[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:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #1241, #2247,
  None/Unknown                       |  #8356, #9103, #9227
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D69    |
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Comment (by goldfire):

 About unpredictability: I think this is quite similar to
 `IncoherentInstances`, where, I believe, similar unpredictability exists.
 A notable difference here is that `DysfunctionalDependencies` would
 introduce compile-time unpredictability. I would say, though, that it is
 up to the user to make sure that their dysfunctional dependencies are
 confluent w.r.t. the equivalence relation that is relevant to them --
 which might not be `(~)`.

 So, I think there's support for this feature. What needs to be done to
 merge?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:52>
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