variance okay for literals, but not expressions ([1, 2] okay for list[int | None] but [1] + [2] is error)

a: list[int | None] = [1] + [2] + [None]
b: list[int | None] = [1] + [2]
c: list[int | None] = [1, 2]

a is okay, b is an error, c is okay

Apologies if this is a repeat / common issue. I couldn't find anything in the docs. This is popping up for me a lot, my interpretation is that mypy allows type invariance on literals that it will not for expressions.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=b999a1bd2e975645b868c0b376c910e8

Expected Behavior

Either b and c should both be errors or (preferably) neither.

Actual Behavior

b is an error but c isn't

Your Environment

• Mypy version used: 1.7.1 and master
• Python version used: 3.11

まず、リンク先の mypy-play の再現コードを mypy 1.7.1 または master と Python 3.11 で実行し、代入 a、b、c の診断を比較します。Issue に記載された期待される動作に従い、[1] + [2] の動作が [1, 2] の動作と一致すれば完了です。