Unexpected DataKind Panic

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

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett
Operating system
Architecture