[GHC] #12756: ghci gives stg_ap_v_ret error

[GHC]

#12756: ghci gives stg_ap_v_ret error
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        Reporter:  ryanreich      |                Owner:
            Type:  bug            |               Status:  new
        Priority:  normal         |            Milestone:
       Component:  GHCi           |              Version:  8.0.1
      Resolution:                 |             Keywords:
Operating System:  Linux          |         Architecture:  x86_64 (amd64)
 Type of failure:  Runtime crash  |            Test Case:
      Blocked By:                 |             Blocking:
 Related Tickets:                 |  Differential Rev(s):
       Wiki Page:                 |
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Comment (by simonpj):

 Writing out of bounds with `unsafeWrite` is surely a library error, no?
 Can't we just fix it?

 Amaxing work to figure out what it was -- thank you AlexET!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12756#comment:3>
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