diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -56,6 +56,15 @@
   related to missing declarations inside namespace ``std`` when updating libc++,
   you are probably missing ``#include <algorithm>`` in a file where you use algorithms.
 
+- Users can now enable assertions inside libc++ by defining ``_LIBCPP_DEBUG=0`` on
+  the command-line, regardless of how the library was built (usually by a vendor).
+  Note that if the library was built with assertions disabled, any function that is
+  compiled into the library won't have assertions enabled, but functions compiled into
+  your own code (usually those defined in the headers) will have assertions enabled.
+
+  This feature requires library support, so it is not enabled when deploying on
+  platforms that have not previously shipped with the ability to turn assertions on.
+
 ABI Changes
 -----------
 
diff --git a/libcxx/include/__assert b/libcxx/include/__assert
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP___ASSERT
 #define _LIBCPP___ASSERT
 
+#include <__availability>
 #include <__config>
 #include <iosfwd> // for std::string
 
@@ -40,6 +41,7 @@
   __libcpp_debug_info(const char* __f, int __l, const char* __p, const char* __m)
     : __file_(__f), __line_(__l), __pred_(__p), __msg_(__m) {}
 
+  _LIBCPP_AVAILABILITY_ASSERTIONS
   _LIBCPP_FUNC_VIS string what() const;
 
   const char* __file_;
@@ -53,15 +55,16 @@
 
 /// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
 ///    fails.
+_LIBCPP_AVAILABILITY_ASSERTIONS
 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
+_LIBCPP_AVAILABILITY_ASSERTIONS _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
+_LIBCPP_AVAILABILITY_ASSERTIONS _LIBCPP_FUNC_VIS
 bool __libcpp_set_debug_function(__libcpp_debug_function_type __func);
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__availability b/libcxx/include/__availability
--- a/libcxx/include/__availability
+++ b/libcxx/include/__availability
@@ -81,6 +81,10 @@
 
 #if defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
+    // This controls whether users are allowed to turn on assertions
+    // when using the library.
+#   define _LIBCPP_AVAILABILITY_ASSERTIONS
+
     // This controls the availability of std::shared_mutex and std::shared_timed_mutex,
     // which were added to the dylib later.
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX
@@ -151,6 +155,9 @@
 
 #elif defined(__APPLE__)
 
+#   define _LIBCPP_AVAILABILITY_ASSERTIONS                                      \
+        __attribute__((unavailable))
+
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
         __attribute__((availability(macosx,strict,introduced=10.12)))           \
         __attribute__((availability(ios,strict,introduced=10.0)))               \
diff --git a/libcxx/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
@@ -526,6 +526,7 @@
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_thousands_sepEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__118__time_get_storageIcE15__do_date_orderEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__118__time_get_storageIwE15__do_date_orderEv', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNKSt3__119__libcpp_debug_info4whatEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__119__shared_weak_count13__get_deleterERKSt9type_info', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__120__codecvt_utf8_utf16IDiE10do_unshiftER11__mbstate_tPcS4_RS4_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__120__codecvt_utf8_utf16IDiE11do_encodingEv', 'type': 'FUNC'}
@@ -1533,6 +1534,7 @@
 {'is_defined': True, 'name': '__ZNSt3__123__cxx_atomic_notify_oneEPVKv', 'type': 'FUNC'}
 {'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__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__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'}
@@ -1549,6 +1551,8 @@
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIwwEEPwEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIxxEEPxEEbT0_S5_T_', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIyyEEPyEEbT0_S5_T_', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__127__libcpp_set_debug_functionEPFvRKNS_19__libcpp_debug_infoEE', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNSt3__129__libcpp_abort_debug_functionERKNS_19__libcpp_debug_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -4,6 +4,7 @@
 set(LIBCXX_SOURCES
   algorithm.cpp
   any.cpp
+  assert.cpp
   atomic.cpp
   barrier.cpp
   bind.cpp
@@ -65,7 +66,6 @@
 
 if (LIBCXX_ENABLE_DEBUG_MODE_SUPPORT)
   list(APPEND LIBCXX_SOURCES
-    assert.cpp
     debug.cpp
     )
 endif()