diff --git a/README.chromium b/README.chromium
index 9723fe1..2f3624f 100644
--- a/README.chromium
+++ b/README.chromium
@@ -4,7 +4,7 @@
 License: Apache 2.0
 License File: LICENSE
 Version: 0
-Revision: 3204cc0625230e9876f0310a6dea0014210ab325
+Revision: 6f43f5bb398b6685575b36874e36cf1695734df1
 Security Critical: yes
 
 Description:
diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 85a9488..b37c21d 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -151,7 +151,9 @@
         "internal/direct_mmap.h",
         "internal/low_level_alloc.h",
     ],
-    copts = ABSL_DEFAULT_COPTS,
+    copts = ABSL_DEFAULT_COPTS + select({
+        "//conditions:default": [],
+    }),
     linkopts = select({
         "//absl:msvc_compiler": [],
         "//absl:clang-cl_compiler": [],
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index c7233cb..7e600f0 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -16,6 +16,7 @@
 
 find_library(LIBRT rt)
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     atomic_hook
@@ -28,6 +29,7 @@
     ${ABSL_DEFAULT_COPTS}
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     errno_saver
@@ -52,6 +54,7 @@
     ${ABSL_DEFAULT_COPTS}
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     raw_logging_internal
@@ -68,6 +71,7 @@
     ${ABSL_DEFAULT_COPTS}
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     spinlock_wait
@@ -131,6 +135,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     malloc_internal
@@ -151,6 +156,7 @@
     Threads::Threads
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     base_internal
@@ -207,6 +213,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     throw_delegate
@@ -221,6 +228,7 @@
     absl::raw_logging_internal
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     exception_testing
@@ -234,6 +242,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     pretty_function
@@ -243,6 +252,7 @@
     ${ABSL_DEFAULT_COPTS}
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     exception_safety_testing
@@ -276,6 +286,7 @@
     GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     atomic_hook_test_helper
@@ -375,6 +386,7 @@
     GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     spinlock_test_common
@@ -409,6 +421,7 @@
     GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     endian
@@ -519,6 +532,7 @@
     GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     scoped_set_env
@@ -570,6 +584,7 @@
     GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     strerror
@@ -601,6 +616,7 @@
     GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     fast_type_id
diff --git a/absl/cleanup/CMakeLists.txt b/absl/cleanup/CMakeLists.txt
index 26a6d0d..f5af40b 100644
--- a/absl/cleanup/CMakeLists.txt
+++ b/absl/cleanup/CMakeLists.txt
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cleanup_internal
diff --git a/absl/container/BUILD.bazel b/absl/container/BUILD.bazel
index 2095e57..d733cb2 100644
--- a/absl/container/BUILD.bazel
+++ b/absl/container/BUILD.bazel
@@ -217,11 +217,6 @@
     ],
 )
 
-NOTEST_TAGS_NONMOBILE = [
-    "no_test_darwin_x86_64",
-    "no_test_loonix",
-]
-
 NOTEST_TAGS_MOBILE = [
     "no_test_android_arm",
     "no_test_android_arm64",
@@ -229,8 +224,6 @@
     "no_test_ios_x86_64",
 ]
 
