When using Dotty, the sourcePath of the reported positions is absolute path, and so the BuildServerReporter was not working.

Which file paths are we passing to Dotty?

In the BuildServerReporter I now use the pos.sourceFile rather than the pos.sourcePath to store the diagnostics.

We should avoid the use of java.io.File as much as possible. Not just for virtual file, but this also relate to other abstract paths like paths in JAR files and Java modules.

Which file paths are we passing to Dotty?

We are passing VirtualFile.

But the Dotty sbt-bridge is sending absolute paths back to us. It get them from the pos.sourceFile:
https://github.com/lampepfl/dotty/blob/b2dc1016976558c6fbc67970139266486f2ad9d7/sbt-bridge/src/xsbt/DelegatingReporter.java#L77-L91

We should avoid the use of java.io.File as much as possible.

Ok. I changed the implementation so that it relies on pos.sourcePath and not pos.sourceFile. It uses the FileConverter to handle both cases: virtual file ids and absolute paths.

But the Dotty sbt-bridge is sending absolute paths back to us. It get them from the pos.sourceFile:
https://github.com/lampepfl/dotty/blob/b2dc1016976558c6fbc67970139266486f2ad9d7/sbt-bridge/src/xsbt/DelegatingReporter.java#L77-L91

Since I made changes to the compiler bridge in sbt/zinc but did not send PRs to Dotty it makes sense that there are some differences in the implementation. I'd suggest we send PR to make that side consistent then.

I'd suggest we send PR to make that side consistent then.

Agreed.

I still think this current PR makes much sense since it enables compatibility on previous versions of sbt-dotty. IMO it is a pity that I cannot use BSP in sbt 1.4.1 on a Dotty project. It used to work pretty well on 1.4.0.

Also I think this implementation will be a lot more robust on future changes since it relies on the file converter.