Thanks for the PR, and sorry this went so long unacknowledged! This looks good, but I think we can improve the error a little more still. We should be able to always include information about the names of the relevant classes, I think. Presumably if we get an SCC constructor with exactly one element in the list, that means we have a class which has itself as a superclass? In that case it might be best to print something like

A type class may not have itself as a superclass.

In the case where we have more than one class in the SCC, I think it would be better to print something like

A cycle appears in a set of type class definitions:
  {A, B, C}

where A, B, and C are the names of the classes appearing in the SCC constructor. I think ideally we'd be able to print an actual cycle like A -> B -> C -> A, but looking at the Data.Graph docs I think this might be a little more awkward to achieve so I think just printing the set of classes in a strongly-connected component should be fine.

If you're unlikely to pick this up again (which is totally understandable at this point) I'd appreciate it if you could let us know so that someone else can.

@hdgarrood Thanks for your response. I have made changes as you suggested.