set.union not working correctly

[za3k]

Code:
a = set().union(*[[1],[2],[3]]) # type: Set[int]

Expected:This should work fine
Result:

error: Argument 1 to "union" of "set" has incompatible type *List[List[int]]; expected Iterable[None]
error: Incompatible types in assignment (expression has type Set[None], variable has type Set[int])

I'm fairly new to mypy, so please let me know if this is user error.

[za3k]

Worth noting that this works:

base_set = set()  # type: Set[int]
a = base_set.union(*[[1],[2],[3]])```

[gnprice]

Thanks for the report! I think this is a case where our type inference within a statement/expression is a little too dependent on specific rules and consequently only able to reason through N layers of indirection but not N+1. We've talked about this before (/cc especially @ddfisher), but I'm not sure we have an issue squarely directed at it -- #1055 appears to be closest.

I'd certainly like to fix this kind of situation, preferably in a way that involves reducing the number of special cases, but I don't think we yet know precisely how to do that. For now, the workaround in your follow-up comment is the recommended approach -- fortunately it's usually not particularly cumbersome.

[djrochford]

Note that this:

a = set.union(*[{1},{2},{3}])

works fine. This may be a nice workaround in some situations.

[AlexWaygood]

Can't reproduce on 0.940

https://mypy-play.net/?mypy=0.940&python=3.10&flags=strict&gist=40e9a86b91e4e4727bdf743b544847dd