PR libstdc++/90920 restore previous checks for empty ranges

The change in r263433 broke the contract of the __rotate functions, by no
longer accepting empty ranges. That means that callers which inlined the
old version of std::rotate (without checks) that end up linking to a new
definition of std::__rotate (also without checks) could perform a divide
by zero and crash.

This restores the old contract of the __rotate overloads.

	PR libstdc++/90920 partially revert r263433
	* include/bits/stl_algo.h (__rotate): Restore checks for empty ranges.
	(rotate): Remove checks.
	* testsuite/25_algorithms/rotate/90920.cc: New test.

From-SVN: r272489

libstdc++-v3/ChangeLog

@@ -1,5 +1,10 @@
 2019-06-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/90920 partially revert r263433
+	* include/bits/stl_algo.h (__rotate): Restore checks for empty ranges.
+	(rotate): Remove checks.
+	* testsuite/25_algorithms/rotate/90920.cc: New test.
+
 	* include/std/numeric (reduce(Iter, Iter, T, BinOp)): Fix value
 	category used in invocable check.
 	(reduce(Iter, Iter, T)): Pass initial value as rvalue.

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

@@ -1251,6 +1251,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	     _ForwardIterator __last,
 	     forward_iterator_tag)
     {
+      if (__first == __middle)
+	return __last;
+      else if (__last == __middle)
+	return __first;
+
       _ForwardIterator __first2 = __middle;
       do
 	{
@@ -1291,6 +1296,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_function_requires(_Mutable_BidirectionalIteratorConcept<
 				  _BidirectionalIterator>)
 
+      if (__first == __middle)
+	return __last;
+      else if (__last == __middle)
+	return __first;
+
       std::__reverse(__first,  __middle, bidirectional_iterator_tag());
       std::__reverse(__middle, __last,   bidirectional_iterator_tag());
 
@@ -1324,6 +1334,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_function_requires(_Mutable_RandomAccessIteratorConcept<
 				  _RandomAccessIterator>)
 
+      if (__first == __middle)
+	return __last;
+      else if (__last == __middle)
+	return __first;
+
       typedef typename iterator_traits<_RandomAccessIterator>::difference_type
 	_Distance;
       typedef typename iterator_traits<_RandomAccessIterator>::value_type
@@ -1425,11 +1440,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_requires_valid_range(__first, __middle);
       __glibcxx_requires_valid_range(__middle, __last);
 
-      if (__first == __middle)
-	return __last;
-      else if (__last  == __middle)
-	return __first;
-
       return std::__rotate(__first, __middle, __last,
 			   std::__iterator_category(__first));
     }

libstdc++-v3/testsuite/25_algorithms/rotate/90920.cc

@@ -0,0 +1,48 @@
+// 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-do run }
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+
+namespace gnu_test
+{
+  // This is the definition from GCC 8.x, with no checks for first==middle
+  // or middle==last.
+  template<typename _ForwardIterator>
+    inline _ForwardIterator
+    rotate(_ForwardIterator __first, _ForwardIterator __middle,
+	   _ForwardIterator __last)
+    {
+      return std::__rotate(__first, __middle, __last,
+			   std::__iterator_category(__first));
+    }
+}
+
+void
+test01()
+{
+  int i = 0;
+  gnu_test::rotate(&i, &i, &i+1);
+}
+
+int
+main()
+{
+  test01();
+}