libstdc++: Make istreambuf_iterator base class consistent (PR92285)

Since LWG 445 was implemented for GCC 4.7, the std::iterator base class
of std::istreambuf_iterator changes type depending on the -std mode
used. This creates an ABI incompatibility between different -std modes.

This change ensures the base class always has the same type. This makes
layout for C++98 compatible with the current -std=gnu++14 default, but
no longer compatible with C++98 code from previous releases. In practice
this is unlikely to cause real problems, because it only affects the
layout of types with two std::iterator base classes, one of which comes
from std::istreambuf_iterator. Such types are expected to be vanishingly
rare.

	PR libstdc++/92285
	* include/bits/streambuf_iterator.h (istreambuf_iterator): Make type
	of base class independent of __cplusplus value.
	[__cplusplus < 201103L] (istreambuf_iterator::reference): Override the
	type defined in the base class
	* testsuite/24_iterators/istreambuf_iterator/92285.cc: New test.
	* testsuite/24_iterators/istreambuf_iterator/requirements/
	base_classes.cc: Adjust expected base class for C++98.

From-SVN: r280116

libstdc++-v3/ChangeLog

@@ -1,3 +1,14 @@
+2020-01-10  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/92285
+	* include/bits/streambuf_iterator.h (istreambuf_iterator): Make type
+	of base class independent of __cplusplus value.
+	[__cplusplus < 201103L] (istreambuf_iterator::reference): Override the
+	type defined in the base class
+	* testsuite/24_iterators/istreambuf_iterator/92285.cc: New test.
+	* testsuite/24_iterators/istreambuf_iterator/requirements/
+	base_classes.cc: Adjust expected base class for C++98.
+
 2020-01-09  Olivier Hainque  <hainque@adacore.com>
 
 	* doc/xml/manual/appendix_contributing.xml: Document _C2

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

@@ -49,23 +49,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _CharT, typename _Traits>
     class istreambuf_iterator
     : public iterator<input_iterator_tag, _CharT, typename _Traits::off_type,
-		      _CharT*,
-#if __cplusplus >= 201103L
-    // LWG 445.
-		      _CharT>
-#else
-		      _CharT&>
-#endif
+		      _CharT*, _CharT>
     {
     public:
       // Types:
       //@{
       /// Public typedefs
-#if __cplusplus > 201703L
+#if __cplusplus < 201103L
+      typedef _CharT& reference; // Changed to _CharT by LWG 445
+#elif __cplusplus > 201703L
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 3188. istreambuf_iterator::pointer should not be unspecified
       using pointer = void;
 #endif
+
       typedef _CharT					char_type;
       typedef _Traits					traits_type;
       typedef typename _Traits::int_type		int_type;

libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/92285.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/>.
+
+#include <iterator>
+#include <iostream>
+#include <testsuite_hooks.h>
+
+// PR libstdc++/92285
+// See https://gcc.gnu.org/ml/libstdc++/2019-10/msg00129.html
+
+typedef std::input_iterator_tag category;
+typedef std::char_traits<char>::off_type off_type;
+typedef std::iterator<category, char, off_type, char*, char> good;
+typedef std::iterator<category, char, off_type, char*, char&> bad;
+
+bool check(good&) { return true; }
+void check(bad&) { }
+
+void
+test01()
+{
+  typedef std::istreambuf_iterator<char> I;
+  I it;
+  VERIFY( check(it) );
+#if __cplusplus < 201103L
+  char c = 'c';
+  I::reference r = c;
+  VERIFY( &r == &c );
+#else
+  static_assert( std::is_same<I::reference, char>::value, "LWG 445" );
+#endif
+}
+
+int main()
+{
+  test01();
+}

libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/requirements/base_classes.cc

@@ -32,12 +32,8 @@ void test01()
   typedef istreambuf_iterator<char> test_iterator;
   typedef char_traits<char>::off_type off_type;
 
-  typedef iterator<input_iterator_tag, char, off_type, char*,
-#if __cplusplus >= 201103L
-    char>
-#else
-    char&>
-#endif
+  // This is the base class required since LWG 445, which differs from C++03:
+  typedef iterator<input_iterator_tag, char, off_type, char*, char>
     base_iterator;
 
   istringstream isstream("this tag");