How to create self-referential toctree for single file Sphinx project in 7.1+?

Question

My project is simple so I have a single index.rst file with everything defined in it. Originally, at the top of index.rst, I had a toctree like this:

.. toctree::
   :hidden:
   :maxdepth: 5

   index

For HTML output, in my theme, this resulted in no table of contents at the top of the page but instead it was included in the sidebar under navigation.

Some change in the 7.1 or 7.2 series of Sphinx has broken this. I now get

index.rst:1: WARNING: toctree contains reference to nonexisting document 'index'

when trying to build the documentation. The resulting file has an empty "Navigation" section in the sidebar.

Is there a 7.1/7.2-approved way to get the self-referential TOC in the sidebar back?