I think the solver portion looks good. As for tests, I think it's fine to keep them in this module, but maybe something like:

solveL :: forall l r u. Union l r u => RProxy r -> RProxy u -> RProxy l
solveL _ _ = RProxy

solveUnionBackwardsNil :: RProxy ()
solveUnionBackwardsNil = solveL (RProxy :: _ ()) (RProxy :: _ ())

...etc

Fix the conflict and add an entry to the changelog and we can merge this, yeah?

Sounds good to me 👍

@MonoidMusician Just FYI. I've submitted a PR that fixes the merge conflict and adds a changelog entry in MonoidMusician#1

Thanks!! I'll take a look in a week after I'm done with my semester.

Will this PR will make it into the next release?

I'm not trying to rush this, but just to clarify, the merge conflict is quite simple (spaces added to better compare differences):

<<<<<<< solve-union-backwards
import Data.List (             minimumBy, groupBy, nubBy, sortBy, elem, delete)
=======
import Data.List (findIndices, minimumBy, groupBy, nubBy, sortBy)
>>>>>>> master

If we merge the two lists together and remove the unneeded import elem, then we get this import list:

import Data.List (findIndices, minimumBy, groupBy, nubBy, sortBy, delete)

While the conflict could be resolved here, I wasn't sure whether that would be rude to do so (hence my PR).

Even if the conflict is solved here, a commit adding this change to the changelog would still need to be pushed.

Lastly, I'm not sure how this would be affected by #4090.

Any updates on this?

Thanks for your patience! I'm sorry I got busy, but I feel good about this now. I ran HLint and fixed the one linter issue I introduced. I also ran more test cases in the REPL and it works exactly like I would expect, include failure cases.

Do you want to include this in the next release or merge it after?