-NOTEST_TAGS = NOTEST_TAGS_MOBILE + NOTEST_TAGS_NONMOBILE
-
 cc_library(
     name = "flat_hash_map",
     hdrs = ["flat_hash_map.h"],
@@ -251,7 +244,7 @@
     srcs = ["flat_hash_map_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":flat_hash_map",
         ":hash_generator_testing",
@@ -285,7 +278,7 @@
     srcs = ["flat_hash_set_test.cc"],
     copts = ABSL_TEST_COPTS + ["-DUNORDERED_SET_CXX17"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":flat_hash_set",
         ":hash_generator_testing",
@@ -321,7 +314,7 @@
     srcs = ["node_hash_map_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":hash_generator_testing",
         ":node_hash_map",
@@ -354,7 +347,7 @@
     srcs = ["node_hash_set_test.cc"],
     copts = ABSL_TEST_COPTS + ["-DUNORDERED_SET_CXX17"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":node_hash_set",
         ":unordered_set_constructor_test",
@@ -383,7 +376,7 @@
     srcs = ["internal/container_memory_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":container_memory",
         ":test_instance_tracker",
@@ -410,7 +403,7 @@
     srcs = ["internal/hash_function_defaults_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS,
+    tags = NOTEST_TAGS_MOBILE + ["no_test_loonix"],
     deps = [
         ":hash_function_defaults",
         "//absl/hash",
@@ -608,7 +601,7 @@
     srcs = ["internal/raw_hash_set_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkstatic = 1,
-    tags = NOTEST_TAGS,
+    tags = NOTEST_TAGS_MOBILE + ["no_test_loonix"],
     deps = [
         ":container_memory",
         ":hash_function_defaults",
@@ -698,7 +691,7 @@
     srcs = ["internal/layout_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS,
+    tags = NOTEST_TAGS_MOBILE + ["no_test_loonix"],
     visibility = ["//visibility:private"],
     deps = [
         ":layout",
@@ -845,7 +838,7 @@
     srcs = ["internal/unordered_set_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":unordered_set_constructor_test",
         ":unordered_set_lookup_test",
@@ -860,7 +853,7 @@
     srcs = ["internal/unordered_map_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":unordered_map_constructor_test",
         ":unordered_map_lookup_test",
@@ -875,7 +868,7 @@
     srcs = ["sample_element_size_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     visibility = ["//visibility:private"],
     deps = [
         ":flat_hash_map",
diff --git a/absl/container/CMakeLists.txt b/absl/container/CMakeLists.txt
index 7176907..aad69fa 100644
--- a/absl/container/CMakeLists.txt
+++ b/absl/container/CMakeLists.txt
@@ -42,6 +42,7 @@
     absl::utility
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     btree_test_common
@@ -84,6 +85,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     compressed_tuple
@@ -162,6 +164,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     inlined_vector_internal
@@ -194,6 +197,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     counting_allocator
@@ -240,6 +244,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     test_instance_tracker
@@ -411,6 +416,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     container_memory
@@ -440,6 +446,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_function_defaults
@@ -472,6 +479,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_generator_testing
@@ -489,6 +497,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_policy_testing
@@ -514,6 +523,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_policy_traits
@@ -538,6 +548,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hashtablez_sampler
@@ -569,6 +580,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hashtable_debug
@@ -580,6 +592,7 @@
     absl::hashtable_debug_hooks
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hashtable_debug_hooks
@@ -592,6 +605,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     node_slot_policy
@@ -617,6 +631,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     raw_hash_map
@@ -631,6 +646,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     container_common
@@ -642,6 +658,7 @@
     absl::type_traits
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     raw_hash_set
@@ -704,6 +721,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     layout
@@ -737,6 +755,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     tracked
@@ -749,6 +768,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_constructor_test
@@ -763,6 +783,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_lookup_test
@@ -777,6 +798,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_members_test
@@ -790,6 +812,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_modifiers_test
@@ -804,6 +827,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_constructor_test
@@ -818,6 +842,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_lookup_test
@@ -832,6 +857,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_members_test
@@ -845,6 +871,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_modifiers_test
diff --git a/absl/debugging/CMakeLists.txt b/absl/debugging/CMakeLists.txt
index b16fa00..5850bdd 100644
--- a/absl/debugging/CMakeLists.txt
+++ b/absl/debugging/CMakeLists.txt
@@ -93,6 +93,7 @@
     GTest::gmock
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     examine_stack
@@ -147,6 +148,7 @@
     GTest::gmock
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     debugging_internal
@@ -168,6 +170,7 @@
     absl::raw_logging_internal
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     demangle_internal
@@ -298,6 +301,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     stack_consumption
diff --git a/absl/debugging/internal/vdso_support.cc b/absl/debugging/internal/vdso_support.cc
index c655cf4..e63ac4a 100644
--- a/absl/debugging/internal/vdso_support.cc
+++ b/absl/debugging/internal/vdso_support.cc
@@ -33,7 +33,7 @@
 #endif
 #include <unistd.h>
 
