Union interacts poorly with `isinstance`-based unreachability

Bug Report

This is probably already a bug report but the only place I know where this was mentioned was https://github.com/python/mypy/pull/13607#issuecomment-1236357236.

Including | None in a type makes isinstance type narrowing incorrectly report unreachability. This isn't only a problem with | None.

To Reproduce

class SymbolTableNode: pass
class FuncBase: pass

def bad(node: SymbolTableNode | None) -> None:
    assert isinstance(node, FuncBase)
    reveal_type(node)  # unreachable

def good(node: SymbolTableNode) -> None:
    assert isinstance(node, FuncBase)
    reveal_type(node)  # <subclass of SymbolTableNode and FuncBase>

Expected Behavior

No unreachable statements.

Actual Behavior

Unreachable reveal_type(node).

Your Environment

Reproduced in mypy-play.

• Mypy version used: 1.18.2
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.12

Inizia con il riproduttore fornito in mypy-play usando mypy 1.18.2 e --warn-unreachable, confrontando il caso Union | None con il caso senza Union. Traccia il comportamento del narrowing basato su isinstance; il lavoro è completato quando reveal_type(node) non viene segnalato come irraggiungibile nell’esempio con Union, mentre il narrowing previsto rimane intatto.