diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -82,7 +82,10 @@
 include(HandleCompilerRT)
 
 # Basic options ---------------------------------------------------------------
-option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." OFF)
+option(LIBCXX_ENABLE_ASSERTIONS
+  "Enable assertions inside the compiled library, and at the same time make it the
+   default when compiling user code. Note that assertions can be enabled or disabled
+   by users in their own code regardless of this option." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
@@ -675,9 +678,6 @@
 endif()
 
 # Assertion flags =============================================================
-define_if(LIBCXX_ENABLE_ASSERTIONS -UNDEBUG)
-define_if_not(LIBCXX_ENABLE_ASSERTIONS -DNDEBUG)
-define_if(LIBCXX_ENABLE_ASSERTIONS -D_LIBCPP_DEBUG=0)
 define_if(LIBCXX_DEBUG_BUILD -D_DEBUG)
 if (LIBCXX_ENABLE_ASSERTIONS AND NOT LIBCXX_DEBUG_BUILD)
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.
@@ -879,6 +879,11 @@
 config_define_if_not(LIBCXX_ENABLE_UNICODE _LIBCPP_HAS_NO_UNICODE)
 config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTERS)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
+if (LIBCXX_ENABLE_ASSERTIONS)
+  config_define(1 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT)
+else()
+  config_define(0 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT)
+endif()
 # Incomplete features get their own specific disabling flags. This makes it
 # easier to grep for target specific flags once the feature is complete.
 config_define_if_not(LIBCXX_ENABLE_INCOMPLETE_FEATURES _LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
diff --git a/libcxx/benchmarks/algorithms.bench.cpp b/libcxx/benchmarks/algorithms.bench.cpp
--- a/libcxx/benchmarks/algorithms.bench.cpp
+++ b/libcxx/benchmarks/algorithms.bench.cpp
@@ -151,7 +151,6 @@
 
 template <class T>
 void sortValues(T& V, Order O) {
-  assert(std::is_sorted(V.begin(), V.end()));
   switch (O) {
   case Order::Random: {
     std::random_device R;
diff --git a/libcxx/cmake/caches/Generic-assertions.cmake b/libcxx/cmake/caches/Generic-assertions.cmake
--- a/libcxx/cmake/caches/Generic-assertions.cmake
+++ b/libcxx/cmake/caches/Generic-assertions.cmake
@@ -1 +1,3 @@
 set(LIBCXX_ENABLE_ASSERTIONS ON CACHE BOOL "")
+set(LIBCXX_TEST_PARAMS "enable_assertions=True" CACHE STRING "")
+set(LIBCXXABI_TEST_PARAMS "enable_assertions=True" CACHE STRING "")
diff --git a/libcxx/docs/BuildingLibcxx.rst b/libcxx/docs/BuildingLibcxx.rst
--- a/libcxx/docs/BuildingLibcxx.rst
+++ b/libcxx/docs/BuildingLibcxx.rst
@@ -216,7 +216,10 @@
 
   **Default**: ``OFF``
 
-  Build libc++ with assertions enabled.
+  Build libc++ with assertions enabled in the compiled library, and enable assertions
+  by default when building user code as well. Assertions can be turned off by users
+  by defining ``_LIBCPP_ENABLE_ASSERTIONS=0``. For details, see
+  :ref:`the documentation <assertions-mode>`.
 
 .. option:: LIBCXX_ENABLE_SHARED:BOOL
 
diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -69,6 +69,12 @@
 - The C++14 function ``std::quoted(const char*)`` is no longer supported in
   C++03 or C++11 modes.
 
+- Libc++ now supports a variety of assertions that can be turned on to help catch
+  undefined behavior in user code. This new support is now separate from the old
+  (and incomplete) Debug Mode. Vendors can select whether the library they ship
+  should include assertions or not by default. For details, see
+  :ref:`the documentation <assertions-mode>` about this new feature.
+
 ABI Changes
 -----------
 
diff --git a/libcxx/docs/UsingLibcxx.rst b/libcxx/docs/UsingLibcxx.rst
--- a/libcxx/docs/UsingLibcxx.rst
+++ b/libcxx/docs/UsingLibcxx.rst
@@ -121,6 +121,72 @@
         <args>
 
