autodoc isn't able to resolve struct.Struct type annotations

[anselor]

Describe the bug
If struct.Struct is declared in any type annotations, I get class reference target not found: Struct

To Reproduce
Simple index.rst

Hello World
===========

code docs
=========

.. automodule:: helloworld.helloworld

Simple helloworld.py

import struct
import pathlib

def consume_struct(_: struct.Struct) -> None:
    pass

def make_struct() -> struct.Struct:
    mystruct = struct.Struct('HH')
    return mystruct

def make_path() -> pathlib.Path:
    return pathlib.Path()

Command line:

python3 -m sphinx -b html docs/ doc-out -nvWT

Expected behavior
If you comment out the 2 functions that have Struct type annotations, you'll see that pathlib.Path resolves fine and shows up in the resulting documentation. I'd expect that Struct would also resolve correctly.

Your project
n/a

Screenshots
n/a

Environment info

• OS: Ubuntu 18.04, 20.04
• Python version: 3.8.2
• Sphinx version: 3.2.1
• Sphinx extensions: 'sphinx.ext.autodoc',
  'sphinx.ext.autosectionlabel',
  'sphinx.ext.intersphinx',
  'sphinx.ext.doctest',
  'sphinx.ext.todo'
• Extra tools:

Additional context

• [e.g. URL or Ticket]

[tk0miya]

Unfortunately, the struct.Struct class does not have the correct module-info. So it is difficult to support.

Python 3.8.2 (default, Mar  2 2020, 00:44:41)
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.Struct.__module__
'builtins'

Note: In python3.9, it returns the correct module-info. But it answers the internal module name: _struct.

Python 3.9.1 (default, Dec 18 2020, 00:18:40)
[Clang 11.0.3 (clang-1103.0.32.59)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.Struct.__module__
'_struct'

So it would better to use autodoc_type_aliases to correct it forcedly.

# helloworld.py
from __future__ import annotations  # important!
from struct import Struct

def consume_struct(_: Struct) -> None:
    pass

# conf.py
autodoc_type_aliases = {
    'Struct': 'struct.Struct',
}

Then, it working fine.

[tk0miya]

I think it is not good to do a workaround because of a bug of pythons. I think it's better to add a workaround to Sphinx side. Then, nobody will do it on each project. #8627 will resolve this automatically.