-#if defined(__GLIBC__) && \
+#if !defined(__UCLIBC__) && defined(__GLIBC__) && \
     (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 16))
 #define ABSL_HAVE_GETAUXVAL
 #endif
diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index 020b791..91c6e98 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel
@@ -100,6 +100,7 @@
         "//absl/base:log_severity",
         "//absl/strings",
         "//absl/strings:str_format",
+        "//absl/types:optional",
     ],
 )
 
@@ -113,6 +114,9 @@
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//visibility:private",
+    ],
     deps = [
         "//absl/base:config",
         "//absl/base:fast_type_id",
diff --git a/absl/flags/BUILD.gn b/absl/flags/BUILD.gn
index 7cf60e8..f98c71c 100644
--- a/absl/flags/BUILD.gn
+++ b/absl/flags/BUILD.gn
@@ -59,10 +59,12 @@
     "//third_party/abseil-cpp/absl/base:log_severity",
     "//third_party/abseil-cpp/absl/strings",
     "//third_party/abseil-cpp/absl/strings:str_format",
+    "//third_party/abseil-cpp/absl/types:optional",
   ]
 }
 
 absl_source_set("commandlineflag_internal") {
+  visibility = [":*"]
   public = [ "internal/commandlineflag.h" ]
   sources = [ "internal/commandlineflag.cc" ]
   deps = [
diff --git a/absl/flags/CMakeLists.txt b/absl/flags/CMakeLists.txt
index 29c85ad..79e51a1 100644
--- a/absl/flags/CMakeLists.txt
+++ b/absl/flags/CMakeLists.txt
@@ -87,6 +87,7 @@
     absl::config
     absl::core_headers
     absl::log_severity
+    absl::optional
     absl::strings
     absl::str_format
 )
diff --git a/absl/flags/flag_test.cc b/absl/flags/flag_test.cc
index ced332d..05ec79e 100644
--- a/absl/flags/flag_test.cc
+++ b/absl/flags/flag_test.cc
@@ -990,3 +990,177 @@
   absl::SetFlag(&FLAGS_prefix_test_macro_named_flag, 1);
   EXPECT_EQ(absl::GetFlag(FLAGS_prefix_test_macro_named_flag), 1);
 }
+
+// --------------------------------------------------------------------
+
+#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ <= 5
+#define ABSL_SKIP_OPTIONAL_BOOL_TEST_DUE_TO_GCC_BUG
+#endif
+
+#ifndef ABSL_SKIP_OPTIONAL_BOOL_TEST_DUE_TO_GCC_BUG
+ABSL_FLAG(absl::optional<bool>, optional_bool, absl::nullopt, "help");
+#endif
+ABSL_FLAG(absl::optional<int>, optional_int, {}, "help");
+ABSL_FLAG(absl::optional<double>, optional_double, 9.3, "help");
+ABSL_FLAG(absl::optional<std::string>, optional_string, absl::nullopt, "help");
+ABSL_FLAG(absl::optional<absl::Duration>, optional_duration, absl::nullopt,
+          "help");
+ABSL_FLAG(absl::optional<absl::optional<int>>, optional_optional_int,
+          absl::nullopt, "help");
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+ABSL_FLAG(std::optional<int64_t>, std_optional_int64, std::nullopt, "help");
+#endif
+
+namespace {
+
+#ifndef ABSL_SKIP_OPTIONAL_BOOL_TEST_DUE_TO_GCC_BUG
+TEST_F(FlagTest, TestOptionalBool) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), absl::nullopt);
+
+  absl::SetFlag(&FLAGS_optional_bool, false);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), false);
+
+  absl::SetFlag(&FLAGS_optional_bool, true);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), true);
+
+  absl::SetFlag(&FLAGS_optional_bool, absl::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), absl::nullopt);
+}
+
+// --------------------------------------------------------------------
+#endif
+
+TEST_F(FlagTest, TestOptionalInt) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), absl::nullopt);
+
+  absl::SetFlag(&FLAGS_optional_int, 0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), 0);
+
+  absl::SetFlag(&FLAGS_optional_int, 10);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), 10);
+
+  absl::SetFlag(&FLAGS_optional_int, absl::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), absl::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalDouble) {
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_DOUBLE_EQ(*absl::GetFlag(FLAGS_optional_double), 9.3);
+
+  absl::SetFlag(&FLAGS_optional_double, 0.0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_double), 0.0);
+
+  absl::SetFlag(&FLAGS_optional_double, 1.234);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_DOUBLE_EQ(*absl::GetFlag(FLAGS_optional_double), 1.234);
+
+  absl::SetFlag(&FLAGS_optional_double, absl::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_double), absl::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalString) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), absl::nullopt);
+
+  // Setting optional string to "" leads to undefined behavior.
+
+  absl::SetFlag(&FLAGS_optional_string, " ");
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), " ");
+
+  absl::SetFlag(&FLAGS_optional_string, "QWERTY");
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), "QWERTY");
+
+  absl::SetFlag(&FLAGS_optional_string, absl::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), absl::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalDuration) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), absl::nullopt);
+
+  absl::SetFlag(&FLAGS_optional_duration, absl::ZeroDuration());
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), absl::Seconds(0));
+
+  absl::SetFlag(&FLAGS_optional_duration, absl::Hours(3));
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), absl::Hours(3));
+
+  absl::SetFlag(&FLAGS_optional_duration, absl::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), absl::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalOptional) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), absl::nullopt);
+
+  absl::optional<int> nullint{absl::nullopt};
+
+  absl::SetFlag(&FLAGS_optional_optional_int, nullint);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_NE(absl::GetFlag(FLAGS_optional_optional_int), nullint);
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int),
+            absl::optional<absl::optional<int>>{nullint});
+
+  absl::SetFlag(&FLAGS_optional_optional_int, 0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), 0);
+
+  absl::SetFlag(&FLAGS_optional_optional_int, absl::optional<int>{0});
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), 0);
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), absl::optional<int>{0});
+
+  absl::SetFlag(&FLAGS_optional_optional_int, absl::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), absl::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+
+TEST_F(FlagTest, TestStdOptional) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), std::nullopt);
+
+  absl::SetFlag(&FLAGS_std_optional_int64, 0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), 0);
+
+  absl::SetFlag(&FLAGS_std_optional_int64, 0xFFFFFFFFFF16);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), 0xFFFFFFFFFF16);
+
+  absl::SetFlag(&FLAGS_std_optional_int64, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), std::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+#endif
+
+}  // namespace
diff --git a/absl/flags/marshalling.h b/absl/flags/marshalling.h
index 7cbc136..0f63cdc 100644
--- a/absl/flags/marshalling.h
+++ b/absl/flags/marshalling.h
@@ -162,14 +162,27 @@
 #ifndef ABSL_FLAGS_MARSHALLING_H_
 #define ABSL_FLAGS_MARSHALLING_H_
 
