Nested if Optional checking for class attributes fails

Checking optional attributes in a class does not persist in a nested if:

(test.py)

#!/usr/bin/env python3

import typing


class Example:
    def __init__(self):
        self.a: typing.Optional[int]


def check_class(foo: Example, bar: Example):
    reveal_type(foo.a)
    if foo.a is not None:
        reveal_type(foo.a)
        if foo.a > 0:
            reveal_type(foo.a)
            foo = bar
        elif foo.a < 0:
            reveal_type(foo.a)
            foo.a += 1

Output:

$ mypy test.py
test.py:12: note: Revealed type is "Union[builtins.int, None]"
test.py:14: note: Revealed type is "builtins.int"
test.py:16: note: Revealed type is "builtins.int"
test.py:18: error: Unsupported operand types for > ("int" and "None")
test.py:18: note: Left operand is of type "Optional[int]"
test.py:19: note: Revealed type is "Union[builtins.int, None]"
test.py:20: error: Unsupported operand types for + ("None" and "int")
test.py:20: note: Left operand is of type "Optional[int]"

Environment

• Python 3.9.9
• mypy == 0.971
• Linux x86_64
• No other mypy config

まず、提供された test.py の例を mypy で実行し、ネストされた分岐での reveal_type の出力と報告されたエラーを調べます。foo の再代入後に誤った narrowing を導入することなく、意図した non-None の narrowing がそこで一貫して処理されれば、作業は完了です。