[GHC] #12768: 8.0.2 derives invalid code when class method is constrained by itself

[GHC]

#12768: 8.0.2 derives invalid code when class method is constrained by itself
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        Reporter:  jophish           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         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 RyanGlScott):

 I'm disinclined to fix this, too, for the reasons you articulated.

 The only place I'm aware of at the moment that exploited this trick was in
 the [https://github.com/ekmett/trifecta trifecta] library, and, as jophish
 noted in comment:2, the HEAD version of trifecta has already been patched
 to avoid this bug. But there could be other occurrences lurking in the
 wild, too... we'd probably need a build of Stackage with 8.0.2 to be sure.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12768#comment:7>
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