Mypy misses conditional name not defined errors

[Dreamsorcerer]

if False:
    bar = "a"

print(bar)

Mypy passes successfully on this code. Python obviously errors with NameError: name 'bar' is not defined at runtime.

Mypy should ensure that a variable when referenced has been defined in all possible paths of execution up to that point.

[erictraut]

This is related to #2400 and #686

[Dreamsorcerer]

Thanks