[GHC] #15231: UndecidableInstances validity checking is wrong in the presence of QuantifiedConstraints

[GHC]

#15231: UndecidableInstances validity checking is wrong in the presence of
QuantifiedConstraints
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        Reporter:  RyanGlScott       |                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 RyanGlScott):

 Ah, gotcha! That error message does sound like an improvement, yes.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15231#comment:2>
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