Both @thomashoneyman and I have run into this - it can be very difficult to figure out why instance resolution is failing, so having a warning about it would save many hours of wasted time for the few people that encounter it!

The case we encountered it is creating an "app monad" that also has a parallel version as a newtype, and then forgetting to export the parallel version - the Parallel instance exists, but any attempts to resolve the Parallel constraint for the app monad outside of the defining module fails with a missing instance error.