+ (n_silent, dfun_ty) = mkDictFunTy tvs theta clas tys
+
+mkDictFunTy :: [TyVar] -> ThetaType -> Class -> [Type] -> (Int, Type)
+mkDictFunTy tvs theta clas tys
+ = (length silent_theta, dfun_ty)
+ where
+ dfun_ty = mkSigmaTy tvs (silent_theta ++ theta) (mkDictTy clas tys)
+ silent_theta = filterOut discard $
+ substTheta (zipTopTvSubst (classTyVars clas) tys)
+ (classSCTheta clas)
+ -- See Note [Silent Superclass Arguments]
+ discard pred = isEmptyVarSet (tyVarsOfPred pred)
+ || any (`eqPred` pred) theta
+ -- See the DFun Superclass Invariant in TcInstDcls