False negative: isinstance() guard removes parameterisation by `TypeVar`

To Reproduce

mypy does not reject the following:

from collections.abc import Mapping
from typing import TypeVar

T = TypeVar("T")
S = TypeVar("S")

def swap(x: Mapping[T, S]) -> Mapping[S, T]:
    if isinstance(x, dict):
        return x
    else:
        raise Exception

Expected Behavior

After the type guard, reveal_type indicates that x has type dict[Any, Any]. As we're already in swap then it should actually have type dict[T, S].

As a result, no error is raised despite the above code being wrong.

Your Environment

• Mypy version used: 0.95.0
• Python version used: 3.10.0
• Operating system and version: Ubuntu 20.04 on WSL2 on Windows 10

Inizia eseguendo l’esempio di swap segnalato e ispeziona il controllo isinstance(x, dict) e il relativo risultato reveal_type. Segui come mypy restringe il Mapping parametrizzato con TypeVar, quindi verifica che il tipo ristretto conservi T e S e che il ritorno errato venga rifiutato.