Here's a simplified version of a panic I'm getting:

{-# LANGUAGE RankNTypes, GADTs, DataKinds, PolyKinds, TypeOperators, TypeFamilies #-}
import Data.Proxy
data Want :: (i,j) -> * where
  Want :: (a ~ '(b,c) => Proxy b) -> Want a

GHCi, version 7.8.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-apple-darwin):
	tcTyVarDetails k{tv aAv} [tv]

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