Fix non-standard behaviour of std::istream_iterator

diff --git a/libstdc++-v3/include/bits/stream_iterator.h b/libstdc++-v3/include/bits/stream_iterator.h
index bc3353ab10f..247ad5376dd 100644
--- a/libstdc++-v3/include/bits/stream_iterator.h
+++ b/libstdc++-v3/include/bits/stream_iterator.h
@@ -57,6 +57,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     private:
       istream_type*	_M_stream;
       _Tp		_M_value;
+      // This bool becomes false at end-of-stream. It should be sufficient to
+      // check _M_stream != nullptr instead, but historically we did not set
+      // _M_stream to null when reaching the end, so we need to keep this flag.
       bool		_M_ok;
 
     public:
@@ -66,7 +69,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       ///  Construct start of input stream iterator.
       istream_iterator(istream_type& __s)
-      : _M_stream(std::__addressof(__s))
+      : _M_stream(std::__addressof(__s)), _M_ok(true)
       { _M_read(); }
 
       istream_iterator(const istream_iterator& __obj)
@@ -76,6 +79,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cplusplus >= 201103L
       istream_iterator& operator=(const istream_iterator&) = default;
+      ~istream_iterator() = default;
 #endif
 
       const _Tp&
@@ -111,37 +115,38 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return __tmp;
       }
 
+    private:
       bool
       _M_equal(const istream_iterator& __x) const
-      { return (_M_ok == __x._M_ok) && (!_M_ok || _M_stream == __x._M_stream); }
+      {
+	// Ideally this would just return _M_stream == __x._M_stream,
+	// but code compiled with old versions never sets _M_stream to null.
+	return (_M_ok == __x._M_ok) && (!_M_ok || _M_stream == __x._M_stream);
+      }
 
-    private:
       void
       _M_read()
       {
-	_M_ok = (_M_stream && *_M_stream) ? true : false;
-	if (_M_ok)
-	  {
-	    *_M_stream >> _M_value;
-	    _M_ok = *_M_stream ? true : false;
-	  }
+        if (_M_stream && !(*_M_stream >> _M_value))
+          {
+            _M_stream = 0;
+            _M_ok = false;
+          }
       }
+
+      /// Return true if the iterators refer to the same stream,
+      /// or are both at end-of-stream.
+      friend bool
+      operator==(const istream_iterator& __x, const istream_iterator& __y)
+      { return __x._M_equal(__y); }
+
+      /// Return true if the iterators refer to different streams,
+      /// or if one is at end-of-stream and the other is not.
+      friend bool
+      operator!=(const istream_iterator& __x, const istream_iterator& __y)
+      { return !__x._M_equal(__y); }
     };
 
-  ///  Return true if x and y are both end or not end, or x and y are the same.
-  template<typename _Tp, typename _CharT, typename _Traits, typename _Dist>
-    inline bool
-    operator==(const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __x,
-	       const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __y)
-    { return __x._M_equal(__y); }
-
-  ///  Return false if x and y are both end or not end, or x and y are the same.
-  template <class _Tp, class _CharT, class _Traits, class _Dist>
-    inline bool
-    operator!=(const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __x,
-	       const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __y)
-    { return !__x._M_equal(__y); }
-
   /**
    *  @brief  Provides output iterator semantics for streams.
    *
@@ -171,6 +176,11 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       const _CharT*	_M_string;
 
     public:
+#if __cplusplus > 201703L
+      constexpr ostream_iterator() noexcept
+      : _M_stream(nullptr), _M_string(nullptr) { }
+#endif
+
       /// Construct from an ostream.
       ostream_iterator(ostream_type& __s)
       : _M_stream(std::__addressof(__s)), _M_string(0) {}
@@ -186,7 +196,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __c  CharT delimiter string to insert.
       */
       ostream_iterator(ostream_type& __s, const _CharT* __c)
-      : _M_stream(&__s), _M_string(__c)  { }
+      : _M_stream(std::__addressof(__s)), _M_string(__c)  { }
 
