[GHC] #14861: QuantifiedConstraints: Can't use forall'd variable in context

[GHC]

#14861: QuantifiedConstraints: Can't use forall'd variable in context
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints wipT2893
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 simonpj):

 > I would have expected the (forall p. c p => Foldable' f) quantified
 constraint to kick in there.

 Can you say why?  We are trying to solve `c beta`. How does the quantified
 constraint help with that?  It tells how to solve `Foldable` constraints.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14861#comment:1>
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