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)
@@ -694,7 +697,6 @@
 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.
@@ -896,6 +898,11 @@
 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 +121,92 @@
         <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.
+As a user, you can consult your vendor to know whether assertions are enabled by default.
+
+Furthermore, 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 (we recommend passing ``-D_LIBCPP_ENABLE_ASSERTIONS=X`` to the
+compiler). 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 following function:
+
+.. code-block:: cpp
+
+  void __libcpp_assertion_handler(char const* file, int line, char const* expression, char const* message)
+
+This mechanism is similar to how one can replace the default definition of ``operator new``
+and ``operator delete``. For example:
+
+.. code-block:: cpp
+
+  // In HelloWorldHandler.cpp
+  #include <__assert> // must include <__assert> 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.
+
+Back-deploying with a custom assertion handler
+----------------------------------------------
+When deploying to an older platform that does not provide a default assertion handler, the
+compiler will diagnose the usage of ``std::__libcpp_assertion_handler`` with an error. This
+is done to avoid the load-time error that would otherwise happen if the code was being deployed
+on the older system.
+
+If you are providing a custom assertion handler, this error is effectively a false positive.
+To let the library know that you are providing a custom assertion handler in back-deployment
+scenarios, you must define the ``_LIBCPP_AVAILABILITY_CUSTOM_ASSERTION_HANDLER_PROVIDED`` macro,
+and the library will assume that you are providing your own definition. If no definition is
+provided and the code is back-deployed to the older platform, it will fail to load when the
+dynamic linker fails to find a definition for ``std::__libcpp_assertion_handler``, so you
+should only remove the guard rails if you really mean it!
+
 Libc++ Configuration Macros
 ===========================
 
diff --git a/libcxx/include/__assert b/libcxx/include/__assert
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -10,52 +10,39 @@
 #ifndef _LIBCPP___ASSERT
 #define _LIBCPP___ASSERT
 
+#include <__availability>
 #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 _LIBCPP_AVAILABILITY_ASSERTION_HANDLER
+void __libcpp_assertion_handler(char const* __file, int __line, char const* __expression, char const* __message);
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__availability b/libcxx/include/__availability
--- a/libcxx/include/__availability
+++ b/libcxx/include/__availability
@@ -159,6 +159,23 @@
 #   define _LIBCPP_AVAILABILITY_FORMAT
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format
 
+    // This controls whether the std::__libcpp_assertion_handler default
+    // assertion handler is provided by the library.
+    //
+    // Note that when users provide their own custom assertion handler,
+    // it doesn't matter whether the dylib provides a default handler,
+    // and the availability markup can actually give a false positive
+    // diagnostic (it will think that no handler is provided, when in
+    // reality the user has provided their own).
+    //
+    // Users can pass -D_LIBCPP_AVAILABILITY_CUSTOM_ASSERTION_HANDLER_PROVIDED
+    // to the compiler to tell the library to ignore the fact that the
+    // default handler isn't available on their deployment target. Note that
+    // defining this macro but failing to define a custom assertion handler
+    // will lead to a load-time error on back-deployment targets, so it
+    // should be avoided.
+#   define _LIBCPP_AVAILABILITY_DEFAULT_ASSERTION_HANDLER
+
 #elif defined(__APPLE__)
 
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
@@ -260,6 +277,9 @@
 #   define _LIBCPP_AVAILABILITY_FORMAT                                          \
         __attribute__((unavailable))
 #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format
+
+#   define _LIBCPP_AVAILABILITY_DEFAULT_ASSERTION_HANDLER                       \
+        __attribute__((unavailable))
 #else
 
 // ...New vendors can add availability markup here...
@@ -283,4 +303,14 @@
 #   define _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS  _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS
 #endif
 
