False positive with Dict passed to kwargs

[mthuurne]

When I run mypy on the following code:

from typing import Dict, Mapping

def f(*vargs: int, **kwargs: object) -> None:
	pass

d = {} # type: Dict[str, object]
f(**d)

m = {} # type: Mapping[str, object]
f(**m)

It reports:

7: error: Argument 1 to "f" has incompatible type "**Dict[str, object]"; expected "int"

Line 7 contains f(**d).

I don't understand why mypy expects int as the type of an argument passed using **: as far as I know, there is no way for a value passed using ** to be captured by an argument declared with *. Even if the passed map contains a key named vargs, its value would still end up in kwargs.

Note that arguments passed with ** are not positional, so the error message stating "Argument 1" is either poorly worded or perhaps the result of the same problem that causes the false positive.

What is confusing to me is that this false positive only occurs if I pass a Dict, not a Mapping. However, Python will create a new dictionary instance for kwargs in either case, so only the key and value types of the expression after ** should matter, I think, not the mapping type.

I'm using mypy 0.650 with Python 3.6.5.

[JukkaL]

This is clearly a false positive. Thanks for reporting this!

The bug has apparently been around for a long time.

[ilevkivskyi]

The bug has apparently been around for a long time.

Yes, I think this is a duplicate of #1969