Почему этот код, который был установлен в GHC 7.10, больше не регистрируется в GHC 8.0.1?

У меня есть следующий код:

{-# LANGUAGE DefaultSignatures#-}

import Control.Monad.Trans.Class
import Control.Monad.Trans.Maybe

class Monad m => MonadMaybe m where
  liftMaybe :: Maybe a -> m a
  default liftMaybe :: (MonadTrans t, MonadMaybe m) => Maybe a -> t m a
  liftMaybe = lift . liftMaybe

instance MonadMaybe m => MonadMaybe (MaybeT m)

Используя систему компиляции Glorious Glasgow Haskell, версия 8.0.1.20161117, это не скомпилируется:

foo.hs:11:10: error:
    • Couldn't match type ‘m’ with ‘MaybeT m’
      ‘m’ is a rigid type variable bound by
        the instance declaration at foo.hs:11:10
      Expected type: Maybe a -> MaybeT m a
        Actual type: Maybe a -> MaybeT (MaybeT m) a
    • In the expression: Main.$dmliftMaybe @MaybeT m
      In an equation for ‘liftMaybe’:
          liftMaybe = Main.$dmliftMaybe @MaybeT m
      In the instance declaration for ‘MonadMaybe (MaybeT m)’
    • Relevant bindings include
        liftMaybe :: Maybe a -> MaybeT m a (bound at foo.hs:11:10)

Однако в GHC 7.10 это компилируется без проблем.

Правильно ли GHC 8, или я нашел ошибку?

Ответ 1