+#include "absl/base/config.h"
+
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+#include <optional>
+#endif
 #include <string>
 #include <vector>
 
-#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
+
+// Forward declaration to be used inside composable flag parse/unparse
+// implementations
+template <typename T>
+inline bool ParseFlag(absl::string_view input, T* dst, std::string* error);
+template <typename T>
+inline std::string UnparseFlag(const T& v);
+
 namespace flags_internal {
 
 // Overloads of `AbslParseFlag()` and `AbslUnparseFlag()` for fundamental types.
@@ -189,6 +202,36 @@
 bool AbslParseFlag(absl::string_view, std::vector<std::string>*, std::string*);
 
 template <typename T>
+bool AbslParseFlag(absl::string_view text, absl::optional<T>* f,
+                   std::string* err) {
+  if (text.empty()) {
+    *f = absl::nullopt;
+    return true;
+  }
+  T value;
+  if (!absl::ParseFlag(text, &value, err)) return false;
+
+  *f = std::move(value);
+  return true;
+}
+
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+template <typename T>
+bool AbslParseFlag(absl::string_view text, std::optional<T>* f,
+                   std::string* err) {
+  if (text.empty()) {
+    *f = std::nullopt;
+    return true;
+  }
+  T value;
+  if (!absl::ParseFlag(text, &value, err)) return false;
+
+  *f = std::move(value);
+  return true;
+}
+#endif
+
+template <typename T>
 bool InvokeParseFlag(absl::string_view input, T* dst, std::string* err) {
   // Comment on next line provides a good compiler error message if T
   // does not have AbslParseFlag(absl::string_view, T*, std::string*).
@@ -202,6 +245,18 @@
 std::string AbslUnparseFlag(const std::vector<std::string>&);
 
