Nested empty container gets inferred to object

Using mypy 0.600, running mypy on

x = [[], [1, 2], [3, 4]]
reveal_type(x)

shows

test.py:2: error: Revealed type is 'builtins.list[builtins.object*]'

If this doesn't have the empty list, then mypy infers 'builtins.list[builtins.list*[builtins.int]]' for

x = [[1, 2], [3, 4]]
reveal_type(x)

Ideally, mypy would be able to infer List[List[int]] even when there's an empty list.

Yes, looks like a bug in join.

Closing as a duplicate of #2255

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