T15290a.hs:25:12: error:
    • Couldn't match representation of type: m (Int, IntStateT m a1)
                               with that of: m (Int, StateT Int m a1)
        arising from a use of ‘coerce’
      NB: We cannot know what roles the parameters to ‘m’ have;
        we must assume that the role is nominal
    • In the expression:
        coerce
          @(forall a. StateT Int m (StateT Int m a) -> StateT Int m a)
          @(forall a. IntStateT m (IntStateT m a) -> IntStateT m a) join
      In an equation for ‘join’:
          join
            = coerce
                @(forall a. StateT Int m (StateT Int m a) -> StateT Int m a)
                @(forall a. IntStateT m (IntStateT m a) -> IntStateT m a) join
      In the instance declaration for ‘Monad (IntStateT m)’
    • Relevant bindings include
        join :: IntStateT m (IntStateT m a) -> IntStateT m a
          (bound at T15290a.hs:25:5)