Misleading error when using a dict of keyword args

[canepan]

Bug Report

When passing at least part of the function parameter as an ** expanded dictionary, if one or more of the parameters is missing, a misleading "incomaptible type" is shown, instead of "missing parameter".
See also stackoverflow for a way to get a better (but still not correct, since it lists all the parameters as missing) message

To Reproduce

def my_func(a: str = 'Hello', b: bool = False, c: int = 1):
    return a, b, c


input_kwargs = {'b': True}
result = my_func(**input_kwargs)

or see the gist

Actual Behavior

main.py:14: error: Argument 1 to "fib" has incompatible type "**Dict[str, int]"; expected "str"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.812 (or 0.981 on mypy-play)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):

check_untyped_defs = True
show_error_codes = True
pretty = True
ignore_missing_imports = True

• Python version used: 3.8 (or 3.10 on mypy-play)

[ikonst]

This report is a bit misleading since it might appear that in the example, the problem is that mypy understands that input_kwargs provides parameter b but not a and c, which in practice input_kwargs is seen as an opaque dict[str, bool].

A simpler case is:

def f(*, foo: int, bar: bool) -> None:
    raise Exception
    

kw: dict[str, str] = {}
f(**kw)

Actual:

main.py:6: error: Argument 1 to "f" has incompatible type "**Dict[str, str]"; expected "int"  [arg-type]
main.py:6: error: Argument 1 to "f" has incompatible type "**Dict[str, str]"; expected "bool"  [arg-type]

Now, as to what's "expected", maybe something like this?

main.py:6: error: Argument 1 to "f" has incompatible type "**Dict[str, str]"; expected named arguments "(foo: int, bar: bool)"

or maybe, if we want to nudge users towards TypedDict:

... or a TypedDict:

  class ...(TypedDict):
     foo: int
     bar: bool

Playground URL: https://mypy-play.net/?gist=3d6875bd1447fe1b1259c3455155bea5