SynTyConRhs(..),
isFunTyCon, isUnLiftedTyCon, isProductTyCon,
- isAlgTyCon, isDataTyCon, isSynTyCon, isNewTyCon, isPrimTyCon,
+ isAlgTyCon, isDataTyCon, isSynTyCon, isNewTyCon, isClosedNewTyCon,
+ isPrimTyCon,
isEnumerationTyCon, isGadtSyntaxTyCon, isOpenTyCon,
assocTyConArgPoss_maybe, isTyConAssoc, setTyConArgPoss,
isTupleTyCon, isUnboxedTupleTyCon, isBoxedTupleTyCon, tupleTyConBoxity,
_ -> False
isNewTyCon other = False
+-- This is an important refinement as typical newtype optimisations do *not*
+-- hold for newtype families. Why? Given a type `T a', if T is a newtype
+-- family, there is no unique right hand side by which `T a' can be replaced
+-- by a cast.
+--
+isClosedNewTyCon :: TyCon -> Bool
+isClosedNewTyCon tycon = isNewTyCon tycon && not (isOpenTyCon tycon)
+
isProductTyCon :: TyCon -> Bool
-- A "product" tycon
-- has *one* constructor,