+// Define the special assertion handler availability attribute, which can be silenced by
+// users if they provide their own custom assertion handler. The rest of the code should
+// not use the *_DEFAULT_* macro directly, since that would make it ignore the fact that
+// the user provided a custom handler.
+#if defined(_LIBCPP_AVAILABILITY_CUSTOM_ASSERTION_HANDLER_PROVIDED)
+#   define _LIBCPP_AVAILABILITY_ASSERTION_HANDLER /* nothing */
+#else
+#   define _LIBCPP_AVAILABILITY_ASSERTION_HANDLER _LIBCPP_AVAILABILITY_DEFAULT_ASSERTION_HANDLER
+#endif
+
 #endif // _LIBCPP___AVAILABILITY
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
@@ -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/assertions_disabled.pass.cpp b/libcxx/test/libcxx/assertions/assertions_disabled.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/assertions_disabled.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 _LIBCPP_ASSERT doesn't do anything when assertions are disabled.
+// We need to use -Wno-macro-redefined because the test suite defines
+// _LIBCPP_ENABLE_ASSERTIONS=1 under some configurations.
+
+// ADDITIONAL_COMPILE_FLAGS: -Wno-macro-redefined -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/customize_handler.backdeployment.pass.cpp b/libcxx/test/libcxx/assertions/customize_handler.backdeployment.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/customize_handler.backdeployment.pass.cpp
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Make sure that we can enable assertions when we back-deploy to older platforms
+// if we define _LIBCPP_AVAILABILITY_CUSTOM_ASSERTION_HANDLER_PROVIDED.
+//
+// Note that this test isn't really different from customize_handler.pass.cpp when
+// run outside of back-deployment scenarios, but we still run it all the time.
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1 -D_LIBCPP_AVAILABILITY_CUSTOM_ASSERTION_HANDLER_PROVIDED
+
+#include <__assert>
+#include <cassert>
+
+bool handler_called = false;
+void std::__libcpp_assertion_handler(char const*, int, char const*, char const*) {
+  handler_called = true;
+}
+
+int main(int, char**) {
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}
diff --git a/libcxx/test/libcxx/assertions/customize_handler.pass.cpp b/libcxx/test/libcxx/assertions/customize_handler.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/customize_handler.pass.cpp
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// We flag uses of the assertion handler in older dylibs at compile-time to avoid runtime
+// failures when back-deploying.
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+
+#include <__assert>
+#include <cassert>
+
+bool handler_called = false;
+void std::__libcpp_assertion_handler(char const*, int, char const*, char const*) {
+  handler_called = true;
+}
+
+int main(int, char**) {
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}
diff --git a/libcxx/test/libcxx/assertions/debug_mode_compatibility.pass.cpp b/libcxx/test/libcxx/assertions/debug_mode_compatibility.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/debug_mode_compatibility.pass.cpp
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test ensures that assertions are still enabled when _LIBCPP_DEBUG=0 is
+// defined, for backwards compatibility with code that might have been using
+// it to enable assertions previously.
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+
+// We flag uses of the assertion handler in older dylibs at compile-time to avoid runtime
+// failures when back-deploying.
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+
+#include <__assert>
+#include <cassert>
+
+bool handler_called = false;
+void std::__libcpp_assertion_handler(char const*, int, char const*, char const*) {
+  handler_called = true;
+}
+
+int main(int, char**) {
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}
diff --git a/libcxx/test/libcxx/debug/debug_abort.pass.cpp b/libcxx/test/libcxx/assertions/default_handler.abort.pass.cpp
rename from libcxx/test/libcxx/debug/debug_abort.pass.cpp
rename to libcxx/test/libcxx/assertions/default_handler.abort.pass.cpp
--- a/libcxx/test/libcxx/debug/debug_abort.pass.cpp
+++ b/libcxx/test/libcxx/assertions/default_handler.abort.pass.cpp
@@ -6,26 +6,25 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
+// Test that the default assertion handler aborts the program.
 
-// Test that the default debug handler aborts the program.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// We flag uses of the assertion handler in older dylibs at compile-time to avoid runtime
+// failures when back-deploying.
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
 
 #include <__assert>
 #include <csignal>
 #include <cstdlib>
 
-#include "test_macros.h"
-
-void signal_handler(int signal)
-{
-    if (signal == SIGABRT)
-      std::_Exit(EXIT_SUCCESS);
-    std::_Exit(EXIT_FAILURE);
+void signal_handler(int signal) {
+  if (signal == SIGABRT)
+    std::_Exit(EXIT_SUCCESS);
+  std::_Exit(EXIT_FAILURE);
 }
 
