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)
@@ -675,7 +678,6 @@
 endif()
 
 # Assertion flags =============================================================
-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.
@@ -877,6 +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/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
@@ -46,6 +46,12 @@
    "heapsort with bounce" to reduce the number of comparisons, and rearranges
    elements using move-assignment instead of `swap`.
 
+ - 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.
+
 API Changes
 -----------
 
@@ -74,6 +80,10 @@
 - The C++14 function ``std::quoted(const char*)`` is no longer supported in
   C++03 or C++11 modes.
 
+- Setting a custom debug handler with ``std::__libcpp_debug_function`` is not
+  supported anymore. Please migrate to using the new support for
+  :ref:`assertions <assertions-mode>` instead.
+
 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 +121,69 @@
         <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 override that
+assertion handler with their own, 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 ``void __libcpp_assertion_handler(__libcpp_assertion_info)``
+function, similarly to how ``operator new`` and ``operator delete`` can be replaced. For example:
+
+.. code-block:: cpp
+
+  // In HelloWorldHandler.cpp
+  #include <version> // must include any libc++ header before defining the handler
+
+  void std::__libcpp_assertion_handler(char const* file, int line, char const* expression, char const* message) {
+    std::printf("Assertion %s failed at %s:%d, more info: %s", expression, file, line, message);
+    std::abort();
+  }
+
+  // In HelloWorld.cpp
+  #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 and does return.
+
+Furthermore, throwing an exception from the assertion handler is not recommended. Indeed, many
+functions in the library are ``noexcept``, and any exception thrown from the assertion handler
+will result in ``std::terminate`` being called.
+
 Libc++ Configuration Macros
 ===========================
 
diff --git a/libcxx/include/__assert b/libcxx/include/__assert
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -11,51 +11,37 @@
 #define _LIBCPP___ASSERT
 
 #include <__config>
-#include <iosfwd> // for std::string
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  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
+#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
+
+#if _LIBCPP_ENABLE_ASSERTIONS
+# define _LIBCPP_ASSERT(expression, message) ((expression) ? (void)0 : ::std::__libcpp_assertion_handler(__FILE__, __LINE__, #expression, message))
 #else
-#   define _LIBCPP_ASSERT(x, m) ((void)0)
+# define _LIBCPP_ASSERT(x, m) ((void)0)
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
-  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-  __libcpp_debug_info()
-      : __file_(nullptr), __line_(-1), __pred_(nullptr), __msg_(nullptr) {}
-  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-  __libcpp_debug_info(const char* __f, int __l, const char* __p, const char* __m)
-    : __file_(__f), __line_(__l), __pred_(__p), __msg_(__m) {}
-
-  _LIBCPP_FUNC_VIS string what() const;
-
-  const char* __file_;
-  int __line_;
-  const char* __pred_;
-  const char* __msg_;
-};
-
-/// __libcpp_debug_function_type - The type of the assertion failure handler.
-typedef void(*__libcpp_debug_function_type)(__libcpp_debug_info const&);
-
-/// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
-///    fails.
-extern _LIBCPP_EXPORTED_FROM_ABI __libcpp_debug_function_type __libcpp_debug_function;
-
-/// __libcpp_abort_debug_function - A debug handler that aborts when called.
-_LIBCPP_NORETURN _LIBCPP_FUNC_VIS
-void __libcpp_abort_debug_function(__libcpp_debug_info const&);
-
-/// __libcpp_set_debug_function - Set the debug handler to the specified
-///    function.
-_LIBCPP_FUNC_VIS
-bool __libcpp_set_debug_function(__libcpp_debug_function_type __func);
+_LIBCPP_OVERRIDABLE_FUNC_VIS
+void __libcpp_assertion_handler(char const* __file, int __line, char const* __expression, char const* __message);
 
 _LIBCPP_END_NAMESPACE_STD
 
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/include/version b/libcxx/include/version
--- a/libcxx/include/version
+++ b/libcxx/include/version
@@ -193,6 +193,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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
@@ -1561,6 +1561,7 @@
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_debug_functionE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -1534,6 +1534,7 @@
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -1561,6 +1561,7 @@
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_debug_functionE', 'size': 0, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '__ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -1534,6 +1534,7 @@
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -1252,6 +1252,7 @@
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_debug_functionE', 'size': 8, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -1249,6 +1249,7 @@
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_debug_functionE', 'size': 8, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -1228,6 +1228,7 @@
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_atomic_monitorEPVKNS_17__cxx_atomic_implIiNS_22__cxx_atomic_base_implIiEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -1224,6 +1224,7 @@
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__123__libcpp_debug_functionE', 'size': 8, 'type': 'OBJECT'}
 {'is_defined': True, 'name': '_ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIccEEPcEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIddEEPdEEbT0_S5_T_', '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
@@ -65,8 +66,8 @@
 
 if (LIBCXX_ENABLE_DEBUG_MODE_SUPPORT)
   list(APPEND LIBCXX_SOURCES
-    assert.cpp
     debug.cpp
+    legacy_debug_handler.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;
+_LIBCPP_WEAK
+void __libcpp_assertion_handler(char const* __file, int __line, char const* __expression, char const* __message) {
+  std::fprintf(stderr, "%s:%d: libc++ assertion '%s' failed. %s\n", __file, __line, __expression, __message);
+  std::abort();
 }
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/src/assert.cpp b/libcxx/src/legacy_debug_handler.cpp
copy from libcxx/src/assert.cpp
copy to libcxx/src/legacy_debug_handler.cpp
--- a/libcxx/src/assert.cpp
+++ b/libcxx/src/legacy_debug_handler.cpp
@@ -6,14 +6,26 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include <__assert>
 #include <__config>
 #include <cstdio>
 #include <cstdlib>
 #include <string>
 
