[patch] Add std::void_t

[Jonathan Wakely <jwakely at redhat dot com>]

Define std::void_t for -std=c++17 and -std=gnu++11 (like Ville's
uncaught_exceptions() patch I think this is useful to define as an
extension pre-C++17, and the feature-test macro makes it easy for
users to test for the presence/absence of it in different modes).

Tested ppc64le-linux, committed to trunk.

commit 16eaf21080bb603abbfd1098c9c867bc7cefe96a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Apr 27 14:50:19 2015 +0100

    	* include/std/type_traits (void_t): Add.
    	* testsuite/20_util/void_t/1.cc: New.

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 03e198a..3ed1a7b 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2406,6 +2406,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename...> using __void_t = void;
 
+#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
+#define __cpp_lib_void_t 201411
+  /// A metafunction that always yields void, used for detecting valid types.
+  template<typename...> using void_t = void;
+#endif
+
   /// @} group metaprogramming
 
   /**
diff --git a/libstdc++-v3/testsuite/20_util/void_t/1.cc b/libstdc++-v3/testsuite/20_util/void_t/1.cc
new file mode 100644
index 0000000..fe5c507
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/void_t/1.cc
@@ -0,0 +1,45 @@
+// Copyright (C) 2015 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++1z" }
+// { dg-do compile }
+
+#include <type_traits>
+
+#ifndef __cpp_lib_void_t
+# error "Feature-test macro for void_t missing"
+#elif __cpp_lib_void_t != 201411
+# error "Feature-test macro for void_t has wrong value"
+#endif
+
+static_assert( std::is_same<std::void_t<int, long, double>, void>::value,
+               "void_t is a synonym for void" );
+
+struct X { };
+X operator+(const X& x) { return x; }
+
+template<typename T, typename U = std::void_t<>>
+struct has_unary_plus
+: std::false_type
+{ };
+
+template<typename T>
+struct has_unary_plus<T, std::void_t<decltype(+std::declval<X>())>>
+: std::true_type
+{ };
+
+static_assert( has_unary_plus<X>::value, "void_t detects valid expressions" );