[GHC] #13474: GHC HEAD regression: Prelude.!!: index too large

[GHC]

#13474: GHC HEAD regression: Prelude.!!: index too large
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 `git-annex-6.20170321` from Hackage currently fails to build with GHC
 8.2.1 and HEAD because it triggers this panic. Here's a simplified example
 that exhibits the issue:

 {{{#!hs
 {-# LANGUAGE BangPatterns #-}
 module Command.Vicfg where

 import qualified Data.Map as M

 class Default a where
   def :: a

 instance Default Int where
   def = 0

 data Cfg = Cfg
     { cfgTrustMap :: M.Map Int Int
     , cfgGroupMap :: M.Map Int Int
     }

 defCfg :: Cfg -> Cfg
 defCfg curcfg = Cfg
     { cfgTrustMap = mapdef $ cfgTrustMap curcfg
     , cfgGroupMap = mapdef $ cfgGroupMap curcfg
     }
   where
     mapdef :: Default v => M.Map k v -> M.Map k v
     mapdef = M.map (const def)
 }}}

 {{{
 $ /opt/ghc/8.2.1/bin/ghc Bug.hs -O1
 [1 of 1] Compiling Command.Vicfg    ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170321 for x86_64-unknown-linux):
         Prelude.!!: index too large
 }}}

 Note:

 * You need optimization (`-O1` or `-O2`) enabled.
 * You need to import `Map`/`map` for this panic to trigger, it seems, as
 redefining `Map`/`map` locally makes it work again.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13474>
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