[GHC] #15359: Quantified constraints do not work with equality constraints

[GHC]

#15359: Quantified constraints do not work with equality constraints
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
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 goldfire):

 Oh dear. I see trouble a-brewing.

 I agree that `Coercible` and `~` should either both be allowed or both be
 rejected. But the implications in my OP are unimplementable, I think. So
 I'm lost as to how to specify this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15359#comment:2>
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