Infer type of empty collection from sibling

[lincolnq]

The following example makes it clear:

my_dict = {
    'a': [1,2],
    'b': []
}
reveal_type(my_dict)  # error: Revealed type is 'builtins.dict[builtins.str*, builtins.object*]'

I expected my_dict to have type Dict[str, List[int]] rather than Dict[str, object].

Possibly related to #1055.

[ilevkivskyi]

This may be not as easy as it looks, mypy's inference is already exponentially complex, see #3796 so we need to be careful not to slow it down even more. Potentially we can special case built-in collections and their "literals".

[JukkaL]

This looks basically the same as #230. Can we close this as a duplicate?

[ilevkivskyi]

Oh, yes, this is a duplicate.