[GHC] #13649: RebindableSyntax causes type errors when 'fail' is not defined, even if not used.

[GHC]

#13649: RebindableSyntax causes type errors when 'fail' is not defined, even if not
used.
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        Reporter:  AaronFriel        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc1
      Resolution:                    |             Keywords:
                                     |  RebindableSyntax
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 AaronFriel):

 Interesting, I thought that the lack of the constraint meant that GHC here
 was not following that spec. How does the `fail` get removed such that it
 doesn't entail a constraint?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13649#comment:2>
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