"Cannot determine type" if first assignment to attribute is unreachable

To Reproduce

Run mypy on the following code:

class C:
    def __init__(self, a: str) -> None:
        if a is None:
            self.x = 123
        else:
            self.x = 456

It will output:

testcase.py:6: error: Cannot determine type of 'x'

Line 6 contains the second assignment to self.x.

Expected Behavior

mypy should be able to determine that the type of C.x is int.

Actual Behavior

The fact that a is annotated as str means that mypy determines that a is None is never true and therefore the first assignment to self.x is unreachable. The problem disappears if a is annotated as Optional[str].

So it looks like mypy half-ignores the first assignment to self.x: it doesn't use the type information available there since the code is unreachable, but it still considers it the assignment that determines the type of self.x.

Your Environment

mypy 0.790 with default configuration, running on Python 3.8.5 on openSUSE Linux.

Start with the provided Python reproducer and trace mypy's type inference for the first attribute assignment when control flow marks that branch unreachable. Confirm the fix by rerunning mypy on the example: C.x should be inferred as int without an error, while the Optional[str] case should continue to work.