Incorrect and misleading error message when `.mypy_cache` is hot (`[override]` error on `__prepare__()`)

[finite-state-machine]

Bug Report

Mypy outputs the wrong error message when analyzing the code given below, but only when its on-disk cache is hot.

I infer the cache is the issue based on:

• the issue doesn't manifest the 1st time mypy is run with any given configuration (version, flags, etc.), but occurs every time thereafter
• deleting .mypy_cache allows mypy to operate correctly for one run only
• the issue never manifests if --no-incremental is included on the command-line

To Reproduce

[zip] bug.py is:

from __future__ import annotations
from typing import *

class SomeMeta(type):

    def __prepare__(
            metacls, name: str, bases: Tuple[type, ...], /, **kwds: Any
            ) -> Mapping[str, object]:

        return super().__prepare__(name, bases, **kwds)

Run mypy on this script twice.
(mypy-play.net cannot reproduce this issue)

Actual behaviour

Run results (other than the 1st run):

$ python3 -m mypy bug.py
bug.py:6: error: Signature of "__prepare__" incompatible with supertype "type"  [override]
bug.py:6: note:      Superclass:
bug.py:6: note:          def __prepare__(metacls, str, Tuple[type, ...], /, **kwds: Any) -> Mapping[str, object]
bug.py:6: note:      Subclass:
bug.py:6: note:          def __prepare__(metacls, str, Tuple[type, ...], /, **kwds: Any) -> Mapping[str, object]
Found 1 error in 1 file (checked 1 source file)

Note that mypy's output reads roughly:

Superclass:
    <some signature>
Subclass:
    <exactly the same signature>

Expected Behavior

Mypy should output:

bug.py:6: error: Signature of "__prepare__" incompatible with supertype "type"  [override]
bug.py:6: note:      Superclass:
bug.py:6: note:          @classmethod
bug.py:6: note:          def __prepare__(metacls, str, Tuple[type, ...], /, **kwds: Any) -> Mapping[str, object]
bug.py:6: note:      Subclass:
bug.py:6: note:          def __prepare__(metacls, str, Tuple[type, ...], /, **kwds: Any) -> Mapping[str, object]
Found 1 error in 1 file (checked 1 source file)

Note the appearance of @classmethod under Superclass.

Related issues

This might relate to #10366.

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags: none necessary, but changing flags will result in the expected behaviour one time only
• Mypy configuration options from mypy.ini (and other config files): (presumably the same as command-line flags)
• Python version used: 3.8, 3.12