       /// Copy constructor.
       ostream_iterator(const ostream_iterator& __obj)
@@ -205,7 +215,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				_M_message(__gnu_debug::__msg_output_ostream)
 				._M_iterator(*this));
 	*_M_stream << __value;
-	if (_M_string) *_M_stream << _M_string;
+	if (_M_string)
+          *_M_stream << _M_string;
 	return *this;
       }
 
diff --git a/libstdc++-v3/testsuite/24_iterators/istream_iterator/1.cc b/libstdc++-v3/testsuite/24_iterators/istream_iterator/1.cc
new file mode 100644
index 00000000000..78b5f7df0c5
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/istream_iterator/1.cc
@@ -0,0 +1,77 @@ 
+// Copyright (C) 2019 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/>.
+
+// C++98 24.5.1 Template class istream_iterator
+
+#include <iterator>
+#include <sstream>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+void test01()
+{
+  using namespace std;
+
+  const int N = 6;
+  int arr[N]  = { };
+  using __gnu_test::test_container;
+  using __gnu_test::output_iterator_wrapper;
+  test_container<int, output_iterator_wrapper> con(arr, arr+N);
+  output_iterator_wrapper<int> out = con.begin();
+
+  istringstream ss("1 2 3 4 5");
+  std::istream_iterator<int> iter(ss), end;
+  while (iter != end)
+    *out++ = *iter++;
+  VERIFY( iter == end );
+  for (int i = 0; i < N; ++i)
+    VERIFY( arr[i] == (i + 1) % N );
+
+  std::istream_iterator<int> iter2(ss);
+  VERIFY( iter2 == end );
+
+  ss.clear();
+  ss.str("-1 -2 -3");
+  VERIFY( iter == end );
+
+#ifndef _GLIBCXX_DEBUG
+  // This is undefined, so aborts under debug mode.
+  // Without debug mode, it should not extract anything from the stream,
+  // and the iterator should remain at end-of-stream.
+  ++iter;
+  VERIFY( iter == end );
+#endif
+
+  std::istream_iterator<int> iter3(ss);
+  VERIFY( iter3 != end );
+  VERIFY( iter3 != iter );
+  VERIFY( *iter3 == -1 );
+  ++iter3;
+  VERIFY( *iter3 == -2 );
+
+  iter2 = iter3;
+  VERIFY( *iter2 == -2 );
+  ++iter2;
+  VERIFY( *iter2 == -3 );
+  ++iter2;
+  VERIFY( iter2 == end );
+}
+
+int main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/24_iterators/ostream_iterator/1.cc b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/1.cc
new file mode 100644
index 00000000000..6d12bfbef4e
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/1.cc
@@ -0,0 +1,48 @@ 
+// Copyright (C) 2016-2019 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>
+#include <sstream>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::ostringstream ss;
+  std::ostream_iterator<int> iter(ss);
+  for (int i = 0; i < 5; ++i)
+    *iter++ = i;
+  VERIFY( ss.str() == "01234" );
+}
+
+void
+test02()
+{
+  std::ostringstream ss;
+  std::ostream_iterator<int> iter(ss, " - ");
+  for (int i = 0; i < 5; ++i)
+    *iter++ = i;
+  VERIFY( ss.str() == "0 - 1 - 2 - 3 - 4 - " );
+}
+
+int main()
+{
+  test01();
+  test02();
+}
diff --git a/libstdc++-v3/testsuite/24_iterators/ostream_iterator/70766.cc b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/70766.cc
index cc0aa21f50e..c2e069e8e22 100644
--- a/libstdc++-v3/testsuite/24_iterators/ostream_iterator/70766.cc
+++ b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/70766.cc
@@ -33,4 +33,5 @@  test01()
 {
   std::basic_ostream<char, adl::traits> os(nullptr);
   std::ostream_iterator<int, char, adl::traits> oi(os);
+  std::ostream_iterator<int, char, adl::traits> oi2(os, "");
 }
diff --git a/libstdc++-v3/testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc
new file mode 100644
index 00000000000..0fcc8f752cc
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc
@@ -0,0 +1,24 @@ 
+// Copyright (C) 2019 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>
+
+constexpr std::ostream_iterator<int> iter1;
+constexpr std::ostream_iterator<int> iter2{};