When compiled, this program:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeInType #-}
{-# OPTIONS_GHC -fno-print-explicit-kinds #-}
module Bug where

import Data.Kind

data Foo (a :: Type) :: forall b. (a -> b -> Type) -> Type where
  MkFoo :: Foo a f

f :: Foo a f -> String
f = show

Gives the following error:

$ /opt/ghc/8.4.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:13:5: error:
    • No instance for (Show (Foo a b f)) arising from a use of ‘show’
    • In the expression: show
      In an equation for ‘f’: f = show
   |
13 | f = show
   |     ^^^^

This error message is slightly incorrect, however. In "No instance for (Show (Foo a b f))", it claims that Foo has three visible type parameters, but it only has two. (I've even made sure to enable -fno-print-explicit-kinds at the type to ensure that the invisible b kind shouldn't get printed, but it was anyway.)

This is a regression that was apparently introduced between GHC 8.0 and 8.2, since in GHC 8.0.2, it prints the correct thing:

$ /opt/ghc/8.0.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:13:5: error:
    • No instance for (Show (Foo a f)) arising from a use of ‘show’
    • In the expression: show
      In an equation for ‘f’: f = show

But it does not in GHC 8.2.1:

$ /opt/ghc/8.2.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:13:5: error:
    • No instance for (Show (Foo a b f)) arising from a use of ‘show’
    • In the expression: show
      In an equation for ‘f’: f = show
   |
13 | f = show
   |     ^^^^