Superclass Cycle via Associated Type

The following code doesn't compile, but it seems sensible enough to me. Is this a limitation of GHC or is there something I'm missing? class C (A x) => C x where type A x :: * instance C Int where type A Int = String instance C String where type A String = Int The error I get is: SuperclassCycle.hs:1:1: Cycle in class declarations (via superclasses): SuperclassCycle.hs:(1,1)-(2,15): class C (A x) => C x where { type family A x :: *; } Ryan