sphinx-build -M builds into different directory than sphinx-build -b

[dvorapa]

Describe the bug
sphinx-build -M . ./_build builds into ./_build/html, but sphinx-build -b . ./_build builds into ./_build. This is an inconsistency between make html and sphinx-build -b marked as equivalent in https://www.sphinx-doc.org/en/master/usage/quickstart.html.

To Reproduce
Steps to reproduce the behavior:

$ git clone https://github.com/wikimedia/pywikibot
$ cd pywikibot
$ pip install -r requirements.txt .[security] -r docs/requirements-py3.txt
$ cd docs
$ make html
$ sphinx-build -M . ./_build
$ sphinx-build -b . ./_build
compare all three results

make html and sphinx-build -M builds into _build/html, but sphinx-build -b builds into _build

Expected behavior
The build folder should be the same for all three options.

Your project
https://github.com/wikimedia/pywikibot (just a mirror from https://phabricator.wikimedia.org/diffusion/PWBC/)

Environment info

• OS: Debian Linux
• Python version: 3.5
• Sphinx version: 2.2.2
• Sphinx extensions: autodoc, apidoc, rstcheck, epytext, numpydoc or napoleon, alabaster

[tk0miya]

Thank you for reporting. As you mentioned, make-mode (-M) is different with non-make-mode (default). This is intentionally designed behavior. So I'd not like to change this. Anyway, the change will cause incompatibility and troubles.

[dvorapa]

Perhaps this should be mentioned in docs then?

[tk0miya]

You're right. We have to mention about this behavior.

[introt]

Related: #6603

[AA-Turner]

Closing in favour of #5618.

A