Index: include/functional
===================================================================
--- include/functional
+++ include/functional
@@ -1623,7 +1623,10 @@
     function(const function&);
     function(function&&) _NOEXCEPT;
     template<class _Fp, class = typename enable_if<
-        __callable<_Fp>::value && !is_same<_Fp, function>::value
+        is_same<typename enable_if<!is_same<_Fp, function>::value>::type,
+                void
+        >::value &&
+        __callable<_Fp>::value
     >::type>
     function(_Fp);
 
@@ -1648,8 +1651,11 @@
     template<class _Fp>
       typename enable_if
       <
-        __callable<typename decay<_Fp>::type>::value &&
-        !is_same<typename remove_reference<_Fp>::type, function>::value,
+        is_same<typename enable_if<
+         !is_same<typename remove_reference<_Fp>::type, function>::value>::type,
+         void
+        >::value &&
+        __callable<typename decay<_Fp>::type>::value,
         function&
       >::type
       operator=(_Fp&&);
@@ -1857,8 +1863,11 @@
 template <class _Fp>
 typename enable_if
 <
-    function<_Rp(_ArgTypes...)>::template __callable<typename decay<_Fp>::type>::value &&
-    !is_same<typename remove_reference<_Fp>::type, function<_Rp(_ArgTypes...)>>::value,
+    is_same<typename enable_if<
+     !is_same<typename remove_reference<_Fp>::type, function<_Rp(_ArgTypes...)>>::value>::type,
+     void
+    >::value &&
+    function<_Rp(_ArgTypes...)>::template __callable<typename decay<_Fp>::type>::value,
     function<_Rp(_ArgTypes...)>&
 >::type
 function<_Rp(_ArgTypes...)>::operator=(_Fp&& __f)
Index: test/libcxx/utilities/function.objects/func.require/incomplete_return_type.pass.cpp
===================================================================
--- /dev/null
+++ test/libcxx/utilities/function.objects/func.require/incomplete_return_type.pass.cpp
@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// Non-standard extension:
+// std::function is allowed to take a function with an incomplete return type.
+
+// [func.require]
+
+#include <functional>
+
+struct IncompleteReturnType {
+  std::function<IncompleteReturnType ()> fn;
+};
+
+int main() {
+  return 0;
+}