autodoc.type_comments throw IndexError

[gpotter2]

Describe the bug
Since 2.4.0, we get a failure on our test suites:

Warning, treated as error:
1116error while formatting arguments for scapy.contrib.http2.HPackHdrTable.parse_txt_hdrs: list index out of range

I mocked sphinx to display the stack trace:

Exception occurred:
2914  File "/home/travis/build/secdev/scapy/.tox/docs/lib/python3.8/site-packages/Sphinx-2.4.2.dev20200213-py3.8.egg/sphinx/ext/autodoc/type_comment.py", line 62, in update_annotations_using_type_comments
2915    annotation = ast_unparse(function.argtypes[i])  # type: ignore

Here's what the function looks like:
https://github.com/secdev/scapy/blob/0dd974e46b3678f02e422dacafc507f32cacde89/scapy/contrib/http2.py#L2636

The issue happens whenever there is a mismatch between the amount of arguments and the amount of type hints comments. This happens when using the (...)

This triggers it:

def pony(a,
         b,  # type: str
         c=None  # type: Optional[str]
         ):
    # type: (...) -> str
    return b + (c or "")

This is an issue when using functions within class. You don't put a type hint for self (https://mypy.readthedocs.io/en/stable/cheat_sheet.html#classes)

Full travis logs

https://travis-ci.com/secdev/scapy/jobs/286619573
Backup:
https://pastebin.com/raw/vxhT1cGq

To Reproduce

Steps to reproduce the behavior:

$ git clone https://github.com/gpotter2/issues-sphinx.git
$ cd issues-sphinx
$ make html

Your project
https://github.com/secdev/scapy

Environment info

• OS: linux
• Python version: 3.8.1
• Sphinx version: 2.4.1
• Sphinx extensions: sphinx.ext.autodoc

Thanks for the time.

[gpotter2]

Another note: type_comments is enabled by default on 2.4.0+ even if typehints is disabled. This has no effect on the final output but creates bugs such as this one on existing code