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
Test case
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changed weight to 5

added Tbug Trac import labels

mentioned in commit 47640ca4

I get

T9305.hs:8:48:
    No instance for (Functor Event)
      arising from the first field of ‘EventF’ (type ‘F (Event a)’)
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (Functor EventF)

when trying with GHC head. I’m not quite sure if its the best error message, but it looks better than what you report, hence adding it as a (non-broken) test case.

If convenient, can you check if it still happens with 7.8.3? Might be an already fixed and merged bug. Otherwise we should identify the fix and merge it for 7.8.4.

Trac metadata

Trac field	Value
Test case	→ T9305

closed

This is already fixed in 7.8.3 (#9071 (closed), #9255 (closed))

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate

Yes, appears to be fixed in 7.8.3. I get the same output as you. Thanks.

added compiler crash label

added Pnormal label