 template <typename T>
+std::string AbslUnparseFlag(const absl::optional<T>& f) {
+  return f.has_value() ? absl::UnparseFlag(*f) : "";
+}
+
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+template <typename T>
+std::string AbslUnparseFlag(const std::optional<T>& f) {
+  return f.has_value() ? absl::UnparseFlag(*f) : "";
+}
+#endif
+
+template <typename T>
 std::string Unparse(const T& v) {
   // Comment on next line provides a good compiler error message if T does not
   // have UnparseFlag.
diff --git a/absl/flags/marshalling_test.cc b/absl/flags/marshalling_test.cc
index 4a64ce1..691cd2f 100644
--- a/absl/flags/marshalling_test.cc
+++ b/absl/flags/marshalling_test.cc
@@ -659,6 +659,88 @@
 
 // --------------------------------------------------------------------
 
+TEST(MarshallingTest, TestOptionalBoolParsing) {
+  std::string err;
+  absl::optional<bool> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag("true", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_TRUE(*value);
+
+  EXPECT_TRUE(absl::ParseFlag("false", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_FALSE(*value);
+
+  EXPECT_FALSE(absl::ParseFlag("nullopt", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalIntParsing) {
+  std::string err;
+  absl::optional<int> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag("10", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, 10);
+
+  EXPECT_TRUE(absl::ParseFlag("0x1F", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, 31);
+
+  EXPECT_FALSE(absl::ParseFlag("nullopt", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalDoubleParsing) {
+  std::string err;
+  absl::optional<double> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag("1.11", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, 1.11);
+
+  EXPECT_TRUE(absl::ParseFlag("-0.12", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, -0.12);
+
+  EXPECT_FALSE(absl::ParseFlag("nullopt", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalStringParsing) {
+  std::string err;
+  absl::optional<std::string> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag(" ", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, " ");
+
+  EXPECT_TRUE(absl::ParseFlag("aqswde", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, "aqswde");
+
+  EXPECT_TRUE(absl::ParseFlag("nullopt", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, "nullopt");
+}
+
+// --------------------------------------------------------------------
+
 TEST(MarshallingTest, TestBoolUnparsing) {
   EXPECT_EQ(absl::UnparseFlag(true), "true");
   EXPECT_EQ(absl::UnparseFlag(false), "false");
@@ -808,6 +890,86 @@
 
