I'm using GHC 6.6, but I see the same inferred types with 6.4.1.

> {-# OPTIONS -fglasgow-exts #-}
> {-# OPTIONS -fallow-undecidable-instances #-} -- for the coverage condition
> 
> module FunDepEx where


A plain ole' isomorphism class.

> class Iso a b | a -> b, b -> a where
>     rite :: a -> b
>     left :: b -> a


Isomorphism lifts through the sum bifunctor.

> bifmap_either f g = either (Left . f) (Right . g)
>
> instance ( Iso f f', Iso g g'
>          ) => Iso (Either f g) (Either f' g') where
>     rite = bifmap_either rite rite
>     left = bifmap_either left left


Some types to play around with.

> newtype MyChar = MyChar Char deriving (Show, Eq)
> 
> instance Iso MyChar Char where
>     rite (MyChar c) = c
>     left c = MyChar c
> instance Iso Char MyChar where
>     rite c = MyChar c
>     left (MyChar c) = c


My type inference confusion follows; the unit arguments are just to
suppress the monomorphism restriction.

> t1 :: Either Char a
> t1 = Left 'c'
>
> rite_t1 () = rite t1

The inferred type for rite_t1 is
rite_t1 :: (Iso (Either Char a) (Either f' g')) => () -> Either f' g'

Why isn't the inferred type of rite_t1 the same as the ascribed type
of rite_t1'?

> rite_t1' :: Iso b b' => () -> Either MyChar b'
> rite_t1' () = rite t1