Overload ambiguity is ignored in `self`-annotated methods

Bug Report

Per the title: mypy does not check for overload ambiguity in self-annotated methods.
Mypy will instead, effectively, ignore self and then return the first matching overload,
thus circumventing the usual Any that is returned when overload ambiguity is involved.
This has lead to a number of false positives in https://github.com/numpy/numpy/issues/20099 and (in part) https://github.com/numpy/numpy/issues/20072.

The conditions for triggering this bug are as following:

• The callable is a method (things work fine for functions).
• The method is overloaded.
• self must be annotated with a parametrized generic.

To Reproduce

from typing import Any, Generic, overload, TypeVar, TYPE_CHECKING
import numpy as np

_CharType = TypeVar("_CharType", np.str_, np.bytes_)


class CharArray(Generic[_CharType]):
    @overload
    def strip(self: CharArray[np.str_], chars: str = ...) -> CharArray[np.str_]: ...
    @overload
    def strip(self: CharArray[np.bytes_], chars: bytes = ...) -> CharArray[np.bytes_]: ...


ar_any: CharArray[Any]
ar_str: CharArray[np.str_]
ar_bytes: CharArray[np.bytes_]

if TYPE_CHECKING:
    # The good
    reveal_type(ar_str.strip())  # E: Revealed type is "__main__.CharArray[numpy.str_]"
    reveal_type(ar_bytes.strip())  # E: Revealed type is "__main__.CharArray[numpy.bytes_]"

    # The bad
    # "__main__.CharArray[Any]" would be expected here due to overload ambiguity
    reveal_type(ar_any.strip())  # E: Revealed type is "__main__.CharArray[numpy.str_]"

    # Calling the method directly from the class does do the trick
    reveal_type(CharArray.strip(ar_any))  # E: Revealed type is "test.CharArray[Any]"

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: n.a.
• Mypy configuration options from mypy.ini (and other config files): n.a.
• Python version used: 3.9.6
• Operating system and version: macOS 11.4

まず、提供された Python の再現コードを mypy で実行し、オーバーロードされたメソッド呼び出しについて明らかになった型を比較します。self パラメーターがパラメーター化されたジェネリックであるメソッドのオーバーロード解決を追跡します。曖昧な CharArray[Any].strip() 呼び出しが Any を返し、一方で具体的な self 型がそれぞれの固有の結果を保持すれば完了です。