Type class instances not picked up on reload

[georgefst]

When I make an edit to a cabal file, and HLS thus reloads the session, I often see spurious type class instance errors.

See here for a minimal example, where an instance C Bool in Lib.hs is used in Main.hs. If for example I remove -Wall from the cabal file (just saving it without editing is not enough), and only have Main.hs open in VSCode, I see No instance for (C Bool) once HLS is done reloading. It can take some serious kicking to get HLS to pick up the instance. Sometimes it's enough to make some non-trivial edit to Lib.hs, but other times I've had to delete the contents of Lib.hs, edit Main.hs, then restore Lib.hs to get the errors to go away. This has become quite painful in a larger project.

Note that this doesn't happen on initial load. I can restart the LSP server, only load Main.hs and get no error. Even with a completely fresh load, i.e. no entry in ~/.cache/hie-bios.

I haven't bisected, but I'm pretty sure this is a recent regression (2.1 or 2.2).

[fendor]

Thanks for the bug report!

While this sounds like an issue with multiple home units, whether this is a known bug or not depends on the GHC version.
What GHC versions are you using to produce this bug?

[georgefst]

Ah, my bad. 9.4 and 9.6.

[fendor]

Then this issue is most likely related by multiple home unit support #3738.

Your bug report might also be related to our reload logic when file dependencies (e.g. cabal file changes).

[michaelpj]

I think this should be fixed by the proper multi-home-unit support.