Wrong type narrowing when using type without an argument in isinstance call

[twoertwein]

To Reproduce

from typing import TypeVar

T = TypeVar("T")


def fun(x: T, klass: type) -> T:
    if not isinstance(x, klass):
        raise TypeError
    return x

Expected Behavior

No error.

Actual Behavior

error: Incompatible return value type (got "object", expected "T") [return-value]

Your Environment

• Mypy version used: 0.971

[kellerza]

Adding a constraint to the TypeVar seems to help

T = TypeVar("T", str, str)

def fun(val: T) -> T:
    if not isinstance(val, str):
        raise TypeError
    return val