[autodoc] Private (double-underscore) inner class not documented

[ipilcher]

Describe the bug

I am attempting to document a private (name begins with 2 underscores) inner class.

"""A module to test Sphinx.

.. autoclass:: _Foo
   :members:

.. autoclass:: sphinxtest._Foo._Foo__Bar
   :members:
"""

import enum

class _Foo(object):
    """A class to test Sphinx.

    :param name: The name of the Foo.
    :type name: str
    """

    def __init__(self, name):
        self.__name = name

    class __Bar(enum.Enum):
        """A private inner class."""
        BAZ = None
        """A ``BAZ`` is a baz."""

Note that the class name in the autoclass directive is the mangled name.

The only mention of the inner class in the generated HTML is:

<dl class="py attribute">
<dt class="sig sig-object py" id="sphinxtest._Foo._Foo__Bar">
<span class="sig-prename descclassname"><span class="pre">sphinxtest._Foo.</span></span><span class="sig-name descname"><span class="pre">_Foo__Bar</span></span><a class="headerlink" href="#sphinxtest._Foo._Foo__Bar" title="Permalink to this definition">¶</a></dt>
<dd><p>alias of <code class="xref py py-class docutils literal notranslate"><span class="pre">__Bar</span></code></p>
</dd></dl>

Neither the docstring of the inner class nor its member are documented.

How to Reproduce

See Python module (sphinxtest.py) above.

index.rst:

.. SphinxTest documentation master file, created by
   sphinx-quickstart on Mon Feb  6 10:28:42 2023.
   You can adapt this file completely to your liking, but it should at least
   contain the root `toctree` directive.

Welcome to SphinxTest's documentation!
======================================

.. toctree::
   :maxdepth: 2
   :caption: Contents:



Indices and tables
==================

* :ref:`genindex`
* :ref:`modindex`
* :ref:`search`

conf.py:

# Configuration file for the Sphinx documentation builder.
#
# This file only contains a selection of the most common options. For a full
# list see the documentation:
# https://www.sphinx-doc.org/en/master/usage/configuration.html

# -- Path setup --------------------------------------------------------------

# If extensions (or modules to document with autodoc) are in another directory,
# add these directories to sys.path here. If the directory is relative to the
# documentation root, use os.path.abspath to make it absolute, like shown here.
#
import os
import sys
sys.path.insert(0, os.path.abspath('../..'))


# -- Project information -----------------------------------------------------

project = 'SphinxTest'
copyright = '2023, Ian Pilcher'
author = 'Ian Pilcher'


# -- General configuration ---------------------------------------------------

# Add any Sphinx extension module names here, as strings. They can be
# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
# ones.
extensions = [ 'sphinx.ext.autodoc' ]

# Add any paths that contain templates here, relative to this directory.
templates_path = [ '_templates' ]

# List of patterns, relative to source directory, that match files and
# directories to ignore when looking for source files.
# This pattern also affects html_static_path and html_extra_path.
exclude_patterns = []


# -- Options for HTML output -------------------------------------------------

# The theme to use for HTML and HTML Help pages.  See the documentation for
# a list of builtin themes.
#
html_theme = 'alabaster'
#import sphinx_py3doc_enhanced_theme
#html_theme = 'sphinx_py3doc_enhanced_theme'
#html_theme_path = [ sphinx_py3doc_enhanced_theme.get_html_theme_path() ]

# Add any paths that contain custom static files (such as style sheets) here,
# relative to this directory. They are copied after the builtin static files,
# so a file named "default.css" will overwrite the builtin "default.css".
html_static_path = [ '_static' ]

Environment Information

Platform:              linux; (Linux-6.1.9-200.fc37.x86_64-x86_64-with-glibc2.36)
Python version:        3.11.1 (main, Jan  6 2023, 00:00:00) [GCC 12.2.1 20221121 (Red Hat 12.2.1-4)])
Python implementation: CPython
Sphinx version:        6.1.3
Docutils version:      0.19
Jinja2 version:        3.1.2
Pygments version:      2.14.0

Sphinx extensions

extensions = [ 'sphinx.ext.autodoc' ]

Additional context

No response