Only allow splatting iterables if function takes at least one positional arg

[piotrfilipiuk]

Bug Report

At runtime the following code snippet:

def f(*, x: int = 9, y:int = 8) -> None:
    pass

kwargs = {"x": 1, "y": 2}
f(*kwargs)

results in TypeError: f() takes 0 positional arguments but 2 were given. However, mypy does not report any issues.

To Reproduce

1. Execute python interpreter for the code snipped above.
2. Run mypy for the code snippet above.

Expected Behavior

Mypy should yield an error. For example pytype yields the following error:

Function f expects 0 arg(s), got 2 [wrong-arg-count]
         Expected: (*, x, y)
  Actually passed: (_, _)

Actual Behavior

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: None. Simply mypy <FILENAME>.
• Mypy configuration options from mypy.ini (and other config files): N/A (defaults)
• Python version used: 3.8.2
• Operating system and version: Mac OS Big Sur

[ktbarrett]

As I stated on the gitter, pytype is doing a lot keeping the number of elements in the literal dict around, I think the suggestion is overkill for such a corner case (though it might be a good long term goal) and only works for this minimal example and not in the common case. An appropriate solution might be to check when splatting that the function being called has no positional (or keyword in the other case) arguments and error, since it is likely not what the user intended.

[hauntsaninja]

Changing the title to Kaleb's suggested action item.