Inherit tp_traverse and tp_clear from the base class

We used to just ignore these, but we can call these from
the proxy_to_tp_traverse and proxy_to_tp_clear adapters.

src/runtime/objmodel.cpp

@@ -364,6 +364,13 @@ BoxedClass::BoxedClass(BoxedClass* base, gcvisit_func gc_visit, int attrs_offset
 }
 
 void BoxedClass::finishInitialization() {
+    assert(!tp_traverse);
+    assert(!tp_clear);
+    if (tp_base) {
+        tp_traverse = tp_base->tp_traverse;
+        tp_clear = tp_base->tp_clear;
+    }
+
     commonClassSetup(this);
 }
 
@@ -2825,7 +2832,7 @@ Box* callCLFunc(CLFunction* f, CallRewriteArgs* rewrite_args, int num_output_arg
     else
         r = chosen_cf->call(oarg1, oarg2, oarg3, oargs);
 
-    ASSERT(chosen_cf->spec->rtn_type->isFitBy(r->cls), "%s (%p) %s %s",
+    ASSERT(chosen_cf->spec->rtn_type->isFitBy(r->cls), "%s (%p) was supposed to return %s, but gave a %s",
            g.func_addr_registry.getFuncNameAtAddress(chosen_cf->code, true, NULL).c_str(), chosen_cf->code,
            chosen_cf->spec->rtn_type->debugName().c_str(), r->cls->tp_name);
     return r;

src/runtime/types.cpp

@@ -511,10 +511,13 @@ static int call_gc_visit(PyObject* val, void* arg) {
 
 static void proxy_to_tp_traverse(GCVisitor* v, Box* b) {
     boxGCHandler(v, b);
+
+    assert(b->cls->tp_traverse);
     b->cls->tp_traverse(b, call_gc_visit, v);
 }
 
 static void proxy_to_tp_clear(Box* b) {
+    assert(b->cls->tp_clear);
     b->cls->tp_clear(b);
 }
 

test/tests/weakref_ref_subclass.py

+# Make sure we can subclass from weakref.ref, since the weakref module itself does this
+
+from weakref import ref
+class MyRef(ref):
+    pass
+
+for i in xrange(100):
+    m = MyRef(MyRef)
+
+import gc
+gc.collect()