[GHC] #8714: ExistentialQuantification plus DeriveDataTypeable leads to core lint error

[GHC]

#8714: ExistentialQuantification plus DeriveDataTypeable leads to core lint error
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        Reporter:  CoreyOConnor      |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.8.1-rc1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by CoreyOConnor):

 Thanks for the investigation!
 Is there a situation where this would cause a runtime crash? I ran across
 this investigating a crash in the yi editor application. According to
 valgrind the GC performs an invalid write: writes into a block not
 allocated AFAICT.

 If that is too much to conjecture then no worries. I'm still breaking the
 crash down into a smaller test case. This just fell out of that process.
 :-)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8714#comment:8>
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