An instance of a final class should be treated as satisfying a TypeVar bound to it, after being narrowed

Bug Report

See title, I think this is the cause of the remaining issue for https://github.com/python/mypy/issues/9424. I think this is a followup to https://github.com/python/mypy/pull/19183.

To Reproduce

from typing import TypeVar, final

@final
class A:
    pass

U = TypeVar('U', bound=A | int)

def f(u: U) -> U:
    if isinstance(u, A):
        return A()
    else:
        return u

Expected Behavior

No errors

Actual Behavior

There's an error:

main.py:11: error: Incompatible return value type (got "A", expected "U")  [return-value]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Reproduced on mypy play.

• Mypy version used: v1.16
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.12

まず、提供された Python の例を mypy v1.16 で、mypy playground またはローカルの checkout を使って実行します。final class A のインスタンスの narrowing が、A | int に bound された TypeVar U とどのように相互作用するかを調査します。例が incompatible return value error なしで型チェックに通れば完了です。