Unexpected Errors When Using Invariant, Covariant, and Contravariant Type Variables in Protocols for `__class_getitem__`

Bug Report

I encountered an issue while defining a protocol with type variables that are invariant, covariant, and contravariant for the __class_getitem__ method. My goal was to create a protocol that supports different variance for the key parameter. I expected that at least one of the variance options (invariant, covariant, or contravariant) would work without errors. However, all three approaches result in type errors:

1. Invariant Type Variable: Using an invariant type variable T results in an error because the protocol expects a covariant type variable in this context.
2. Covariant Type Variable: When using a covariant type variable T_co, I cannot use it as a parameter for the __class_getitem__ method, which expects invariance or contravariance.
3. Contravariant Type Variable: Using a contravariant type variable T_contra also leads to an error since it is expected to be covariant in this context.

I was expecting that at least one of these cases would not raise an error.

To Reproduce

from abc import abstractmethod
from typing import Protocol, Self, TypeVar

T = TypeVar("T")
T_co = TypeVar("T_co", covariant=True)
T_contra = TypeVar("T_contra", contravariant=True)
R = TypeVar("R")
R_co = TypeVar("R_co", covariant=True)
R_contra = TypeVar("R_contra", contravariant=True)

class SupportsClassGetItemInvariant(Protocol[T, R_co]):
    @classmethod
    @abstractmethod
    def __class_getitem__(cls: type[Self], key: T, /) -> R_co: ...

class SupportsClassGetItemCovariant(Protocol[T_co, R_co]):
    @classmethod
    @abstractmethod
    def __class_getitem__(cls: type[Self], key: T_co, /) -> R_co: ...

class SupportsClassGetItemContravariant(Protocol[T_contra, R_co]):
    @classmethod
    @abstractmethod
    def __class_getitem__(cls: type[Self], key: T_contra, /) -> R_co: ...

Expected Behavior

I expected that at least one of these variations (invariant, covariant, or contravariant) would work without causing a type error in Mypy, allowing me to use the type variable as the key parameter in the __class_getitem__ method.

Actual Behavior

Mypy raises errors for all three cases, indicating that none of the variance annotations are accepted for the key parameter in the __class_getitem__ method within the protocol.

Your Environment

• Mypy version used: mypy 1.11.2
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini: None
• Python version used: Python 3.12.2

I would appreciate any insights or suggestions on how to approach this issue, as I was expecting at least one of these scenarios to work.

Riproduci le tre definizioni di protocollo dell'issue con Python 3.12.2 e mypy 1.11.2, quindi confronta le diagnosi di varianza riportate. Esamina la gestione della varianza dei protocolli da parte di mypy e i test esistenti per class_getitem; il lavoro è completato quando è stato stabilito il comportamento di varianza valido previsto e viene aggiunta una copertura di regressione per questi casi.