meliae hits type_traverse assertion when dumping ctypes objects under Py_DEBUG

This is probably a fairly obscure bug, but it's worth posting about in case it can be fixed (or the information is useful to someone else).

In several versions of Python, type_traverse in Objects/typeobject.c contains the assertion

assert(type->tp_flags & Py_TPFLAGS_HEAPTYPE);

meliae developers know about it, so they made a check in _scanner_core.c which tries to avoid tickling this assertion:

    if (Py_TYPE(c_obj)->tp_traverse == NULL
        || (Py_TYPE(c_obj)->tp_traverse == PyType_Type.tp_traverse
            && !PyType_HasFeature((PyTypeObject*)c_obj, Py_TPFLAGS_HEAPTYPE)))
    {
        /* Obviously we don't traverse if there is no traverse function. But
         * also, if this is a 'Type' (class definition), then
         * PyTypeObject.tp_traverse has an assertion about whether this type is
         * a HEAPTYPE. In debug builds, this can trip and cause failures, even
         * though it doesn't seem to hurt anything.
         * See: https://bugs.launchpad.net/bugs/586122
         */
        do_traverse = 0;
    }

This isn't a complete solution because type_traverse can be called indirectly from some custom tp_traverse. In fact, this is precisely what ctypes.PyCPointerType does:

static int
CDataType_traverse(PyTypeObject *self, visitproc visit, void *arg)
{
    StgDictObject *dict = PyType_stgdict((PyObject *)self);
    if (dict)
        Py_VISIT(dict->proto);
    return PyType_Type.tp_traverse((PyObject *)self, visit, arg);
}
/*...*/
    (traverseproc)CDataType_traverse, /* tp_traverse */
    (inquiry)CDataType_clear, /* tp_clear */

To trigger this bug requires the following steps:
1) Build Python 3.2 using Py_DEBUG
2) Build meliae against this Python
3) Run the following script:
import ctypes
import sys
import meliae.scanner
meliae.scanner.dump_all_referenced('/tmp/foo', sys.getobjects(0))

Here's what I get from running that:

Python 3.2.1 (default, Nov 20 2011, 14:49:05)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
[60256 refs]
>>> import sys
[60258 refs]
>>> import meliae.scanner
[61527 refs]
>>> meliae.scanner.dump_all_referenced('/tmp/foo', sys.getobjects(0))
Assertion failed: (type->tp_flags & Py_TPFLAGS_HEAPTYPE), function type_traverse, file Objects/typeobject.c, line 2600.
Abort trap

If you don't import ctypes, the dump succeeds without incident.

Workaround:
I just compiled typeobject.c with
    if(!(type->tp_flags & Py_TPFLAGS_HEAPTYPE))
        return -1;
instead of the assert.