Do you have a reproducer that does not use cloudpickle?

Pickling functions seems to work correctly.

>>> import pickle
>>> def func():
...     return len([])
... 
>>> func2 = pickle.loads(pickle.dumps(func))
>>> 
>>> func2()
0

How is cloudpickle supposed to work? It looks like the globals dict passed to FunctionType(...) lacks a __builtins__.