mypy failed to narrow down union types including Self with isinstance

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

isinstance(other, self.__class__) doesn't work too.

Simpler repro:

[case testNarrowSelfType]
from typing import Self, Union
class A: ...
class B:
    def f(self, v: Union[Self, A]) -> A:
        if isinstance(v, B):
            return A()
        else:
            return v  # false error: Incompatible return value type (got "Union[Self, A]", expected "A")
[builtins fixtures/isinstancelist.pyi]

Fix narrowing union types that include Self with isinstance (#14923)

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

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

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mon-jai commented on Mar 17, 2023

tyralla commented on Mar 18, 2023

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