 
+.. _assertions-mode:
+
+Enabling the "safe libc++" mode
+===============================
+
+Libc++ contains a number of assertions whose goal is to catch undefined behavior in the
+library, usually caused by precondition violations. Those assertions do not aim to be
+exhaustive -- instead they aim to provide a good balance between safety and performance.
+In particular, these assertions do not change the complexity of algorithms. However, they
+might, in some cases, interfere with compiler optimizations.
+
+By default, these assertions are turned off. Vendors can decide to turn them on while building
+the compiled library by defining ``LIBCXX_ENABLE_ASSERTIONS=ON`` at CMake configuration time.
+When ``LIBCXX_ENABLE_ASSERTIONS`` is used, the compiled library will be built with assertions
+enabled, **and** user code will be built with assertions enabled by default. If ``LIBCXX_ENABLE_ASSERTIONS=OFF``
+at CMake configure time, the compiled library will not contain assertions and the default when
+building user code will be to have assertions disabled.
+
+Independently of any vendor-selected default, users can always control whether assertions
+are enabled in their code by defining ``_LIBCPP_ENABLE_ASSERTIONS=0|1`` before including any
+libc++ header. Note that if the compiled library was built by the vendor without assertions,
+functions compiled inside the static or shared library won't have assertions enabled even if
+the user defines ``_LIBCPP_ENABLE_ASSERTIONS=1`` (the same is true for the inverse case where
+the static or shared library was compiled **with** assertions but the user tries to disable
+them). However, most of the code in libc++ is in the headers, so the user-selected value for
+``_LIBCPP_ENABLE_ASSERTIONS`` (if any) will usually be respected.
+
+When an assertion fails, an assertion handler function is called. The library provides a default
+assertion handler that prints an error message and calls ``std::abort()``. Note that this assertion
+handler is provided by the static or shared library, so it is only available when deploying to a
+platform where the compiled library is sufficiently recent. However, users can also define their
+own assertion handler to replace the default one, which can be useful to provide custom behavior,
+or when deploying to older platforms where the default assertion handler isn't available.
+
+Replacing the default assertion handler is done by defining the ``_LIBCPP_ASSERTION_HANDLER`` macro,
+which is called with an instance of ``std::__libcpp_assertion_info``. Generally speaking,
+``_LIBCPP_ASSERTION_HANDLER`` should expand to a fully qualified function name. For example:
+
+.. code-block:: cpp
+
+  // In HelloWorldHandler.h
+  #define _LIBCPP_ASSERTION_HANDLER ::HelloWorldAssertionHandler
+  #include <__assert>
+
+  // This could also be defined in another translation unit, but we're defining it inline
+  // for simplicity in this example.
+  inline [[noreturn]] void HelloWorldAssertionHandler(std::__libcpp_assertion_info info) {
+      std::printf("Assertion %s failed at %s:%d, more info: %s",
+                  info.__predicate(), info.file(), info.__line(), info.__message());
+      std::abort();
+  }
+
+  // In HelloWorld.cpp
+  // Note: You must add `-include path/to/HelloWorldHandler.h` to your compiler flags.
+  #include <vector>
+
+  int main() {
+    std::vector<int> v;
+    int& x = v[0]; // Your assertion handler will be called here if _LIBCPP_ENABLE_ASSERTIONS=1
+  }
+
+Also note that the assertion handler should usually not return. Since the assertions in libc++
+catch undefined behavior, your code will proceed with undefined behavior if your assertion
+handler is called but does return.
+
+
 Libc++ Configuration Macros
 ===========================
 
diff --git a/libcxx/include/__assert b/libcxx/include/__assert
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP___ASSERT
 #define _LIBCPP___ASSERT
 
+#include <__availability>
 #include <__config>
 #include <iosfwd> // for std::string
 
