False positive with list comprehensions and --warn-return-any

[srittau]

Given the following code in test.py:

def bar() -> "FooClass":
    list1 = [FooClass()]
    list2 = [m for m in list1 if m.matches]
    return list2[0]


class FooClass:

    def __init__(self) -> None:
        self.matches = True

Testing it with mypy --warn-return-any test.py (mypy 0.550, Python 3.6.3) gives the following error:

test.py:4: warning: Returning Any from function declared to return "FooClass"

Any of the following "fixes" this error:

• Move FooClass declaration in front of the declaration of bar()
• Remove the m.matches check inside the list comprehension
• Remove --warn-return-any from the command line
• Adding assert isinstance(list2[0], FooClass) before the return

[elazarg]

Duplicate of #4296, fixed on master (#4295).