Incorrect type inference in "if ham and spam is None: spam = ham; spam.startswith('/')"

[elprans]

Given the following code (test.py):

import typing
import urllib.parse


def _parse_dsn(*, dsn, database) -> dict:
    if dsn:
        parsed = urllib.parse.urlparse(dsn)
        if parsed.path and database is None:
            database = parsed.path
            if database.startswith('/'):  # this is line 10
                database = database[1:]

    return dict(database=database)

$ mypy test.py fails with:

test.py:10: error: "None" has no attribute "startswith"
test.py:11: error: Value of type "None" is not indexable

despite database provably not being None.

[ilevkivskyi]

I think this is essentially a duplicate of #3526 (although a bit special since in this case initial type is also Any).

[emmatyping]

Agreed. Closing as duplicate of #3526.