libstdc++: Add test for using istreambuf_iterator with sentinel

This test was supposed to be added two months ago as part of commit
120e8734 but was omitted by mistake.

	* testsuite/24_iterators/istreambuf_iterator/sentinel.cc: New test.

libstdc++-v3/ChangeLog

 2020-04-20  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/24_iterators/istreambuf_iterator/sentinel.cc: New test.
+
 	* testsuite/20_util/is_constructible/51185.cc: Make test class a
 	non-aggregate so that the test verifies the same thing in all -std
 	modes.

libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/sentinel.cc

+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+// 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 <sstream>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  static_assert( std::sentinel_for<std::default_sentinel_t,
+				   std::istreambuf_iterator<char>> );
+
+  std::istreambuf_iterator<char> i = std::default_sentinel;
+  VERIFY( i == std::default_sentinel );
+  VERIFY( std::default_sentinel == i );
+}
+
+void
+test02()
+{
+  std::istringstream in("abc");
+  std::istreambuf_iterator<char> iter(in);
+  VERIFY( iter != std::default_sentinel );
+  VERIFY( std::default_sentinel != iter );
+
+  std::next(iter, 3);
+  VERIFY( iter == std::default_sentinel );
+  VERIFY( std::default_sentinel == iter );
+}
+
+int main()
+{
+  test01();
+  test02();
+}