Protocol incorrectly matches when positional or keyword parameter matches *args

[danielknell]

Bug Report

When a protocol defines a function with a keyword or positional parameter (a: int) -> None, a concrete function where said parameter is missing, but matches the *args parameter (*args: int) -> None will be seen as implementing the protocol.

I would expect (a: int) -> None to also need matching **kwargs such as (*args: int, **kwargs: int) -> None and for it to only match *args alone when specified as a positional only argument (e.g. (a: int, /) -> None)

Otherwise it cannot be called safely, pyright and ty both successfully raise errors for this case.

To Reproduce

https://mypy-play.net/?mypy=master&python=3.14&flags=strict&gist=a5f86e32e70cbc84c45a53db5426f78f

Expected Behavior

main.py:13: error: Argument 1 to "f" has incompatible type "C"; expected "P"  [arg-type]

because calling the method as foo(a=1) would fail.

Actual Behavior

mypy:

Success: no issues found in 1 source file

pyright:

Argument of type "C" cannot be assigned to parameter "o" of type "P" in function "f"
  "C" is incompatible with protocol "P"
    "foo" is an incompatible type
      Type "(*args: int) -> None" is not assignable to type "(a: int) -> None"
        Missing keyword parameter "a"  (reportArgumentType)

ty:

Argument to function `f` is incorrect: Expected `P`, found `C` (invalid-argument-type) [Ln 17, Col 3]

python:

Traceback (most recent call last):
  File "xxx.py", line 17, in <module>
    f(C())
    ~^^^^^
  File "xxx.py", line 14, in f
    o.foo(a=1)
    ~~~~~^^^^^
TypeError: C.foo() got an unexpected keyword argument 'a'

Your Environment

• Mypy version used: 1.19.1 and master
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 1.14

[ilevkivskyi]

I am quite sure this is a duplicate, but can't find original after a quick search. This is a very old intentional exception that we should get rid of at some point, at least with --strict. This is not specific to protocols, nor to star arguments, mypy currently allows something like this:

class B:
    def foo(self, x: int) -> None: ...
class C(B):
    def foo(self, arg: int) -> None: ...

even though it is not safe. This is because of tons of code written in days predating mypy.

[hauntsaninja]

For LSP, I had opened #18355. Based on primer it will need to be off by default.

The code paths for LSP and this are different though, so that PR won't fix this.

[ilevkivskyi]

Yeah. Btw for protocols it is also just removing ignore_pos_arg_names=True from is_protocol_implementation(). Btw I guess fallout may be smaller for protocols.