GADT - fundep interaction

GADTs and fundeps don't seem to interact in the way
that I (perhaps naively) expect. I expected that for
each case, the type variables would be instantiated
according to the type of the constructors, and then the
fundep would be used to figure out the result type. 

{-# OPTIONS_GHC -fglasgow-exts #-}

data Succ n
data Zero

class Plus x y z | x y -> z
instance Plus Zero x x
instance Plus x y z => Plus (Succ x) y (Succ z)

infixr 5 :::

data List :: * -> * -> * where
              Nil :: List a Zero
              (:::) :: a -> List a n -> List a (Succ n)

append :: Plus x y z => List a x -> List a y -> List a z
append Nil ys = ys
append (x ::: xs) ys = x ::: append xs ys

{-
  GHC 6.4 says:

    Couldn't match the rigid variable `y' against `Succ z'
      `y' is bound by the type signature for `append'
      Expected type: List a y
      Inferred type: List a (Succ z)
    In the expression: x ::: (append xs ys)
    In the definition of `append': append (x ::: xs) ys
= x ::: (append xs ys)
-}