[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)                           |
      Resolution:                    |             Keywords:
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):

 Best guess: the solver doesn't check for relevant quantified constraints
 before reducing `Yeah a_a11k[sk:1] ~R# Yeah b_a11l[sk:1]` to `a_a11k[sk:1]
 ~R# b_a11l[sk:1]`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15639#comment:2>
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