re PR libstdc++/79141 (std::pair<int,int> p = {}; fails to compile due to ambiguous overload)

PR libstdc++/79141
* include/bits/stl_pair.h (__nonesuch_no_braces): New.
(operator=(typename conditional<
__and_<is_copy_assignable<_T1>,
is_copy_assignable<_T2>>::value,
const pair&, const __nonesuch&>::type)): Change __nonesuch
to __nonesuch_no_braces.
(operator=(typename conditional<
__not_<__and_<is_copy_assignable<_T1>,
is_copy_assignable<_T2>>>::value,
const pair&, const __nonesuch&>::type)): Likewise.
(operator=(typename conditional<
__and_<is_move_assignable<_T1>,
is_move_assignable<_T2>>::value,
pair&&, __nonesuch&&>::type)): Likewise.
* testsuite/20_util/pair/79141.cc: New.

From-SVN: r246653

libstdc++-v3/ChangeLog

 2017-04-03  Ville Voutilainen  <ville.voutilainen@gmail.com>
 
+	PR libstdc++/79141
+	* include/bits/stl_pair.h (__nonesuch_no_braces): New.
+	(operator=(typename conditional<
+	__and_<is_copy_assignable<_T1>,
+	is_copy_assignable<_T2>>::value,
+	const pair&, const __nonesuch&>::type)): Change __nonesuch
+	to __nonesuch_no_braces.
+	(operator=(typename conditional<
+	__not_<__and_<is_copy_assignable<_T1>,
+	is_copy_assignable<_T2>>>::value,
+	const pair&, const __nonesuch&>::type)): Likewise.
+	(operator=(typename conditional<
+	__and_<is_move_assignable<_T1>,
+	is_move_assignable<_T2>>::value,
+	pair&&, __nonesuch&&>::type)): Likewise.
+	* testsuite/20_util/pair/79141.cc: New.
+
+2017-04-03  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
 	Implement std::is_aggregate.
 	* include/std/type_traits (is_aggregate, is_aggregate_v): New.
 	* testsuite/20_util/is_aggregate/requirements/explicit_instantiation.cc:

libstdc++-v3/include/bits/stl_pair.h

@@ -179,6 +179,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
   };
 
+  // PR libstdc++/79141, a utility type for preventing
+  // initialization of an argument of a disabled assignment
+  // operator from a pair of empty braces.
+  struct __nonesuch_no_braces : std::__nonesuch {
+    explicit __nonesuch_no_braces(const __nonesuch&) = delete;
+  };
+
 #endif
 
  /**
@@ -360,7 +367,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator=(typename conditional<
 		__and_<is_copy_assignable<_T1>,
 		       is_copy_assignable<_T2>>::value,
-		const pair&, const __nonesuch&>::type __p)
+		const pair&, const __nonesuch_no_braces&>::type __p)
       {
 	first = __p.first;
 	second = __p.second;
@@ -371,13 +378,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator=(typename conditional<
 		__not_<__and_<is_copy_assignable<_T1>,
 		              is_copy_assignable<_T2>>>::value,
-		const pair&, const __nonesuch&>::type __p) = delete;
+		const pair&, const __nonesuch_no_braces&>::type __p) = delete;
 
       pair&
       operator=(typename conditional<
 		__and_<is_move_assignable<_T1>,
 		       is_move_assignable<_T2>>::value,
-		pair&&, __nonesuch&&>::type __p)
+		pair&&, __nonesuch_no_braces&&>::type __p)
       noexcept(__and_<is_nothrow_move_assignable<_T1>,
 	              is_nothrow_move_assignable<_T2>>::value)
       {

libstdc++-v3/testsuite/20_util/pair/79141.cc

+// { dg-do compile { target c++11 } }
+
+// 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 <utility>
+
+int main() {
+    std::pair<int,int> p;
+    p = {};
+}