mypy failed to narrow down union types including Self with isinstance #14912

Bug Report

mypy failed to narrow down union types including Self with isinstance.

To Reproduce

Consider the following code:

from dataclasses import dataclass
from typing import Self, reveal_type


@dataclass
class Foo:
    i: int

    def bar(self, other: Self | int) -> int:
        if isinstance(other, Foo):
            return self.i * other.i
        else:
            reveal_type(other)  # "int" expected, gets "Self | int"
            return self.i * other # Unsupported operand types for * ("int" and "Self")  [operator]mypy(error)

Expected Behavior

Other should be infered as an int and mypy reports no error.

Actual Behavior

mypy failed to narrow type of other

Revealed type is "Union[Self`0, builtins.int]"mypy(note)

Also, mypy reports the following error.

Unsupported operand types for * ("int" and "Self")  [operator]mypy(error)

Your Environment

• Mypy version used: 1.1.1
• Mypy command-line flags: --follow-imports=silent --ignore-missing-imports --show-column-numbers --no-pretty (VSCode defaults)
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.1