type_traits (void_t): Add.
* include/std/type_traits (void_t): Add. * testsuite/20_util/void_t/1.cc: New. From-SVN: r222522
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2015-04-28 Jonathan Wakely <jwakely@redhat.com>
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* include/std/type_traits (void_t): Add.
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* testsuite/20_util/void_t/1.cc: New.
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2015-04-28 Tim Shen <timshen@google.com>
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* include/bits/regex.tcc: Handle regex_constants::__polynomial.
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@ -2406,6 +2406,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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template<typename...> using __void_t = void;
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#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
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#define __cpp_lib_void_t 201411
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/// A metafunction that always yields void, used for detecting valid types.
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template<typename...> using void_t = void;
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#endif
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/// @} group metaprogramming
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/**
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// Copyright (C) 2015 Free Software Foundation, Inc.
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//
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// This file is part of the GNU ISO C++ Library. This library is free
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// software; you can redistribute it and/or modify it under the
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// terms of the GNU General Public License as published by the
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// Free Software Foundation; either version 3, or (at your option)
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// any later version.
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// This library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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// You should have received a copy of the GNU General Public License along
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// with this library; see the file COPYING3. If not see
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// <http://www.gnu.org/licenses/>.
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// { dg-options "-std=gnu++1z" }
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// { dg-do compile }
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#include <type_traits>
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#ifndef __cpp_lib_void_t
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# error "Feature-test macro for void_t missing"
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#elif __cpp_lib_void_t != 201411
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# error "Feature-test macro for void_t has wrong value"
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#endif
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static_assert( std::is_same<std::void_t<int, long, double>, void>::value,
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"void_t is a synonym for void" );
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struct X { };
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X operator+(const X& x) { return x; }
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template<typename T, typename U = std::void_t<>>
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struct has_unary_plus
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: std::false_type
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{ };
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template<typename T>
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struct has_unary_plus<T, std::void_t<decltype(+std::declval<X>())>>
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: std::true_type
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{ };
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static_assert( has_unary_plus<X>::value, "void_t detects valid expressions" );
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