@@ -17,14 +18,39 @@
 #  pragma GCC system_header
 #endif
 
+// This is for backwards compatibility with code that might have been enabling
+// assertions through the Debug mode previously.
 #if _LIBCPP_DEBUG_LEVEL >= 1
-#   define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : ::std::__libcpp_debug_function(::std::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
+# ifndef _LIBCPP_ENABLE_ASSERTIONS
+#   define _LIBCPP_ENABLE_ASSERTIONS 1
+# endif
+# ifndef _LIBCPP_ASSERTION_HANDLER
+#   define _LIBCPP_ASSERTION_HANDLER ::std::__libcpp_debug_function
+# endif
+#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
+
+#ifndef _LIBCPP_ASSERTION_HANDLER
+# define _LIBCPP_ASSERTION_HANDLER ::std::__libcpp_default_assertion_handler
+#endif
+
+#if _LIBCPP_ENABLE_ASSERTIONS
+# define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : _LIBCPP_ASSERTION_HANDLER(::std::__libcpp_assertion_info(__FILE__, __LINE__, #x, m)))
 #else
-#   define _LIBCPP_ASSERT(x, m) ((void)0)
+# define _LIBCPP_ASSERT(x, m) ((void)0)
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+//////////////////////////////////////////////////////////////////
+// For backwards compatibility with clients of the Debug mode
 struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
   __libcpp_debug_info()
@@ -56,6 +82,41 @@
 ///    function.
 _LIBCPP_FUNC_VIS
 bool __libcpp_set_debug_function(__libcpp_debug_function_type __func);
+//////////////////////////////////////////////////////////////////
+
+struct __libcpp_assertion_info {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  explicit __libcpp_assertion_info(char const* __file, int __line, char const* __predicate, char const* __message)
+    : __file_(__file), __line_(__line), __predicate_(__predicate), __message_(__message)
+  { }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  char const* __file() const { return __file_; }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  int __line() const { return __line_; }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  char const* __predicate() const { return __predicate_; }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  char const* __message() const { return __message_; }
+
+  // For backwards compatibility
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  operator __libcpp_debug_info() const {
+    return __libcpp_debug_info(__file_, __line_, __predicate_, __message_);
+  }
+
+private:
+  char const* __file_;
+  int __line_;
+  char const* __predicate_;
+  char const* __message_;
+};
+
+_LIBCPP_NORETURN _LIBCPP_AVAILABILITY_ASSERTION_HANDLER _LIBCPP_EXPORTED_FROM_ABI
+void __libcpp_default_assertion_handler(__libcpp_assertion_info);
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__availability b/libcxx/include/__availability
--- a/libcxx/include/__availability
+++ b/libcxx/include/__availability
@@ -81,6 +81,11 @@
 
 #if defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
+    // This controls whether the default assertion handler is provided by the library.
+    // When the default assertion handler is not provided, users either can't enable
+    // assertions in the library, or must provide their own assertion handler.
+#   define _LIBCPP_AVAILABILITY_ASSERTION_HANDLER
+
     // This controls the availability of std::shared_mutex and std::shared_timed_mutex,
     // which were added to the dylib later.
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX
@@ -151,6 +156,9 @@
 
 #elif defined(__APPLE__)
 
+#   define _LIBCPP_AVAILABILITY_ASSERTION_HANDLER                               \
+        __attribute__((unavailable))
+
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
         __attribute__((availability(macosx,strict,introduced=10.12)))           \
         __attribute__((availability(ios,strict,introduced=10.0)))               \
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
@@ -32,6 +32,7 @@
 #cmakedefine _LIBCPP_HAS_NO_WIDE_CHARACTERS
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_FORMAT
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_RANGES
+#cmakedefine01 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT
 
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__
diff --git a/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist b/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
--- a/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
+++ b/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
@@ -1581,6 +1581,7 @@
 {'is_defined': True, 'name': '__ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__13cinE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist b/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist
--- a/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist
+++ b/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist
@@ -1552,6 +1552,7 @@
 {'is_defined': True, 'name': '__ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__13cinE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist b/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
--- a/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
+++ b/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
@@ -1581,6 +1581,7 @@
 {'is_defined': True, 'name': '__ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__13cinE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist b/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist
--- a/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist
+++ b/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist
@@ -1552,6 +1552,7 @@
 {'is_defined': True, 'name': '__ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__13cinE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
--- a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
+++ b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist
@@ -1272,6 +1272,7 @@
 {'is_defined': True, 'name': '_ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__13cinE', 'size': 168, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.noincomplete.abilist b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.noincomplete.abilist
--- a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.noincomplete.abilist
+++ b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.noincomplete.abilist
@@ -1269,6 +1269,7 @@
 {'is_defined': True, 'name': '_ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__13cinE', 'size': 168, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.nodebug.incomplete.abilist b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.nodebug.incomplete.abilist
--- a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.nodebug.incomplete.abilist
+++ b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.nodebug.incomplete.abilist
@@ -1246,6 +1246,7 @@
 {'is_defined': True, 'name': '_ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__13cinE', 'size': 168, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.debug.incomplete.abilist b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.debug.incomplete.abilist
--- a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.debug.incomplete.abilist
+++ b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.debug.incomplete.abilist
@@ -1244,6 +1244,7 @@
 {'is_defined': True, 'name': '_ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__13cinE', 'size': 168, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10__absoluteERKNS1_4pathEPNS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__14__fs10filesystem10hash_valueERKNS1_4pathE', 'type': 'FUNC'}
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -4,6 +4,7 @@
 set(LIBCXX_SOURCES
   algorithm.cpp
   any.cpp
+  assert.cpp
   atomic.cpp
   barrier.cpp
   bind.cpp
@@ -65,7 +66,6 @@
 
