type synonyms confuse generalized newtype deriving role checking

Below is a small Haskell file showing the problem. It defines a simple MonadTrans typeclass, but using a type operator ~>. When using the type operator, GHC complains that generalized newtype deriving gets stuck on a nominal role, but when the type operator is not used then GND works just fine.

Perhaps this is just expected behavior with mixing RankNTypes and GND (and roles), but it's rather unfortunate.

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}

module GND where

type m ~> n = forall a. m a -> n a

class MonadTrans t where
  -- > this line works:
  -- lift :: (Monad m) => m a -> t m a
  -- > this line doesn't:
  lift :: (Monad m) => m ~> t m

data StateT s m a = StateT { runStateT :: s -> m (a, s) }

instance MonadTrans (StateT s) where
  lift xM = StateT $ \ s -> do { x <- xM ; return (x,s) }

newtype OtherStateT s m a = OtherStateT { runOtherStateT :: StateT s m a }
  deriving (MonadTrans)

The error message is:

GND.hs:23:13: error:
    • Couldn't match representation of type ‘m1 ~> StateT s m1’
                               with that of ‘m1 a1 -> OtherStateT s m1 a1’
        arising from a use of ‘GHC.Prim.coerce’
    • In the expression:
          GHC.Prim.coerce (lift :: (~>) m (StateT s m)) ::
            forall (m :: TYPE GHC.Types.PtrRepLifted
                         -> TYPE GHC.Types.PtrRepLifted).
            Monad m => (~>) m (OtherStateT s m)
      In an equation for ‘lift’:
          lift
            = GHC.Prim.coerce (lift :: (~>) m (StateT s m)) ::
                forall (m :: TYPE GHC.Types.PtrRepLifted
                             -> TYPE GHC.Types.PtrRepLifted).
                Monad m => (~>) m (OtherStateT s m)
      When typechecking the code for ‘lift’
        in a derived instance for ‘MonadTrans (OtherStateT s)’:
        To see the code I am typechecking, use -ddump-deriv
      In the instance declaration for ‘MonadTrans (OtherStateT s)’
    • Relevant bindings include
        lift :: m ~> OtherStateT s m (bound at GND.hs:23:13)

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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