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Constrain std::variant constructor for class template argument deduction

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2017-07-14  Jason Merrill  <jason@redhat.com>
	    Jonathan Wakely  <jwakely@redhat.com>

	* include/std/variant (variant::variant(_Tp&&)): Constrain to remove
	the constructor for empty variants from the candidate functions
	during class template argument deduction.
	* testsuite/20_util/variant/deduction.cc: New.

Co-Authored-By: Jonathan Wakely <jwakely@redhat.com>

From-SVN: r250213
This commit is contained in:
Jason Merrill 2017-07-14 15:13:49 -04:00 • committed by Jonathan Wakely
parent 3b80566699
commit 20316b9b2e
3 changed files with 48 additions and 0 deletions
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8
libstdc++-v3/ChangeLog
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@ -1,3 +1,11 @@
2017-07-14 Jason Merrill <jason@redhat.com>
Jonathan Wakely <jwakely@redhat.com>
* include/std/variant (variant::variant(_Tp&&)): Constrain to remove
the constructor for empty variants from the candidate functions
during class template argument deduction.
* testsuite/20_util/variant/deduction.cc: New.
2017-07-11 Jonathan Wakely <jwakely@redhat.com>
* doc/xml/faq.xml: Update several old entries. Improve

1
libstdc++-v3/include/std/variant
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@ -1101,6 +1101,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp,
typename = enable_if_t<!is_same_v<decay_t<_Tp>, variant>>,
typename = enable_if_t<(sizeof...(_Types)>0)>,
typename = enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
&& is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>>>
constexpr

39
libstdc++-v3/testsuite/20_util/variant/deduction.cc Normal file
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@ -0,0 +1,39 @@
// { dg-options "-std=gnu++17" }
// { dg-do compile { target c++1z } }
// 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 <variant>
template<typename T, typename U> struct require_same;
template<typename T> struct require_same<T, T> { using type = void; };
template<typename T, typename U>
typename require_same<T, U>::type
check_type(U&) { }
void
test01()
{
std::variant<int, double> x(3);
std::variant copy = x;
check_type<decltype(x)>(copy);
std::variant move = std::move(x);
check_type<decltype(x)>(move);
}