TypeVar mentioned in type annotation is not found, causing an unexpected warning on Sphinx 3.x

[pquentin]

Describe the bug

TypeVar mentioned in type annotation is not found, causing an unexpected warning on Sphinx 3.x

To Reproduce

Write conf.py as follows:

import os
import sys

sys.path.insert(0, os.path.abspath('.'))
extensions = ['sphinx.ext.autodoc']

Write index.rst as follows:

Welcome to repro's documentation!
=================================

.. autoclass:: repro.SendChannel
   :members:

Write repro.py as follows:

from typing import TypeVar

SendType = TypeVar("SendType")


class SendChannel():
    async def send(self, value: SendType):
        """send"""
        pass

Run the following command:

sphinx-build -nW -b html . _build/html

Expected behavior

Like with Sphinx 2.x, the SendType type hint is ignored.

Actual behavior

The build fails, since we treat warnings as errors:

docstring of repro.SendChannel.send::py:class reference target not found: SendType

Environment info

• OS: macOS 10.15 and Linux
• Python version: 3.8.2
• Sphinx version: 3.1.0+ (302c23f)
• Sphinx extensions: sphinx.ext.autodoc

Additional context
Add any other context about the problem here.

• Bump sphinx from 2.4.4 to 3.0.3 python-trio/trio#1465

[pquentin]

I just ran git bisect, and realized that this issue is a duplicate of #7493. Closing

[tk0miya]

I think this is similar to #7493, but not the same problem.
The warning was caused from that the type annotation SendType is not documented. So you need to make a document for it by .. autoclass:: repro.SendType. But autoclass for TypeVar will generate a long text document unexpectedly. It comes from a docstring of TypeVar class. In addition, autodoc generates a broken type annotation reference for TypeVars. In this case, the absolute pathname of the annotation should be repro.SendType. But it refers SendType instead. So it will be never resolved.

[tk0miya]

I noticed #7930 also fixed this case. It helps to resolve the location of SendType.

I confirmed the fix with adding a comment to SendType and using the HEAD of 3.x branch:

from typing import TypeVar

#: SendType
SendType = TypeVar("SendType")


class SendChannel():
    async def send(self, value: SendType):
        """send"""
        pass