[Haskell-beginners] Is this a GHC bug? Problem inferring type

This is confusing me, and I'm wondering if it's a compiler bug. I have this: foo = (++) `on` show I'm expecting the type to be: foo :: Show a => a -> a -> String But GHCI is inferring: foo :: () -> () -> String If I explicitly provide a type annotation, or if I use -XNoMonormorphismRestriction, then it all works fine. I would expect a compile error if the type couldn't be inferred due to the Monomorphism Restriction, but instead it has silently chosen the least useful type. Peter