Mypy error on assigning an empty dict to Union variable

[sharmaeklavya2]

This fails:

d = {} # type: Union[Dict[str, int], List[int]]

with error message:

error: Incompatible types in assignment (expression has type Dict[None, None],
variable has type "Union[Dict[str, int], List[int]]")

This also fails with a similar error message:

d = [] # type: Union[Dict[str, int], List[int]]

error: Incompatible types in assignment (expression has type List[None],
variable has type "Union[Dict[str, int], List[int]]")

This works however:

d = {"abc":123} # type: Union[Dict[str, int], List[int]]

I was expecting empty dict to work with Union, since the following 2 statements work:

• d = {} # type: Dict[str, int]
• d = 3 # type: Union[int, str]

[rwbarton]

This is related to inference for unions like #1430 (but simpler because a Dict is not a List). I'll add support for this case to #1408.