Returning Undefined value doesn't get flagged as error

[gvanrossum]

This gets no errors:

def words(s: str) -> Dict[str, int]:
    x = Undefined(Dict[str, int])
    return x

Shouldn't it be an error to return an undefined value?

[JukkaL]

Yeah, it would be good to track undefined values and flag an error at least if a program definitely uses an undefined value. This can be implemented using some fairly simple data flow analysis.

[JukkaL]

We are getting rid of Undefined so this is no longer relevant.