This is arguably not our responsibility to document, but due to various pip behaviors (at least on some platforms), it seems like specifically installs of a local copy of the project in editable development mode does not function correctly. Installing using a non-file based install such as from git+https://github.com/amaranth-lang/amaranth#egg=amaranth[builtin-yosys] does seem to work now that a workaround has been implemented.

As far as I know this also affects venv setups which poses an annoying problem. It seems however that it's not too bad to work around by just manually setting PYTHONPATH to include the relevant repositories in it.

The only question here is given that this isn't specific to this project and seems to primarily affect people that are working on the source itself, is this something we really need to document? Pros: seem to mostly be saving time, cons: this is probably not the best place to learn things about how Python works.