fix: take count into account when using underlying hint

Author: KowalskiThomas

Lib/test/test_itertools.py

@@ -1231,6 +1231,17 @@ def __index__(self):
         list(it)  # exhaust
         self.assertEqual(it.__length_hint__(), 0)
 
+        it = islice(iter(range(10)), 2, None, 2)
+        self.assertEqual(it.__length_hint__(), 4)  # indices 2, 4, 6, 8
+        next(it)  # yields 2
+        self.assertEqual(it.__length_hint__(), 3)  # indices 4, 6, 8
+        next(it)  # yields 4
+        self.assertEqual(it.__length_hint__(), 2)  # indices 6, 8
+        next(it)  # yields 6
+        self.assertEqual(it.__length_hint__(), 1)  # index 8
+        next(it)  # yields 8
+        self.assertEqual(it.__length_hint__(), 0)
+
     def test_takewhile(self):
         data = [1, 3, 5, 20, 2, 4, 6, 8]
         self.assertEqual(list(takewhile(underten, data)), [1, 3, 5])

Modules/itertoolsmodule.c

@@ -1676,7 +1676,7 @@ islice_length_hint(PyObject *op, PyObject *Py_UNUSED(dummy))
             /* propagate exception */
             return NULL;
         }
-        remaining = hint - lz->next;
+        remaining = hint - (lz->next - lz->cnt);
     } else {
         remaining = lz->stop - lz->next;
     }