mypy confused by ternary operator

[dvarrazzo]

The two functions error_from_state_[ok|bad]() below are pretty much equivalent; one use an if statement, the other uses a conditional expression.

Mypy reports an error only in the expression case. I would expect the result to be the same.

from typing import Type, Optional

def class_for_state(state: bytes) -> Type[Exception]:
    return TypeError

def error_from_state_ok(state: Optional[bytes]) -> Exception:
    if state is not None:
        cls = class_for_state(state)
    else:
        cls = ValueError

    return cls("message")

def error_from_state_bad(state: Optional[bytes]) -> Exception:
    cls = class_for_state(state) if state is not None else ValueError
    return cls("message")

mypy --strict reports:

$ mypy --strict mypyerr.py 
mypyerr.py:16: error: Returning Any from function declared to return "Exception"

Tested with Python 3.6 and mypy 0.770 and master.

[JelleZijlstra]

This is basically #4134.