foreach(f ${files})
   set(src "${CMAKE_CURRENT_SOURCE_DIR}/${f}")
   set(dst "${LIBCXX_GENERATED_INCLUDE_DIR}/${f}")
diff --git a/libcxx/include/csignal b/libcxx/include/csignal
index 81ca89091c1bcc..c1b58f818b6242 100644
--- a/libcxx/include/csignal
+++ b/libcxx/include/csignal
@@ -41,7 +41,10 @@ int raise(int sig);
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
-#include <signal.h>
+
+#if __has_include(<signal.h>)
+# include <signal.h>
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap.in
similarity index 97%
rename from libcxx/include/module.modulemap
rename to libcxx/include/module.modulemap.in
index 01946120462dcf..b89983bcfd9d1d 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap.in
@@ -38,6 +38,7 @@ module std [system] {
     // <iso646.h> provided by compiler.
     // <limits.h> provided by compiler or C library.
     module locale_h {
+      @requires_LIBCXX_ENABLE_LOCALIZATION@
       header "locale.h"
       export *
     }
@@ -50,6 +51,7 @@ module std [system] {
       export *
     }
     module stdatomic_h {
+      @requires_LIBCXX_ENABLE_THREADS@
       requires cplusplus23
       header "stdatomic.h"
       export *
@@ -90,11 +92,13 @@ module std [system] {
     }
     // <time.h> provided by C library.
     module wchar_h {
+      @requires_LIBCXX_ENABLE_WIDE_CHARACTERS@
       // <wchar.h>'s __need_* macros require textual inclusion.
       textual header "wchar.h"
       export *
     }
     module wctype_h {
+      @requires_LIBCXX_ENABLE_WIDE_CHARACTERS@
       header "wctype.h"
       export *
     }
@@ -155,6 +159,7 @@ module std [system] {
       export *
     }
     module clocale {
+      @requires_LIBCXX_ENABLE_LOCALIZATION@
       header "clocale"
       export *
     }
@@ -215,11 +220,13 @@ module std [system] {
       export *
     }
     module cwchar {
+      @requires_LIBCXX_ENABLE_WIDE_CHARACTERS@
       header "cwchar"
       export depr.stdio_h
       export *
     }
     module cwctype {
+      @requires_LIBCXX_ENABLE_WIDE_CHARACTERS@
       header "cwctype"
       export *
     }
@@ -382,6 +389,7 @@ module std [system] {
     export *
   }
   module barrier {
+    @requires_LIBCXX_ENABLE_THREADS@
     header "barrier"
     export *
   }
@@ -425,7 +433,11 @@ module std [system] {
       module duration              { private header "__chrono/duration.h" }
       module file_clock            { private header "__chrono/file_clock.h" }
       module hh_mm_ss              { private header "__chrono/hh_mm_ss.h" }
-      module high_resolution_clock { private header "__chrono/high_resolution_clock.h" }
+      module high_resolution_clock {
+        private header "__chrono/high_resolution_clock.h"
+        export steady_clock
+        export system_clock
+      }
       module literals              { private header "__chrono/literals.h" }
       module month                 { private header "__chrono/month.h" }
       module month_weekday         { private header "__chrono/month_weekday.h" }
@@ -441,6 +453,7 @@ module std [system] {
     }
   }
   module codecvt {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "codecvt"
     export *
   }
@@ -528,6 +541,7 @@ module std [system] {
     export *
   }
   module filesystem {
+    @requires_LIBCXX_ENABLE_FILESYSTEM@
     header "filesystem"
     export *
 
