Type inference of list comprehension and `Optional[t]`

[JukkaL]

The type of b is incorrectly List[Union[None, int]] instead of List[int] when using --strict-optional:

from typing import Optional, List
a = [] # type: List[Optional[int]]
b = [x for x in a if x]
reveal_type(b)

Generators and other comprehensions are related (not checked them).

[ddfisher]

Rather than being strict optional specific, this is a general problem with Unions:

from typing import Optional, List, Union
a = [] # type: List[Union[int, str]]
b = [x for x in a if isinstance(x, int)]
reveal_type(b)  # E: Revealed type is 'builtins.list[Union[builtins.int, builtins.str]]'