[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 goldfire):

 I have a terrible feeling this one is my fault. (I wrote code using `!!`
 in !TcIface.) But I can't build HEAD (see #8744) to find out.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8743#comment:2>
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