[PATCH] PR 79798 Fix incorrect use of std::result_of in std::bind

[Jonathan Wakely]

Another case of problems caused by incorrect use of result_of. Because
functions can't have top-level const on parameters result_of<F(const
T)> is result_of<F(T)> so doesn't give you the answer for the question
you meant to ask.

        PR libstdc++/79798
        * include/std/functional (bind::_Res_type_impl): Fix incorrect use of
        result_of that loses top-level cv-qualifiers.
        * testsuite/20_util/bind/79798.cc: New test.


Tested powerpc64le-linux, committed to trunk.

commit d2d652a7e40288a89259bae17eddec9cde0e177c
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Mar 2 01:33:10 2017 +0000

    PR 79798 Fix incorrect use of std::result_of in std::bind
    
        PR libstdc++/79798
        * include/std/functional (bind::_Res_type_impl): Fix incorrect use of
        result_of that loses top-level cv-qualifiers.
        * testsuite/20_util/bind/79798.cc: New test.

diff --git a/libstdc++-v3/include/std/functional 
b/libstdc++-v3/include/std/functional
index 4f3d8b3..ea36dd0 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -502,7 +502,7 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       template<typename _Fn, typename _CallArgs, typename... _BArgs>
        using _Res_type_impl
-         = typename result_of< _Fn&(_Mu_type<_BArgs, _CallArgs>...) >::type;
+         = typename result_of< _Fn&(_Mu_type<_BArgs, _CallArgs>&&...) >::type;
 
       template<typename _CallArgs>
        using _Res_type = _Res_type_impl<_Functor, _CallArgs, _Bound_args...>;
diff --git a/libstdc++-v3/testsuite/20_util/bind/79798.cc 
b/libstdc++-v3/testsuite/20_util/bind/79798.cc
new file mode 100644
index 0000000..9780ff4
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/bind/79798.cc
@@ -0,0 +1,33 @@
+// 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 <functional>
+
+// PR libstdc++/79798
+
+struct X { };
+const X f(int);
+
+struct Y {
+  void operator()(X&&) = delete;
+  int operator()(const X&&);
+};
+
+auto b = std::bind(Y(), std::bind(f, std::placeholders::_1));
+auto i = b(1);