#15318: Core Lint error involving newtype family instances with wrappers -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 (Type checker) | Keywords: TypeFamilies | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following program gives a Core Lint error on GHC 8.4 and later: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} module Bug where data family Sn a newtype instance Sn (Either a b) where SnC :: forall b a. Char -> Sn (Either a b) }}} {{{ $ /opt/ghc/8.4.3/bin/ghc -dcore-lint Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) *** Core Lint errors : in result of Tidy Core *** <no location info>: warning: [in body of lambda with binder dt_aZm :: Char] From-type of Cast differs from type of enclosed expression From-type: R:SnEither a_auS b_auR Type of enclosed expr: Sn (Either a_auS b_auR) Actual enclosed expr: dt_aZm `cast` (Sym (N:R:SnEither[0] _N _N) ; Sym (D:R:SnEither0[0] _N _N) :: (Char :: *) ~R# (Sn (Either a_auS b_auR) :: *)) Coercion used in cast: Sym (D:R:SnEither0[0] _N _N) *** Offending Program *** $WSnC [InlPrag=INLINE[2]] :: forall b a. Char -> Sn (Either a b) [GblId[DataConWrapper], Arity=1, Caf=NoCafRefs, Str=, Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=True) Tmpl= \ (@ b_auR) (@ a_auS) (dt_aZm [Occ=Once] :: Char) -> (dt_aZm `cast` (Sym (N:R:SnEither[0] _N _N) ; Sym (D:R:SnEither0[0] _N _N) :: (Char :: *) ~R# (Sn (Either a_auS b_auR) :: *))) `cast` (Sym (D:R:SnEither0[0] _N _N) :: (R:SnEither a_auS b_auR :: *) ~R# (Sn (Either a_auS b_auR) :: *))}] $WSnC = \ (@ b_auR) (@ a_auS) (dt_aZm [Occ=Once] :: Char) -> (dt_aZm `cast` (Sym (N:R:SnEither[0] _N _N) ; Sym (D:R:SnEither0[0] _N _N) :: (Char :: *) ~R# (Sn (Either a_auS b_auR) :: *))) `cast` (Sym (D:R:SnEither0[0] _N _N) :: (R:SnEither a_auS b_auR :: *) ~R# (Sn (Either a_auS b_auR) :: *)) $trModule1_r10g :: Addr# [GblId, Caf=NoCafRefs] $trModule1_r10g = "main"# $trModule2_r10D :: TrName [GblId, Caf=NoCafRefs] $trModule2_r10D = TrNameS $trModule1_r10g $trModule3_r10E :: Addr# [GblId, Caf=NoCafRefs] $trModule3_r10E = "Bug"# $trModule4_r10F :: TrName [GblId, Caf=NoCafRefs] $trModule4_r10F = TrNameS $trModule3_r10E $trModule :: Module [GblId, Caf=NoCafRefs] $trModule = Module $trModule2_r10D $trModule4_r10F $krep_r10G :: KindRep [GblId] $krep_r10G = KindRepTyConApp $tcChar ([] @ KindRep) $krep1_r10H :: KindRep [GblId, Caf=NoCafRefs] $krep1_r10H = KindRepVar 1# $krep2_r10I :: KindRep [GblId, Caf=NoCafRefs] $krep2_r10I = KindRepVar 0# $krep3_r10J :: [KindRep] [GblId, Caf=NoCafRefs] $krep3_r10J = : @ KindRep $krep2_r10I ([] @ KindRep) $krep4_r10K :: [KindRep] [GblId, Caf=NoCafRefs] $krep4_r10K = : @ KindRep $krep1_r10H $krep3_r10J $krep5_r10L :: KindRep [GblId] $krep5_r10L = KindRepTyConApp $tcEither $krep4_r10K $tcSn1_r10M :: Addr# [GblId, Caf=NoCafRefs] $tcSn1_r10M = "Sn"# $tcSn2_r10N :: TrName [GblId, Caf=NoCafRefs] $tcSn2_r10N = TrNameS $tcSn1_r10M $tcSn :: TyCon [GblId] $tcSn = TyCon 461968091845555535## 16320521938866097056## $trModule $tcSn2_r10N 0# krep$*Arr* $krep6_r10O :: [KindRep] [GblId] $krep6_r10O = : @ KindRep $krep5_r10L ([] @ KindRep) $krep7_r10P :: KindRep [GblId] $krep7_r10P = KindRepTyConApp $tcSn $krep6_r10O $krep8_r10Q :: KindRep [GblId] $krep8_r10Q = KindRepFun $krep_r10G $krep7_r10P $tc'SnC1_r10R :: Addr# [GblId, Caf=NoCafRefs] $tc'SnC1_r10R = "'SnC"# $tc'SnC2_r10S :: TrName [GblId, Caf=NoCafRefs] $tc'SnC2_r10S = TrNameS $tc'SnC1_r10R $tc'SnC :: TyCon [GblId] $tc'SnC = TyCon 3818830880305712792## 17484539998814842835## $trModule $tc'SnC2_r10S 2# $krep8_r10Q *** End of Offense *** <no location info>: error: Compilation had errors }}} If we look at the Core for `$WSnC`, we see the culprit: {{{ $WSnC = \ (@ b_auR) (@ a_auS) (dt_aZm [Occ=Once] :: Char) -> (dt_aZm `cast` (Sym (N:R:SnEither[0] _N _N) ; Sym (D:R:SnEither0[0] _N _N) :: (Char :: *) ~R# (Sn (Either a_auS b_auR) :: *))) `cast` (Sym (D:R:SnEither0[0] _N _N) :: (R:SnEither a_auS b_auR :: *) ~R# (Sn (Either a_auS b_auR) :: *)) }}} The inner cast goes from `Char` to `Sn (Either a b)`, but then the outer cast goes from `R:SnEither a b` to `Sn (Either a b)`, which is not transitive. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15318 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler