WARNING: duplicate object description

[mezhaka]

A Python class with type annotation produces a warning: WARNING: duplicate object description during the sphinx-build if the sphinx.ext.napoleon extension is used. This used to not produce any warnings on version 1.8.5 of sphinx, but produces the warnings in version 3.4.3.

To Reproduce Steps to reproduce the behavior:

#!/usr/bin/env bash
set -e
set -x

mkdir repro && cd repro
mkdir -p docs/source

cat << EOF > generate_warning.py
class Test:
    """
    Attributes:
        attr: a doc
    """

    attr: bool
EOF

cat << EOF > docs/source/conf.py
import os
import sys
sys.path.insert(0, os.path.abspath('../../'))

extensions = [
    'sphinx.ext.napoleon',
]

source_suffix = '.rst'

# The master toctree document.
master_doc = 'index'
EOF

cat << EOF > docs/source/index.rst
.. toctree::

   apidoc/modules
EOF

# I use conda to create environemnts, but if you don't create a virtual env with
# sphinx 3.4.3 instead of the following block
cat << EOF > environment.yml
name: sphinx-duplicate-warning
channels:
  - conda-forge
  - nodefaults

dependencies:
  - sphinx==3.4.3
EOF

conda env create

Then activate the environment and run sphinx build:

cd repro
conda activate sphinx-duplicate-warning

sphinx-apidoc --force --separate -o docs/source/apidoc . && sphinx-build -M html "docs/source" "build"

Expected behavior No warning

Your project repro.zip

Environment info

• OS: MacOS 10.15.7
• Python version: 3.8.6
• Sphinx version: 3.4.3
• Sphinx extensions: sphinx.ext.napoleon
• Extra tools: [e.g. Browser, tex or something else]