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

[GHC]

#13474: GHC HEAD regression: Prelude.!!: index too large
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             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:                    |
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Comment (by simonpj):

 Stunning.  Here's an even small test case
 {{{
 module T13474 where

 import qualified Data.Map as M

 class Default a where
   def :: a

 foo :: Default a => b -> a
 foo x = def

 mapdef :: Default v => M.Map k v -> M.Map k v
 mapdef = M.map foo
 }}}
 With a debug compiler I get
 {{{
 matchN
   map/coerce
   [TYPE: a_a1eq, TYPE: b_a1er, TYPE: k_a1ep,
    (\ (v_a1et :: a_a1eq) -> v_a1et)
    `cast` (<a_a1eq>_R -> $r$dCoercible_a1es
            :: ((a_a1eq -> a_a1eq) :: *) ~R# ((a_a1eq -> b_a1er) :: *))]
   [TYPE: v_aJ1, TYPE: v_aJ1, TYPE: k_aJ2,
    (\ _ [Occ=Dead] -> $dDefault_aJ4)
    `cast` (<v_aJ1>_R -> N:Default[0] <v_aJ1>_N
            :: ((v_aJ1 -> Default v_aJ1) :: *) ~R# ((v_aJ1 -> v_aJ1) ::
 *))]
 ghc-stage1.exe: panic! (the 'impossible' happened)
   (GHC version 8.3.20170316 for x86_64-unknown-mingw32):
         ASSERT failed!
   0
   Constraint
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler\utils\Outputable.hs:1191:58 in ghc:Outputable
         callStackDoc, called at compiler\utils\Outputable.hs:1246:22 in
 ghc:Outputable
         assertPprPanic, called at compiler\types\Coercion.hs:898:49 in
 ghc:Coercion
         mkNthCo, called at compiler\types\Coercion.hs:430:5 in
 ghc:Coercion
         mkRuntimeRepCo, called at compiler\types\Coercion.hs:343:36 in
 ghc:Coercion
         splitTyConAppCo_maybe, called at
 compiler\specialise\Rules.hs:806:10 in ghc:Rules
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler\utils\Outputable.hs:1191:58 in ghc:Outputable
         callStackDoc, called at compiler\utils\Outputable.hs:1195:37 in
 ghc:Outputable
         pprPanic, called at compiler\utils\Outputable.hs:1244:5 in
 ghc:Outputable
         assertPprPanic, called at compiler\types\Coercion.hs:898:49 in
 ghc:Coercion
         mkNthCo, called at compiler\types\Coercion.hs:430:5 in
 ghc:Coercion
         mkRuntimeRepCo, called at compiler\types\Coercion.hs:343:36 in
 ghc:Coercion
         splitTyConAppCo_maybe, called at
 compiler\specialise\Rules.hs:806:10 in ghc:Rules
 }}}
 I'm still not sure what's actually wrong, but something is very wrong.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13474#comment:2>
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