Index: libcxx/docs/DesignDocs/DebugMode.rst
===================================================================
--- libcxx/docs/DesignDocs/DebugMode.rst
+++ libcxx/docs/DesignDocs/DebugMode.rst
@@ -56,6 +56,25 @@
     // control flow doesn't return
   }
 
+The above example overrides the bug handler at run-time, but it is also
+possible to override it at compile time using the
+``_LIBCPP_DEBUG_FUNCTION_INVOCATION`` macro. This macro takes 2 arguments: the
+expression ``x`` to trigger the assertion, and an error message ``m``. The
+default invocation uses these arguments to construct the
+``std::__libcpp_debug_info`` argument, but you can do whatever you like with
+them (including nothing):
+
+.. code-block:: cpp
+
+  #define _LIBCPP_DEBUG_FUNCTION_INVOCATION(x, m) abort()
+  #include <string>
+  int main(int, char**) {
+    std::string::iterator bad_it;
+    std::string str("hello world");
+    str.insert(bad_it, '!'); // causes debug assertion
+    // control flow doesn't return
+  }
+
 Debug Mode Checks
 =================
 
@@ -66,11 +85,11 @@
 Basic Checks
 ============
 
-These checks are enabled when ``_LIBCPP_DEBUG`` is defined to either 0 or 1.
-
-The following checks are enabled by ``_LIBCPP_DEBUG``:
+These checks are enabled when ``_LIBCPP_DEBUG`` is defined to either 0 or 1:
 
-  * FIXME: Update this list
+  * Bounds checks in containers (e.g ``operator[]``)
+  * Validity of dereferencing (e.g. ``operator*``, ``operator->``)
+  * Invariants and preconditions (e.g. ``has_value`` for ``std::optional``)
 
 Iterator Debugging Checks
 =========================
@@ -82,10 +101,11 @@
   * ``std::string``
   * ``std::vector<T>`` (``T != bool``)
   * ``std::list``
+  * ``std::locale``
   * ``std::unordered_map``
   * ``std::unordered_multimap``
   * ``std::unordered_set``
   * ``std::unordered_multiset``
 
-The remaining containers do not currently support iterator debugging.
-Patches welcome.
+The remaining includes do not currently support iterator debugging. Patches
+welcome.
Index: libcxx/include/__debug
===================================================================
--- libcxx/include/__debug
+++ libcxx/include/__debug
@@ -27,15 +27,19 @@
 #   include <cstddef>
 #endif
 
+#if !defined(_LIBCPP_DEBUG_FUNCTION_INVOCATION)
+#   define _LIBCPP_DEBUG_FUNCTION_INVOCATION(x, m) _VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, #x, m))
+#endif
+
 #if _LIBCPP_DEBUG_LEVEL == 0
 #   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)
 #   define _LIBCPP_ASSERT_IMPL(x, m) ((void)0)
 #elif _LIBCPP_DEBUG_LEVEL == 1
 #   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)
-#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
+#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _LIBCPP_DEBUG_FUNCTION_INVOCATION((x), m))
 #elif _LIBCPP_DEBUG_LEVEL == 2
 #   define _LIBCPP_DEBUG_ASSERT(x, m) _LIBCPP_ASSERT(x, m)
-#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
+#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _LIBCPP_DEBUG_FUNCTION_INVOCATION((x), m))
 #else
 #   error _LIBCPP_DEBUG_LEVEL must be one of 0, 1, 2
 #endif