-int main(int, char**)
-{
+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/default_handler.availability.verify.cpp b/libcxx/test/libcxx/assertions/default_handler.availability.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/assertions/default_handler.availability.verify.cpp
@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Make sure that we diagnose any usage of the default assertion handler on a platform
+// that doesn't support it at compile-time.
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// REQUIRES: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+
+#include <__assert>
+
+void f() {
+  _LIBCPP_ASSERT(true, "message"); // expected-error {{'__libcpp_assertion_handler' is unavailable}}
+}
diff --git a/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.back.pass.cpp b/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.back.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.back.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.back.pass.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // test that array<T, 0>::back() triggers an assertion
 
diff --git a/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.front.pass.cpp b/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.front.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.front.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.front.pass.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // test that array<T, 0>::back() triggers an assertion
 
diff --git a/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.subscript.pass.cpp b/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.subscript.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.subscript.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/array/array.zero/assert.subscript.pass.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // test that array<T, 0>::operator[] triggers an assertion
 
diff --git a/libcxx/test/libcxx/containers/sequences/deque/assert.pop_back.empty.pass.cpp b/libcxx/test/libcxx/containers/sequences/deque/assert.pop_back.empty.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/deque/assert.pop_back.empty.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/deque/assert.pop_back.empty.pass.cpp
@@ -10,8 +10,9 @@
 
 // pop_back() more than the number of elements in a deque
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <deque>
 
diff --git a/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.erase_iter.end.pass.cpp b/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.erase_iter.end.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.erase_iter.end.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.erase_iter.end.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call erase(const_iterator position) with end()
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <list>
 
diff --git a/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.pop_back.empty.pass.cpp b/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.pop_back.empty.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.pop_back.empty.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/list/list.modifiers/assert.pop_back.empty.pass.cpp
@@ -10,8 +10,9 @@
 
 // void pop_back();
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <list>
 #include <cassert>
diff --git a/libcxx/test/libcxx/containers/sequences/vector/assert.back.empty.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/assert.back.empty.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/assert.back.empty.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/assert.back.empty.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call back() on empty container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <vector>
 #include <cassert>
diff --git a/libcxx/test/libcxx/containers/sequences/vector/assert.cback.empty.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/assert.cback.empty.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/assert.cback.empty.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/assert.cback.empty.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call back() on empty const container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <vector>
 
diff --git a/libcxx/test/libcxx/containers/sequences/vector/assert.cfront.empty.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/assert.cfront.empty.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/assert.cfront.empty.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/assert.cfront.empty.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call front() on empty const container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <vector>
 
diff --git a/libcxx/test/libcxx/containers/sequences/vector/assert.cindex.oob.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/assert.cindex.oob.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/assert.cindex.oob.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/assert.cindex.oob.pass.cpp
@@ -10,8 +10,9 @@
 
 // Index const vector out of bounds.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <vector>
 #include <cassert>
diff --git a/libcxx/test/libcxx/containers/sequences/vector/assert.front.empty.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/assert.front.empty.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/assert.front.empty.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/assert.front.empty.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call front() on empty container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <vector>
 #include <cassert>
diff --git a/libcxx/test/libcxx/containers/sequences/vector/assert.index.oob.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/assert.index.oob.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/assert.index.oob.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/assert.index.oob.pass.cpp
@@ -10,8 +10,9 @@
 
 // Index vector out of bounds.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <vector>
 #include <cassert>
diff --git a/libcxx/test/libcxx/containers/sequences/vector/assert.pop_back.empty.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/assert.pop_back.empty.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/assert.pop_back.empty.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/assert.pop_back.empty.pass.cpp
@@ -10,8 +10,9 @@
 
 // pop_back() more than the number of elements in a vector
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <vector>
 
diff --git a/libcxx/test/libcxx/containers/sequences/vector/robust_against_adl.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/robust_against_adl.pass.cpp
--- a/libcxx/test/libcxx/containers/sequences/vector/robust_against_adl.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/robust_against_adl.pass.cpp
@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
-// UNSUPPORTED: debug_level=0, debug_level=1
+// UNSUPPORTED: debug_level=0, debug_level=1, libcpp-has-assertions
 
 // <vector>
 
diff --git a/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket.pass.cpp
@@ -10,8 +10,9 @@
 
 // size_type bucket(const key_type& __k) const;
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_map>
 #include <string>
diff --git a/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket_size.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket_size.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket_size.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.map/assert.bucket_size.pass.cpp
@@ -14,8 +14,9 @@
 
