In bpo-40142, I tried to modify _PyObject_GC_TRACK() to visit the object before tracking it, as done by PyObject_GC_Track().

Problem: PyType_GenericAlloc() cannot traverse the object since the object members are not initialized yet. For example, dict_traverse() can only be called at dict_new() exit.

Here I propose a different approach. First, fix built-in types to only track instances when they are fully initialized. Avoid PyType_GenericAlloc().