Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -908,6 +908,13 @@
 #define _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
 #endif
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+_LIBCPP_FUNC_VIS _LIBCPP_WEAK int __libcpp_library_version();
+_LIBCPP_END_NAMESPACE_STD
+
+#define _LIBCPP_LIBRARY_VERSION \
+ (_VSTD::__libcpp_library_version ? _VSTD::__libcpp_library_version() : -1)
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
Index: include/__libcpp_version
===================================================================
--- /dev/null
+++ include/__libcpp_version
@@ -0,0 +1 @@
+4000
\ No newline at end of file
Index: src/libcpp_version.cpp
===================================================================
--- /dev/null
+++ src/libcpp_version.cpp
@@ -0,0 +1,14 @@
+#include "__config"
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// Test that _LIBCPP_VERSION and __libcpp_version are in sync.
+// The __libcpp_version file stores only a number representing the libc++
+// version so it can be easily parsed by clang.
+static_assert(_LIBCPP_VERSION ==
+#include "__libcpp_version"
+    , "version file does not match");
+
+int __libcpp_library_version() { return _LIBCPP_VERSION; }
+
+_LIBCPP_END_NAMESPACE_STD
Index: test/libcxx/version.pass.cpp
===================================================================
--- /dev/null
+++ test/libcxx/version.pass.cpp
@@ -0,0 +1,29 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Test the _LIBCPP_VERSION and _LIBCPP_LIBRARY_VERSION macros
+
+#include <__config>
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION must be defined
+#endif
+
+#ifndef _LIBCPP_LIBRARY_VERSION
+#error _LIBCPP_LIBRARY_VERSION must be defined
+#endif
+
+#include <cassert>
+
+int main() {
+  assert(_LIBCPP_VERSION == _LIBCPP_LIBRARY_VERSION);
+  assert(std::__libcpp_library_version);
+  assert(_LIBCPP_LIBRARY_VERSION == std::__libcpp_library_version());
+}