 // --------------------------------------------------------------------
 
+TEST(MarshallingTest, TestOptionalBoolUnparsing) {
+  absl::optional<bool> value;
+
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = true;
+  EXPECT_EQ(absl::UnparseFlag(value), "true");
+  value = false;
+  EXPECT_EQ(absl::UnparseFlag(value), "false");
+  value = absl::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalIntUnparsing) {
+  absl::optional<int> value;
+
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = 0;
+  EXPECT_EQ(absl::UnparseFlag(value), "0");
+  value = -12;
+  EXPECT_EQ(absl::UnparseFlag(value), "-12");
+  value = absl::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalDoubleUnparsing) {
+  absl::optional<double> value;
+
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = 1.;
+  EXPECT_EQ(absl::UnparseFlag(value), "1");
+  value = -1.23;
+  EXPECT_EQ(absl::UnparseFlag(value), "-1.23");
+  value = absl::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalStringUnparsing) {
+  absl::optional<std::string> value;
+
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = "asdfg";
+  EXPECT_EQ(absl::UnparseFlag(value), "asdfg");
+  value = " ";
+  EXPECT_EQ(absl::UnparseFlag(value), " ");
+  value = "";  // This is UB to set optional string flag to ""
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = absl::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+}
+
+// --------------------------------------------------------------------
+
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+
+TEST(MarshallingTest, TestStdOptionalUnparsing) {
+  std::optional<std::string> strvalue;
+
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "");
+  strvalue = "asdfg";
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "asdfg");
+  strvalue = std::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "");
+
+  std::optional<int> intvalue(10);
+
+  EXPECT_EQ(absl::UnparseFlag(intvalue), "10");
+  intvalue = std::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(intvalue), "");
+}
+
+// --------------------------------------------------------------------
+
+#endif
+
 template <typename T>
 void TestRoundtrip(T v) {
   T new_v;
diff --git a/absl/hash/CMakeLists.txt b/absl/hash/CMakeLists.txt
index 34434fa..423b74b 100644
--- a/absl/hash/CMakeLists.txt
+++ b/absl/hash/CMakeLists.txt
@@ -80,6 +80,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     spy_hash_state
@@ -94,6 +95,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     city
@@ -121,6 +123,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     low_level_hash
diff --git a/absl/memory/BUILD.bazel b/absl/memory/BUILD.bazel
index c16bf8a..389aedf 100644
--- a/absl/memory/BUILD.bazel
+++ b/absl/memory/BUILD.bazel
@@ -29,6 +29,9 @@
     name = "memory",
     hdrs = ["memory.h"],
     copts = ABSL_DEFAULT_COPTS,
+    defines = select({
+        "//conditions:default": [],
+    }),
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         "//absl/base:core_headers",
diff --git a/absl/random/internal/randen_detect.cc b/absl/random/internal/randen_detect.cc
index 9bb58fc..6dababa 100644
--- a/absl/random/internal/randen_detect.cc
+++ b/absl/random/internal/randen_detect.cc
@@ -24,6 +24,11 @@
 
 #include "absl/random/internal/platform.h"
 
+#if !defined(__UCLIBC__) && defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 16))
+#define ABSL_HAVE_GETAUXVAL
+#endif
+
 #if defined(ABSL_ARCH_X86_64)
 #define ABSL_INTERNAL_USE_X86_CPUID
 #elif defined(ABSL_ARCH_PPC) || defined(ABSL_ARCH_ARM) || \
@@ -31,7 +36,7 @@
 #if defined(__ANDROID__)
 #define ABSL_INTERNAL_USE_ANDROID_GETAUXVAL
 #define ABSL_INTERNAL_USE_GETAUXVAL
-#elif defined(__linux__)
+#elif defined(__linux__) && defined(ABSL_HAVE_GETAUXVAL)
 #define ABSL_INTERNAL_USE_LINUX_GETAUXVAL
 #define ABSL_INTERNAL_USE_GETAUXVAL
 #endif
diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 5f122ee..813aef4 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -736,7 +736,6 @@
         "no_test_android_arm",
         "no_test_android_arm64",
         "no_test_android_x86",
-        "no_test_darwin_x86_64",
         "no_test_ios_x86_64",
         "no_test_loonix",
         "no_test_msvc_x64",
diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index 5d418c8..d8715ed 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -68,6 +68,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     strings_internal
@@ -385,6 +386,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     str_format_internal
@@ -523,6 +525,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     pow10_helper
@@ -550,6 +553,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cord_internal
@@ -588,6 +592,7 @@
     absl::type_traits
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_update_tracker
@@ -614,6 +619,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_functions
@@ -642,6 +648,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_statistics
@@ -656,6 +663,7 @@
     absl::synchronization
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_handle
@@ -689,6 +697,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_info
@@ -756,6 +765,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_sample_token
@@ -794,6 +804,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_update_scope
@@ -859,6 +870,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cord_rep_test_util
@@ -889,6 +901,7 @@
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     cordz_test_helpers
diff --git a/absl/strings/cord_ring_reader_test.cc b/absl/strings/cord_ring_reader_test.cc
index d9a9a76..8e7183b 100644
--- a/absl/strings/cord_ring_reader_test.cc
+++ b/absl/strings/cord_ring_reader_test.cc
@@ -126,7 +126,7 @@
 
