GHC 6.8.* doesn't recognize type variables

The module

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{- # LANGUAGE FlexibleInstances # -}
module Leandro where

data Abb a b = Branch a b (Abb a b) (Abb a b) | Leaf

data ListAssoc a b = Node a b (ListAssoc a b) | Empty

class Container c a b |c -> a, c -> b where
    empty :: c
    add :: c -> a -> b -> c
    search :: c -> a -> Maybe b
    del :: c -> a -> c
    toListPair :: c -> [(a,b)]

instance (Ord a) => Container (Abb a b) a b where
    empty = Leaf
    add Leaf x y = Branch x y Leaf Leaf
    add arb@(Branch ni nd ri rd) x y
        |x == ni = arb
        |x > ni = Branch ni nd ri (add rd x y)
        |otherwise = Branch ni nd (add ri x y) rd
    search Leaf x = Nothing
    search (Branch ni nd ri rd) x
        |x == ni = Just nd
        |x > ni = search rd x
        |x < ni = search ri x

triggers the following error in ghci (6.8.1 and 6.8.2):

dafis@linux:~/Documents/haskell/move> ghci Leandro
GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Leandro          ( Leandro.hs, interpreted )

Leandro.hs:17:0:
    Illegal instance declaration for `Container (Abb a b) a b'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are distinct type *variables*
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Container (Abb a b) a b'
Failed, modules loaded: none.

To the best of my knowledge, a and b in 'instance Container (Abb a b) a b where' are distinct type variables and neither hugs nor ghci-6.6.1 complain. Converting the third line to a language pragma makes the code acceptable to 6.8.*.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture