Type confusion with protocol attributes when explicitly subclassing protocol

Bug Report

When type hinting an attribute on a protocol, and then explicitly subclassing the protocol in a class and performing assignment on the attribute, mypy appears to lose the type information on the attribute. In one case, mypy determines the type is whatever can be inferred from the assignment, and in another case, mypy cannot determine the type.

I've looked at Python's documentation for explicitly subclassing a Protocol (docs) as well as the documentation in mypy for Protocols (docs) and I believe this should work.

To Reproduce

Case 1: Incompatible types in assignment

from typing import Protocol


class InventoryItemProtocol(Protocol):
    val: int | None


class InventoryItemMixin(InventoryItemProtocol):
    def set(self) -> None:
        self.val = 1


class InventoryItem(InventoryItemMixin):
    def __init__(self) -> None:
        self.val: int | None = None

Case 2: Cannot determine type

from typing import Protocol


class ValProtocol(Protocol):
    val: int | None


class Runner(ValProtocol):
    def run(self) -> None:
        if not self.val:
            raise ValueError

        self.val = 1

Expected Behavior

Both programs pass type checks.

Actual Behavior

Case 1

sandbox_protocol1.py:15: error: Incompatible types in assignment (expression has type "int | None", base class "InventoryItemMixin" defined the type as "int")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Case 2

sandbox_protocol2.py:10: error: Cannot determine type of "val"  [has-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

mypy --version
mypy 1.17.1 (compiled: yes)

python --version
Python 3.12.2

まず、issue にある 2 つの Protocol の例を mypy 1.17.1 で実行し、エラーを期待される動作と比較します。明示的にサブクラス化された Protocol 属性がどのように推論され、検証されるかを追跡します。完了条件は、宣言された属性の型を失わずに、両方の例が型チェックを通過することです。