 // size_type bucket_size(size_type n) const
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_map>
 #include <string>
diff --git a/libcxx/test/libcxx/containers/unord/unord.map/assert.max_load_factor.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.map/assert.max_load_factor.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.map/assert.max_load_factor.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.map/assert.max_load_factor.pass.cpp
@@ -15,8 +15,9 @@
 // float max_load_factor() const;
 // void max_load_factor(float mlf);
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_map>
 #include <string>
diff --git a/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket.pass.cpp
@@ -14,8 +14,9 @@
 
 // size_type bucket(const key_type& __k) const;
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_map>
 #include <string>
diff --git a/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket_size.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket_size.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket_size.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.multimap/assert.bucket_size.pass.cpp
@@ -14,8 +14,9 @@
 
 // size_type bucket_size(size_type n) const
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_map>
 #include <string>
diff --git a/libcxx/test/libcxx/containers/unord/unord.multimap/assert.max_load_factor.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.multimap/assert.max_load_factor.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.multimap/assert.max_load_factor.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.multimap/assert.max_load_factor.pass.cpp
@@ -15,8 +15,9 @@
 // float max_load_factor() const;
 // void max_load_factor(float mlf);
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_map>
 #include <string>
diff --git a/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket.pass.cpp
@@ -14,8 +14,9 @@
 
 // size_type bucket(const key_type& __k) const;
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_set>
 
diff --git a/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket_size.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket_size.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket_size.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.multiset/assert.bucket_size.pass.cpp
@@ -14,8 +14,9 @@
 
 // size_type bucket_size(size_type n) const
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_set>
 
diff --git a/libcxx/test/libcxx/containers/unord/unord.multiset/assert.max_load_factor.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.multiset/assert.max_load_factor.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.multiset/assert.max_load_factor.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.multiset/assert.max_load_factor.pass.cpp
@@ -15,8 +15,9 @@
 // float max_load_factor() const;
 // void max_load_factor(float mlf);
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_set>
 
diff --git a/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket.pass.cpp
@@ -14,8 +14,9 @@
 
 // size_type bucket(const key_type& __k) const;
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_set>
 
diff --git a/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket_size.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket_size.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket_size.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.set/assert.bucket_size.pass.cpp
@@ -14,8 +14,9 @@
 
 // size_type bucket_size(size_type n) const
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_set>
 
diff --git a/libcxx/test/libcxx/containers/unord/unord.set/assert.max_load_factor.pass.cpp b/libcxx/test/libcxx/containers/unord/unord.set/assert.max_load_factor.pass.cpp
--- a/libcxx/test/libcxx/containers/unord/unord.set/assert.max_load_factor.pass.cpp
+++ b/libcxx/test/libcxx/containers/unord/unord.set/assert.max_load_factor.pass.cpp
@@ -15,8 +15,9 @@
 // float max_load_factor() const;
 // void max_load_factor(float mlf);
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <unordered_set>
 
diff --git a/libcxx/test/libcxx/debug/containers/string.pass.cpp b/libcxx/test/libcxx/debug/containers/string.pass.cpp
--- a/libcxx/test/libcxx/debug/containers/string.pass.cpp
+++ b/libcxx/test/libcxx/debug/containers/string.pass.cpp
@@ -82,7 +82,7 @@
     EXPECT_DEATH( C1.erase(it1) );
     C1.erase(C1.begin(), C1.end());
     assert(C1.size() == 0);
-    EXPECT_DEATH_MATCHES(DebugInfoMatcher("string::pop_back(): string is already empty"), C1.pop_back() );
+    TEST_LIBCPP_ASSERT_FAILURE(C1.pop_back(), "string::pop_back(): string is already empty");
   }
 };
 
