[GHC] #12040: Code that builds on 7.8.4 and 7.10.3 but fails with requiring UndecidableInstances on 8.0.1-rc4

[GHC]

#12040: Code that builds on 7.8.4 and 7.10.3 but fails with requiring
UndecidableInstances on 8.0.1-rc4
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        Reporter:  dmcclean          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         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 dmcclean):

 Won't-fix is fine by me. I reported it because I had no idea what was
 going on and was surprised by the change. In the intervening time we have
 changed our approach to this part of the code anyway, and as it happens
 the new code works on GHC 8 without UndecidableInstances anyway.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12040#comment:7>
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