GHC accepts these two definitions:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeInType #-}
module Bug where

import Data.Proxy

f :: forall (x :: forall a. a -> Int). Proxy x
f = Proxy

g :: forall (x :: forall a. a -> Int). Proxy (x :: forall b. b -> Int)
g = Proxy

However, it does not accept this one, which (AFAICT) should be equivalent to the two above:

h :: forall x. Proxy (x :: forall b. b -> Int)
h = Proxy

GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:13:23: error:
    • Expected kind ‘forall b. b -> Int’, but ‘x’ has kind ‘k0’
    • In the first argument of ‘Proxy’, namely
        ‘(x :: forall b. b -> Int)’
      In the type signature:
        h :: forall x. Proxy (x :: forall b. b -> Int)
   |
13 | h :: forall x. Proxy (x :: forall b. b -> Int)
   |                       ^