@@ -588,6 +602,7 @@ module std [system] {
     export *
   }
   module fstream {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "fstream"
     export *
   }
@@ -626,6 +641,7 @@ module std [system] {
     }
   }
   module future {
+    @requires_LIBCXX_ENABLE_THREADS@
     header "future"
     export *
   }
@@ -634,10 +650,12 @@ module std [system] {
     export *
   }
   module iomanip {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "iomanip"
     export *
   }
   module ios {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "ios"
     export iosfwd
     export *
@@ -651,6 +669,7 @@ module std [system] {
     export *
   }
   module iostream {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "iostream"
     export ios
     export streambuf
@@ -659,6 +678,7 @@ module std [system] {
     export *
   }
   module istream {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "istream"
     // FIXME: should re-export ios, streambuf?
     export *
@@ -711,6 +731,7 @@ module std [system] {
     }
   }
   module latch {
+    @requires_LIBCXX_ENABLE_THREADS@
     header "latch"
     export *
   }
@@ -724,6 +745,7 @@ module std [system] {
     export *
   }
   module locale {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "locale"
     export *
   }
@@ -761,6 +783,7 @@ module std [system] {
     }
   }
   module mutex {
+    @requires_LIBCXX_ENABLE_THREADS@
     header "mutex"
     export *
   }
@@ -797,6 +820,7 @@ module std [system] {
     export *
   }
   module ostream {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "ostream"
     // FIXME: should re-export ios, streambuf?
     export *
@@ -909,6 +933,7 @@ module std [system] {
     export *
   }
   module regex {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "regex"
     export initializer_list
     export *
@@ -918,6 +943,7 @@ module std [system] {
     export *
   }
   module semaphore {
+    @requires_LIBCXX_ENABLE_THREADS@
     header "semaphore"
     export *
   }
@@ -927,6 +953,7 @@ module std [system] {
     export *
   }
   module shared_mutex {
+    @requires_LIBCXX_ENABLE_THREADS@
     header "shared_mutex"
     export version
   }
@@ -937,6 +964,7 @@ module std [system] {
     module span_fwd { private header "__fwd/span.h" }
   }
   module sstream {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "sstream"
     // FIXME: should re-export istream, ostream, ios, streambuf, string?
     export *
@@ -951,6 +979,7 @@ module std [system] {
     export *
   }
   module streambuf {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "streambuf"
     export *
   }
@@ -969,6 +998,7 @@ module std [system] {
     module string_view_fwd { private header "__fwd/string_view.h" }
   }
   module strstream {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
     header "strstream"
     export *
   }
@@ -977,6 +1007,7 @@ module std [system] {
     export *
   }
   module thread {
+    @requires_LIBCXX_ENABLE_THREADS@
     header "thread"
     export *
 
@@ -1118,7 +1149,6 @@ module std [system] {
     export *
   }
 
-  // __config not modularised due to a bug in Clang
   // FIXME: These should be private.
   module __assert            {         header "__assert"            export * }
   module __availability      { private header "__availability"      export * }
@@ -1127,12 +1157,18 @@ module std [system] {
   module __debug             {         header "__debug"             export * }
   module __errc              { private header "__errc"              export * }
   module __hash_table        {         header "__hash_table"        export * }
-  module __locale            { private header "__locale"            export * }
+  module __locale            {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
+    private header "__locale" export *
+  }
   module __mbstate_t         { private header "__mbstate_t.h"       export * }
   module __mutex_base        { private header "__mutex_base"        export * }
   module __node_handle       { private header "__node_handle"       export * }
   module __split_buffer      { private header "__split_buffer"      export * }
-  module __std_stream        { private header "__std_stream"        export * }
+  module __std_stream        {
+    @requires_LIBCXX_ENABLE_LOCALIZATION@
+    private header "__std_stream" export *
+  }
   module __string            { private header "__string"            export * }
   module __threading_support {         header "__threading_support" export * }
   module __tree              {         header "__tree"              export * }
@@ -1184,6 +1220,7 @@ module std [system] {
       export *
     }
     module regex {
+      @requires_LIBCXX_ENABLE_LOCALIZATION@
       header "experimental/regex"
       export *
     }
diff --git a/libcxx/test/libcxx/lint/lint_headers.sh.py b/libcxx/test/libcxx/lint/lint_headers.sh.py
index 9075044dc7df92..9113b1aecf8ad3 100644
--- a/libcxx/test/libcxx/lint/lint_headers.sh.py
+++ b/libcxx/test/libcxx/lint/lint_headers.sh.py
@@ -11,7 +11,7 @@
 def exclude_from_consideration(path):
     return (
         path.endswith('.txt') or
-        path.endswith('.modulemap') or
+        path.endswith('.modulemap.in') or
         os.path.basename(path) == '__config' or
         os.path.basename(path) == '__config_site.in' or
         not os.path.isfile(path)
diff --git a/libcxx/test/libcxx/lint/lint_modulemap.sh.py b/libcxx/test/libcxx/lint/lint_modulemap.sh.py
index e167813cfc099a..bbb5885273b791 100755
--- a/libcxx/test/libcxx/lint/lint_modulemap.sh.py
+++ b/libcxx/test/libcxx/lint/lint_modulemap.sh.py
@@ -1,6 +1,6 @@
 # RUN: %{python} %s
 
-# Verify that each list of private submodules in libcxx/include/module.modulemap
+# Verify that each list of private submodules in libcxx/include/module.modulemap.in
 # is maintained in alphabetical order.
 
