list of lists not inferred correctly

[Dr-Irv]

Bug Report

This came up in testing for pandas-stubs.

If you have a list of lists of different types, mypy infers it as a list[object] rather than a list of the union of the lists of the inner types.

To Reproduce

class NAType:
    pass

class NaTType:
    pass

NA: NAType = NAType()
NaT: NaTType =  NaTType()

foo = [[None], [NA], [NaT]]
reveal_type(foo)

Expected Behavior

Revealed type is list[list[None] | list[NAType] | list[NaTType]]

This is what pyright returns with the setting strictListInference=True

Actual Behavior

Revealed type is "builtins.list[builtins.object]

Your Environment

• Mypy version used: 1.19.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11

[A5rocks]

This is just join v union behavior. If you attach a type hint to foo it'll work.

[Dr-Irv]

Well, maybe, but if I do:

goo = [None, NA, NaT]
reveal_type(goo)

then mypy correctly returns builtins.list[tstmypylistofflist.NaTType | tstmypylistofflist.NAType | None]"

And while a type hint would work, the goal is to be able to pass something like list[list[None], list[NA], list[NaT]] to a function/method, so adding the type hint gunks up the code.

[A5rocks]

-snip- somehow misread again

Ok, I'm not sure of the divergence, even if I suspect join-v-union. I'll remove that tag!