Lambda inference

[hasufell]

Sorry I didn't look through all issues, so this migh very well be a duplicate.

This should be rejected by MyPy (which is true for pytype btw):

def foo(a : int) -> str:
    return (lambda x: x + 1)(a)

x in the lambda can only ever be of type int anyway, so the result of the whole expression can only be int as well, never str.

[JukkaL]

Yes, this seems like a mypy bug. The technical explanation is that mypy doesn't use information from the surrounding call expression as part of the type context for the lambda expression.