   reader.Reset(ring);
   size_t consumed = 0;
-  size_t remaining = ring->length;;
+  size_t remaining = ring->length;
   for (int i = 0; i < flats.size(); ++i) {
     CordRepRing::index_type index = ring->advance(head, i);
     size_t offset = consumed;
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index 2087ffe..5ca5e58 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -156,7 +156,7 @@
   // used for the StringConstant constructor to avoid collecting immutable
   // constant cords.
   // kReservedFlag is reserved for future use.
-  enum {
+  enum Flags {
     kNumFlags = 2,
 
     kImmortalFlag = 0x1,
diff --git a/absl/strings/internal/cord_rep_btree_navigator.h b/absl/strings/internal/cord_rep_btree_navigator.h
index 971b92e..3d581c8 100644
--- a/absl/strings/internal/cord_rep_btree_navigator.h
+++ b/absl/strings/internal/cord_rep_btree_navigator.h
@@ -143,8 +143,8 @@
   // `index_` and `node_` contain the navigation state as the 'path' to the
   // current data edge which is at `node_[0]->Edge(index_[0])`. The contents
   // of these are undefined until the instance is initialized (`height_ >= 0`).
-  uint8_t index_[CordRepBtree::kMaxHeight];
-  CordRepBtree* node_[CordRepBtree::kMaxHeight];
+  uint8_t index_[CordRepBtree::kMaxDepth];
+  CordRepBtree* node_[CordRepBtree::kMaxDepth];
 };
 
 // Returns true if this instance is not empty.
@@ -173,6 +173,7 @@
 inline CordRep* CordRepBtreeNavigator::Init(CordRepBtree* tree) {
   assert(tree != nullptr);
   assert(tree->size() > 0);
+  assert(tree->height() <= CordRepBtree::kMaxHeight);
   int height = height_ = tree->height();
   size_t index = tree->index(edge_type);
   node_[height] = tree;
@@ -206,6 +207,7 @@
 inline CordRepBtreeNavigator::Position CordRepBtreeNavigator::InitOffset(
     CordRepBtree* tree, size_t offset) {
   assert(tree != nullptr);
+  assert(tree->height() <= CordRepBtree::kMaxHeight);
   if (ABSL_PREDICT_FALSE(offset >= tree->length)) return {nullptr, 0};
   height_ = tree->height();
   node_[height_] = tree;
diff --git a/absl/strings/internal/cord_rep_btree_navigator_test.cc b/absl/strings/internal/cord_rep_btree_navigator_test.cc
index ce09b19..4f9bd4e 100644
--- a/absl/strings/internal/cord_rep_btree_navigator_test.cc
+++ b/absl/strings/internal/cord_rep_btree_navigator_test.cc
@@ -319,6 +319,27 @@
   ASSERT_THAT(result.tree, Eq(nullptr));
 }
 
