[GHC] #14796: Pretty-printer bug

[GHC]

#14796: Pretty-printer bug
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 From Ryan's [https://ryanglscott.github.io/2018/02/11/how-to-derive-
 generic-for-some-gadts/ How to derive Generic for (some) GADTs using
 QuantifiedConstraints]:

 {{{#!hs
 data ECC ctx f a where
   ECC :: ctx => f a -> ECC ctx f a
 }}}

 When instantiating types using `-XTypeApplications` something weird
 happens

 {{{
 $ ~/code/qc-ghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -XGADTs -XConstraintKinds -XTypeApplications
 Prelude> data ECC ctx f a where ECC :: ctx => f a -> ECC ctx f a
 Prelude> :t ECC @[] @() @()
 ECC @[] @() @() :: [()] -> ECC () :: Constraint [] ()
 }}}

 Doesn't seem to happen in other cases:

 {{{
 Prelude> :t ECC @[] @() @((), ())
 ECC @[] @() @((), ())
   :: [()] -> ECC (() :: Constraint, () :: Constraint) [] ()

 Prelude> :t ECC @[] @() @(Eq Int)
 ECC @[] @() @(Eq Int) :: [()] -> ECC (Eq Int) [] ()
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14796>
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