René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Incorrect type narrowing for select class hierarchies ","articleBody":"**Bug Report**\r\n\r\nI've found that in an exhaustive `isinstance` if block, mypy still leaves a more general type resolution than it should. In the snippet of code, mypy should be able to understand that there is no attr-defined error.\r\n\r\n**To Reproduce**\r\n\r\n```python\r\nfrom __future__ import annotations\r\n\r\n\r\nclass A:\r\n    pass\r\n\r\n\r\nclass B(A):\r\n    def get_x(self):\r\n        pass\r\n\r\n\r\nclass C(A):\r\n    def get_x(self):\r\n        pass\r\n\r\n\r\nclass D(C):\r\n    pass\r\n\r\n\r\nclass E(C):\r\n    pass\r\n\r\n\r\nclass F(C):\r\n    pass\r\n\r\n\r\ndef x(obj: A):\r\n    value = \"\"\r\n    if isinstance(obj, B):\r\n        value = obj.__class__.__name__\r\n    elif isinstance(obj, D):\r\n        value = obj.__class__.__name__\r\n    elif isinstance(obj, E):\r\n        value = obj.__class__.__name__\r\n    elif isinstance(obj, F):\r\n        value = obj.__class__.__name__\r\n    else:\r\n        raise RuntimeError(\"unhandled type\")\r\n\r\n    reveal_type(obj)\r\n    obj.get_x()\r\n\r\n```\r\n\r\n**Expected Behavior**\r\n\r\n```\r\nx.py:43: note: Revealed type is \"B | D | E | F\"\r\n```\r\n\r\n**Actual Behavior**\r\n```\r\nx.py:43: note: Revealed type is \"x.A\"\r\nx.py:44: error: \"A\" has no attribute \"get_x\"  [attr-defined]\r\n```\r\n\r\n**Your Environment**\r\n\r\n\r\n- Mypy version used: mypy 1.6.1 (compiled: yes)\r\n- Mypy command-line flags: default\r\n- Mypy configuration options from `mypy.ini` (and other config files): default\r\n- Python version used: 3.10.13\r\n\r\n","author":{"url":"https://github.com/Redoubts","@type":"Person","name":"Redoubts"},"datePublished":"2023-10-31T17:32:51.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/16379/mypy/issues/16379"}