Bug: the following code (Python 3.6.7, mypy 0.701)

o = [[1, 2], ["foo", "bar"]]
reveal_type(o)

Outputs a type of builtins.list[builtins.object*] instead of something like builtins.list[builtins.list[Any]]. This in turn makes mypy raise an error (in my case) when I try to apply a len operation on elements of that list:

[len(x) for x in o]

Outputs: error: Argument 1 to "len" has incompatible type "object"; expected "Sized"

This is probably related to #4975 but only the union of Tuple was evoked there, and it seems that there is the same problem on lists.

Thanks!