What happens if we derive Ord but not Eq while making a data type?

Question

I understand that in haskell if we make a data type deriving Ord it should be deriving Eq as well, but why do we have to explicitly write both of them in the definition?

Answer 1

One could plausibly derive Ord but instantiate Eq by hand:

data Foo = Foo deriving Ord
instance Eq Foo where
    _ == _ = True

I can't think of a place off-hand where this would be useful, but there's no reason to prevent it.