Change filtertuple() to use tp_as_sequence->sq_item

instead of PyTuple_GetItem, so an overwritten __getitem__
in a tuple subclass works. SF bug #665835.

Lib/test/test_builtin.py

@@ -419,7 +419,6 @@ class BuiltinTest(unittest.TestCase):
     def test_filter_subclasses(self):
         # test, that filter() never returns tuple, str or unicode subclasses
         # and that the result always go's through __getitem__
-        # FIXME: For tuple currently it doesn't go through __getitem__
         funcs = (None, lambda x: True)
         class tuple2(tuple):
             def __getitem__(self, index):
@@ -428,7 +427,7 @@ class BuiltinTest(unittest.TestCase):
             def __getitem__(self, index):
                 return 2*str.__getitem__(self, index)
         inputs = {
-            tuple2: {(): (), (1, 2, 3): (1, 2, 3)}, # FIXME
+            tuple2: {(): (), (1, 2, 3): (2, 4, 6)},
             str2:   {"": "", "123": "112233"}
         }
         if have_unicode:

Python/bltinmodule.c

@@ -1888,8 +1888,13 @@ filtertuple(PyObject *func, PyObject *tuple)
 		PyObject *item, *good;
 		int ok;
 
-		if ((item = PyTuple_GetItem(tuple, i)) == NULL)
+		if (tuple->ob_type->tp_as_sequence &&
+		    tuple->ob_type->tp_as_sequence->sq_item) {
+			item = tuple->ob_type->tp_as_sequence->sq_item(tuple, i);
+		} else {
+			PyErr_SetString(PyExc_TypeError, "unsubscriptable object");
 			goto Fail_1;
+		}
 		if (func == Py_None) {
 			Py_INCREF(item);
 			good = item;