GNTD regressions in GHC HEAD. This typechecks on 7.6.3 but not on 7.7.20131217 unless ImpredicativeTypes are enabled.

error.txt

[1 of 1] Compiling Repro            ( repro.hs, interpreted )

repro.hs:24:13:
    Cannot instantiate unification variable ‛b0’
    with a type involving foralls:
      (forall r. (a1 -> IO r) -> IO r) -> DecodeAST a1
      Perhaps you want ImpredicativeTypes
    In the expression:
        GHC.Prim.coerce
          (anyContToM ::
             (forall (r :: *). (a -> IO r) -> IO r)
             -> AnyContT (StateT DecodeState IO) a) ::
          forall (a :: *).
          (forall (r :: *). (a -> IO r) -> IO r) -> DecodeAST a
    In an equation for ‛anyContToM’:
        anyContToM
          = GHC.Prim.coerce
              (anyContToM ::
                 (forall (r :: *). (a -> IO r) -> IO r)
                 -> AnyContT (StateT DecodeState IO) a) ::
              forall (a :: *).
              (forall (r :: *). (a -> IO r) -> IO r) -> DecodeAST a
Failed, modules loaded: none.

gistfile1.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
-- Uncomment to compile on GHC 7.8
-- {-# LANGUAGE ImpredicativeTypes #-}

module Repro where

import Control.Monad.Trans.Cont
import Control.Monad.Trans.State.Lazy

newtype AnyContT m a = AnyContT { unAnyContT :: forall r . ContT r m a }

class MonadAnyCont b m where
  anyContToM :: (forall r . (a -> b r) -> b r) -> m a

instance MonadAnyCont b (AnyContT m) where
  anyContToM _ = error "foo"

data DecodeState = DecodeState

newtype DecodeAST a = DecodeAST { unDecodeAST :: AnyContT (StateT DecodeState IO) a }
  deriving (MonadAnyCont IO)