[GHC] #8743: The impossible happened : Prelude.(!!): index too large

[GHC]

#8743: The impossible happened : Prelude.(!!): index too large
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        Reporter:  erikd               |            Owner:
            Type:  bug                 |           Status:  new
        Priority:  highest             |        Milestone:  7.8.1
       Component:  Compiler            |          Version:  7.8.1-rc1
      Resolution:                      |         Keywords:
Operating System:  Unknown/Multiple    |     Architecture:  x86_64 (amd64)
 Type of failure:  Compile-time crash  |       Difficulty:  Unknown
       Test Case:                      |       Blocked By:
        Blocking:                      |  Related Tickets:
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Comment (by rwbarton):

 Okay, success.

 {{{
 *** Exception (reporting due to +RTS -xc): (THUNK_1_0), stack trace:
   GHC.List.CAF
   --> evaluated by: WwLib.deepSplitCprType_maybe.con,
   called from WwLib.mkWWcpr,
   called from WwLib.mkWwBodies,
   called from WorkWrap.splitFun,
   called from WorkWrap.splitFun,
   called from WorkWrap.tryWW.doSplit,
   called from WorkWrap.wwBind,
   called from WorkWrap.wwTopBinds,
   called from WorkWrap.wwTopBinds,
   called from UniqSupply.initUs_,
   called from WorkWrap.wwTopBinds,
   called from WorkWrap.wwTopBinds,
   called from SimplCore.doPassDFU,
   called from ...
 }}}

 The offending code is

 {{{#!haskell
 deepSplitCprType_maybe fam_envs con_tag ty
   | let (co, ty1) = topNormaliseType_maybe fam_envs ty
                     `orElse` (mkReflCo Representational ty, ty)
   , Just (tc, tc_args) <- splitTyConApp_maybe ty1
   , isDataTyCon tc
   , let cons = tyConDataCons tc
         con = ASSERT( cons `lengthAtLeast` con_tag ) cons !! (con_tag -
 fIRST_TAG)
   = Just (con, tc_args, dataConInstArgTys con tc_args, co)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8743#comment:4>
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