autodoc_typehints='signature' doesn't handle None

[godlygeek]

Describe the bug
Given a Python file containing:

def foo() -> None: pass

And a conf.py containing:

extensions = ["sphinx.ext.autodoc", "sphinx.ext.intersphinx"]
intersphinx_mapping = {"python": ("https://docs.python.org/3", None)}

The latest Sphinx installed from PyPI generates:

.. py:function:: foo() -> None
   :module: type_hint_test

And then winds up generating a warning, if -n is enabled:

type_hint_test.py:docstring of type_hint_test.foo::
WARNING: py:class reference target not found: None

Which of course fails the build if -W is enabled.

This works fine if the return type is int instead of None - I'm guessing that the issue is the type is resolved using py:class, but that None is special, as per PEP-484:

When used in a type hint, the expression None is considered equivalent to type(None).

To Reproduce

mkdir -p sphinx_type_hint_none
cd sphinx_type_hint_none

printf "%s\n" > type_hint_test.py \
    "def foo() -> None: pass"     \

mkdir -p docs

printf "%s\n" > docs/conf.py                                                \
    'extensions = ["sphinx.ext.autodoc", "sphinx.ext.intersphinx"]'         \
    'intersphinx_mapping = {"python": ("https://docs.python.org/3", None)}' \

printf "%s\n" > docs/index.rst       \
    '.. automodule:: type_hint_test' \
    '.. autofunction:: foo'          \

mkdir -p html
python3.8 -m sphinx -vvv -nW -b html --keep-going docs html

Expected behavior
Documentation generation completes without an error, and includes a link to https://docs.python.org/3/library/constants.html#None

Environment info

• OS: Linux 4.4.0
• Python version: 3.8.1
• Sphinx version: 3.0.0
• Sphinx extensions: sphinx.ext.autodoc, sphinx.ext.intersphinx

Additional context
Appears to have been introduced in 3.0.0; our CI had no issue with the latest version on PyPI as of Friday.

[tk0miya]

+1; Reasonable.
I worked to suppress warnings for built-in types in 3.0 (see #6895). But I forgot to handle None.

[godlygeek]

Note that if I use autodoc_typehints='description', the generated HTML contains:

<dt class="field-even">Return type</dt>
<dd class="field-even"><p><a class="reference external" href="https://docs.python.org/3/library/constants.html#None" title="(in Python v3.8)">None</a></p>
</dd>

That is, it generates a clickable link to the None singleton from the Return type section. It seems inconsistent for autodoc_typehints='description' to generate a link to the None singleton's documentation but for autodoc_typehints='signature' to not generate a link.

[tk0miya]

Fixed by #7432
Thank you for reporting.

@godlygeek I think it is another topic. So please file another issue. Thanks,