[GHC] #15639: Surprising failure combining QuantifiedConstraints with Coercible

[GHC]

#15639: Surprising failure combining QuantifiedConstraints with Coercible
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
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 dfeuer):

 malo, I don't think the problem you're encountering is related at all.
 You're hoping that `forall i. Coercible (f i) (g i)` will satisfy the
 `type role Bar representational nominal` requirement. But we don't have
 such an extensionality rule. Perhaps we could add one somehow, but that
 would be quite independent.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15639#comment:9>
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