libstdc++: Constrain reverse_iterator and move_iterator conversions [LWG 3435]

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (reverse_iterator): Constrain
	converting constructor and converting assignment operator.
	Access source iterator's data member directly instead of
	calling base().
	(move_iterator): Likewise.
	* testsuite/24_iterators/move_iterator/dr3435.cc: New test.
	* testsuite/24_iterators/reverse_iterator/dr3435.cc: New test.

3 files changed, 125 insertions, 3 deletions

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 6d2d19e..60bb40a 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -129,6 +129,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		      typename iterator_traits<_Iterator>::pointer,
                       typename iterator_traits<_Iterator>::reference>
     {
+#if __cplusplus >= 201103L
+      template<typename _Iter>
+	friend class reverse_iterator;
+#endif
+
+#if __cpp_lib_concepts
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3435. three_way_comparable_with<reverse_iterator<int*>, [...]>
+      template<typename _Iter>
+	static constexpr bool __convertible = !is_same_v<_Iter, _Iterator>
+	    && convertible_to<const _Iter&, _Iterator>;
+#endif
+
     protected:
       _Iterator current;
 
@@ -182,9 +195,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  underlying %iterator can be converted to the type of @c current.
       */
       template<typename _Iter>
+#if __cpp_lib_concepts
+	requires __convertible<_Iter>
+#endif
 	_GLIBCXX17_CONSTEXPR
         reverse_iterator(const reverse_iterator<_Iter>& __x)
-	: current(__x.base()) { }
+	: current(__x.current) { }
+
+#if __cplusplus >= 201103L
+      template<typename _Iter>
+#if __cpp_lib_concepts
+	requires __convertible<_Iter>
+	  && assignable_from<_Iterator&, const _Iter&>
+#endif
+	_GLIBCXX17_CONSTEXPR
+	reverse_iterator&
+	operator=(const reverse_iterator<_Iter>& __x)
+	{
+	  current = __x.current;
+	  return *this;
+	}
+#endif
 
       /**
        *  @return  @c current, the %iterator used for underlying work.
@@ -1270,6 +1301,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using __base_ref = typename __traits_type::reference;
 #endif
 
+      template<typename _Iter2>
+	friend class move_iterator;
+
+#if __cpp_lib_concepts
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3435. three_way_comparable_with<reverse_iterator<int*>, [...]>
+      template<typename _Iter2>
+	static constexpr bool __convertible = !is_same_v<_Iter2, _Iterator>
+	    && convertible_to<const _Iter2&, _Iterator>;
+#endif
+
     public:
       using iterator_type = _Iterator;
 
@@ -1303,15 +1345,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _M_current(std::move(__i)) { }
 
       template<typename _Iter>
+#if __cpp_lib_concepts
+	requires __convertible<_Iter>
+#endif
 	_GLIBCXX17_CONSTEXPR
 	move_iterator(const move_iterator<_Iter>& __i)
-	: _M_current(__i.base()) { }
+	: _M_current(__i._M_current) { }
 
       template<typename _Iter>
+#if __cpp_lib_concepts
+	requires __convertible<_Iter>
+	  && assignable_from<_Iterator&, const _Iter&>
+#endif
 	_GLIBCXX17_CONSTEXPR
 	move_iterator& operator=(const move_iterator<_Iter>& __i)
 	{
-	  _M_current = __i.base();
+	  _M_current = __i._M_current;
 	  return *this;
 	}
 
diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/dr3435.cc b/libstdc++-v3/testsuite/24_iterators/move_iterator/dr3435.cc
new file mode 100644
index 0000000..170d497
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/dr3435.cc
@@ -0,0 +1,37 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <iterator>
+#include <compare>
+
+// LWG 3435. three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>
+// and
+// LWG 3265. move_iterator's conversions are more broken after P1207
+
+using RI = std::move_iterator<int*>;
+using CRI = std::move_iterator<const int*>;
+
+static_assert( std::three_way_comparable_with<RI, CRI> );
+
+static_assert( std::is_convertible_v<const RI&, CRI> );
+static_assert( ! std::is_convertible_v<const CRI&, RI> );
+
+static_assert( std::is_assignable_v<CRI, const RI&> );
+static_assert( ! std::is_assignable_v<RI, const CRI&> );
diff --git a/libstdc++-v3/testsuite/24_iterators/reverse_iterator/dr3435.cc b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/dr3435.cc
new file mode 100644
index 0000000..77b0f39
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/dr3435.cc
@@ -0,0 +1,36 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <iterator>
+#include <compare>
+
+// LWG 3435.
+// three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>
+
+using RI = std::reverse_iterator<int*>;
+using CRI = std::reverse_iterator<const int*>;
+
+static_assert( std::three_way_comparable_with<RI, CRI> );
+
+static_assert( std::is_convertible_v<const RI&, CRI> );
+static_assert( ! std::is_convertible_v<const CRI&, RI> );
+
+static_assert( std::is_assignable_v<CRI, const RI&> );
+static_assert( ! std::is_assignable_v<RI, const CRI&> );