re PR libstdc++/56785 (std::tuple of two elements does not apply empty base…

re PR libstdc++/56785 (std::tuple of two elements does not apply empty base class optimization when one of its elements is a std::tuple with two elements)

	PR libstdc++/56785
	* include/std/tuple (_Tuple_impl): Remove zero-element specialization
	and define one-element specialization.
	* testsuite/20_util/tuple/56785.cc: New.

From-SVN: r219785

libstdc++-v3/ChangeLog

 2015-01-17  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/56785
+	* include/std/tuple (_Tuple_impl): Remove zero-element specialization
+	and define one-element specialization.
+	* testsuite/20_util/tuple/56785.cc: New.
+
+2015-01-17  Jonathan Wakely  <jwakely@redhat.com>
+
 	* testsuite/22_locale/codecvt/codecvt_utf8/requirements/1.cc:
 	Remove unused header.
 	* testsuite/22_locale/codecvt/codecvt_utf16/requirements/1.cc:

libstdc++-v3/include/std/tuple

@@ -158,30 +158,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<std::size_t _Idx, typename... _Elements>
     struct _Tuple_impl; 
 
-  /**
-   * Zero-element tuple implementation. This is the basis case for the 
-   * inheritance recursion.
-   */
-  template<std::size_t _Idx>
-    struct _Tuple_impl<_Idx>
-    {
-      template<std::size_t, typename...> friend class _Tuple_impl;
-
-      _Tuple_impl() = default;
-
-      template<typename _Alloc>
-        _Tuple_impl(allocator_arg_t, const _Alloc&) { }
-
-      template<typename _Alloc>
-        _Tuple_impl(allocator_arg_t, const _Alloc&, const _Tuple_impl&) { }
-
-      template<typename _Alloc>
-        _Tuple_impl(allocator_arg_t, const _Alloc&, _Tuple_impl&&) { }
-
-    protected:
-      void _M_swap(_Tuple_impl&) noexcept { /* no-op */ }
-    };
-
   template<typename _Tp>
     struct __is_empty_non_tuple : is_empty<_Tp> { };
 
@@ -358,6 +334,130 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
     };
 
+  // Basis case of inheritance recursion.
+  template<std::size_t _Idx, typename _Head>
+    struct _Tuple_impl<_Idx, _Head>
+    : private _Head_base<_Idx, _Head, __empty_not_final<_Head>::value>
+    {
+      template<std::size_t, typename...> friend class _Tuple_impl;
+
+      typedef _Head_base<_Idx, _Head, __empty_not_final<_Head>::value> _Base;
+
+      static constexpr _Head&
+      _M_head(_Tuple_impl& __t) noexcept { return _Base::_M_head(__t); }
+
+      static constexpr const _Head&
+      _M_head(const _Tuple_impl& __t) noexcept { return _Base::_M_head(__t); }
+
+      constexpr _Tuple_impl()
+      : _Base() { }
+
+      explicit
+      constexpr _Tuple_impl(const _Head& __head)
+      : _Base(__head) { }
+
+      template<typename _UHead>
+        explicit
+        constexpr _Tuple_impl(_UHead&& __head)
+	: _Base(std::forward<_UHead>(__head)) { }
+
+      constexpr _Tuple_impl(const _Tuple_impl&) = default;
+
+      constexpr
+      _Tuple_impl(_Tuple_impl&& __in)
+      noexcept(is_nothrow_move_constructible<_Head>::value)
+      : _Base(std::forward<_Head>(_M_head(__in))) { }
+
+      template<typename _UHead>
+        constexpr _Tuple_impl(const _Tuple_impl<_Idx, _UHead>& __in)
+	: _Base(_Tuple_impl<_Idx, _UHead>::_M_head(__in)) { }
+
+      template<typename _UHead>
+        constexpr _Tuple_impl(_Tuple_impl<_Idx, _UHead>&& __in)
+	: _Base(std::forward<_UHead>(_Tuple_impl<_Idx, _UHead>::_M_head(__in)))
+	{ }
+
+      template<typename _Alloc>
+	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a)
+	: _Base(__tag, __use_alloc<_Head>(__a)) { }
+
+      template<typename _Alloc>
+	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
+		    const _Head& __head)
+	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a), __head) { }
+
+      template<typename _Alloc, typename _UHead>
+	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
+	            _UHead&& __head)
+	: _Base(__use_alloc<_Head, _Alloc, _UHead>(__a),
+	        std::forward<_UHead>(__head)) { }
+
+      template<typename _Alloc>
+        _Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
+	            const _Tuple_impl& __in)
+	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a), _M_head(__in)) { }
+
+      template<typename _Alloc>
+	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
+	            _Tuple_impl&& __in)
+	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
+	        std::forward<_Head>(_M_head(__in))) { }
+
+      template<typename _Alloc, typename _UHead>
+	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
+	            const _Tuple_impl<_Idx, _UHead>& __in)
+	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
+		_Tuple_impl<_Idx, _UHead>::_M_head(__in)) { }
+
+      template<typename _Alloc, typename _UHead>
+	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
+	            _Tuple_impl<_Idx, _UHead>&& __in)
+	: _Base(__use_alloc<_Head, _Alloc, _UHead>(__a),
+                std::forward<_UHead>(_Tuple_impl<_Idx, _UHead>::_M_head(__in)))
+	{ }
+
+      _Tuple_impl&
+      operator=(const _Tuple_impl& __in)
+      {
+	_M_head(*this) = _M_head(__in);
+	return *this;
+      }
+
+      _Tuple_impl&
+      operator=(_Tuple_impl&& __in)
+      noexcept(is_nothrow_move_assignable<_Head>::value)
+      {
+	_M_head(*this) = std::forward<_Head>(_M_head(__in));
+	return *this;
+      }
+
+      template<typename _UHead>
+        _Tuple_impl&
+        operator=(const _Tuple_impl<_Idx, _UHead>& __in)
+        {
+	  _M_head(*this) = _Tuple_impl<_Idx, _UHead>::_M_head(__in);
+	  return *this;
+	}
+
+      template<typename _UHead>
+        _Tuple_impl&
+        operator=(_Tuple_impl<_Idx, _UHead>&& __in)
+        {
+	  _M_head(*this)
+	    = std::forward<_UHead>(_Tuple_impl<_Idx, _UHead>::_M_head(__in));
+	  return *this;
+	}
+
+    protected:
+      void
+      _M_swap(_Tuple_impl& __in)
+      noexcept(noexcept(swap(std::declval<_Head&>(), std::declval<_Head&>())))
+      {
+	using std::swap;
+	swap(_M_head(*this), _M_head(__in));
+      }
+    };
+
   /// Primary class template, tuple
   template<typename... _Elements> 
     class tuple : public _Tuple_impl<0, _Elements...>

libstdc++-v3/testsuite/20_util/tuple/56785.cc

+// Copyright (C) 2015 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-options "-std=gnu++11" }
+// { dg-do compile }
+
+#include <tuple>
+
+class Empty { };
+
+using std::tuple;
+using char_pair = tuple<char, char>;
+
+static_assert( sizeof(tuple<Empty, char_pair>) == sizeof(char_pair),
+               "Nested tuple tuple<Empty, tuple<T,T>> is too big");
+
+static_assert( sizeof(tuple<char_pair, char_pair>) == (2 * sizeof(char_pair)),
+               "Nested tuple<tuple<T,T, tuple<T,T>> is too big" );