panic: attempt to subtract with overflow in validate_type_guard_positional_argument_count

[correctmost]

Describe the Bug

Pyrefly debug builds crash when checking this code:

from typing import TypeIs

class C:
    def f() -> TypeIs[bool]: 
        pass

ERROR Thread panicked, shutting down: panicked at pyrefly/lib/alt/function.rs:1190:35:
attempt to subtract with overflow

Version info

20cce72d22

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

[DanielNoord]

https://github.com/facebook/pyrefly/blob/d555b28cab00389e73a9e18b4d65c35f2a36570a/pyrefly/lib/alt/function.rs#L1190-L1198

This incorrectly assumes that if a method is not a staticmethod it will have either cls or self as its first argument. But of course, users can write code that is incorrect. That should be handled here.