TH reification prints invisible arguments to rank-2-kinded type as visible

If you run the following program:

{-# 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.

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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