+// This file defines the legacy default debug handler and related mechanisms
+// to set it. This is for backwards ABI compatibility with code that has been
+// using this debug handler previously.
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
+  _LIBCPP_EXPORTED_FROM_ABI string what() const;
+
+  const char* __file_;
+  int __line_;
+  const char* __pred_;
+  const char* __msg_;
+};
+
 std::string __libcpp_debug_info::what() const {
   string msg = __file_;
   msg += ":" + std::to_string(__line_) + ": _LIBCPP_ASSERT '";
@@ -23,13 +35,17 @@
   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();
+_LIBCPP_NORETURN _LIBCPP_EXPORTED_FROM_ABI void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
+  std::fprintf(stderr, "%s\n", info.what().c_str());
+  std::abort();
 }
 
+typedef void (*__libcpp_debug_function_type)(__libcpp_debug_info const&);
+
+_LIBCPP_EXPORTED_FROM_ABI
 constinit __libcpp_debug_function_type __libcpp_debug_function = __libcpp_abort_debug_function;
 
+_LIBCPP_EXPORTED_FROM_ABI
 bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
   __libcpp_debug_function = __func;
   return true;
diff --git a/libcxx/test/libcxx/assertions/assertion_handler.abort.pass.cpp b/libcxx/test/libcxx/assertions/assertion_handler.abort.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/assertion_handler.abort.pass.cpp
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 the default assertion handler aborts the program.
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <__assert>
+#include <csignal>
+#include <cstdlib>
+
+void signal_handler(int signal) {
+    if (signal == SIGABRT)
+      std::_Exit(EXIT_SUCCESS);
+    std::_Exit(EXIT_FAILURE);
+}
+
+int main(int, char**) {
+  if (std::signal(SIGABRT, signal_handler) != SIG_ERR)
+    _LIBCPP_ASSERT(false, "foo");
+  return EXIT_FAILURE;
+}
diff --git a/libcxx/test/libcxx/assertions/assertion_handler.no-abort.pass.cpp b/libcxx/test/libcxx/assertions/assertion_handler.no-abort.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/assertion_handler.no-abort.pass.cpp
@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 _LIBCPP_ASSERT doesn't do anything when assertions are disabled.
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=0
+
+#include <__assert>
+#include <cassert>
+
+bool executed_condition = false;
+bool f() { executed_condition = true; return false; }
+
+int main(int, char**) {
+  _LIBCPP_ASSERT(f(), "message"); // should not execute anything
+  assert(!executed_condition); // really make sure we did not execute anything at all
+  return 0;
+}
diff --git a/libcxx/test/libcxx/assertions/assertion_handler.pass.cpp b/libcxx/test/libcxx/assertions/assertion_handler.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/assertion_handler.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 can set a custom assertion handler.
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+#include <version>
+
+bool handler_called = false;
+void std::__libcpp_assertion_handler(char const*, int, char const*, char const*) {
+  handler_called = true;
+}
+
+#include <cassert>
+
+int main(int, char**) {
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}
diff --git a/libcxx/test/support/check_assertion.h b/libcxx/test/support/check_assertion.h
--- a/libcxx/test/support/check_assertion.h
+++ b/libcxx/test/support/check_assertion.h
@@ -9,16 +9,6 @@
 #ifndef TEST_SUPPORT_CHECK_ASSERTION_H
 #define TEST_SUPPORT_CHECK_ASSERTION_H
 
-#ifndef _LIBCPP_DEBUG
-#error _LIBCPP_DEBUG must be defined before including this header
-#endif
-
-#include <ciso646>
-#ifndef _LIBCPP_VERSION
-#error "This header may only be used for libc++ tests"
-#endif
-
-#include <__debug>
 #include <cassert>
 #include <cstddef>
 #include <cstdio>
@@ -33,28 +23,31 @@
 #include "test_macros.h"
 #include "test_allocator.h"
 
+#ifndef _LIBCPP_VERSION
+# error "This header may only be used for libc++ tests"
+#endif
+
 #if TEST_STD_VER < 11
 # error "C++11 or greater is required to use this header"
 #endif
 
-struct DebugInfoMatcher {
+struct AssertionInfoMatcher {
   static const int any_line = -1;
   static constexpr const char* any_file = "*";
   static constexpr const char* any_msg = "*";
 
