Can't assign result of list addition to list variable covariantly

Bug Report

I cannot assign a list[int] to a list[int | None] if I make it using addition. However, it works fine if I don't use addition. See the tiny example below.

In this example, of course, I can avoid the problem using the list literal [1, 2], but in the application code where I'm facing this issue I actually need to add two comprehensions and store the result as a variable. It shows the same unexpected behavior.

To Reproduce

a: list[int | None] = [1]         # allowed
b: list[int | None] = [1] + [2]   # not allowed

https://mypy-play.net/?mypy=latest&python=3.12&gist=800cfe12c8689a108440e71e641ad240

Expected Behavior

I expect mypy to behave in one of two ways:

• Both lines are allowed 👈 this would be very convenient
• Both lines are not allowed 👈 this would be correct
  • How can the first line be allowed if list is invariant?
  • In this case I would need a way to type-hint a list comprehension

Note:

I expect both lines to be treated the same way. This expectation is reinforced because reveal_type([1]) reports the same type as reveal_type([1] + [2]). If that was not the case the different treatment would be easier to understand.

Actual Behavior

main.py:2: error: Incompatible types in assignment (expression has type "list[int]", variable has type "list[int | None]")  [assignment]
main.py:2: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
main.py:2: note: Consider using "Sequence" instead, which is covarian

Your Environment

• Mypy version used: 1.8.0 and 1.5.1 (latest and version in use on my project)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12

Inizia con la riproduzione minima in main.py ed eseguila con l’esempio mypy-play collegato usando le versioni di Python e mypy indicate. Traccia il modo in cui vengono inferite le espressioni di letterale di lista e di somma di liste, quindi rendi coerente il loro comportamento di assegnazione preservando l’invarianza delle liste; aggiungi o aggiorna un test di regressione per entrambe le forme.