| otherwise = rm_dups (b:acc) bs
- rn_inst_info (InstInfo { iSpec = inst, iBinds = NewTypeDerived co })
- = return (InstInfo { iSpec = inst, iBinds = NewTypeDerived co }, emptyFVs)
+ rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
+ rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
+ = return ( info { iBinds = NewTypeDerived coi tc }
+ , mkFVs (map dataConName (tyConDataCons tc)))
+ -- See Note [Newtype deriving and unused constructors]
rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
= -- Bring the right type variables into
-- The predicate tyConHasGenerics finds both of these
\end{code}
+Note [Newtype deriving and unused constructors]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this (see Trac #1954):
+
+ module Bug(P) where
+ newtype P a = MkP (IO a) deriving Monad
+
+If you compile with -fwarn-unused-binds you do not expect the warning
+"Defined but not used: data consructor MkP". Yet the newtype deriving
+code does not explicitly mention MkP, but it should behave as if you
+had written
+ instance Monad P where
+ return x = MkP (return x)
+ ...etc...
+
+So we want to signal a user of the data constructor 'MkP'. That's
+what we do in rn_inst_info, and it's the only reason we have the TyCon
+stored in NewTypeDerived.
+
%************************************************************************
%* *
genInst standalone_deriv oflag spec
| ds_newtype spec
= return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
- , iBinds = NewTypeDerived co }, [])
+ , iBinds = NewTypeDerived co rep_tycon }, [])
| otherwise
= do { let loc = getSrcSpan (ds_name spec)
-- specialised instances
Bool -- True <=> This code came from a standalone deriving clause
- | NewTypeDerived -- Used for deriving instances of newtypes, where the
- CoercionI -- witness dictionary is identical to the argument
- -- dictionary. Hence no bindings, no pragmas.
- -- The coercion maps from newtype to the representation type
- -- (mentioning type variables bound by the forall'd iSpec variables)
- -- E.g. newtype instance N [a] = N1 (Tree a)
- -- co : N [a] ~ Tree a
+ | NewTypeDerived -- Used for deriving instances of newtypes, where the
+ -- witness dictionary is identical to the argument
+ -- dictionary. Hence no bindings, no pragmas.
+
+ CoercionI -- The coercion maps from newtype to the representation type
+ -- (mentioning type variables bound by the forall'd iSpec variables)
+ -- E.g. newtype instance N [a] = N1 (Tree a)
+ -- co : N [a] ~ Tree a
+
+ TyCon -- The TyCon is the newtype N. If it's indexed, then it's the
+ -- representation TyCon, so that tyConDataCons returns [N1],
+ -- the "data constructor".
+ -- See Note [Newtype deriving and unused constructors]
+ -- in TcDeriv
pprInstInfo :: InstInfo a -> SDoc
pprInstInfo info = vcat [ptext (sLit "InstInfo:") <+> ppr (idType (iDFunId info))]
pprInstInfoDetails info = pprInstInfo info $$ nest 2 (details (iBinds info))
where
details (VanillaInst b _ _) = pprLHsBinds b
- details (NewTypeDerived _) = text "Derived from the representation type"
+ details (NewTypeDerived {}) = text "Derived from the representation type"
simpleInstInfoClsTy :: InstInfo a -> (Class, Type)
simpleInstInfoClsTy info = case instanceHead (iSpec info) of
-- If there are no superclasses, matters are simpler, because we don't need the case
-- see Note [Newtype deriving superclasses] in TcDeriv.lhs
-tc_inst_decl2 dfun_id (NewTypeDerived coi)
+tc_inst_decl2 dfun_id (NewTypeDerived coi _)
= do { let rigid_info = InstSkol
origin = SigOrigin rigid_info
inst_ty = idType dfun_id