[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:
       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 jophish):

 Yes, as 8.1 suggests, this constraint is redundant.

 I encountered this in the trifecta library. It can be seen in the `rend`
 and `restOfLine` member functions of the `DeltaParsing` class here
 https://hackage.haskell.org/package/trifecta-1.6/docs/src/Text.Trifecta.Combinators.html#DeltaParsing
 (this is fixed in trifecta's HEAD now).

 I imagine that this can happen when a top level constrained function is
 moved to be a member of a class.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12768#comment:2>
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