[PATCH] Constrain std::variant constructor for class template argument deduction

[Jonathan Wakely]

This patch from Jason solves the problem pointed out at the end of
D0702R1 (not public yet):

std::variant<int, double> v1(3);
std::variant v2 = v1;

Tested powerpc64le-linux, committed to trunk.


-------------- next part --------------
commit 54e520fc40bff5fe441715f23b64c2c788db23dc
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jul 14 19:38:51 2017 +0100

    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.

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 06646f6..23db6d2 100644
--- a/libstdc++-v3/include/std/variant
+++ b/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
diff --git a/libstdc++-v3/testsuite/20_util/variant/deduction.cc b/libstdc++-v3/testsuite/20_util/variant/deduction.cc
new file mode 100644
index 0000000..33bc9d2
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/variant/deduction.cc
@@ -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);
+}