diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -133,7 +133,7 @@
     "Whether to enable support for incomplete library features. Incomplete features
     are new library features under development. These features don't guarantee
     ABI stability nor the quality of completed library features. Vendors
-    shipping the library may want to disable this option." ON)
+    shipping the library may want to disable this option." OFF)
 set(LIBCXX_TEST_CONFIG "legacy.cfg.in" CACHE STRING
     "The path to the Lit testing configuration to use when running the tests.
      If a relative path is provided, it is assumed to be relative to '<monorepo>/libcxx/test/configs'.")
diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -166,6 +166,11 @@
 # Register Benchmark tests
 #==============================================================================
 file(GLOB BENCHMARK_TESTS "*.bench.cpp")
+
+if (NOT LIBCXX_ENABLE_INCOMPLETE_FEATURES)
+  list(FILTER BENCHMARK_TESTS EXCLUDE REGEX "(format_to_n|format_to|format|formatted_size|formatter_float|std_format_spec_string_unicode).bench.cpp")
+endif()
+
 foreach(test_path ${BENCHMARK_TESTS})
   get_filename_component(test_file "${test_path}" NAME)
   string(REPLACE ".bench.cpp" "" test_name "${test_file}")
diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -43,8 +43,9 @@
   functions are known to be inefficient, both in memory usage and performance.
   The implementation isn't API- or ABI-stable and therefore considered
   experimental. (Some not-yet-implemented papers require an API-break.)
-  Vendors can still disable this header by turning the CMake option
-  `LIBCXX_ENABLE_INCOMPLETE_FEATURES` off.
+  As a result, it is disabled by default, however vendors can enable the
+  header by using ``-DLIBCXX_ENABLE_INCOMPLETE_FEATURES=ON`` when
+  configuring their build.
 
 - There's a new CMake option ``LIBCXX_ENABLE_UNICODE`` to disable Unicode
   support in the ``<format>`` header. This only affects the estimation of the
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -2,7 +2,8 @@
 // since __config may be included from C headers which may create an
 // include cycle.
 module std_config [system] [extern_c] {
-    header "__config"
+  textual header "__config"
+  textual header "__config_site"
 }
 
