libstdc++: Make deque iterator operator- usable with value-init iterators

N3644 implies that operator- can be used on value-init iterators. We now return
0 if both iterators are value initialized. If only one is value initialized we
keep the UB by returning the result of a normal computation which is a meaningless
value.

libstdc++-v3/ChangeLog:

	PR libstdc++/70303
	* include/bits/stl_deque.h (std::deque<>::operator-(iterator, iterator)):
	Return 0 if both iterators are value-initialized.
	* testsuite/23_containers/deque/70303.cc: New test.
	* testsuite/23_containers/vector/70303.cc: New test.

3 files changed, 146 insertions, 6 deletions

diff --git a/libstdc++-v3/include/bits/stl_deque.h b/libstdc++-v3/include/bits/stl_deque.h
index d41c2771..04b70b7 100644
--- a/libstdc++-v3/include/bits/stl_deque.h
+++ b/libstdc++-v3/include/bits/stl_deque.h
@@ -352,9 +352,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       friend difference_type
       operator-(const _Self& __x, const _Self& __y) _GLIBCXX_NOEXCEPT
       {
-	return difference_type(_S_buffer_size())
-	  * (__x._M_node - __y._M_node - 1) + (__x._M_cur - __x._M_first)
-	  + (__y._M_last - __y._M_cur);
+	if (__builtin_expect(__x._M_node || __y._M_node, true))
+	  return difference_type(_S_buffer_size())
+	    * (__x._M_node - __y._M_node - 1) + (__x._M_cur - __x._M_first)
+	    + (__y._M_last - __y._M_cur);
+
+	return 0;
       }
 
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
@@ -366,9 +369,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	operator-(const _Self& __x,
 		  const _Deque_iterator<_Tp, _RefR, _PtrR>& __y) _GLIBCXX_NOEXCEPT
 	{
-	  return difference_type(_S_buffer_size())
-	    * (__x._M_node - __y._M_node - 1) + (__x._M_cur - __x._M_first)
-	    + (__y._M_last - __y._M_cur);
+	  if (__builtin_expect(__x._M_node || __y._M_node, true))
+	    return difference_type(_S_buffer_size())
+	      * (__x._M_node - __y._M_node - 1) + (__x._M_cur - __x._M_first)
+	      + (__y._M_last - __y._M_cur);
+
+	  return 0;
 	}
 
       friend _Self
diff --git a/libstdc++-v3/testsuite/23_containers/deque/70303.cc b/libstdc++-v3/testsuite/23_containers/deque/70303.cc
new file mode 100644
index 0000000..e0e6369
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/deque/70303.cc
@@ -0,0 +1,67 @@
+// Copyright (C) 2021 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 <deque>
+#include <testsuite_hooks.h>
+
+// PR libstdc++/70303
+
+void test01()
+{
+  typedef typename std::deque<int>::iterator It;
+  It it = It();
+  VERIFY(it == it);
+  VERIFY(!(it != it));
+  VERIFY(it - it == 0);
+  VERIFY(!(it < it));
+  VERIFY(!(it > it));
+  VERIFY(it <= it);
+  VERIFY(it >= it);
+
+  typedef typename std::deque<int>::const_iterator CIt;
+  CIt cit = CIt();
+  VERIFY(cit == cit);
+  VERIFY(!(cit != cit));
+  VERIFY(cit - cit == 0);
+  VERIFY(!(cit < cit));
+  VERIFY(!(cit > cit));
+  VERIFY(cit <= cit);
+  VERIFY(cit >= cit);
+
+  VERIFY(it == cit);
+  VERIFY(!(it != cit));
+  VERIFY(cit == it);
+  VERIFY(!(cit != it));
+  VERIFY(it - cit == 0);
+  VERIFY(cit - it == 0);
+  VERIFY(!(it < cit));
+  VERIFY(!(it > cit));
+  VERIFY(it <= cit);
+  VERIFY(it >= cit);
+  VERIFY(!(cit < it));
+  VERIFY(!(cit > it));
+  VERIFY(cit <= it);
+  VERIFY(cit >= it);
+}
+
+int main()
+{
+  test01();
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/23_containers/vector/70303.cc b/libstdc++-v3/testsuite/23_containers/vector/70303.cc
new file mode 100644
index 0000000..af18a05
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/70303.cc
@@ -0,0 +1,67 @@
+// Copyright (C) 2021 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 <vector>
+#include <testsuite_hooks.h>
+
+// PR libstdc++/70303
+
+void test01()
+{
+  typedef typename std::vector<int>::iterator It;
+  It it = It();
+  VERIFY(it == it);
+  VERIFY(!(it != it));
+  VERIFY(it - it == 0);
+  VERIFY(!(it < it));
+  VERIFY(!(it > it));
+  VERIFY(it <= it);
+  VERIFY(it >= it);
+
+  typedef typename std::vector<int>::const_iterator CIt;
+  CIt cit = CIt();
+  VERIFY(cit == cit);
+  VERIFY(!(cit != cit));
+  VERIFY(cit - cit == 0);
+  VERIFY(!(cit < cit));
+  VERIFY(!(cit > cit));
+  VERIFY(cit <= cit);
+  VERIFY(cit >= cit);
+
+  VERIFY(it == cit);
+  VERIFY(!(it != cit));
+  VERIFY(cit == it);
+  VERIFY(!(cit != it));
+  VERIFY(it - cit == 0);
+  VERIFY(cit - it == 0);
+  VERIFY(!(it < cit));
+  VERIFY(!(it > cit));
+  VERIFY(it <= cit);
+  VERIFY(it >= cit);
+  VERIFY(!(cit < it));
+  VERIFY(!(cit > it));
+  VERIFY(cit <= it);
+  VERIFY(cit >= it);
+}
+
+int main()
+{
+  test01();
+  return 0;
+}