At the moment the Trees that Grow implementation in GHC master makes use of massive constraint types to provide Data instances for the hsSyn AST. I am trying to remove the need for this, and have hit a problem. The example I have reduced it to is here [1] The essence of the problem is ------------------- data Experiment p = Experiment { e_ext :: (XEOverLit p), e_val :: Int } | XExperiment (XXOverLit p) deriving instance (Data (GhcPass p)) => Data (Experiment (GhcPass p)) type family XEOverLit x type family XXOverLit x -- The following line works -- type instance XEOverLit (GhcPass _) = PlaceHolder -- The following 3 lines do noe type instance XEOverLit (GhcPass 'Parsed ) = PlaceHolder type instance XEOverLit (GhcPass 'Renamed ) = PlaceHolder type instance XEOverLit (GhcPass 'Typechecked) = PlaceHolder type instance XXOverLit (GhcPass _) = PlaceHolder ------------------- Where specifying the type instance with a wild card keeps GHC happy (the XXOverLit case), but specifying for each of the three constructors for pass does not (the XEOverLit case) The exact error message is ------------------------------ Experiment.hs:34:1: error: • Could not deduce (Data (XEOverLit (GhcPass p))) arising from a use of ‘k’ from the context: Data (GhcPass p) bound by the instance declaration at Experiment.hs:34:1-69 • In the first argument of ‘k’, namely ‘(z Experiment `k` a1)’ In the expression: ((z Experiment `k` a1) `k` a2) In an equation for ‘gfoldl’: gfoldl k z Experiment a1 a2 = ((z Experiment `k` a1) `k` a2) When typechecking the code for ‘gfoldl’ in a derived instance for ‘Data (Experiment (GhcPass p))’: To see the code I am typechecking, use -ddump-deriv | 34 | deriving instance (Data (GhcPass p)) => Data (Experiment (GhcPass p)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ------------------------------ Alan [1] http://lpaste.net/360017