[GHC] #12660: singletons doesn't compile with GHC 8.0.2 snapshot

[GHC]

#12660: singletons doesn't compile with GHC 8.0.2 snapshot
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Reverting 2c3b77e44fd1f982a6416db5edc212f22c3dbcbf appears to allow
 `singletons` to build.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12660#comment:3>
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