[v3 PATCH] Implement C++17 make_from_tuple.

[Ville Voutilainen]

I was in the middle of doing this, so here's the patch before I commit
the string_view one.

Tested on Linux-x64.

2016-08-11  Ville Voutilainen  <ville.voutilai...@gmail.com>

    Implement C++17 make_from_tuple.
    * include/std/tuple (__make_from_tuple_impl, make_from_tuple): New.
    * testsuite/20_util/tuple/make_from_tuple/1.cc: Likewise.

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index b9074cb..3403048 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1655,6 +1655,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __apply_impl(std::forward<_Fn>(__f), std::forward<_Tuple>(__t),
                          _Indices{});
     }
+
+  template <typename _Tp, typename _Tuple, size_t... _Idx>
+    constexpr _Tp
+    __make_from_tuple_impl(_Tuple&& __t, index_sequence<_Idx...>)
+    { return _Tp(get<_Idx>(std::forward<_Tuple>(__t))...); }
+
+  template <typename _Tp, typename _Tuple>
+    constexpr _Tp
+    make_from_tuple(_Tuple&& __t)
+    {
+      return __make_from_tuple_impl<_Tp>(
+        std::forward<_Tuple>(__t),
+       make_index_sequence<tuple_size_v<decay_t<_Tuple>>>{});
+    }
 #endif // C++17
 
   /// @}
diff --git a/libstdc++-v3/testsuite/20_util/tuple/make_from_tuple/1.cc 
b/libstdc++-v3/testsuite/20_util/tuple/make_from_tuple/1.cc
new file mode 100644
index 0000000..7963ea1
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/make_from_tuple/1.cc
@@ -0,0 +1,55 @@
+// 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 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" }
+
+#include <tuple>
+#include <testsuite_hooks.h>
+#include <functional>
+
+template <class T, class U, class V>
+struct ThreeParam
+{
+  T t;
+  U u;
+  V v;
+  ThreeParam(const T& t, const U& u, const V& v)
+    : t(t),
+      u(u),
+      v(v) {}
+};
+
+void
+test01()
+{
+  auto x = std::make_tuple(1024, 'x', 2048);
+  ThreeParam<int, char, int> y
+    = std::make_from_tuple<ThreeParam<int, char, int>>(x);
+  VERIFY(y.t == 1024 && y.u == 'x' && y.v == 2048);
+  auto x2 = std::make_tuple(4096, 'z');
+  std::pair<int, char> z = std::make_from_tuple<std::pair<int, char>>(x2);
+  VERIFY(z.first == 4096 && z.second == 'z');
+  auto x3 = std::make_tuple(8192);
+  int i = std::make_from_tuple<int>(x3);
+  VERIFY(i == 8192);
+}
+
+int
+main()
+{
+  test01();
+}