libstdc++: make negative count safe with std::for_each_n

The Library Working Group have approved a change to std::for_each_n that
requires it to handle negative N gracefully, which we were not doing for
random access iterators.

	* include/bits/stl_algo.h (for_each_n): Handle negative count.
	* testsuite/25_algorithms/for_each/for_each_n_debug.cc: New test.

From-SVN: r277932

libstdc++-v3/ChangeLog

+2019-11-07  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/stl_algo.h (for_each_n): Handle negative count.
+	* testsuite/25_algorithms/for_each/for_each_n_debug.cc: New test.
+
 2019-11-07  Jason Merrill  <jason@redhat.com>
 
 	* libsupc++/compare: Remove strong_equality and weak_equality.

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

@@ -4011,6 +4011,8 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
       using _Cat = typename iterator_traits<_InputIterator>::iterator_category;
       if constexpr (is_base_of_v<random_access_iterator_tag, _Cat>)
 	{
+	  if (__n2 <= 0)
+	    return __first;
 	  auto __last = __first + __n2;
 	  std::for_each(__first, __last, std::move(__f));
 	  return __last;

libstdc++-v3/testsuite/25_algorithms/for_each/for_each_n_debug.cc

+// 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++17" }
+// { dg-do run { target c++17 } }
+
+#include <algorithm>
+#include <debug/vector>
+#include <debug/list>
+
+void
+test01()
+{
+  __gnu_debug::vector<int> v{1, 2, 3};
+  std::for_each_n(v.begin(), -2, [](int){});
+}
+
+void
+test02()
+{
+  __gnu_debug::list<int> l{1, 2, 3};
+  std::for_each_n(l.begin(), -2, [](int){});
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}