diff --git a/libcxx/docs/UsingLibcxx.rst b/libcxx/docs/UsingLibcxx.rst
index 1d2cc9e..1fd1d49 100644
--- a/libcxx/docs/UsingLibcxx.rst
+++ b/libcxx/docs/UsingLibcxx.rst
@@ -155,3 +155,30 @@
   Defining this macro and then building libc++ with hidden visibility gives a
   build of libc++ which does not export any symbols, which can be useful when
   building statically for inclusion into another library.
+
+**_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION**:
+  This macro is used to re-enable an extension in `std::tuple` which allowed
+  it to be implicitly constructed from fewer initializers than contained
+  elements. Elements without an initializer are default constructed. For example:
+
+  .. code-block:: cpp
+
+    std::tuple<std::string, int, std::error_code> foo() {
+      return {"hello world", 42}; // default constructs error_code
+    }
+
+
+  Since libc++ 4.0 this extension has been disabled by default. This macro
+  may be defined to re-enable it in order to support existing code that depends
+  on the extension. New use of this extension should be discouraged.
+  See `PR 27374 <http://llvm.org/PR27374>`_ for more information.
+
+  Note: The "reduced-arity-initialization" extension is still offered but only
+  for explicit conversions. Example:
+
+  .. code-block:: cpp
+
+    auto foo() {
+      using Tup = std::tuple<std::string, int, std::error_code>;
+      return Tup{"hello world", 42}; // explicit constructor called. OK.
+    }