diff --git a/lib/libcxx/include/__config b/lib/libcxx/include/__config
index 5286165f49..fc76e6bd13 100644
--- a/lib/libcxx/include/__config
+++ b/lib/libcxx/include/__config
@@ -208,19 +208,16 @@
 
 // HARDENING {
 
-// TODO(hardening): remove this in LLVM 18.
-// This is for backward compatibility -- make enabling `_LIBCPP_ENABLE_ASSERTIONS` (which predates hardening modes)
-// equivalent to setting the hardened mode.
-#  ifdef _LIBCPP_ENABLE_ASSERTIONS
-#    warning "_LIBCPP_ENABLE_ASSERTIONS is deprecated, please use _LIBCPP_ENABLE_HARDENED_MODE instead."
-#    if _LIBCPP_ENABLE_ASSERTIONS != 0 && _LIBCPP_ENABLE_ASSERTIONS != 1
-#      error "_LIBCPP_ENABLE_ASSERTIONS must be set to 0 or 1"
-#    endif
-#    if _LIBCPP_ENABLE_ASSERTIONS
-#      define _LIBCPP_ENABLE_HARDENED_MODE 1
-#    endif
+#  ifndef _LIBCPP_ENABLE_ASSERTIONS
+#    define _LIBCPP_ENABLE_ASSERTIONS _LIBCPP_ENABLE_ASSERTIONS_DEFAULT
+#  endif
+#  if _LIBCPP_ENABLE_ASSERTIONS != 0 && _LIBCPP_ENABLE_ASSERTIONS != 1
+#    error "_LIBCPP_ENABLE_ASSERTIONS must be set to 0 or 1"
 #  endif
 
+// NOTE: These modes are experimental and are not stable yet in LLVM 17. Please refrain from using them and use the
+// documented libc++ "safe" mode instead.
+//
 // Enables the hardened mode which consists of all checks intended to be used in production. Hardened mode prioritizes
 // security-critical checks that can be done with relatively little overhead in constant time. Mutually exclusive with
 // `_LIBCPP_ENABLE_DEBUG_MODE`.
@@ -275,6 +272,11 @@
 #    error "Only one of _LIBCPP_ENABLE_HARDENED_MODE and _LIBCPP_ENABLE_DEBUG_MODE can be enabled."
 #  endif
 
+#  if _LIBCPP_ENABLE_ASSERTIONS && (_LIBCPP_ENABLE_HARDENED_MODE || _LIBCPP_ENABLE_DEBUG_MODE)
+#    error                                                                                                             \
+        "_LIBCPP_ENABLE_ASSERTIONS is mutually exclusive with _LIBCPP_ENABLE_HARDENED_MODE and _LIBCPP_ENABLE_DEBUG_MODE."
+#  endif
+
 // Hardened mode checks.
 
 // clang-format off
@@ -303,6 +305,18 @@
 #    define _LIBCPP_ASSERT_INTERNAL(expression, message)                  _LIBCPP_ASSERT(expression, message)
 #    define _LIBCPP_ASSERT_UNCATEGORIZED(expression, message)             _LIBCPP_ASSERT(expression, message)
 
+// Safe mode checks.
+
+#  elif _LIBCPP_ENABLE_ASSERTIONS
+
+// All checks enabled.
+#    define _LIBCPP_ASSERT_VALID_INPUT_RANGE(expression, message)         _LIBCPP_ASSERT(expression, message)
+#    define _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(expression, message)      _LIBCPP_ASSERT(expression, message)
+#    define _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(expression, message)    _LIBCPP_ASSERT(expression, message)
+#    define _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(expression, message)      _LIBCPP_ASSERT(expression, message)
+#    define _LIBCPP_ASSERT_INTERNAL(expression, message)                  _LIBCPP_ASSERT(expression, message)
+#    define _LIBCPP_ASSERT_UNCATEGORIZED(expression, message)             _LIBCPP_ASSERT(expression, message)
+
 // Disable all checks if hardening is not enabled.
 
 #  else
diff --git a/lib/libcxx/include/__format/format_functions.h b/lib/libcxx/include/__format/format_functions.h
index 27ec0a295f..bb62c1ce10 100644
--- a/lib/libcxx/include/__format/format_functions.h
+++ b/lib/libcxx/include/__format/format_functions.h
@@ -245,6 +245,9 @@ __handle_replacement_field(_Iterator __begin, _Iterator __end,
   using _CharT = iter_value_t<_Iterator>;
   __format::__parse_number_result __r = __format::__parse_arg_id(__begin, __end, __parse_ctx);
 
+  if (__r.__last == __end)
+    std::__throw_format_error("The argument index should end with a ':' or a '}'");
+
   bool __parse = *__r.__last == _CharT(':');
   switch (*__r.__last) {
   case _CharT(':'):
diff --git a/lib/libcxx/include/__locale_dir/locale_base_api/locale_guard.h b/lib/libcxx/include/__locale_dir/locale_base_api/locale_guard.h
index 0e2e91af7d..5946ed698e 100644
--- a/lib/libcxx/include/__locale_dir/locale_base_api/locale_guard.h
+++ b/lib/libcxx/include/__locale_dir/locale_base_api/locale_guard.h
@@ -10,6 +10,7 @@
 #define _LIBCPP___LOCALE_LOCALE_BASE_API_LOCALE_GUARD_H
 
 #include <__config>
+#include <__locale> // for locale_t
 #include <clocale>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
diff --git a/lib/libcxx/include/__type_traits/is_nothrow_constructible.h b/lib/libcxx/include/__type_traits/is_nothrow_constructible.h
index d4686d89fd..4949062433 100644
--- a/lib/libcxx/include/__type_traits/is_nothrow_constructible.h
+++ b/lib/libcxx/include/__type_traits/is_nothrow_constructible.h
@@ -22,7 +22,8 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__is_nothrow_constructible)
+// GCC is disabled due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106611
+#if __has_builtin(__is_nothrow_constructible) && !defined(_LIBCPP_COMPILER_GCC)
 
 template < class _Tp, class... _Args>
 struct _LIBCPP_TEMPLATE_VIS is_nothrow_constructible
diff --git a/lib/libcxx/src/chrono.cpp b/lib/libcxx/src/chrono.cpp
index 0990d8dc18..f159613202 100644
--- a/lib/libcxx/src/chrono.cpp
+++ b/lib/libcxx/src/chrono.cpp
@@ -31,7 +31,7 @@
 # include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if defined(__APPLE__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+#if defined(__APPLE__) || defined (__gnu_hurd__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
 # define _LIBCPP_HAS_CLOCK_GETTIME
 #endif
 
diff --git a/lib/libcxx/src/filesystem/filesystem_clock.cpp b/lib/libcxx/src/filesystem/filesystem_clock.cpp
index d00cdc6df3..fbb19ac68d 100644
--- a/lib/libcxx/src/filesystem/filesystem_clock.cpp
+++ b/lib/libcxx/src/filesystem/filesystem_clock.cpp
@@ -29,7 +29,7 @@
 # include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if defined(__APPLE__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+#if defined(__APPLE__) || defined (__gnu_hurd__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
 # define _LIBCPP_HAS_CLOCK_GETTIME
 #endif