Incompatible type when expanded-dict-as-kwargs used in named default argument position

[briancurtin]

(Please feel free to re-title this as necessary if I've misunderstood the short explanation)

I believe I've found a bug that lies somewhere between a named default argument and keyword arguments that follow it, but specifically in the case where the kwargs come from a ** expanded dict.

Here is a minimal example of the function causing this:

from typing import List

def fn(x: str, y: bool=True, **kwargs: List[str]) -> None:
    print(x, y, kwargs)

Here is an example of some calls to this function:

fn("hello", y=False)  # ok
fn("hello", kw1=["a", "b"], kw2=["c", "d"])  # ok
...
kwargs = {"kw1": ["a", "b"], "kw2": ["c", "d"]}
fn("hello", **kwargs)  # functionally same as above, but error

On the third fn call I'll end up receiving the following error when running mypy on that code:

error: Argument 2 to "fn" has incompatible type "**Dict[str, List[str]]"; expected "bool"

My general expectation was that this would work fine. This is seen while running with the following (with no special flags to the mypy run):

$ mypy --version
mypy 0.641
$ python3 --version
Python 3.7.0+

I've also tried with master (mypy 0.650+dev.e09e72775bfeb622e67cab7d5eae0f12ef4adefa) and the behavior is the same.

[ilevkivskyi]

Thanks for reporting! This is a duplicate of #1969 (which is pretty old).