[v3 PATCH] Make optional::reset noexcept, make optional::value work in constant expressions.

[Ville Voutilainen]

These tidbits were reported by Eric Fiselier.
They are fairly simple conformance issues.

Tested on Linux-x64.

2016-09-29  Ville Voutilainen  <ville.voutilai...@gmail.com>

    Make optional::reset noexcept, make optional::value
    work in constant expressions.
    * include/std/optional (_M_get): Make constexpr.
    (reset): Make noexcept.
    * testsuite/20_util/optional/assignment/7.cc: New.
    * testsuite/20_util/optional/observers/6.cc: New.

diff --git a/libstdc++-v3/include/std/optional 
b/libstdc++-v3/include/std/optional
index b14faf1..21210ab 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -378,7 +378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr bool _M_is_engaged() const noexcept
       { return this->_M_engaged; }
 
-      _Tp&
+      constexpr _Tp&
       _M_get() noexcept
       { return _M_payload; }
 
@@ -777,7 +777,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
            ? std::move(this->_M_get())
            : static_cast<_Tp>(std::forward<_Up>(__u));
        }
-      void reset() { this->_M_reset(); }
+      void reset() noexcept { this->_M_reset(); }
     };
 
   template<typename _Tp>
diff --git a/libstdc++-v3/testsuite/20_util/optional/assignment/7.cc 
b/libstdc++-v3/testsuite/20_util/optional/assignment/7.cc
new file mode 100644
index 0000000..d392b40
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/optional/assignment/7.cc
@@ -0,0 +1,31 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do run }
+
+// Copyright (C) 2016 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 moved_to 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 <testsuite_hooks.h>
+
+int main()
+{
+  std::optional<int> o{666};
+  VERIFY(o && *o == 666);
+  o.reset();
+  VERIFY(!o);
+  static_assert(noexcept(std::declval<std::optional<int>>().reset()));
+}
diff --git a/libstdc++-v3/testsuite/20_util/optional/observers/6.cc 
b/libstdc++-v3/testsuite/20_util/optional/observers/6.cc
new file mode 100644
index 0000000..f156a66
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/optional/observers/6.cc
@@ -0,0 +1,39 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do run }
+
+// Copyright (C) 2016 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 moved_to 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 <testsuite_hooks.h>
+
+struct Y
+{
+  constexpr int test() & {return 7;}
+};
+
+constexpr int
+test()
+{
+  std::optional<Y> opt{Y{}};
+  return opt.value().test();
+}
+
+int main()
+{
+  static_assert(test());
+}