 module std [system] {
diff --git a/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist b/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
--- a/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
+++ b/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
@@ -1039,9 +1039,6 @@
 {'is_defined': True, 'name': '__ZNSt3__112ctype_bynameIwED0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112ctype_bynameIwED1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112ctype_bynameIwED2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112format_errorD0Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112format_errorD1Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112format_errorD2Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112future_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112future_errorC2ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112future_errorD0Ev', 'type': 'FUNC'}
@@ -1991,7 +1988,6 @@
 {'is_defined': True, 'name': '__ZTINSt3__112bad_weak_ptrE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112ctype_bynameIcEE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112ctype_bynameIwEE', 'size': 0, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '__ZTINSt3__112format_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112future_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112strstreambufE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112system_errorE', 'size': 0, 'type': 'OBJECT'}
@@ -2186,7 +2182,6 @@
 {'is_defined': True, 'name': '__ZTSNSt3__112bad_weak_ptrE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112ctype_bynameIcEE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112ctype_bynameIwEE', 'size': 0, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '__ZTSNSt3__112format_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112future_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112strstreambufE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112system_errorE', 'size': 0, 'type': 'OBJECT'}
@@ -2366,7 +2361,6 @@
 {'is_defined': True, 'name': '__ZTVNSt3__112bad_weak_ptrE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112ctype_bynameIcEE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112ctype_bynameIwEE', 'size': 0, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '__ZTVNSt3__112format_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112future_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112strstreambufE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112system_errorE', 'size': 0, 'type': 'OBJECT'}
diff --git a/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist b/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
--- a/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
+++ b/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
@@ -1039,9 +1039,6 @@
 {'is_defined': True, 'name': '__ZNSt3__112ctype_bynameIwED0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112ctype_bynameIwED1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112ctype_bynameIwED2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112format_errorD0Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112format_errorD1Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112format_errorD2Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112future_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112future_errorC2ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112future_errorD0Ev', 'type': 'FUNC'}
@@ -2000,7 +1997,6 @@
 {'is_defined': True, 'name': '__ZTINSt3__112codecvt_baseE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112ctype_bynameIcEE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112ctype_bynameIwEE', 'size': 0, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '__ZTINSt3__112format_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112future_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112strstreambufE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTINSt3__112system_errorE', 'size': 0, 'type': 'OBJECT'}
@@ -2210,7 +2206,6 @@
 {'is_defined': True, 'name': '__ZTSNSt3__112codecvt_baseE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112ctype_bynameIcEE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112ctype_bynameIwEE', 'size': 0, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '__ZTSNSt3__112format_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112future_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112strstreambufE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTSNSt3__112system_errorE', 'size': 0, 'type': 'OBJECT'}
@@ -2396,7 +2391,6 @@
 {'is_defined': True, 'name': '__ZTVNSt3__112bad_weak_ptrE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112ctype_bynameIcEE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112ctype_bynameIwEE', 'size': 0, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '__ZTVNSt3__112format_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112future_errorE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112strstreambufE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZTVNSt3__112system_errorE', 'size': 0, 'type': 'OBJECT'}
diff --git a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
--- a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
+++ b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
@@ -730,9 +730,6 @@
 {'is_defined': True, 'name': '_ZNSt3__112ctype_bynameIwED0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112ctype_bynameIwED1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112ctype_bynameIwED2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '_ZNSt3__112format_errorD0Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '_ZNSt3__112format_errorD1Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '_ZNSt3__112format_errorD2Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112future_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112future_errorC2ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112future_errorD0Ev', 'type': 'FUNC'}
@@ -1667,7 +1664,6 @@
 {'is_defined': True, 'name': '_ZTINSt3__112codecvt_baseE', 'size': 16, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTINSt3__112ctype_bynameIcEE', 'size': 24, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTINSt3__112ctype_bynameIwEE', 'size': 24, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '_ZTINSt3__112format_errorE', 'size': 24, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTINSt3__112future_errorE', 'size': 24, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTINSt3__112strstreambufE', 'size': 24, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTINSt3__112system_errorE', 'size': 24, 'type': 'OBJECT'}
@@ -1797,7 +1793,6 @@
 {'is_defined': True, 'name': '_ZTSNSt3__112codecvt_baseE', 'size': 23, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTSNSt3__112ctype_bynameIcEE', 'size': 26, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTSNSt3__112ctype_bynameIwEE', 'size': 26, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '_ZTSNSt3__112format_errorE', 'size': 23, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTSNSt3__112future_errorE', 'size': 23, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTSNSt3__112strstreambufE', 'size': 23, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTSNSt3__112system_errorE', 'size': 23, 'type': 'OBJECT'}
@@ -1931,7 +1926,6 @@
 {'is_defined': True, 'name': '_ZTVNSt3__112bad_weak_ptrE', 'size': 40, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTVNSt3__112ctype_bynameIcEE', 'size': 104, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTVNSt3__112ctype_bynameIwEE', 'size': 136, 'type': 'OBJECT'}
-{'is_defined': True, 'name': '_ZTVNSt3__112format_errorE', 'size': 40, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTVNSt3__112future_errorE', 'size': 40, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTVNSt3__112strstreambufE', 'size': 128, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZTVNSt3__112system_errorE', 'size': 40, 'type': 'OBJECT'}
diff --git a/libcxxabi/test/libcxxabi/test/config.py b/libcxxabi/test/libcxxabi/test/config.py
--- a/libcxxabi/test/libcxxabi/test/config.py
+++ b/libcxxabi/test/libcxxabi/test/config.py
@@ -70,7 +70,7 @@
             cxx_target_headers = os.path.join(path, triple, cxx, version)
             if os.path.isdir(cxx_target_headers):
                 self.cxx.compile_flags += ['-I' + cxx_target_headers]
-        self.cxx.compile_flags += ['-I' + cxx_headers]
+        self.cxx.compile_flags += ['-isystem' + cxx_headers]
         self.cxx.compile_flags += ['-I' + os.path.join(self.libcxx_src_root, 'src')]
 
         libcxxabi_headers = self.get_lit_conf(