[GHC] #12797: Default Rules stop working when providing some constraints

[GHC]

#12797: Default Rules stop working when providing some constraints
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        Reporter:  danilo2           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 danilo2):

 Some notes from #ghc IRC: The error can be reproduced in GHC 7.10.3 (after
 adding `-XDataKinds`). It never even gets to the rewrites, apparently
 (compiling with -O0 so no RULES).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12797#comment:2>
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