Transitive typeclass instances in Haskell

transitive.hs

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
{-# LANGUAGE FunctionalDependencies, TypeApplications #-}

import Data.Proxy

data User

class MyClass a b | b -> a

instance MyClass Char Int
instance MyClass Int Bool
instance MyClass Bool Double
instance MyClass Bool User

instance {-# OVERLAPPABLE #-} (MyClass a b, MyClass b c) => MyClass a c

test :: MyClass a b => Proxy (a, b)
test = Proxy

findTheTop :: (MyClass top a, MyClass a bottom) => Proxy bottom -> Proxy top
findTheTop _ = Proxy

main =
  let a = test @Char @Double
      b = test @Char @User
      c = findTheTop (Proxy :: Proxy User) :: Proxy Char
      d = findTheTop (Proxy :: Proxy User) -- Proxy Int if allowed to infer. Can only, on its own, do one hop
  in return ()