PR libstdc++/78939 make tuple_size<cv T> depend on tuple_size<T>

diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 188fcc2..c18bcb6 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -88,24 +88,26 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct tuple_size;
 
   // _GLIBCXX_RESOLVE_LIB_DEFECTS
-  // 2770. tuple_size<const T> specialization is not SFINAE compatible
-  template<typename _Tp, typename = void>
-    struct __tuple_size_cv_impl { };
-
-  template<typename _Tp>
-    struct __tuple_size_cv_impl<_Tp, __void_t<decltype(tuple_size<_Tp>::value)>>
-    : integral_constant<size_t, tuple_size<_Tp>::value> { };
-
-  // _GLIBCXX_RESOLVE_LIB_DEFECTS
   // 2313. tuple_size should always derive from integral_constant<size_t, N>
-  template<typename _Tp>
-    struct tuple_size<const _Tp> : __tuple_size_cv_impl<_Tp> { };
+  // 2770. tuple_size<const T> specialization is not SFINAE compatible
+
+  template<typename _Tp,
+	   typename _Up = typename remove_cv<_Tp>::type,
+	   typename = typename enable_if<is_same<_Tp, _Up>::value>::type,
+	   size_t = tuple_size<_Tp>::value>
+    using __enable_if_has_tuple_size = _Tp;
 
   template<typename _Tp>
-    struct tuple_size<volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
+    struct tuple_size<const __enable_if_has_tuple_size<_Tp>>
+    : public tuple_size<_Tp> { };
 
   template<typename _Tp>
-    struct tuple_size<const volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
+    struct tuple_size<volatile __enable_if_has_tuple_size<_Tp>>
+    : public tuple_size<_Tp> { };
+
+  template<typename _Tp>
+    struct tuple_size<const volatile __enable_if_has_tuple_size<_Tp>>
+    : public tuple_size<_Tp> { };
 
   /// Gives the type of the ith element of a given tuple type.
   template<std::size_t __i, typename _Tp>
diff --git a/libstdc++-v3/testsuite/20_util/tuple/78939.cc b/libstdc++-v3/testsuite/20_util/tuple/78939.cc
new file mode 100644
index 0000000..bab143b
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/78939.cc
@@ -0,0 +1,49 @@ 
+// Copyright (C) 2017 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/>.
+
+// { dg-options "-std=gnu++17" }
+// { dg-do compile { target c++1z } }
+
+// PR libstdc++/78939
+
+#include <utility>
+
+struct A { int i, j; };
+
+int
+test01()
+{
+  A a{};
+  const auto [i, j] = a;
+  return i + j;
+}
+
+int
+test02()
+{
+  A a{};
+  volatile auto [i, j] = a;
+  return i + j;
+}
+
+int
+test03()
+{
+  A a{};
+  const volatile auto [i, j] = a;
+  return i + j;
+}
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cv_tuple_size_neg.cc b/libstdc++-v3/testsuite/20_util/tuple/cv_tuple_size_neg.cc
new file mode 100644
index 0000000..450ed89
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/cv_tuple_size_neg.cc
@@ -0,0 +1,35 @@ 
+// Copyright (C) 2017 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/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <tuple>
+
+// PR libstdc++/78939
+
+std::tuple_size<const int> ic;		  // { dg-error "incomplete" }
+std::tuple_size<volatile int> iv;	  // { dg-error "incomplete" }
+std::tuple_size<const volatile int> icv;  // { dg-error "incomplete" }
+
+struct A { };
+std::tuple_size<const A> ac;		  // { dg-error "incomplete" }
+std::tuple_size<volatile A> av;		  // { dg-error "incomplete" }
+std::tuple_size<const volatile A> acv;	  // { dg-error "incomplete" }
+
+std::tuple_size<const void> vc;		  // { dg-error "incomplete" }
+std::tuple_size<volatile void> vv;	  // { dg-error "incomplete" }
+std::tuple_size<const volatile void> vcv; // { dg-error "incomplete" }