PR libstdc++/86734 make reverse_iterator::operator-> more robust

Implement the proposed resolution from LWG 1052, which also resolves
DR 2118 by avoiding taking the address in the first place.

	PR libstdc++/86734
	* include/bits/stl_iterator.h (reverse_iterator::operator->): Call
	_S_to_pointer (LWG 1052, LWG 2118).
	(reverse_iterator::_S_to_pointer): Define overloaded helper functions.
	* testsuite/24_iterators/reverse_iterator/dr1052.cc: New test.
	* testsuite/24_iterators/reverse_iterator/dr2188.cc: New test.

From-SVN: r263074

libstdc++-v3/ChangeLog

 2018-07-30  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/86734
+	* include/bits/stl_iterator.h (reverse_iterator::operator->): Call
+	_S_to_pointer (LWG 1052, LWG 2118).
+	(reverse_iterator::_S_to_pointer): Define overloaded helper functions.
+	* testsuite/24_iterators/reverse_iterator/dr1052.cc: New test.
+	* testsuite/24_iterators/reverse_iterator/dr2188.cc: New test.
+
 	* libsupc++/new_opa.cc (operator new(size_t, align_val_t)): Add
 	workaround for aligned_alloc bug on AIX.
 	* testsuite/18_support/new_aligned.cc: New test.

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

@@ -122,6 +122,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       */
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 235 No specification of default ctor for reverse_iterator
+      // 1012. reverse_iterator default ctor should value initialize
       _GLIBCXX17_CONSTEXPR
       reverse_iterator() : current() { }
 
@@ -182,7 +183,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       */
       _GLIBCXX17_CONSTEXPR pointer
       operator->() const
-      { return &(operator*()); }
+      {
+	// _GLIBCXX_RESOLVE_LIB_DEFECTS
+	// 1052. operator-> should also support smart pointers
+	_Iterator __tmp = current;
+	--__tmp;
+	return _S_to_pointer(__tmp);
+      }
 
       /**
        *  @return  @c *this
@@ -286,6 +293,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX17_CONSTEXPR reference
       operator[](difference_type __n) const
       { return *(*this + __n); }
+
+    private:
+      template<typename _Tp>
+	static _GLIBCXX17_CONSTEXPR _Tp*
+	_S_to_pointer(_Tp* __p)
+        { return __p; }
+
+      template<typename _Tp>
+	static _GLIBCXX17_CONSTEXPR pointer
+	_S_to_pointer(_Tp __t)
+        { return __t.operator->(); }
     };
 
   //@{

libstdc++-v3/testsuite/24_iterators/reverse_iterator/dr1052.cc

+// Copyright (C) 2018 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 run { target c++11 } }
+
+// PR libstdc++/86734
+// LWG 1052. reverse_iterator::operator-> should also support smart pointers
+// LWG 2775. reverse_iterator is does not compile for fancy pointers
+
+#include <iterator>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  // Example 1 from LWG 1052
+
+  struct X { int m; };
+
+  static X x;
+
+  struct IterX {
+    typedef std::bidirectional_iterator_tag iterator_category;
+    typedef X& reference;
+    struct pointer
+    {
+      pointer(X& v) : value(v) {}
+      X& value;
+      X* operator->() const {return &value;}
+    };
+    typedef std::ptrdiff_t difference_type;
+    typedef X value_type;
+    // additional iterator requirements not important for this issue
+
+    reference operator*() const { return x; }
+    pointer operator->() const { return pointer(x); }
+    IterX& operator--() {return *this;}
+
+  };
+
+  std::reverse_iterator<IterX> ix;
+  VERIFY( &ix->m == &(*ix).m );
+}
+
+void
+test02()
+{
+  // Example 2 from LWG 1052
+
+  struct P {
+    P() : first(10), second(20.0) { }
+    int first;
+    double second;
+  };
+  P op;
+  std::reverse_iterator<P*> ri(&op + 1);
+  VERIFY( ri->first == 10 );
+}
+
+// N.B. Example 3 from LWG 1052 isn't expected to work,
+// because a caching iterator like IterX is not a forward iterator.
+
+int
+main()
+{
+  test01();
+  test02();
+}

libstdc++-v3/testsuite/24_iterators/reverse_iterator/dr2188.cc

+// Copyright (C) 2018 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 run { target c++11 } }
+
+// PR libstdc++/86734
+
+#include <iterator>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  // LWG DR 2188
+  // Reverse iterator does not fully support targets that overload operator&
+  struct X {
+    int val;
+    int* operator&() { return &val; }
+    const int* operator&() const { return &val; }
+  };
+
+  X x[2] = { {1}, {2} };
+  std::reverse_iterator<X*> rev(x+2);
+  VERIFY( rev->val == 2 );
+  ++rev;
+  VERIFY( rev->val == 1 );
+}
+
+int
+main()
+{
+  test01();
+}