 if (LIBCXX_ENABLE_DEBUG_MODE_SUPPORT)
   list(APPEND LIBCXX_SOURCES
-    assert.cpp
     debug.cpp
     )
 endif()
diff --git a/libcxx/src/assert.cpp b/libcxx/src/assert.cpp
--- a/libcxx/src/assert.cpp
+++ b/libcxx/src/assert.cpp
@@ -10,29 +10,13 @@
 #include <__config>
 #include <cstdio>
 #include <cstdlib>
-#include <string>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-std::string __libcpp_debug_info::what() const {
-  string msg = __file_;
-  msg += ":" + std::to_string(__line_) + ": _LIBCPP_ASSERT '";
-  msg += __pred_;
-  msg += "' failed. ";
-  msg += __msg_;
-  return msg;
-}
-
-_LIBCPP_NORETURN void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
-    std::fprintf(stderr, "%s\n", info.what().c_str());
-    std::abort();
-}
-
-constinit __libcpp_debug_function_type __libcpp_debug_function = __libcpp_abort_debug_function;
-
-bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
-  __libcpp_debug_function = __func;
-  return true;
+void __libcpp_default_assertion_handler(__libcpp_assertion_info __info) {
+  std::fprintf(stderr, "%s:%d: libc++ assertion '%s' failed. %s\n",
+                      __info.__file(), __info.__line(), __info.__predicate(), __info.__message());
+  std::abort();
 }
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/src/debug.cpp b/libcxx/src/debug.cpp
--- a/libcxx/src/debug.cpp
+++ b/libcxx/src/debug.cpp
@@ -24,6 +24,27 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+std::string __libcpp_debug_info::what() const {
+  string msg = __file_;
+  msg += ":" + std::to_string(__line_) + ": _LIBCPP_ASSERT '";
+  msg += __pred_;
+  msg += "' failed. ";
+  msg += __msg_;
+  return msg;
+}
+
+_LIBCPP_NORETURN void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
+    std::fprintf(stderr, "%s\n", info.what().c_str());
+    std::abort();
+}
+
+constinit __libcpp_debug_function_type __libcpp_debug_function = __libcpp_abort_debug_function;
+
+bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
+  __libcpp_debug_function = __func;
+  return true;
+}
+
 _LIBCPP_FUNC_VIS
 __libcpp_db*
 __get_db()
diff --git a/libcxx/test/libcxx/assertion_handler.legacy_debug_mode.pass.cpp b/libcxx/test/libcxx/assertion_handler.legacy_debug_mode.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertion_handler.legacy_debug_mode.pass.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Test that we still honor any legacy debug handler set in user code.
+
+#define _LIBCPP_DEBUG 0
+#include <cassert>
+#include <vector> // any random header that definitely includes assertions
+
+bool handler_called = false;
+inline void HelloWorldDebugHandler(std::__libcpp_debug_info const&) {
+  handler_called = true;
+}
+
+int main(int, char**) {
+  std::__libcpp_set_debug_function(&HelloWorldDebugHandler);
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}
diff --git a/libcxx/test/libcxx/assertion_handler.pass.cpp b/libcxx/test/libcxx/assertion_handler.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertion_handler.pass.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Test that we can set a custom assertion handler.
+
+#define _LIBCPP_ENABLE_ASSERTIONS 1
+#define _LIBCPP_ASSERTION_HANDLER ::HelloWorldAssertionHandler
+#include <__assert>
+
+bool handler_called = false;
+inline void HelloWorldAssertionHandler(std::__libcpp_assertion_info) {
+  handler_called = true;
+}
+
+#include <cassert>
+
+int main(int, char**) {
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}
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
@@ -177,6 +177,11 @@
               AddFeature('libcxx-no-debug-mode')
             ]),
 
+  Parameter(name='enable_assertions', choices=[True, False], type=bool, default=False,
+            help="Whether to enable assertions when compiling the test suite. This is only meaningful when "
+                 "running the tests against libc++.",
+            actions=lambda assertions: [AddCompileFlag('-D_LIBCPP_ENABLE_ASSERTIONS=1')] if assertions else []),
+
   Parameter(name='additional_features', type=list, default=[],
             help="A comma-delimited list of additional features that will be enabled when running the tests. "
                  "This should be used sparingly since specifying ad-hoc features manually is error-prone and "