error in EitherC

EitherC.hs

type CarrierKind = (Type -> Type) -> Type -> Type

data ((t :: CarrierKind) :|: (u :: CarrierKind)) (m :: Type -> Type) (a :: Type) = EitherC
  { runLeft  :: t m a
  , runRight :: u m a }
  deriving Functor

instance
  ( MonadBaseControl b (t m), MonadBaseControl b (u m)
  , forall x . StM (t m) x ~ StM (u m) x )
  => MonadBaseControl b ((t :|: u) m) where
  type StM ((t :|: u) m) a = StM (u m) a
  liftBaseWith f = EitherC left right
    where
      left  = liftBaseWith \h -> f \(EitherC l _) -> h l
      right = liftBaseWith \h -> f \(EitherC _ r) -> h r

-- • Couldn't match type ‘StM (t m) a1’ with ‘StM (u m) a1’
--   Expected type: b (StM ((:|:) t u m) a1)
--     Actual type: b (StM (t m) a1)
--   NB: ‘StM’ is a non-injective type family
-- • In the expression: h l
--   In the second argument of ‘($)’, namely ‘\ (EitherC l _) -> h l’
--   In the expression: f $ \ (EitherC l _) -> h l
-- • Relevant bindings include
--     l :: t m a1 (bound at src/EitherC.hs:??:??)
--     h :: Control.Monad.Trans.Control.RunInBase (t m) b
--       (bound at src/EitherC.hs:??:??)
--     left :: t m a (bound at src/EitherC.hs:??:?)
--     right :: u m a (bound at src/EitherC.hs:??:?)
--     f :: Control.Monad.Trans.Control.RunInBase ((:|:) t u m) b -> b a
--       (bound at src/Metasearch/Aviasales/ABTest.hs:82:16)
--     liftBaseWith :: (Control.Monad.Trans.Control.RunInBase
--                        ((:|:) t u m) b
--                      -> b a)
--                     -> (:|:) t u m a