Optional[TypeVar] does not preserve type of TypeVar when given Union type

Bug Report

When passing a union into an Optional[TypeVar], we expect to preserve the type of the TypeVar.

To Reproduce

from typing import Optional, TypeVar, TypedDict, Union

T = TypeVar("T")


def non_null(var: Optional[T]) -> T:
    assert var is not None
    return var


def test(a_union: Union[str, int]) -> None:
    value: Union[str, int] = non_null(a_union)

Expected Behavior

Success: no issues found in 1 source file

Actual Behavior

test.py:16: error: Incompatible types in assignment (expression has type "object", variable has type "Union[str, int])"  [assignment]

Your Environment

• Mypy version used: 1.8.0
• Python version used: 3.9

Non viene indicato alcun file o test del repository; inizia con il reproducer Python fornito ed esegui mypy su di esso per confermare l’errore object-versus-Union segnalato. Traccia la gestione di Optional[TypeVar] per un argomento Union e considera completato il lavoro quando l’esempio non segnala problemi, come previsto.