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{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE InstanceSigs, RoleAnnotations #-}
module T9123 where
import Data.Coerce
type role Wrap representational nominal
newtype Wrap m a = Wrap (m a)
class Monad' m where
join' :: forall a. m (m a) -> m a
-- Tests the superclass stuff
instance (forall p q. Coercible p q => Coercible (m p) (m q), Monad' m) => Monad' (Wrap m) where
join' :: forall a. Wrap m (Wrap m a) -> Wrap m a
join' = coerce @(m (m a) -> m a)
@(Wrap m (Wrap m a) -> Wrap m a)
join'
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