[GHC] #15632: Undependable Dependencies

[GHC]

#15632: Undependable Dependencies
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        Reporter:  AntC              |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
                                     |  FunctionalDependencies,
                                     |  OverlappingInstances
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  10675, 9210,      |  Differential Rev(s):
  8634                               |
       Wiki Page:                    |
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Comment (by AntC):

 Replying to [comment:2 AntC]:

 I've implemented the suggestion, in a version of Hugs. So throw at me your
 awkward cases/counter-examples.

 (Hugs latest release/Sep 2006 only allows `FunDeps + Overlap` if all the
 overlapping instances are per the strictly unify-to-equal rule. Hugs
 instance heads must overlap in a strict substitution order; none of GHC's
 allowing the potential for overlap.)

 Seems to be working; I have a type-level type equality test; it handles
 the tricky cases I'm aware of, including this ticket. #10675 is rejected.
 Even handles overlap of instances for TRex records.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15632#comment:4>
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