 import os
@@ -10,7 +10,7 @@
 if __name__ == '__main__':
     libcxx_test_libcxx_lint = os.path.dirname(os.path.abspath(__file__))
     libcxx = os.path.abspath(os.path.join(libcxx_test_libcxx_lint, '../../..'))
-    modulemap_name = os.path.join(libcxx, 'include/module.modulemap')
+    modulemap_name = os.path.join(libcxx, 'include/module.modulemap.in')
     assert os.path.isfile(modulemap_name)
 
     okay = True
@@ -31,12 +31,12 @@
                     pass
                 else:
                     okay = False
-                    print("LINE DOESN'T MATCH REGEX in libcxx/include/module.modulemap!")
+                    print("LINE DOESN'T MATCH REGEX in libcxx/include/module.modulemap.in!")
                     print(line)
                 # Check that these lines are alphabetized.
                 if (prevline is not None) and (line < prevline):
                     okay = False
-                    print('LINES OUT OF ORDER in libcxx/include/module.modulemap!')
+                    print('LINES OUT OF ORDER in libcxx/include/module.modulemap.in!')
                     print(prevline)
                     print(line)
                 prevline = line
diff --git a/libcxx/test/libcxx/modules_include.sh.cpp b/libcxx/test/libcxx/modules_include.sh.cpp
index 4dd8a8f73e87c1..4ab12592ff1da0 100644
--- a/libcxx/test/libcxx/modules_include.sh.cpp
+++ b/libcxx/test/libcxx/modules_include.sh.cpp
@@ -19,10 +19,6 @@
 // The Windows headers don't appear to be compatible with modules
 // UNSUPPORTED: windows
 
-// TODO: Some headers produce errors when we include them and the library has been
-//       configured without support for them, which breaks the modules build.
-// UNSUPPORTED: no-localization, no-filesystem, no-threads, no-wide-characters
-
 // Prevent <ext/hash_map> from generating deprecated warnings for this test.
 #if defined(__DEPRECATED)
 #    undef __DEPRECATED
diff --git a/libcxx/utils/generate_header_tests.py b/libcxx/utils/generate_header_tests.py
index fa9ab3cd7dcaae..59b90489f16b13 100755
--- a/libcxx/utils/generate_header_tests.py
+++ b/libcxx/utils/generate_header_tests.py
@@ -113,7 +113,7 @@ def produce(test_file, variables):
 
 def is_header(file):
     """Returns whether the given file is a header (i.e. not a directory or the modulemap file)."""
-    return not file.is_dir() and not file.name == 'module.modulemap'
+    return not file.is_dir() and not file.name == 'module.modulemap.in'
 
 def main():
     monorepo_root = pathlib.Path(os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__)))))
diff --git a/libcxx/utils/libcxx/test/dsl.py b/libcxx/utils/libcxx/test/dsl.py
index f43937a9664d11..e77ea8fa600489 100644
--- a/libcxx/utils/libcxx/test/dsl.py
+++ b/libcxx/utils/libcxx/test/dsl.py
@@ -276,9 +276,11 @@ def compilerMacros(config, flags=''):
   """
   with _makeConfigTest(config) as test:
     with open(test.getSourcePath(), 'w') as sourceFile:
-      # Make sure files like <__config> are included, since they can define
-      # additional macros.
-      sourceFile.write("#include <stddef.h>")
+      sourceFile.write("""
+      #if __has_include(<__config_site>)
+      #  include <__config_site>
+      #endif
+      """)
     unparsedOutput, err, exitCode, _ = _executeScriptInternal(test, [
       "%{{cxx}} %s -dM -E %{{flags}} %{{compile_flags}} {}".format(flags)
     ])
diff --git a/libcxx/utils/libcxx/test/features.py b/libcxx/utils/libcxx/test/features.py
index d7e4c849a0fc75..d805ae6e7ac388 100644
--- a/libcxx/utils/libcxx/test/features.py
+++ b/libcxx/utils/libcxx/test/features.py
@@ -82,7 +82,7 @@ def _hasSuitableClangTidy(cfg):
   # Check for a Windows UCRT bug (fixed in UCRT/Windows 10.0.20348.0):
   # https://developercommunity.visualstudio.com/t/utf-8-locales-break-ctype-functions-for-wchar-type/1653678
   Feature(name='win32-broken-utf8-wchar-ctype',
-          when=lambda cfg: '_WIN32' in compilerMacros(cfg) and not programSucceeds(cfg, """
+          when=lambda cfg: not '_LIBCPP_HAS_NO_LOCALIZATION' in compilerMacros(cfg) and '_WIN32' in compilerMacros(cfg) and not programSucceeds(cfg, """
             #include <locale.h>
             #include <wctype.h>
             int main(int, char**) {
@@ -94,7 +94,7 @@ def _hasSuitableClangTidy(cfg):
   # Check for a Windows UCRT bug (fixed in UCRT/Windows 10.0.19041.0).
   # https://developercommunity.visualstudio.com/t/printf-formatting-with-g-outputs-too/1660837
   Feature(name='win32-broken-printf-g-precision',
-          when=lambda cfg: '_WIN32' in compilerMacros(cfg) and not programSucceeds(cfg, """
+          when=lambda cfg: not '_LIBCPP_HAS_NO_LOCALIZATION' in compilerMacros(cfg) and '_WIN32' in compilerMacros(cfg) and not programSucceeds(cfg, """
             #include <stdio.h>
             #include <string.h>
             int main(int, char**) {