Inconsistent Handling of Self in ClassVar Annotations

Bug Report
When using Self within a ClassVar, mypy raises an arg-type error. This behavior seems inconsistent with the intended use of Self and ClassVar.​

To Reproduce

MyPy playground: https://mypy-play.net/?mypy=latest&python=3.12&gist=dfe06f06e69d157317a667311aa75952

from typing import TypeVar, Generic, ClassVar, Self

T = TypeVar("T", bound="BaseModel")

class BaseModel:
    _meta: "ClassVar[Meta[Self]]"

    class Meta(Generic[T]):
        """Metadata class for BaseModel."""
        def __init__(self, model: type[T]) -> None:
            self.model = model

    @classmethod
    def __init_subclass__(cls) -> None:
        # The following line triggers mypy error "arg-type"
        # bug.py: note: In member "__init_subclass__" of class "BaseModel":
	# bug.py:21:30: error: Argument 1 to "Meta" has incompatible type "type[BaseModel]"; expected "type[Self]"  [arg-type]
        cls._meta = cls.Meta(cls)

# outputs  -->  Revealed type is "bug.BaseModel.Meta[bug.BaseModel]"
reveal_type(BaseModel._meta)

Expected Behavior

The code should type-check without errors, as Self within the ClassVar should correspond to the subclass type, making cls.Meta(cls) a valid instantiation.​

Actual Behavior

mypy raises an arg-type error

bug.py: note: In member "__init_subclass__" of class "BaseModel":
bug.py:21:30: error: Argument 1 to "Meta" has incompatible type "type[BaseModel]"; expected "type[Self]"  [arg-type]

Your Environment:

mypy version used: 1.15.0
Python version used: 3.12.8

Additional Context:

Removing ClassVar, or changing Self to BaseModel both eliminate the mypy error.

This issue may be related to or a duplicate of Issue #17395, where Self types in ClassVar annotations lose specificity through TypeVar. In that case, Self appears to decay to the TypeVar bound instead of retaining the specific subclass type.​ I'm not certain if these are the same issue.

Inizia con il reproducer del playground di MyPy usando mypy 1.15.0 e Python 3.12.8, quindi confronta il comportamento con l’issue #17395. Traccia come viene gestito Self all’interno dell’annotazione ClassVar e della chiamata a init_subclass. Il lavoro è completato quando l’esempio supera il controllo dei tipi senza l’errore arg-type e dispone di copertura di regressione.