Ternary operator + isinstance + typevar confuses mypy

[098799]

Bug Report

Two different results depending on whether a ternary operator or a simple if-else block is used.

To Reproduce

from typing import TypeVar


def handle_str(arg: str) -> str:
    return arg


def handle_list(arg: list) -> list:
    return arg


type_arg = TypeVar('type_arg', str, list[str])


def chooser__standard(arg: type_arg) -> type_arg:
    if isinstance(arg, str):
        return handle_str(arg)
    else:
        return handle_list(arg)


def chooser__ternary(arg: type_arg) -> type_arg:
    return handle_str(arg) if isinstance(arg, str) else handle_list(arg)

raises errors only in the second case.

Expected Behavior

No error in the ternary case. (Well, would be amazing to have singledispatch handle this so I don't have to write code like this, but that's not a discussion for here...)

Actual Behavior

playground.py:23: error: Argument 1 to "handle_str" has incompatible type "List[str]"; expected "str"
playground.py:23: error: Argument 1 to "handle_list" has incompatible type "str"; expected "List[Any]"

Your Environment

• Mypy version used: mypy==0.910
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): ignore_missing_imports = True
• Python version used: Python 3.9.6
• Operating system and version: Manjaro Linux x86_64

[stevenh]

Tripped over this as its a rule that ruff enforces by default, putting it in conflict with mypy.

Ended up using # noqa: SIM108 to disable the ruff rule until this is fixed.

Tested with mypy: 1.8.0

[brianschubert]

Thanks for the report! Closing as duplicate of #4134