Flow-sensitive typing ignores dictionary key mutation

Bug Report

mypy allows runtime type checks for dictionary items. However it ignores subsequent dictionary mutations. This leads to mypy accepting code that fails at runtime.

To Reproduce

from typing import Dict, Optional

foo_dict: Dict[str, Optional[str]] = {'bar': 'baz'}

if foo_dict['bar'] is not None:
    foo_dict['bar'] = None
    foo_dict['bar'].upper()

Expected Behavior

mypy reports .upper() call as error. This is similar to what already happens with object attributes:

from dataclasses import dataclass
from typing import Optional

@dataclass
class Foo:
    bar: Optional[str]

foo = Foo(bar='baz')

if foo.bar is not None:
    foo.bar = None
    foo.bar.upper()

For such code mypy outputs the following error: "None" has no attribute "upper".

Actual Behavior

mypy reports no error.

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.6.8
• Operating system and version: Windows 8.1

まず、mypy --strict を使って Python の再現コードを実行し、dataclass 属性の例と比較して、辞書の場合は受け入れられる一方で属性の場合は拒否されることを確認します。辞書要素式に対するフロー依存の narrowing ロジックを追跡し、ミューテーションのカバレッジを追加します。mypy が .upper() の呼び出しをエラーとして報告すれば完了です。