libstdc++: Define converting assignment operator for std::move_iterator

As clarified by LWG 3265, std::move_iterator is supposed to have an
assignment operator that converts from a different specialization of
std::move_iterator, which performs an assignment. That has always been
missing from libstdc++, so assigning a different type actually performs
a converting construction, then an assignment. This is non-conforming
for the (fairly contrived) case where the converting assignment is
well-formed but the converting construction is not.

	* include/bits/stl_iterator.h (move_iterator::operator=): Define.
	* testsuite/24_iterators/move_iterator/dr3265.cc: New test.

2 files changed, 59 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 7290781..6d2d19e 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1307,6 +1307,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	move_iterator(const move_iterator<_Iter>& __i)
 	: _M_current(__i.base()) { }
 
+      template<typename _Iter>
+	_GLIBCXX17_CONSTEXPR
+	move_iterator& operator=(const move_iterator<_Iter>& __i)
+	{
+	  _M_current = __i.base();
+	  return *this;
+	}
+
 #if __cplusplus <= 201703L
       _GLIBCXX17_CONSTEXPR iterator_type
       base() const
diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/dr3265.cc b/libstdc++-v3/testsuite/24_iterators/move_iterator/dr3265.cc
new file mode 100644
index 0000000..05327e9
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/dr3265.cc
@@ -0,0 +1,51 @@
+// 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 <iterator>
+
+struct Iter
+{
+  using iterator_category = std::forward_iterator_tag;
+  using value_type = int;
+  using pointer = int*;
+  using reference = int&;
+  using difference_type = std::ptrdiff_t;
+
+  Iter();
+
+  // Construction from int* is not valid:
+  Iter(int*) = delete;
+
+  // Assignment from int* is valid:
+  Iter& operator=(int*);
+
+  Iter& operator++();
+  Iter operator++(int);
+  int& operator*() const;
+  int* operator->() const;
+
+  template<int N> friend bool operator==(Iter, Iter);
+};
+
+void test01()
+{
+  std::move_iterator<Iter> m;
+  int i = 0;
+  m = std::make_move_iterator(&i); // Should use assignment not construction
+}