-conDetailsFVs (PrefixCon btys) = plusFVs (map bangTyFVs btys)
-conDetailsFVs (InfixCon bty1 bty2) = bangTyFVs bty1 `plusFV` bangTyFVs bty2
-conDetailsFVs (RecCon flds) = plusFVs [bangTyFVs bty | (HsRecField _ bty _) <- flds]
+conDetailsFVs :: HsConDeclDetails Name -> FreeVars
+conDetailsFVs details = plusFVs (map bangTyFVs (hsConDeclArgTys details))