Type hole(?) causes compiler failure

I tried to understand some Servant code, and added

      bodyCheck :: _

To the following code

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module Servant.Files ( FilesTmp
                     , FilesMem
                     , MultiPartData
                     , MultiPartDataT
                     , Tmp
                     , Mem
                     ) where

import Control.Monad.Trans.Resource
import Data.ByteString.Lazy (ByteString)
import Network.Wai.Parse
import Servant
import Servant.Server.Internal

-- Backends for file upload: in memory or in /tmp ?

data Mem
data Tmp

class KnownBackend b where
  type Storage b :: *

  withBackend :: Proxy b -> (BackEnd (Storage b) -> IO r) -> IO r

instance KnownBackend Mem where
  type Storage Mem = ByteString

  withBackend Proxy f = f lbsBackEnd

instance KnownBackend Tmp where
  type Storage Tmp = FilePath


  withBackend Proxy f = runResourceT . withInternalState $ \s ->
    f (tempFileBackEnd s)

-- * Files combinator, to get all of the uploaded files

data Files b

type MultiPartData b = ([Param], [File (Storage b)])
type MultiPartDataT b = ((MultiPartData b -> IO (MultiPartData b)) -> IO (MultiPartData b))

type FilesMem = Files Mem
type FilesTmp = Files Tmp

instance (KnownBackend b,
          HasServer sublayout config) => HasServer (Files b :> sublayout) config where
  type ServerT (Files b :> sublayout) m =
    MultiPartDataT b -> ServerT sublayout m

  route Proxy config subserver = WithRequest $ \request ->
    route (Proxy :: Proxy sublayout) config (addBodyCheck subserver (bodyCheck request))
    where
      bodyCheck :: _
      bodyCheck request = return $ Route (\f ->
        withBackend (Proxy :: Proxy b) $ \pb -> parseRequestBody pb request >>= f
        )

Output:

> :load src/Servant/Test.hs
[1 of 1] Compiling Servant.Files    ( src/Servant/Test.hs, interpreted )

src/Servant/Test.hs:59:59:
    Couldn't match type ‘r’ with ‘([Param], [File (Storage b)])’
      because type variable ‘b’ would escape its scope
    This (rigid, skolem) type variable is bound by
      the instance declaration
      at src/Servant/Test.hs:(53,10)-(54,80)
    Expected type: Delayed
                     (((([Param], [File (Storage b)]) -> IO r) -> IO r)
                      -> Server sublayout)
      Actual type: Delayed (Server (Files b :> sublayout))
    Relevant bindings include
      bodyCheck :: Network.Wai.Internal.Request
                   -> m (RouteResult
                           ((([Param], [File (Storage b)]) -> IO r) -> IO r))
        (bound at src/Servant/Test.hs:62:7)
      subserver :: Delayed (Server (Files b :> sublayout))
        (bound at src/Servant/Test.hs:58:22)
      route :: Proxy (Files b :> sublayout)
               -> Context config
               -> Delayed (Server (Files b :> sublayout))
               -> Router
        (bound at src/Servant/Test.hs:58:3)
    In the first argument of ‘addBodyCheck’, namely ‘subserver’
    In the third argument of ‘route’, namely
      ‘(addBodyCheck subserver (bodyCheck request))’

src/Servant/Test.hs:59:70:
    Couldn't match type ‘m’ with ‘IO’
      ‘m’ is untouchable
        inside the constraints (KnownBackend b, HasServer sublayout config)
        bound by the instance declaration
        at src/Servant/Test.hs:(53,10)-(54,80)ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.3 for x86_64-unknown-linux):
        No skolem info: m_a5An[sk]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug