GHC panic when resolving Show instance

I'm playing around with rolling my own type defaulting and found a panic. Note: adding a stub Show (W a) instance resolves this (i.e. show _ = "test").

{- Test.hs -}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}

module Test where

import Data.Proxy

data W (a :: k) = Wk | W (MkStar a)

type family MkStar (a :: k) :: *

main = print (W Proxy :: W (Proxy (~)))

$ ghc Test.hs -o test 
[1 of 1] Compiling Test             ( Test.hs, Test.o )

Test.hs:13:8: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-apple-darwin):
	print_equality ~

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

If x = W Proxy :: W (Proxy (~)) is only defined, there is no issue. It seems to occur exactly when resolving a Show instance for W (Proxy (~)) and one does not exist.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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