diff --git a/libcxx/test/libcxx/debug/register_debug_handler.pass.cpp b/libcxx/test/libcxx/debug/register_debug_handler.pass.cpp
deleted file mode 100644
--- a/libcxx/test/libcxx/debug/register_debug_handler.pass.cpp
+++ /dev/null
@@ -1,30 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
-// UNSUPPORTED: libcxx-no-debug-mode
-
-#include <cstdlib>
-#include <string>
-#include <type_traits>
-#include <__debug>
-#include <cassert>
-
-#include "test_macros.h"
-
-void my_debug_function(std::__libcpp_debug_info const& info) {
-  assert(info.__msg_ == std::string("foo"));
-  std::exit(0);
-}
-
-int main(int, char**)
-{
-  std::__libcpp_set_debug_function(&my_debug_function);
-  _LIBCPP_ASSERT(false, "foo");
-  return 1;
-}
diff --git a/libcxx/test/libcxx/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/assert.deallocate.pass.cpp b/libcxx/test/libcxx/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/assert.deallocate.pass.cpp
--- a/libcxx/test/libcxx/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/assert.deallocate.pass.cpp
+++ b/libcxx/test/libcxx/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/assert.deallocate.pass.cpp
@@ -12,8 +12,9 @@
 
 // T* polymorphic_allocator<T>::deallocate(T*, size_t size)
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <experimental/memory_resource>
 #include <type_traits>
diff --git a/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/assert.deallocate.pass.cpp b/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/assert.deallocate.pass.cpp
--- a/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/assert.deallocate.pass.cpp
+++ b/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/assert.deallocate.pass.cpp
@@ -12,8 +12,9 @@
 
 // T* polymorphic_allocator<T>::deallocate(T*, size_t size)
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <experimental/memory_resource>
 #include <type_traits>
diff --git a/libcxx/test/libcxx/input.output/filesystems/class.path/path.itr/assert.iterator.pass.cpp b/libcxx/test/libcxx/input.output/filesystems/class.path/path.itr/assert.iterator.pass.cpp
--- a/libcxx/test/libcxx/input.output/filesystems/class.path/path.itr/assert.iterator.pass.cpp
+++ b/libcxx/test/libcxx/input.output/filesystems/class.path/path.itr/assert.iterator.pass.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // <filesystem>
 
diff --git a/libcxx/test/libcxx/iterators/assert.advance.pass.cpp b/libcxx/test/libcxx/iterators/assert.advance.pass.cpp
--- a/libcxx/test/libcxx/iterators/assert.advance.pass.cpp
+++ b/libcxx/test/libcxx/iterators/assert.advance.pass.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // <list>
 
diff --git a/libcxx/test/libcxx/iterators/assert.next.pass.cpp b/libcxx/test/libcxx/iterators/assert.next.pass.cpp
--- a/libcxx/test/libcxx/iterators/assert.next.pass.cpp
+++ b/libcxx/test/libcxx/iterators/assert.next.pass.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // <list>
 
diff --git a/libcxx/test/libcxx/iterators/assert.prev.pass.cpp b/libcxx/test/libcxx/iterators/assert.prev.pass.cpp
--- a/libcxx/test/libcxx/iterators/assert.prev.pass.cpp
+++ b/libcxx/test/libcxx/iterators/assert.prev.pass.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // <list>
 
diff --git a/libcxx/test/libcxx/strings/basic.string/string.access/assert.back.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.access/assert.back.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.access/assert.back.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.access/assert.back.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call back() on empty container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 
diff --git a/libcxx/test/libcxx/strings/basic.string/string.access/assert.cback.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.access/assert.cback.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.access/assert.cback.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.access/assert.cback.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call back() on empty const container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 
diff --git a/libcxx/test/libcxx/strings/basic.string/string.access/assert.cfront.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.access/assert.cfront.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.access/assert.cfront.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.access/assert.cfront.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call front() on empty const container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 
diff --git a/libcxx/test/libcxx/strings/basic.string/string.access/assert.cindex.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.access/assert.cindex.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.access/assert.cindex.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.access/assert.cindex.pass.cpp
@@ -10,8 +10,9 @@
 
 // Index const string out of bounds.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 #include <cassert>
diff --git a/libcxx/test/libcxx/strings/basic.string/string.access/assert.front.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.access/assert.front.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.access/assert.front.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.access/assert.front.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call front() on empty container.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 #include <cassert>
diff --git a/libcxx/test/libcxx/strings/basic.string/string.access/assert.index.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.access/assert.index.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.access/assert.index.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.access/assert.index.pass.cpp
@@ -10,8 +10,9 @@
 
 // Index string out of bounds.
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 #include <cassert>
diff --git a/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.erase_iter.null.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.erase_iter.null.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.erase_iter.null.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.erase_iter.null.pass.cpp
@@ -10,8 +10,9 @@
 
