Implement LWG 2825, LWG 2756 breaks class template argument deduction for optional.

* include/std/optional: Add a deduction guide.
* testsuite/20_util/optional/cons/deduction_guide.cc: New.

From-SVN: r245024

libstdc++-v3/ChangeLog

@@ -1,3 +1,10 @@
+2017-01-30  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+	Implement LWG 2825, LWG 2756 breaks class template argument
+	deduction for optional.
+	* include/std/optional: Add a deduction guide.
+	* testsuite/20_util/optional/cons/deduction_guide.cc: New.
+
 2017-01-27  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/79254

libstdc++-v3/include/std/optional

@@ -981,6 +981,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// @}
 
+  template <typename _Tp> optional(_Tp) -> optional<_Tp>;
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 

libstdc++-v3/testsuite/20_util/optional/cons/deduction_guide.cc

@@ -0,0 +1,44 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do compile }
+
+// 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/>.
+
+#include <optional>
+#include <type_traits>
+
+struct MoveOnly
+{
+  MoveOnly() = default;
+  MoveOnly(MoveOnly&&) {}
+  MoveOnly& operator=(MoveOnly&&) {}
+};
+
+int main()
+{
+    std::optional x = 5;
+    static_assert(std::is_same_v<decltype(x), std::optional<int>>);
+    int y = 42;
+    std::optional x2 = y;
+    static_assert(std::is_same_v<decltype(x2), std::optional<int>>);
+    const int z = 666;
+    std::optional x3 = z;
+    static_assert(std::is_same_v<decltype(x3), std::optional<int>>);
+    std::optional mo = MoveOnly();
+    static_assert(std::is_same_v<decltype(mo), std::optional<MoveOnly>>);
+    mo = MoveOnly();
+}