Index: docs/DesignDocs/VisibilityMacros.rst
===================================================================
--- docs/DesignDocs/VisibilityMacros.rst
+++ docs/DesignDocs/VisibilityMacros.rst
@@ -22,6 +22,12 @@
   Mark a symbol as being exported by the libc++ library. This attribute must
   be applied to the declaration of all functions exported by the libc++ dylib.
 
+**_LIBCPP_EXTERN_VIS**
+  Mark a symbol as being exported by the libc++ library. This attribute may
+  only be applied to objects defined in the libc++ library. On Windows this
+  macro applies `dllimport`/`dllexport` to the symbol. On all other platforms
+  this macro has no effect.
+
 **_LIBCPP_OVERRIDABLE_FUNC_VIS**
   Mark a symbol as being exported by the libc++ library, but allow it to be
   overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`.
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -583,6 +583,7 @@
 
 #define _LIBCPP_TYPE_VIS            _LIBCPP_DLL_VIS
 #define _LIBCPP_FUNC_VIS            _LIBCPP_DLL_VIS
+#define _LIBCPP_EXTERN_VIS          _LIBCPP_DLL_VIS
 #define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS
 #define _LIBCPP_HIDDEN
 #define _LIBCPP_TEMPLATE_VIS
@@ -636,6 +637,10 @@
 # define _LIBCPP_FUNC_VIS_ONLY _LIBCPP_FUNC_VIS
 #endif
 
+#ifndef _LIBCPP_EXTERN_VIS
+# define _LIBCPP_EXTERN_VIS
+#endif
+
 #ifndef _LIBCPP_OVERRIDABLE_FUNC_VIS
 # define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_FUNC_VIS
 #endif
Index: include/__debug
===================================================================
--- include/__debug
+++ include/__debug
@@ -74,7 +74,7 @@
 
 /// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
 ///    fails.
-extern __libcpp_debug_function_type __libcpp_debug_function;
+extern _LIBCPP_EXTERN_VIS __libcpp_debug_function_type __libcpp_debug_function;
 
 /// __libcpp_abort_debug_function - A debug handler that aborts when called.
 _LIBCPP_NORETURN _LIBCPP_FUNC_VIS