Comparison operators emit "Unsupported operand types" when using `isinstance` to narrow `TypeVar` to union of numeric types

Bug Report

I have a generic class that wraps a bool, float, int, or str. It implements the comparison operator magic methods like __lt__ etc., using isinstance to ensure that callers don't mix bool/float/int with str. With mypy 1.16.1, this does not error, but with mypy 1.17.0, the comparison operators started emitting "error: Unsupported operand types for < (likely involving Union) [operator]"

To Reproduce

Minimal reproduction (using a generic function instead of a generic class): https://mypy-play.net/?mypy=latest&python=3.12&gist=dd4696c3ba71103c49d7a4aaf34b2aef

def f[T: float | int | str](x: T, y: T) -> bool:
    if isinstance(x, (float, int)) and isinstance(y, (float, int)):
        return x < y
    elif isinstance(x, str) and isinstance(y, str):
        return x < y
    else:
        raise TypeError

Expected Behavior

No errors.

Actual Behavior

test3.py:3: error: Unsupported operand types for < (likely involving Union)  [operator]
Found 1 error in 1 file (checked 1 source file)

It does not error if I remove the type variable and use union types directly:
def f(x: float | int | str, y: float | int | str) -> bool:

Your Environment

• Mypy version used: 1.17.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12.9

Inizia con la riproduzione in mypy-play e lo snippet equivalente di test3.py con mypy 1.17.0, quindi confronta il relativo narrowing di TypeVar con la versione con unione diretta. Traccia il percorso del controllo dei tipi per il narrowing di isinstance e gli operatori di confronto. Il lavoro è completato quando l’esempio generico non produce errori, mentre i confronti tra tipi misti continuano a essere rifiutati.