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

Message ID 20200110152732.GA1478923@redhat.com
State New
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  • libstdc++: Make istreambuf_iterator base class consistent (PR92285)
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Commit Message

Jonathan Wakely Jan. 10, 2020, 3:27 p.m.
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.

Tested powerpc64le-linux, committed to trunk.
commit e4e3920f4e834b1a6c6b58bdb282947d3a08bc8c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jan 10 12:02:07 2020 +0000

    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.

Patch

diff --git a/libstdc++-v3/include/bits/streambuf_iterator.h b/libstdc++-v3/include/bits/streambuf_iterator.h
index 67dc3861aa6..fe612f39fe9 100644
--- a/libstdc++-v3/include/bits/streambuf_iterator.h
+++ b/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;
diff --git a/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/92285.cc b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/92285.cc
new file mode 100644
index 00000000000..68b484151d5
--- /dev/null
+++ b/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();
+}
diff --git a/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/requirements/base_classes.cc b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/requirements/base_classes.cc
index 07b0a44a9a1..947b772040b 100644
--- a/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/requirements/base_classes.cc
+++ b/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");