[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):

 Thanks thornie.

 Do you have a position on whether the issue is with 7.10.3 being too
 permissive or 8.0.1 being too restrictive? Beyond "it makes sense to me"
 and "it used to work", I don't have a good handle on how to go about
 deciding whether this code is actually in line with the rules.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12040#comment:3>
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