Thanks for the PR. I'm not feeling comfortable just merging this through. I also wasn't aware that you can use a string as a version. Could you please explain what your workflow is and how you use mike to deploy your docs? We need to make sure that this change does not break other users' docs.

Sure. I have two GitHub Actions workflows named docs_nightly.yaml and docs_stable.yaml, which look like this:

name: Docs nightly
on:
  push:
    branches:
      - main
jobs:
  deploy-docs:

    ...

    - name: Deploy docs
      run: mike deploy --push nightly

and

name: Docs stable
on:
  push:
    tags:
      - 'v[0-9]+.[0-9]+.[0-9]+'
jobs:
  deploy-docs:

    ...

    - name: Deploy docs
      run: |
        mike deploy --push --update-aliases ${{ steps.docs_version.outputs.docs_version }} stable
        mike set-default --push stable

Running these workflows would generate the following version.json:

[
  {
    "version": "nightly",
    "title": "nightly",
    "aliases": [],
  },
  {
    "version": "0.1",
    "title": "0.1",
    "aliases": ["stable"]
  }
]

If extra.version.default only matches against aliases, I would need to set up an additional alias, such as continuous, and use mike deploy --push --update-aliases nightly continuous to publish the nightly docs, which is not very ideal.

If there is any other information you would need, just let me know!

Thanks for the explanation. I don't see why this should break anything for existing users, as we're only adding an additional name to the version strings to check against which must be explicitly listed in extra.version.default. Thus, we'll merge this PR and revisit it if we find out that it breaks something downstream.