Generic type variable not unified with specific bound type.

Bug Report

In g below, argument types of different versions of x.f from different bounds of T cross-contaminate. When T is A, x.f can only accept A = T. Likewise, when T is B, x.f can only accept B = T. Hence the code is correct. However, mypy seems to believe that T can somehow be both A and B at simultaneously.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=a92693b3d619e1a11011b86a7cee1ce2

from typing import TypeVar, Generic

class A:
    def f(self, other: A) -> None:
        ...

class B:
    def f(self, other: B) -> None:
        ...

T = TypeVar('T', bound = A | B)

def g(x: T) -> None:
    x.f(x)

Expected Behavior

g typechecked successfully.

Actual Behavior

g fails typechecking:

main.py:14: error: Argument 1 to "f" of "A" has incompatible type "T"; expected "A"  [arg-type]
main.py:14: error: Argument 1 to "f" of "B" has incompatible type "T"; expected "B"  [arg-type]

Your Environment

• Mypy version used: 1.5.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.11

まず、リンクされた mypy-play の再現例を mypy 1.5.0 とデフォルトのフラグで実行し、報告されたエラーを g で期待される型チェック成功の結果と比較します。呼び出し x.f(x) の背後にある型変数の処理を追跡します。この例が A と B の互換性のない引数エラーなしで型チェックを通過し、回帰テストのカバレッジが追加されれば完了です。