libstdc++: Fix istream::ignore discarding too many chars (PR 94749)

The current code assumes that if the next character in the stream is
equal to the delimiter then we stopped because we saw that delimiter,
and so discards it.  But in the testcase for the PR we stop because we
reached the maximum number of characters, and it's coincidence that the
next character equals the delimiter. We should not discard the next
character in that case.

The fix is to check that we haven't discarded __n characters already,
instead of checking whether the next character equals __delim. Because
we've already checked for EOF, if we haven't discarded __n yet then we
know we stopped because we saw the delimiter. On the other hand, if the
next character is the delimiter we don't know if that's why we stopped.

	PR libstdc++/94749
	* include/bits/istream.tcc (basic_istream::ignore(streamsize, CharT)):
	Only discard an extra character if we didn't already reach the
	maximum number.
	* src/c++98/istream.cc (istream::ignore(streamsiz, char))
	(wistream::ignore(streamsize, wchar_t)): Likewise.
	* testsuite/27_io/basic_istream/ignore/char/94749.cc: New test.
	* testsuite/27_io/basic_istream/ignore/wchar_t/94749.cc: New test.

2 files changed, 152 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/94749.cc b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/94749.cc
new file mode 100644
index 0000000..03b5286
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/94749.cc
@@ -0,0 +1,76 @@
+// 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/>.
+
+// { dg-do run }
+
+// PR libstdc++/94749
+// basic_istream::ignore(n, c) discards n+1 if next character is equal to c.
+
+#include <sstream>
+#include <testsuite_hooks.h>
+
+typedef char C;
+
+void
+test01()
+{
+  std::basic_istringstream<C> s(" +   -");
+  s.ignore(1, '+');
+  VERIFY( s.get() == '+' );
+  s.ignore(3, '-');
+  VERIFY( s.get() == '-' );
+}
+
+void
+test02()
+{
+  std::basic_istringstream<C> s(".+...-");
+  s.ignore(1, '+');
+  VERIFY( s.get() == '+' );
+  s.ignore(3, '-');
+  VERIFY( s.get() == '-' );
+}
+
+void
+test03()
+{
+  std::basic_istringstream<C, __gnu_cxx::char_traits<C> > s(" +   -");
+  s.ignore(1, '+');
+  VERIFY( s.get() == '+' );
+  s.ignore(3, '-');
+  VERIFY( s.get() == '-' );
+}
+
+void
+test04()
+{
+  std::basic_istringstream<C, __gnu_cxx::char_traits<C> > s(".+...-");
+  s.ignore(1, '+');
+  VERIFY( s.get() == '+' );
+  s.ignore(3, '-');
+  VERIFY( s.get() == '-' );
+}
+
+
+int
+main()
+{
+  test01();
+  test02();
+  test03();
+  test04();
+}
diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/94749.cc b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/94749.cc
new file mode 100644
index 0000000..e5ec4e7
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/94749.cc
@@ -0,0 +1,76 @@
+// 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/>.
+
+// { dg-do run }
+
+// PR libstdc++/94749
+// basic_istream::ignore(n, c) discards n+1 if next character is equal to c.
+
+#include <sstream>
+#include <testsuite_hooks.h>
+
+typedef wchar_t C;
+
+void
+test01()
+{
+  std::basic_istringstream<C> s(L" +   -");
+  s.ignore(1, L'+');
+  VERIFY( s.get() == L'+' );
+  s.ignore(3, L'-');
+  VERIFY( s.get() == L'-' );
+}
+
+void
+test02()
+{
+  std::basic_istringstream<C> s(L".+...-");
+  s.ignore(1, L'+');
+  VERIFY( s.get() == L'+' );
+  s.ignore(3, L'-');
+  VERIFY( s.get() == L'-' );
+}
+
+void
+test03()
+{
+  std::basic_istringstream<C, __gnu_cxx::char_traits<C> > s(L" +   -");
+  s.ignore(1, L'+');
+  VERIFY( s.get() == L'+' );
+  s.ignore(3, L'-');
+  VERIFY( s.get() == L'-' );
+}
+
+void
+test04()
+{
+  std::basic_istringstream<C, __gnu_cxx::char_traits<C> > s(L".+...-");
+  s.ignore(1, L'+');
+  VERIFY( s.get() == L'+' );
+  s.ignore(3, L'-');
+  VERIFY( s.get() == L'-' );
+}
+
+
+int
+main()
+{
+  // test01();
+  // test02();
+  test03();
+  test04();
+}