isinstance and TypeVar

[johnburnett]

Bug Report

Run mypy against the following:

from typing import TypeVar

T = TypeVar("T")

def _checkType(arg: T, typ: type) -> T:
    if isinstance(arg, typ):
        return arg
    raise TypeError(f"Argument is not a {typ}.")

reveal_type(_checkType(42, int))

Expected Behavior

Type checks to pass

Actual Behavior

The following error is printed: error: Incompatible return value type (got "object", expected "T")

Additionally, these two variants work around the issue (and pass without error):

def _checkType_cast(arg: T, typ: type) -> T:
    if isinstance(arg, typ):
        return cast(T, arg)
    raise TypeError(f"Argument is not a {typ}.")

def _checkType_backup(arg: T, typ: type) -> T:
    backup = arg
    if isinstance(arg, typ):
        return backup
    raise TypeError(f"Argument is not a {typ}.")

Your Environment

• Mypy version used: 0.971
• Python version used: 3.9.7

[ilinum]

Duplicate of #13462.