Literal infers tuple literal as Union of the items though this syntax is forbidden by PEP 586

The section Illegal parameters for Literal at type check time describes that tuple literals inside Literal are forbidden:

Tuples containing valid literal types like Literal[(1, "foo", "bar")]. The user could always express this type as Tuple[Literal[1], Literal["foo"], Literal["bar"]] instead. Also, tuples are likely to be confused with the Literal[1, 2, 3] shortcut.

Nevertheless mypy seems to be taking this shortcut since it doesn't flag the following code:

from typing_extensions import Literal

a : Literal[(1, 2)] = 1 

Python 3.7, mypy 0.740, typing-extensions 3.7.4.

まず、報告された Python スニペットを Literal[(1, 2)] を使って再現し、mypy が受け入れるか確認してください。Literal の型チェック処理と既存のテストを見つけてください。Literal 内のタプルリテラルが PEP 586 に従って拒否されれば完了です。