mypy throws assignment error for a list created from an empty list

[Felixoid]

Bug Report

mypy fails to proceed sorting of the empty list, considering it's a list[object], see the reproduce script and the output

To Reproduce

a: list[str] | list[int] = []
a = sorted(a)

Expected Behavior

There shouldn't be errors

Actual Behavior

> mypy .
/tmp/test.py:2: error: Incompatible types in assignment (expression has type "list[SupportsDunderLT[Any] | SupportsDunderGT[Any]]", variable has type "list[str] | list[int]")  [assignment]

Your Environment

• Mypy version used: mypy 1.14.0 (compiled: no)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.13.2

In the scope of ClickHouse/ClickHouse#76679

[A5rocks]

This ultimately is equivalent to:

a: list[str] | list[int] = []
b = list(a)
reveal_type(b)  # N: Revealed type is "builtins.list[builtins.object]"
# ^ you want `b` to be `list[str] | list[int]`, but mypy doesn't do that

I think this not something mypy can fix. I think the issue is that list is taking a Sequence[T] (guessing), so mypy says T = object -- at best it would be T = str | int, so join-v-union doesn't help -- and then returns list[T] so list[object]. AFAIK there's no way for mypy to realize it should process the union elements one at a time. You could use type var values if you care quite a bit.

... I wonder what mypy could do. Maybe someone could prototype checking union items seperately but that sounds inefficient.

(if you want to reopen this just let me know and I'll switch this to being a feature request)

[Felixoid]

So, the minimal code with issue is the following:

a: list[str] | list[int] = []
a = sorted(a)

Yeah, I'd like to convert this issue to a feature request. I feel it would be beneficial to have this addressed.