No match in record selector Var.tcTyVarDetails with ghc >= 6.10.2

This seems to have been introduced in 6.10.2:

{-# OPTIONS_GHC -fglasgow-exts #-}
{-# LANGUAGE UndecidableInstances #-}

module Bug where

newtype Trie m k a = Trie (Maybe a, m (SubKey k) (Trie m k a))

type family SubKey k
type instance SubKey [k] = k

deriving instance (Eq (m k (Trie m [k] a))
                  ,Eq a) => Eq (Trie m [k] a)

which fails with:

[m@monire ~]$ ghc -O2 -c --make Bug.hs
[1 of 1] Compiling Bug           ( Bug.hs, Bug.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.3 for x86_64-unknown-linux):
  No match in record selector Var.tcTyVarDetails

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

Matt

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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