[GHC] #15792: TH reification prints invisible arguments to rank-2-kinded type as visible

[GHC]

#15792: TH reification prints invisible arguments to rank-2-kinded type as visible
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Template       |           Version:  8.6.1
  Haskell                            |
           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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 If you run the following program:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 import Data.Kind
 import Language.Haskell.TH hiding (Type)

 newtype T (f :: forall a. a -> Type) = MkT (f Bool)

 $(pure [])

 main :: IO ()
 main = do
   putStrLn $(reify ''T >>= stringE . pprint)
   putStrLn $(reify ''T >>= stringE . show)
 }}}

 You'll get:

 {{{
 $ /opt/ghc/8.6.1/bin/runghc Bug.hs
 newtype Bug.T (f_0 :: forall (a_1 :: *) . a_1 -> *)
   = Bug.MkT (f_0 * GHC.Types.Bool)
 TyConI (NewtypeD [] Bug.T [KindedTV f_6989586621679016168 (ForallT
 [KindedTV a_6989586621679016167 StarT] [] (AppT (AppT ArrowT (VarT
 a_6989586621679016167)) StarT))] Nothing (NormalC Bug.MkT [(Bang
 NoSourceUnpackedness NoSourceStrictness,AppT (AppT (VarT
 f_6989586621679016168) StarT) (ConT GHC.Types.Bool))]) [])
 }}}

 These are the parts that are suspect:

 * `f_0 * GHC.Types.Bool`
 * `AppT (AppT (VarT f_6989586621679016168) StarT) (ConT GHC.Types.Bool)`

 Notice how `f`/`VarT f` accepts `*`/`StarT` as a visible argument, despite
 the fact that its kind `forall a. a -> Type` indicates that this should be
 invisible.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15792>
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