__init_subclass__ with classmethod raises IndexError in stubtest

[rayzchen]

Bug Report

Stubtest raises IndexError: list index out of range when a __init_subclass__ method with the classmethod decorator is used.

To Reproduce

https://github.com/rayzchen/stubtest-bug

Run python -m mypy.stubtest mod

Actual Behavior

init subclass
Traceback (most recent call last):
  File "D:\RayChen\Programs\Python\Python310\lib\runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "D:\RayChen\Programs\Python\Python310\lib\runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 1879, in <module>
    sys.exit(main())
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 1875, in main
    return test_stubs(parse_options(sys.argv[1:]))
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 1748, in test_stubs
    for error in test_module(module):
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 218, in test_module
    yield from verify(stub, runtime, [module_name])
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 396, in verify_mypyfile
    yield from verify(stub_entry, runtime_entry, object_path + [entry])
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 534, in verify_typeinfo
    yield from verify(stub_to_verify, runtime_attr, object_path + [entry])
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 1254, in verify_decorator
    yield from verify(func, runtime, object_path)
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 974, in verify_funcitem
    stub_sig = Signature.from_funcitem(stub)
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 730, in from_funcitem
    stub_args = maybe_strip_cls(stub.name, stub.arguments)
  File "D:\RayChen\Programs\Python\Python310\lib\site-packages\mypy\stubtest.py", line 674, in maybe_strip_cls
    if args[0].variable.name == "cls":
IndexError: list index out of range

Your Environment

• Mypy version used: 1.3.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10.6

[hauntsaninja]

Thanks for the report!