[GHC] #14995: QuantifiedConstraints: Incorrect pretty printing

[GHC]

#14995: QuantifiedConstraints: Incorrect pretty printing
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints, wipT2893    |
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This code from [https://www.microsoft.com/en-us/research/wp-
 content/uploads/2016/02/jfp-outsidein.pdf OutsideIn(X)] compiles

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

 class C a
 class           B a b   where op :: a -> b
 instance C a => B a [a] where op = undefined

 data R a where
   MkR :: C a => a -> R a

 k :: (C a => B a b) => R a -> b
 k (MkR x) = op x
 }}}

 but pretty-printing `k` prints `B a b => ..` instead of `(C a => B a b) =>
 ..`

 {{{
 $ ghci -ignore-dot-ghci Test.hs
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( hs/228-bug.hs, interpreted )
 Ok, one module loaded.
 *Main> :t k
 k :: B a b => R a -> b
 *Main>
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14995>
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