backport of r51950

* regression bug, count_next was coercing a Py_ssize_t to an unsigned Py_size_t
  which breaks negative counts
* added test for negative numbers

Lib/test/test_itertools.py

@@ -58,6 +58,10 @@ class TestBasicOps(unittest.TestCase):
         self.assertEqual(repr(c), 'count(3)')
         c.next()
         self.assertEqual(repr(c), 'count(4)')
+        c = count(-9)
+        self.assertEqual(repr(c), 'count(-9)')
+        c.next()
+        self.assertEqual(c.next(), -8)
 
     def test_cycle(self):
         self.assertEqual(take(10, cycle('abc')), list('abcabcabca'))

Modules/itertoolsmodule.c

@@ -2072,7 +2072,7 @@ count_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 static PyObject *
 count_next(countobject *lz)
 {
-	return PyInt_FromSize_t(lz->cnt++);
+	return PyInt_FromSsize_t(lz->cnt++);
 }
 
 static PyObject *