When we resolve #12679 (closed), it will be possible to implement an abstract type class using a constraint synonym. However, in my implementation, I didn't implement functional dependency checking:

signature H where
  class F a b | a -> b

-- This should be an invalid implementation
module H where
  type F a b = (Eq a, Eq b)

The check is annoying fiddly: we have to descend into the constraint synonym, collect all the implied fundeps, and then see if we have the ones we need, so I didn't implement it. Maybe some time.