libstdc++: Use correct argument type for __use_alloc [PR 96803]

The _Tuple_impl constructor for allocator-extended construction from a
different tuple type uses the _Tuple_impl's own _Head type in the
__use_alloc test. That is incorrect, because the argument tuple could
have a different type. Using the wrong type might select the
leading-allocator convention when it should use the trailing-allocator
convention, or vice versa.

libstdc++-v3/ChangeLog:

	PR libstdc++/96803
	* include/std/tuple
	(_Tuple_impl(allocator_arg_t, Alloc, const _Tuple_impl<U...>&)):
	Replace parameter pack with a type parameter and a pack and pass
	the first type to __use_alloc.
	* testsuite/20_util/tuple/cons/96803.cc: New test.

2 files changed, 46 insertions, 5 deletions

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index d4a35f0..7be9943 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -338,14 +338,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
 	        std::forward<_Head>(_M_head(__in))) { }
 
-      template<typename _Alloc, typename... _UElements>
+      template<typename _Alloc, typename _UHead, typename... _UTails>
 	_GLIBCXX20_CONSTEXPR
 	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
-	            const _Tuple_impl<_Idx, _UElements...>& __in)
+		    const _Tuple_impl<_Idx, _UHead, _UTails...>& __in)
 	: _Inherited(__tag, __a,
-		     _Tuple_impl<_Idx, _UElements...>::_M_tail(__in)),
-	  _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
-		_Tuple_impl<_Idx, _UElements...>::_M_head(__in)) { }
+		     _Tuple_impl<_Idx, _UHead, _UTails...>::_M_tail(__in)),
+	  _Base(__use_alloc<_Head, _Alloc, _UHead>(__a),
+		_Tuple_impl<_Idx, _UHead, _UTails...>::_M_head(__in)) { }
 
       template<typename _Alloc, typename _UHead, typename... _UTails>
 	_GLIBCXX20_CONSTEXPR
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cons/96803.cc b/libstdc++-v3/testsuite/20_util/tuple/cons/96803.cc
new file mode 100644
index 0000000..9d3c07d
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/cons/96803.cc
@@ -0,0 +1,41 @@
+// Copyright (C) 2020 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++11 } }
+
+#include <tuple>
+#include <memory>
+
+struct X
+{
+  using allocator_type = std::allocator<int>;
+
+  X(X&&) { }
+  X(std::allocator_arg_t, const allocator_type&, X&&) { }
+
+  explicit X(int) { }
+  explicit X(int, allocator_type) { }
+};
+
+void
+test01()
+{
+  // PR libstdc++/96803
+  // std::tuple chooses wrong constructor for uses-allocator construction
+  std::tuple<int> o;
+  std::tuple<X> nok(std::allocator_arg, std::allocator<int>(), o);
+}