list of lists not inferred correctly

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

Start by running the issue's Python reproduction with mypy 1.19.0 and inspect the revealed type for foo. Trace the list-inference path responsible for combining nested list types, then verify that the result is list[list[None] | list[NAType] | list[NaTType]] without changing the existing command-line or configuration assumptions.