Haskell functional dependency conflict

Question

Why does this result in a conflict?

class Foo a b | b -> a where
  foo :: a -> b -> Bool

instance Eq a => Foo a a where
  foo = (==)

instance Eq a => Foo a (a -> a) where
  foo x f = f x == x

Note that the code will compile if I remove the functional dependecy.

I was under the impression that functional dependencies should only disallow stuff like the following, when in fact, it compiles!

class Foo a b | b -> a where
  foo :: a -> b -> Bool

instance Eq a => Foo a a where
  foo = (==)

instance Eq a => Foo Bool a where
  foo _ x = x == x

Same b parameter, yet different a parameters. Shouldn't b -> a disallow this, as this means a is uniquely determined by b?