Compiler panic on incorrect code (TcTyFuns.flattenType: synonym family in a rank-n type)

On the following code sample the compiler panics with:

 ghc: panic! (the 'impossible' happened)
  (GHC version 6.11.20090403 for i386-unknown-linux):
	TcTyFuns.flattenType: synonym family in a rank-n type

I found this when working on some code when I made a mistake; the code should not type check, but should probably not crash the compiler either. I simplified the code to:

{-# LANGUAGE TypeFamilies
           , KindSignatures
           , RankNTypes
           #-}

type family PF a :: (* -> *) -> * -> *

class Ix a where
    type Es a :: * -> *
    from  :: a          -> PF a (Es a) a

crash :: (forall n. Es a n) -> a
crash = from

It seems similar to #3101 (closed), but that one was about data types. A similar example also seems to be in #1897 (closed), but this bug doesn't seem to fit that ticket's description.

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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