#14680: GHC 8.4.1-alpha panics when optimizing function using getTag and tagToEnum# -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.4.1 Component: Compiler | Version: 8.4.1-alpha1 Keywords: | 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: -------------------------------------+------------------------------------- I originally observed this panic in [https://travis-ci.org/haskell-compat /deriving-compat/jobs/329948624#L804 this Travis build]. The panic can be reduced to this file: {{{#!hs {-# LANGUAGE MagicHash #-} {-# LANGUAGE TypeFamilies #-} module Bug where import GHC.Base (getTag) import GHC.Exts (Int(..), tagToEnum#) data family TyFamilyEnum data instance TyFamilyEnum = TyFamilyEnum1 | TyFamilyEnum2 | TyFamilyEnum3 suc :: TyFamilyEnum -> TyFamilyEnum suc a_aaf8 = case getTag a_aaf8 of a_aaf9 -> if 2 == I# a_aaf9 then error "succ{TyFamilyEnum}: tried to take `succ' of last tag in enumeration" else case I# a_aaf9 + 1 of I# i_aafa -> tagToEnum# i_aafa :: TyFamilyEnum }}} In GHC 8.2.2, compiling this with optimization works fine. But in GHC 8.4.1-alpha, it panics: {{{ $ /opt/ghc/8.4.1/bin/ghc -fforce-recomp -O1 Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.4.0.20171222 for x86_64-unknown-linux): Prelude.!!: index too large }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14680 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler