[1.20 regression] when same object is narrowed multiple times

Bug Report

There was a regression in narrowing with isinstance checks from 1.19 to 1.20. This seems to only happen when the same object is narrowed multiple times. For example, this works correct if removing the outer is not None guard and the | None from the argument.

To Reproduce

mypy playground link

from __future__ import annotations


def _score_test(
    exc: BaseException | None, expected_excs: set[type[BaseException]]
) -> None:
    if exc is not None:
        reveal_type(exc)
        reveal_type(expected_excs)  # Revealed type is "BaseException"
        reveal_type(tuple(expected_excs))  # Revealed type is "tuple[type[BaseException], ...]"
        if isinstance(exc, tuple(expected_excs)):
            reveal_type(exc)  # Revealed type is "object"
            return logexc(exc)  # Argument 1 to "logexc" has incompatible type "object"; expected "BaseException | None"

def logexc(
    exc: BaseException | None
) -> None:
    ...

This bisects to #20675

While you can patch by making changes in the spot that that code touched, I wonder if the more correct fix is in conditional_types, or maybe even the binder... we shouldn't be widening the type at all

Okay yeah....

So if you look at the following code

from __future__ import annotations
from typing import Iterable
from typing_extensions import TypeAlias
import types

# Regression test for https://github.com/python/mypy/issues/21181
# We don't have the same type context as with the real stubs, so sort of fake it
_ClassInfoLike: TypeAlias = "type | tuple[_ClassInfoLike, ...]"

class A: ...

def fake_type_context(ts: list[type[A]]) -> _ClassInfoLike:
    return tuple(ts)  # E: Too many arguments for "tuple"

def f1(x: A | None, ts: list[type[A]]) -> None:
    if x is not None:
        reveal_type(x)  # N: Revealed type is "__main__.A"
        if isinstance(x, fake_type_context(ts)):
            reveal_type(x)  # N: Revealed type is "__main__.A"

def f2(x: A | None) -> None:
    if x is not None:
        reveal_type(x)  # N: Revealed type is "__main__.A"
        if isinstance(x, object):
            reveal_type(x)  # N: Revealed type is "__main__.A"

def f3(x: A | None, t: type) -> None:
    if x is not None:
        reveal_type(x)  # N: Revealed type is "__main__.A"
        if isinstance(x, t):
            reveal_type(x)  # N: Revealed type is "__main__.A"

f1 is like your case, and it regresses when we go from 1.19 to 1.20
But f3 I claim is the underlying issue and we incorrectly widen the type in both 1.19 and 1.20

Avoid widening types in conditional_types

Avoid widening types in conditional_types (python#21242)

Avoid widening types in conditional_types (#21242)

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[1.20 regression] when same object is narrowed multiple times #21181

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[1.20 regression] when same object is narrowed multiple times #21181

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hauntsaninja commented on Apr 13, 2026

hauntsaninja commented on Apr 13, 2026

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