False negative when lvalue has union type

Mypy generates no error, even though the code is unsafe:

class C[T]:
    a: T

def f(x: C[int] | C[str]) -> None:
    x.a = 1  # No error

c = C[str]()
f(c)
print(c.a, type(c.a))  # 1, int

I'd expect x.a = 1 to generate an error, since the assignment is unsafe.

I suspect that the lvalue type is inferred as int | str, and mypy thinks it's fine. instead, assignment should be valid for each union item separately.

まず、提供された generic-union の例を mypy で再現し、union 型の lvalues への代入に対する型チェックの経路を調べてください。いずれかの union メンバーに対して安全でない場合は代入が拒否され、妥当な代入は引き続き通過すれば完了です。