Errors when calling a function that has **kwargs and default value parameter (non-trivial type)

Hello, when I declare a function that has **kwargs along with another parameter with a default value, mypy reports errors when trying to call this function without the default argument.

Repro

def foo(option: bool = None, **kwargs):
    pass

def bar(option: str = '', **kwargs):
    pass


d = {'test': True}

foo(**{'test': True})
foo(**d)
foo(other_option=True, option=True, **d)
foo(option=True, **{'test': True})

bar(**{'test': True})  # Error
bar(**d)  # Error
bar(other_option=True, **d)  # Error
bar(option='Hello, world!', **{'test': True})

Mypy output:

test.py:15: error: Argument 1 to "bar" has incompatible type "**Dict[str, bool]"; expected "str"
test.py:16: error: Argument 1 to "bar" has incompatible type "**Dict[str, bool]"; expected "str"
test.py:17: error: Argument 2 to "bar" has incompatible type "**Dict[str, bool]"; expected "str"
Found 3 errors in 1 file (checked 1 source file)

Other failing inputs

From my observations: default parameters with {bool, int, float} types work fine (like the foo function). But if the default parameter has type of any of the following types:

• Custom classes
• Enums
• bytes
• str

mypy will report errors.

Expected result

foo and bar should behave the same way (e.g. no errors).

Versions

mypy 0.770 and Python 3.8.2

Start by running the reported reproducer with the current mypy test setup and trace how calls using **kwargs are checked when another parameter has a non-trivial default type. Add regression coverage for the shown bar calls and confirm they produce no errors while incompatible explicit arguments remain diagnosed.