GADT + Type family doesn't unify like I expect

I'd expect this program to compile; the "given" equations in "conn" should add additional constraints to the types determined by the GADT. Instead it looks like (Dual a) is failing to unify with (R _ _), and the same for (Dual b) and (W _ _).

{-# LANGUAGE GADTs, TypeFamilies, EmptyDataDecls #-}
module Dual where

data R a b
data W a b
data Z

type family Dual a
type instance Dual Z = Z
type instance Dual (R a b) = W a (Dual b)
type instance Dual (W a b) = R a (Dual b)

data Comm a where
    Rd :: (a -> Comm b) -> Comm (R a b)
    Wr :: a  -> Comm b  -> Comm (W a b)
    Fin :: Int -> Comm Z

conn :: (Dual a ~ b, Dual b ~ a) => Comm a -> Comm b -> (Int, Int)
conn (Fin x) (Fin y) = (x,y)
conn (Rd k) (Wr a r) = conn (k a) r
conn (Wr a r) (Rd k) = conn r (k a)

Running off of GHC HEAD: The Glorious Glasgow Haskell Compilation System, version 6.11.20080925

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown