Optional types not being narrowed after a logical-and followed by a test of one of the operands

[arusahni]

(Sorry if the title is overly specific)

• Are you reporting a bug, or opening a feature request?

Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Optional

def get_org_id(org_name: str) -> str:
    return 'some_org_id'

def create(org_name: Optional[str], org_id: Optional[str]):
    if not org_name and not org_id:
        raise ValueError('No org specified')
    if not org_id:
        org_id = get_org_id(org_name)

• What is the actual behavior/output?

The inferred type of org_name at the last line is Optional[str] (technically Union[builtins.str, None]).

• What is the behavior/output you expect?

The type should be narrowed to str.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

Python 3.7.3
mypy: 0.701

• What are the mypy flags you are using? (For example --strict-optional)

No flags

[ilevkivskyi]

This is to tricky for mypy to understand. We may support this at some point, but I thin kit is relatively low priority. Se also #6641 which is quite similar.

[ghost]

@ilevkivskyi #10696 might also be a duplicate

maybe I'm mistaken, but i believe this might be a bug?
this only occurs in some cases; i don't understand the issue enough to describe which cases

one example i've seen is some definition of f like f = () -> Optional[str] which terminates with return A and A.b where A is some class, and b is some str attribute of A will result in an error indicating
Incompatible return value type (got "Union[A, str]", expected "Optional[str]")

you know already that the correct type is Union[str, None]

also, this sometimes? works correctly with things like it: str = (A and A.b) or 'nice'

edit: some weird libs make their own falsey types; i.e. Union[Unset, T]...but don't support them, imo