Index: include/std/type_traits
===================================================================
--- include/std/type_traits	(revision 181554)
+++ include/std/type_traits	(working copy)
@@ -745,6 +745,8 @@
   // Implementation for non-reference types. To meet the proper
   // variable definition semantics, we also need to test for
   // is_destructible in this case.
+  // This form should be simplified by a single expression:
+  // ::delete ::new _Tp(declval<_Arg>()), see c++/51222.
   struct __do_is_direct_constructible_impl
   {
     template<typename _Tp, typename _Arg, typename
@@ -778,9 +780,12 @@
     struct remove_reference;
 
   template<typename _From, typename _To, bool
-           = is_reference<_From>::value>
+           = __not_<__or_<is_void<_From>, 
+                          is_function<_From>>>::value>
     struct __is_base_to_derived_ref;
 
+  // Detect whether we have a downcast situation during
+  // reference binding.
   template<typename _From, typename _To>
     struct __is_base_to_derived_ref<_From, _To, true>
     {
@@ -803,6 +808,8 @@
                     is_rvalue_reference<_To>>::value>
     struct __is_lvalue_to_rvalue_ref;
 
+  // Detect whether we have an lvalue of non-function type
+  // bound to a reference-compatible rvalue-reference.
   template<typename _From, typename _To>
     struct __is_lvalue_to_rvalue_ref<_From, _To, true>
     {
@@ -810,8 +817,9 @@
         _From>::type>::type __src_t;
       typedef typename remove_cv<typename remove_reference<
         _To>::type>::type __dst_t;
-      typedef __or_<is_same<__src_t, __dst_t>,
-		    is_base_of<__dst_t, __src_t>> type;
+      typedef __and_<__not_<is_function<__src_t>>, 
+        __or_<is_same<__src_t, __dst_t>,
+		    is_base_of<__dst_t, __src_t>>> type;
       static constexpr bool value = type::value;
     };
 
@@ -823,9 +831,9 @@
   // Here we handle direct-initialization to a reference type as 
   // equivalent to a static_cast modulo overshooting conversions.
   // These are restricted to the following conversions:
-  //    a) A glvalue of a base class to a derived class reference
+  //    a) A base class value to a derived class reference
   //    b) An lvalue to an rvalue-reference of reference-compatible 
-  //       types
+  //       types that are not functions
   template<typename _Tp, typename _Arg>
     struct __is_direct_constructible_ref_cast
     : public __and_<__is_static_castable<_Arg, _Tp>,
@@ -850,7 +858,9 @@
 
   // Since default-construction and binary direct-initialization have
   // been handled separately, the implementation of the remaining
-  // n-ary construction cases is rather straightforward.
+  // n-ary construction cases is rather straightforward. We can use
+  // here a functional cast, because array types are excluded anyway
+  // and this form is never interpreted as a C cast.
   struct __do_is_nary_constructible_impl
   {
     template<typename _Tp, typename... _Args, typename
Index: testsuite/20_util/make_signed/requirements/typedefs_neg.cc
===================================================================
--- testsuite/20_util/make_signed/requirements/typedefs_neg.cc	(revision 181554)
+++ testsuite/20_util/make_signed/requirements/typedefs_neg.cc	(working copy)
@@ -48,5 +48,5 @@
 // { dg-error "required from here" "" { target *-*-* } 40 }
 // { dg-error "required from here" "" { target *-*-* } 42 }
 
-// { dg-error "invalid use of incomplete type" "" { target *-*-* } 1545 }
-// { dg-error "declaration of" "" { target *-*-* } 1509 }
+// { dg-error "invalid use of incomplete type" "" { target *-*-* } 1555 }
+// { dg-error "declaration of" "" { target *-*-* } 1519 }
Index: testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc
===================================================================
--- testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc	(revision 181554)
+++ testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc	(working copy)
@@ -48,5 +48,5 @@
 // { dg-error "required from here" "" { target *-*-* } 40 }
 // { dg-error "required from here" "" { target *-*-* } 42 }
 
-// { dg-error "invalid use of incomplete type" "" { target *-*-* } 1463 }
-// { dg-error "declaration of" "" { target *-*-* } 1427 }
+// { dg-error "invalid use of incomplete type" "" { target *-*-* } 1473 }
+// { dg-error "declaration of" "" { target *-*-* } 1437 }
Index: testsuite/20_util/declval/requirements/1_neg.cc
===================================================================
--- testsuite/20_util/declval/requirements/1_neg.cc	(revision 181554)
+++ testsuite/20_util/declval/requirements/1_neg.cc	(working copy)
@@ -19,7 +19,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// { dg-error "static assertion failed" "" { target *-*-* } 1759 }
+// { dg-error "static assertion failed" "" { target *-*-* } 1769 }
 
 #include <utility>
 
Index: testsuite/20_util/is_constructible/value-2.cc
===================================================================
--- testsuite/20_util/is_constructible/value-2.cc	(revision 181554)
+++ testsuite/20_util/is_constructible/value-2.cc	(working copy)
@@ -817,3 +817,5 @@
 	      "Error");
 static_assert(!std::is_constructible<int&&, ExplicitTo<double&&>>::value,
 	      "Error");
+
+static_assert(std::is_constructible<void(&&)(), void(&)()>::value, "Error");
Index: testsuite/20_util/is_constructible/51185.cc
===================================================================
--- testsuite/20_util/is_constructible/51185.cc	(revision 0)
+++ testsuite/20_util/is_constructible/51185.cc	(revision 0)
@@ -0,0 +1,39 @@
+// { dg-options "-std=gnu++0x" }
+// { dg-do compile }
+
+// Copyright (C) 2011 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <type_traits>
+
+struct A { };
+struct B : A { };
+
+// libstdc++/51185
+void f()
+{
+  static_assert(!std::is_constructible<B &&, A>(), "");
+  static_assert(!std::is_constructible<B const &&, A>(), "");
+  static_assert(!std::is_constructible<B const &&, A const>(), "");
+  static_assert(!std::is_constructible<B volatile &&, A>(), "");
+  static_assert(!std::is_constructible<B volatile &&, A volatile>(), "");
+  static_assert(!std::is_constructible<B const volatile &&, A>(), "");
+  static_assert(!std::is_constructible<B const volatile &&, A const>(), "");
+  static_assert(!std::is_constructible<B const volatile &&, A volatile>(), "");
+  static_assert(!std::is_constructible<B const volatile &&,
+		A const volatile>(), "");
+}
