[GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition")

[GHC]

#8634: Relax functional dependency coherence check ("liberal coverage condition")
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              Reporter:  danilo2     |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:  7.10.1
              Priority:  high        |           Version:  7.7
             Component:  Compiler    |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:  Phab:D69    |   Type of failure:  None/Unknown
          Architecture:              |         Test Case:
  Unknown/Multiple                   |          Blocking:
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:  #1241,      |
  #2247, #8356, #9103, #9227         |
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Comment (by danilo2):

 @sulzmann - great, so `-XDysfunctionalDependencies` are safe for our
 purposes - we never define instances described by your example :) Its very
 good to hear that, because we've got such instance in core system of our
 product, which works great now and we do not imagine how could we handle
 it other way around :)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:29>
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