Martin, I removed the class blocks by accident. In any case, I reject the patch by myself, because to me the definition of "free variable" is not clear. 
The documentation [1] says: "If a variable is used in a code block but not defined there, it is a free variable." According to this description,  it seems to me that ``x`` is free both in ``foo()`` and in ``moo()``:


>>> def foo():
...     print(x)
...     def moo():
...         print(x)
...     return moo


But actually for the code object it is not:


>>> foo.__code__.co_freevars
()
>>> moo.__code__.co_freevars
('x',)


Thank you for your feedback, I will continue the discussion in issue 26683.

[1] https://docs.python.org/3/reference/executionmodel.html#naming-and-binding