Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

Typeguard version

4.5.1

Python version

3.14.4

What happened?

typeguard.check_type(value, SomeProtocol) crashed with IndexError: pop from empty list instead of returning or raising TypeCheckError.

File ".../typeguard/_checkers.py", line 769, in check_signature_compatible
    subject_args.pop(0)
IndexError: pop from empty list

The two pop(0) calls in check_signature_compatible :

If inspect.signature(subject_attr) reports zero positional-or-keyword params and no *args, subject_args is empty and the pop blows up. Callable instances, partials, and other non-staticmethod/non-classmethod descriptors all hit this path.

What I expected was a TypeCheckError. The same arity mismatch with one more positional arg on the subject raises a clean TypeCheckError ("too few positional arguments"). Drop the count by one and it becomes IndexError. Equivalent mismatch, two different exception types, only one of which check_type is documented to raise, so callers catching TypeCheckError leak the IndexError through.

The fix I am running locally is to guard each pop:

if protocol_type == "instance" and protocol_args:
    protocol_args.pop(0)
if subject_type == "instance" and subject_args:
    subject_args.pop(0)

With that, the repro raises TypeCheckError and the rest of the test suite still passes for me.

Happy to PR with a regression test under tests/test_checkers.py::TestProtocol. If you would rather the empty case raise an explicit TypeCheckError earlier with a more specific message, let me know and I will rework.

How can we reproduce the bug?

Stdlib only, no third-party deps:

from typing import Protocol
from typeguard import check_type

class _ZeroArg:
    def __call__(self):
        return None

class Subject:
    method = _ZeroArg()

class P(Protocol):
    def method(self, x: int) -> None: ...

check_type(Subject(), P)
# IndexError: pop from empty list

Run on Python 3.14, typeguard 4.5.1.