GHC 8.4.1-alpha allows the use of kind polymorphism without PolyKinds

This program:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ImpredicativeTypes #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# OPTIONS_GHC -ddump-deriv #-}
module Bug where

import Data.Coerce
import Data.Proxy

class C a b where
  c :: Proxy (x :: a) -> b

newtype I a = MkI a

instance C x a => C x (Bug.I a) where
  c = coerce @(forall (z :: x). Proxy z -> a)
             @(forall (z :: x). Proxy z -> I a)
             c

is rightfully rejected by GHC 8.2.2:

$ /opt/ghc/8.2.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:20:16: error:
    Unexpected kind variables Perhaps you intended to use PolyKinds
    In a type argument
   |
20 |   c = coerce @(forall (z :: x). Proxy z -> a)
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bug.hs:21:16: error:
    Unexpected kind variables Perhaps you intended to use PolyKinds
    In a type argument
   |
21 |              @(forall (z :: x). Proxy z -> I a)
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

But GHC 8.4.1-alpha2 actually //accepts// this program!

$ /opt/ghc/8.4.1/bin/ghci Bug.hs
GHCi, version 8.4.0.20180118: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Ok, one module loaded.

This is almost certainly bogus.

Trac metadata

Trac field	Value
Version	8.4.1-alpha1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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