nonesuch is insufficiently useless (lwg2996)

2019-05-14  Nina Dinka Ranns  <dinka.ranns@gmail.com>

	nonesuch is insufficiently useless (lwg2996)
	* include/std/type_traits (struct __nonesuch): Added private base
	class to make __nonesuch not an aggregate and removed deleted default
	constructor.
	* include/bits/stl_pair.h (struct __nonesuch_no_braces): Removed.
	(operator=(const pair&)): Use __nonesuch instead of
	__nonesuch_no_braces.
	(operator=(pair&&)): Likewise
	* include/std/tuple (operator=(const tuple&)): Use __nonesuch instead
	of __nonesuch_no_braces.
	(operator=(tuple&&)): Likewise
	* include/experimental/type_traits (struct nonesuch): Added private
	base class to make nonesuch not an aggregate and removed deleted
	default constructor.
	* testsuite/20_util/nonesuch/nonesuch.cc: New.
	* testsuite/experimental/type_traits/nonesuch.cc: New.

From-SVN: r271175

libstdc++-v3/ChangeLog

+2019-05-14  Nina Dinka Ranns  <dinka.ranns@gmail.com>
+
+	nonesuch is insufficiently useless (lwg2996)
+	* include/std/type_traits (struct __nonesuch): Added private base
+	class to make __nonesuch not an aggregate and removed deleted default
+	constructor.
+	* include/bits/stl_pair.h (struct __nonesuch_no_braces): Removed.
+	(operator=(const pair&)): Use __nonesuch instead of
+	__nonesuch_no_braces.
+	(operator=(pair&&)): Likewise
+	* include/std/tuple (operator=(const tuple&)): Use __nonesuch instead
+	of __nonesuch_no_braces.
+	(operator=(tuple&&)): Likewise
+	* include/experimental/type_traits (struct nonesuch): Added private
+	base class to make nonesuch not an aggregate and removed deleted
+	default constructor.
+	* testsuite/20_util/nonesuch/nonesuch.cc: New.
+	* testsuite/experimental/type_traits/nonesuch.cc: New.
+
 2019-05-14  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/std_function.h (_Simple_type_wrapper): Remove.

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

@@ -180,13 +180,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return false;
       }
   };
-
-  // 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 // C++11
 
   template<typename _U1, typename _U2> class __pair_base
@@ -393,7 +386,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator=(typename conditional<
 		__and_<is_copy_assignable<_T1>,
 		       is_copy_assignable<_T2>>::value,
-		const pair&, const __nonesuch_no_braces&>::type __p)
+		const pair&, const __nonesuch&>::type __p)
       {
 	first = __p.first;
 	second = __p.second;
@@ -404,7 +397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator=(typename conditional<
 		__and_<is_move_assignable<_T1>,
 		       is_move_assignable<_T2>>::value,
-		pair&&, __nonesuch_no_braces&&>::type __p)
+		pair&&, __nonesuch&&>::type __p)
       noexcept(__and_<is_nothrow_move_assignable<_T1>,
 		      is_nothrow_move_assignable<_T2>>::value)
       {

libstdc++-v3/include/experimental/type_traits

@@ -227,9 +227,9 @@ inline namespace fundamentals_v2
 
 template<typename...> using void_t = void;
 
-struct nonesuch
+struct __nonesuchbase {};
+struct nonesuch : private __nonesuchbase
 {
-  nonesuch() = delete;
   ~nonesuch() = delete;
   nonesuch(nonesuch const&) = delete;
   void operator=(nonesuch const&) = delete;

libstdc++-v3/include/std/tuple

@@ -832,7 +832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       tuple&
       operator=(typename conditional<__assignable<const _Elements&...>(),
 				     const tuple&,
-				     const __nonesuch_no_braces&>::type __in)
+				     const __nonesuch&>::type __in)
       noexcept(__nothrow_assignable<const _Elements&...>())
       {
 	this->_M_assign(__in);
@@ -842,7 +842,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       tuple&
       operator=(typename conditional<__assignable<_Elements...>(),
 				     tuple&&,
-				     __nonesuch_no_braces&&>::type __in)
+				     __nonesuch&&>::type __in)
       noexcept(__nothrow_assignable<_Elements...>())
       {
 	this->_M_assign(std::move(__in));
@@ -1243,7 +1243,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       tuple&
       operator=(typename conditional<__assignable<const _T1&, const _T2&>(),
 				     const tuple&,
-				     const __nonesuch_no_braces&>::type __in)
+				     const __nonesuch&>::type __in)
       noexcept(__nothrow_assignable<const _T1&, const _T2&>())
       {
 	this->_M_assign(__in);
@@ -1253,7 +1253,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       tuple&
       operator=(typename conditional<__assignable<_T1, _T2>(),
 				     tuple&&,
-				     __nonesuch_no_braces&&>::type __in)
+				     __nonesuch&&>::type __in)
       noexcept(__nothrow_assignable<_T1, _T2>())
       {
 	this->_M_assign(std::move(__in));

libstdc++-v3/include/std/type_traits

@@ -2792,8 +2792,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
              __call_is_nothrow_<_Fn, _Args...>>::type
     { };
 
-  struct __nonesuch {
-    __nonesuch() = delete;
+  struct __nonesuchbase {};
+  struct __nonesuch : private __nonesuchbase {
     ~__nonesuch() = delete;
     __nonesuch(__nonesuch const&) = delete;
     void operator=(__nonesuch const&) = delete;

libstdc++-v3/testsuite/20_util/nonesuch/nonesuch.cc

+// 2019-05-14  Nina Dinka Ranns  <dinka.ranns@gmail.com>
+// Copyright (C) 2019 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/>.
+
+// { dg-do compile { target c++14 } }
+
+#include <type_traits>
+// Example taken from LWG2960
+
+using std::__nonesuch;
+struct such {};
+void f(const such&){};
+void f(const std::__nonesuch&);
+
+int main(){
+ static_assert(!std::is_default_constructible<__nonesuch>::value,
+		 "__nonesuch is default constructible");
+ static_assert(!std::is_copy_constructible<__nonesuch>::value,
+		 "__nonesuch is copy constructible");
+ static_assert(!std::is_assignable<__nonesuch, __nonesuch>::value,
+		 "__nonesuch is assignable");
+ static_assert(!std::is_destructible<__nonesuch>::value,
+		 "__nonesuch is destructible");
+ f({});
+}

libstdc++-v3/testsuite/experimental/type_traits/nonesuch.cc

+// 2019-05-14  Nina Dinka Ranns  <dinka.ranns@gmail.com>
+// Copyright (C) 2019 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/>.
+
+// { dg-do compile { target c++14 } }
+
+#include <experimental/type_traits>
+// Example taken from LWG2960
+
+using std::experimental::nonesuch;
+
+struct such {};
+void f(const such&){};
+void f(const nonesuch&);
+
+int main(){
+ static_assert(!std::is_default_constructible<nonesuch>::value,
+		 "nonesuch is default constructible");
+ static_assert(!std::is_copy_constructible<nonesuch>::value,
+		 "nonesuch is copy constructible");
+ static_assert(!std::is_assignable<nonesuch,nonesuch>::value,
+		 "nonesuch is assignable");
+ static_assert(!std::is_destructible<nonesuch>::value,
+		 "nonesuch is destructible");
+ f({});
+}