GHC panic when deriving Functor on a Fixed type

Building the following

{-# LANGUAGE DeriveFunctor#-}
module Main where 

data Event a b = Event a deriving (Functor)

newtype F f = F (f (F f))  

data EventF a = EventF (F (Event a)) deriving (Functor)

results in

Main.hs:7:48:ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-unknown-linux):
	Prelude.(!!): index too large

I am not sure if the above code should actually compile but it probably shouldn't panic.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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