 // Call erase(const_iterator position) with end()
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 
diff --git a/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.pop_back.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.pop_back.pass.cpp
--- a/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.pop_back.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.modifiers/assert.pop_back.pass.cpp
@@ -10,8 +10,9 @@
 
 // void pop_back();
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <string>
 
diff --git a/libcxx/test/libcxx/strings/string.view/assert.ctor.pointer.pass.cpp b/libcxx/test/libcxx/strings/string.view/assert.ctor.pointer.pass.cpp
--- a/libcxx/test/libcxx/strings/string.view/assert.ctor.pointer.pass.cpp
+++ b/libcxx/test/libcxx/strings/string.view/assert.ctor.pointer.pass.cpp
@@ -8,8 +8,9 @@
 
 // UNSUPPORTED: c++11, c++14
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // Construct a string_view from a null pointer
 // constexpr basic_string_view( const CharT* s );
diff --git a/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception.pass.cpp b/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception.pass.cpp
--- a/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception.pass.cpp
+++ b/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception.pass.cpp
@@ -8,8 +8,9 @@
 
 // UNSUPPORTED: libcpp-has-no-threads
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // <future>
 
diff --git a/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception_at_thread_exit.pass.cpp b/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception_at_thread_exit.pass.cpp
--- a/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception_at_thread_exit.pass.cpp
+++ b/libcxx/test/libcxx/thread/futures/futures.promise/assert.set_exception_at_thread_exit.pass.cpp
@@ -8,8 +8,9 @@
 
 // UNSUPPORTED: libcpp-has-no-threads
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // <future>
 
diff --git a/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.dereference.pass.cpp b/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.dereference.pass.cpp
--- a/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.dereference.pass.cpp
+++ b/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.dereference.pass.cpp
@@ -15,8 +15,9 @@
 
 // UNSUPPORTED: c++11, c++14
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <optional>
 
diff --git a/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.op_arrow.pass.cpp b/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.op_arrow.pass.cpp
--- a/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.op_arrow.pass.cpp
+++ b/libcxx/test/libcxx/utilities/optional/optional.object/optional.object.observe/assert.op_arrow.pass.cpp
@@ -13,8 +13,9 @@
 
 // UNSUPPORTED: c++11, c++14
 
-// UNSUPPORTED: c++03, windows, libcxx-no-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #include <optional>
 
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 assertion 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/test/libcxx/debug/check_assertion_test.pass.cpp b/libcxx/test/support/test.support/test_check_assertion.pass.cpp
rename from libcxx/test/libcxx/debug/check_assertion_test.pass.cpp
rename to libcxx/test/support/test.support/test_check_assertion.pass.cpp
--- a/libcxx/test/libcxx/debug/check_assertion_test.pass.cpp
+++ b/libcxx/test/support/test.support/test_check_assertion.pass.cpp
@@ -6,17 +6,18 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: libcxx-no-debug-mode, c++03, windows
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
+// UNSUPPORTED: c++03, windows
+// UNSUPPORTED: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11|12}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
-#include <__debug>
+#include <cassert>
 #include <cstdio>
+#include <string>
 
 #include "check_assertion.h"
-#include "test_macros.h"
 
 template <class Func>
-inline bool TestDeathTest(const char* stmt, Func&& func, DeathTest::ResultKind ExpectResult, DebugInfoMatcher Matcher = AnyMatcher) {
+inline bool TestDeathTest(const char* stmt, Func&& func, DeathTest::ResultKind ExpectResult, AssertionInfoMatcher Matcher = AnyMatcher) {
   DeathTest DT(Matcher);
   DeathTest::ResultKind RK = DT.Run(func);
   auto OnFailure = [&](std::string msg) {
@@ -42,7 +43,7 @@
 }
 
 void test_no_match_found() {
-  DebugInfoMatcher ExpectMatch("my message");
+  AssertionInfoMatcher ExpectMatch("my message");
   TEST_DEATH_TEST_MATCHES(DeathTest::RK_MatchFailure, ExpectMatch, my_libcpp_assert());
 }
 
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 +183,14 @@
               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'),
+              AddFeature('libcpp-has-assertions')
+            ] 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 "