Constrain std::variant constructor for class template argument deduction

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

libstdc++-v3/ChangeLog

+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

libstdc++-v3/include/std/variant

@@ -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

libstdc++-v3/testsuite/20_util/variant/deduction.cc

+// { 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);
+}