Type is lost after merge with `Union[Any, None]`

Bug Report

from typing import Any

def func(d: dict[int, Any]) -> None:
    x: int | None
    x = d.get(1)
    reveal_type(x)  # typing is lost -> Union[Any, None]

d.get get's correctly inferred as Any | None. However, merging the existing type int | None with Any | None results in Any | None. I.e. mypy looses the original type information. That is unexpected.

Comparing it to a type merge with just Any:

def f(a: Any) -> None:
    x: int | None
    x = a
    reveal_type(x)  # int | None

In this case x retains the type information, as expected.

--
Another example which doesn't rely on dict.get

def g(a: Any | None) -> None:
    x: int | None
    x = a
    reveal_type(x)  # typing is lost -> Union[Any, None]

Expected Behavior

Mypy should retain the original type information even after merging it with Any | None.

Your Environment

• Mypy version used: 0.971

issue にある 3 つの再現関数から始め、mypy と reveal_type を使って実行し、現在の動作を確認します。既存の int | None 型の値と Any | None を組み合わせる場合の、型のマージと代入のロジックを追跡します。完了条件は、示されているすべての例で推論される型が int | None のままであり、これらのケースに対するリグレッションカバレッジがあることです。