explicit `x is None` should narrow union candidates like `isinstance(x, type(None))`

[bwo]

from typing import Optional, List  # noqa


def union_test1(x):
    # type: (Optional[List[int]]) -> Optional[int]
    if x is None:
        return x
    else:
        return len(x)


def union_test2(x):
    # type: (Optional[List[int]]) -> Optional[int]
    if isinstance(x, type(None)):
        return x
    else:
        return len(x)

union_test1 with default options gives the error Incompatible return value type (got "Optional[List[int]]", expected "Optional[int]") (with --strict-optional, it does not give this error). union_test2 typechecks just fine. This strikes me as highly counterintuitive; even though (as the docs say) None is a valid value for each type, if one of the above checks allows the typechecker to learn that x is None and therefore not a list, it's hard to see why the other doesn't.

Failing that, I would suggest at least a documentation change to point out both the gotcha embodied in union_test1 and the workaround in union_test2.

[gvanrossum]

Agreed, I don't see why the first one isn't treated the same as the second.

[ilevkivskyi]

Agreed, I don't see why the first one isn't treated the same as the second.

This is not the first time I see weird behaviour of None without --strict-optional.

[msullivan]

It looks like the code in find_isinstance_check to refine optionals based on comparisons is explicitly gated on --strict-optional: https://github.com/python/mypy/blob/master/mypy/checker.py#L2865

Is there any reason to not just change that?