Code does not compile without ScopedTypeVariables

Feel free to change the summary, I have no idea how to summarize this issue.

The code below fails to compile with Couldn't match type A with B and The function X is applied to one argument, but its type Y has none and some more (full output is attached below the code). However, simply enabling a language feature (ScopedTypeVariable) makes the code compile. If this is not a bug in the compiler then the message should be improved, because nothing in it points to the solution.

I always thought of ScopedTypeVariables as allowing me to sprinkle :: T throughout the code, in more places than GHC would normally allow. I did not expect that merely enabling this language feature without any other changes in the source code would have any effect on the output.

Dependencies: servant, servant-server

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE TypeFamilies      #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeOperators     #-}

-- {-# LANGUAGE ScopedTypeVariables  #-}

module Main where

import Data.Proxy
import Servant.API
import Servant.Server


data Credentials = Credential !String


instance (HasServer sublayout) => HasServer (Credentials :> sublayout) where

    type ServerT (Credentials :> sublayout) m =
        Credentials -> ServerT sublayout m

    route Proxy subserver request respond = do
        let mbSessionIdString = lookup "cookie" [("cookie", "session id")]
            mbCredentials     = fmap Credential mbSessionIdString

        case mbCredentials of
            Nothing -> error "No credentials supplied"
            Just cred -> route (Proxy :: Proxy sublayout) (subserver cred) request respond

[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:30:60:
    Couldn't match type ‘ServerT
                           sublayout
                           (either-4.4.1:Control.Monad.Trans.Either.EitherT ServantErr IO)’
                   with ‘ServerT
                           layout0
                           (either-4.4.1:Control.Monad.Trans.Either.EitherT ServantErr IO)’
    NB: ‘ServerT’ is a type function, and may not be injective
    The type variable ‘layout0’ is ambiguous
    Expected type: Credentials -> Server layout0
      Actual type: ServerT
                     (Credentials :> sublayout)
                     (either-4.4.1:Control.Monad.Trans.Either.EitherT ServantErr IO)
    Relevant bindings include
      subserver :: Server (Credentials :> sublayout)
        (bound at Bug.hs:24:17)
      route :: Proxy (Credentials :> sublayout)
               -> Server (Credentials :> sublayout)
               -> Servant.Server.Internal.RoutingApplication
        (bound at Bug.hs:24:5)
    The function ‘subserver’ is applied to one argument,
    but its type ‘Server (Credentials :> sublayout)’ has none
    In the second argument of ‘route’, namely ‘(subserver cred)’
    In the expression:
      route (Proxy :: Proxy sublayout) (subserver cred) request respond
Failed, modules loaded: none.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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