[patch] libstdc++/62039 Add concept checks to std::next and std::prev.

[Jonathan Wakely <jwakely at redhat dot com>]

Marc suggested adding concept checks to std::prev, I've also done so
for std::next. Even though these checks are deprecated, they give us
somewhere to consider putting C++17 concept requirements.

Tested powerpx64le-linux, committed to trunk.

commit 48d52e4f02e6c392086dce8832319c68ebec68b9
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 3 12:07:01 2015 +0100

    Add concept checks to std::next and std::prev.
    
    	PR libstdc++/62039
    	* include/bits/stl_iterator_base_funcs.h (next, prev): Add concept
    	checks.
    	* testsuite/24_iterators/operations/prev_neg.cc: New.
    	* testsuite/24_iterators/operations/next_neg.cc: New.

diff --git a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
index 516f8fc..0f77329 100644
--- a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
+++ b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
@@ -205,6 +205,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     next(_ForwardIterator __x, typename
 	 iterator_traits<_ForwardIterator>::difference_type __n = 1)
     {
+      // concept requirements
+      __glibcxx_function_requires(_ForwardIteratorConcept<
+				  _ForwardIterator>)
       std::advance(__x, __n);
       return __x;
     }
@@ -214,6 +217,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     prev(_BidirectionalIterator __x, typename
 	 iterator_traits<_BidirectionalIterator>::difference_type __n = 1) 
     {
+      // concept requirements
+      __glibcxx_function_requires(_BidirectionalIteratorConcept<
+				  _BidirectionalIterator>)
       std::advance(__x, -__n);
       return __x;
     }
diff --git a/libstdc++-v3/testsuite/24_iterators/operations/next_neg.cc b/libstdc++-v3/testsuite/24_iterators/operations/next_neg.cc
new file mode 100644
index 0000000..881307e
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/operations/next_neg.cc
@@ -0,0 +1,42 @@
+// Copyright (C) 2015 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++11 -D_GLIBCXX_CONCEPT_CHECKS" }
+// { dg-do compile }
+
+#include <iterator>
+
+struct X {};
+
+namespace std
+{
+  template<>
+    struct iterator_traits<const X*> : iterator_traits<X*>
+    {
+      using iterator_category = input_iterator_tag;
+      using reference = const X&;
+      using pointer = const X*;
+    };
+}
+
+void
+test01()
+{
+  const X array[1] = { };
+  std::next(array);
+  // { dg-error "input_iterator" "" { target *-*-* } 220 }
+}
diff --git a/libstdc++-v3/testsuite/24_iterators/operations/prev_neg.cc b/libstdc++-v3/testsuite/24_iterators/operations/prev_neg.cc
new file mode 100644
index 0000000..513e0e8
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/operations/prev_neg.cc
@@ -0,0 +1,42 @@
+// Copyright (C) 2015 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++11 -D_GLIBCXX_CONCEPT_CHECKS" }
+// { dg-do compile }
+
+#include <iterator>
+
+struct Y {};
+
+namespace std
+{
+  template<>
+    struct iterator_traits<const Y*> : iterator_traits<Y*>
+    {
+      using iterator_category = forward_iterator_tag;
+      using reference = const Y&;
+      using pointer = const Y*;
+    };
+}
+
+void
+test02()
+{
+  const Y array[1] = { };
+  std::prev(array + 1);
+  // { dg-error "forward_iterator" "" { target *-*-* } 220 }
+}