standalone deriving for typeable fails

import Data.Typeable

data D a = C1 a | C2 (D a)
deriving instance Typeable a => Typeable (D a)
--           deriving Typeable

x = show $ typeOf (undefined::D Int)

main = print x

yields:

[1 of 1] Compiling Main             ( T.hs, T.o )
ghc-6.8.1: panic! (the 'impossible' happened)
  (GHC version 6.8.1 for i386-unknown-linux):
        No match in record selector Var.tcTyVarDetails

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown