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

[MatejKastak]

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