[GHC] #14833: QuantifiedConstraints: GHC can't deduce (() :: Constraint)?

[GHC]

#14833: QuantifiedConstraints: GHC can't deduce (() :: Constraint)?
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14835            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Same as?

 {{{#!hs
 {-# Language QuantifiedConstraints, GADTs, FlexibleInstances,
 MultiParamTypeClasses, ConstraintKinds, UndecidableInstances #-}

 import Data.Kind
 import Data.Proxy
 import Unsafe.Coerce

 data Dict c where
   Dict :: c => Dict c

 class    (a => b) => Entails a b
 instance (a => b) => Entails a b

 wit :: Dict a -> Dict b -> Dict (Entails (a, b) xx) -> Dict xx
 wit Dict Dict Dict = Dict
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14833#comment:3>
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