+TEST(CordRepBtreeNavigatorTest, NavigateMaximumTreeDepth) {
+  CordRepFlat* flat1 = MakeFlat("Hello world");
+  CordRepFlat* flat2 = MakeFlat("World Hello");
+
+  CordRepBtree* node = CordRepBtree::Create(flat1);
+  node = CordRepBtree::Append(node, flat2);
+  while (node->height() < CordRepBtree::kMaxHeight) {
+    node = CordRepBtree::New(node);
+  }
+
+  CordRepBtreeNavigator nav;
+  CordRep* edge = nav.InitFirst(node);
+  EXPECT_THAT(edge, Eq(flat1));
+  EXPECT_THAT(nav.Next(), Eq(flat2));
+  EXPECT_THAT(nav.Next(), Eq(nullptr));
+  EXPECT_THAT(nav.Previous(), Eq(flat1));
+  EXPECT_THAT(nav.Previous(), Eq(nullptr));
+
+  CordRep::Unref(node);
+}
+
 }  // namespace
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
diff --git a/absl/strings/internal/cordz_sample_token_test.cc b/absl/strings/internal/cordz_sample_token_test.cc
index 9f54301..6be1770 100644
--- a/absl/strings/internal/cordz_sample_token_test.cc
+++ b/absl/strings/internal/cordz_sample_token_test.cc
@@ -167,7 +167,7 @@
           if (cord.data.is_profiled()) {
             // 1) Untrack
             cord.data.cordz_info()->Untrack();
-            cord.data.clear_cordz_info();;
+            cord.data.clear_cordz_info();
           } else {
             // 2) Track
             CordzInfo::TrackCord(cord.data, kTrackCordMethod);
diff --git a/absl/strings/str_join.h b/absl/strings/str_join.h
index 3353453..ee5ae7e 100644
--- a/absl/strings/str_join.h
+++ b/absl/strings/str_join.h
@@ -72,21 +72,15 @@
 // functions. You may provide your own Formatter to enable `absl::StrJoin()` to
 // work with arbitrary types.
 //
-// The following is an example of a custom Formatter that simply uses
-// `std::to_string()` to format an integer as a std::string.
+// The following is an example of a custom Formatter that uses
+// `absl::FormatDuration` to join a list of `absl::Duration`s.
 //
-//   struct MyFormatter {
-//     void operator()(std::string* out, int i) const {
-//       out->append(std::to_string(i));
-//     }
-//   };
-//
-// You would use the above formatter by passing an instance of it as the final
-// argument to `absl::StrJoin()`:
-//
-//   std::vector<int> v = {1, 2, 3, 4};
-//   std::string s = absl::StrJoin(v, "-", MyFormatter());
-//   EXPECT_EQ("1-2-3-4", s);
+//   std::vector<absl::Duration> v = {absl::Seconds(1), absl::Milliseconds(10)};
+//   std::string s =
+//       absl::StrJoin(v, ", ", [](std::string* out, absl::Duration dur) {
+//         absl::StrAppend(out, absl::FormatDuration(dur));
+//       });
+//   EXPECT_EQ("1s, 10ms", s);
 //
 // The following standard formatters are provided within this file:
 //
diff --git a/absl/strings/string_view_test.cc b/absl/strings/string_view_test.cc
index 2c13dd1..9d5463a 100644
--- a/absl/strings/string_view_test.cc
+++ b/absl/strings/string_view_test.cc
@@ -1189,7 +1189,7 @@
   EXPECT_LT("hello", std::string("world"));
 }
 
-TEST(ComparisonOpsTest, HeterogenousStringViewEquals) {
+TEST(ComparisonOpsTest, HeterogeneousStringViewEquals) {
   EXPECT_EQ(absl::string_view("hello"), std::string("hello"));
   EXPECT_EQ("hello", absl::string_view("hello"));
 }
diff --git a/absl/synchronization/BUILD.bazel b/absl/synchronization/BUILD.bazel
index d719547..a0492c5 100644
--- a/absl/synchronization/BUILD.bazel
+++ b/absl/synchronization/BUILD.bazel
@@ -34,7 +34,9 @@
     hdrs = [
         "internal/graphcycles.h",
     ],
-    copts = ABSL_DEFAULT_COPTS,
+    copts = ABSL_DEFAULT_COPTS + select({
+        "//conditions:default": [],
+    }),
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
diff --git a/absl/synchronization/CMakeLists.txt b/absl/synchronization/CMakeLists.txt
index 605efe2..9335c26 100644
--- a/absl/synchronization/CMakeLists.txt
+++ b/absl/synchronization/CMakeLists.txt
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     graphcycles_internal
@@ -32,6 +33,7 @@
     absl::raw_logging_internal
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     kernel_timeout_internal
@@ -125,6 +127,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     thread_pool
@@ -170,6 +173,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     per_thread_sem_test_common
diff --git a/absl/time/CMakeLists.txt b/absl/time/CMakeLists.txt
index f6ff8bd..debab3b 100644
--- a/absl/time/CMakeLists.txt
+++ b/absl/time/CMakeLists.txt
@@ -87,6 +87,7 @@
     $<$<PLATFORM_ID:Darwin>:${CoreFoundation}>
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     time_internal_test_util
diff --git a/absl/types/CMakeLists.txt b/absl/types/CMakeLists.txt
index d7e8614..830953a 100644
--- a/absl/types/CMakeLists.txt
+++ b/absl/types/CMakeLists.txt
@@ -43,6 +43,7 @@
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     bad_any_cast_impl
@@ -239,6 +240,7 @@
     GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     conformance_testing