isinstance on runtime-checkable protocols does not correctly narrow unions inferred by TypeGuard

Bug Report

from __future__ import annotations

from collections.abc import Sized
from typing import (
    Iterator,
    Sequence,
)

from typing_extensions import TypeGuard


def is_list_like(x: object) -> TypeGuard[Iterator | Sequence]:  # not the best example but it demonstrates the issue
    ...


def is_nested_list(xs: object) -> bool:
    return (
        is_list_like(xs)
        and isinstance(xs, Sized)  # after this xs is assumed to be Sized but it should be Sized while simultaneously also being an Iterator or a Sequence
        and len(xs) > 0
        and all(is_list_like(x) for x in xs)  # "Sized" has no attribute "__iter__" (not iterable)  [attr-defined]
    )

Your Environment

• Mypy version used: 0.981 (same for 0.971)
• Python version used: 3.10.4

Comienza con el reproducer proporcionado e inspecciona cómo mypy gestiona las comprobaciones isinstance sobre protocolos comprobables en tiempo de ejecución después de que un TypeGuard estreche una unión. El problema está resuelto cuando xs conserva la unión estrechada por TypeGuard y, al mismo tiempo, se estrecha a Sized, de modo que la iteración y len(xs) ya no producen el error de tipo indicado.