Nested dictionaries/lists not supported by `sphinx-build --define`

Describe the bug

Trying to customize the values of intersphinx_mapping with sphinx-build --define, I only get errors.

How to Reproduce

Any conf.py will work. Run:

sphinx-build --define intersphinx_mapping.python=http://localhost:8001,None -b html . _build/html

The expected result is that it corresponds to something like:

intersphinx_mapping = {"python": ["http://localhost:8001", "None"]}

But I get this error:

ERROR: Invalid value `'http://localhost:8001,None'` in intersphinx_mapping['python']. Expected a two-element tuple or list.

With an already populated intersphinx_mapping, I also tried to override only the first item of the tuple with this value: intersphinx_mapping.python.0=http://localhost:8001 It does not work.

Environment Information

Platform:              linux; (Linux-6.18.35-1.qubes.fc41.x86_64-x86_64-with-glibc2.41)
Python version:        3.13.5 (main, Jun 13 2026, 14:18:01) [GCC 14.2.0])
Python implementation: CPython
Sphinx version:        9.1.0
Docutils version:      0.21.2
Jinja2 version:        3.1.6
Pygments version:      2.20.0

Sphinx extensions

Additional context

No response

Start by reproducing the issue with any conf.py using sphinx-build --define intersphinx_mapping.python=http://localhost:8001,None, then trace how the --define entry is parsed and validated. Done means nested dictionaries and lists, including the intersphinx_mapping example and indexed tuple item, can be configured without the reported validation error.