-  constexpr DebugInfoMatcher() : is_empty_(true), msg_(any_msg, __builtin_strlen(any_msg)), file_(any_file, __builtin_strlen(any_file)), line_(any_line) { }
-  constexpr DebugInfoMatcher(const char* msg, const char* file = any_file, int line = any_line)
+  constexpr AssertionInfoMatcher() : is_empty_(true), msg_(any_msg, __builtin_strlen(any_msg)), file_(any_file, __builtin_strlen(any_file)), line_(any_line) { }
+  constexpr AssertionInfoMatcher(const char* msg, const char* file = any_file, int line = any_line)
     : is_empty_(false), msg_(msg, __builtin_strlen(msg)), file_(file, __builtin_strlen(file)), line_(line) {}
 
-  bool Matches(std::__libcpp_debug_info const& got) const {
+  bool Matches(char const* file, int line, char const* message) const {
     assert(!empty() && "empty matcher");
 
-    if (CheckLineMatches(got.__line_) && CheckFileMatches(got.__file_) &&
-        CheckMessageMatches(got.__msg_))
+    if (CheckLineMatches(line) && CheckFileMatches(file) && CheckMessageMatches(message))
         return true;
     // Write to stdout because that's the file descriptor captured by the parent
     // process.
-    std::printf("Failed to match debug info!\n%s\nVS\n%s\n", ToString().data(), got.what().data());
+    std::printf("Failed to match debug info!\n%s\nVS\n%s:%d (%s)\n", ToString().data(), file, line, message);
     return false;
   }
 
@@ -108,10 +101,10 @@
   int line_;
 };
 
-static constexpr DebugInfoMatcher AnyMatcher(DebugInfoMatcher::any_msg);
+static constexpr AssertionInfoMatcher AnyMatcher(AssertionInfoMatcher::any_msg);
 
-inline DebugInfoMatcher& GlobalMatcher() {
-  static DebugInfoMatcher GMatch;
+inline AssertionInfoMatcher& GlobalMatcher() {
+  static AssertionInfoMatcher GMatch;
   return GMatch;
 }
 
@@ -136,15 +129,7 @@
     return val >= RK_DidNotDie && val <= RK_Unknown;
   }
 
-  TEST_NORETURN static void DeathTestDebugHandler(std::__libcpp_debug_info const& info) {
-    assert(!GlobalMatcher().empty());
-    if (GlobalMatcher().Matches(info)) {
-      std::exit(RK_MatchFound);
-    }
-    std::exit(RK_MatchFailure);
-  }
-
-  DeathTest(DebugInfoMatcher const& Matcher) : matcher_(Matcher) {}
+  DeathTest(AssertionInfoMatcher const& Matcher) : matcher_(Matcher) {}
 
   template <class Func>
   ResultKind Run(Func&& f) {
@@ -180,7 +165,6 @@
     DupFD(GetStdErrWriteFD(), STDERR_FILENO);
 
     GlobalMatcher() = matcher_;
-    std::__libcpp_set_debug_function(&DeathTestDebugHandler);
     f();
     std::exit(RK_DidNotDie);
   }
@@ -242,7 +226,7 @@
     return stderr_pipe_fd_[1];
   }
 private:
-  DebugInfoMatcher matcher_;
+  AssertionInfoMatcher matcher_;
   pid_t child_pid_ = -1;
   int exit_code_ = -1;
   int stdout_pipe_fd_[2];
@@ -251,8 +235,16 @@
   std::string stderr_from_child_;
 };
 
+void std::__libcpp_assertion_handler(char const* file, int line, char const* /*expression*/, char const* message) {
+  assert(!GlobalMatcher().empty());
+  if (GlobalMatcher().Matches(file, line, message)) {
+    std::exit(DeathTest::RK_MatchFound);
+  }
+  std::exit(DeathTest::RK_MatchFailure);
+}
+
 template <class Func>
-inline bool ExpectDeath(const char* stmt, Func&& func, DebugInfoMatcher Matcher) {
+inline bool ExpectDeath(const char* stmt, Func&& func, AssertionInfoMatcher Matcher) {
   DeathTest DT(Matcher);
   DeathTest::ResultKind RK = DT.Run(func);
   auto OnFailure = [&](const char* msg) {
@@ -293,6 +285,6 @@
 
 #define EXPECT_DEATH_MATCHES(Matcher, ...) assert((ExpectDeath(#__VA_ARGS__, [&]() { __VA_ARGS__; }, Matcher)))
 
-#define TEST_LIBCPP_ASSERT_FAILURE(expr, message) assert((ExpectDeath(#expr, [&]() { (void)(expr); }, DebugInfoMatcher(message))))
+#define TEST_LIBCPP_ASSERT_FAILURE(expr, message) assert((ExpectDeath(#expr, [&]() { (void)(expr); }, AssertionInfoMatcher(message))))
 
 #endif // TEST_SUPPORT_CHECK_ASSERTION_H
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -937,6 +937,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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
@@ -183,6 +183,11 @@
               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 "