ScopedTypeVariable and RankNTypes don't scope throughout function body

I'm writing a DSL that uses TypeApplications, and I noticed the following weird behavior with ScopedTypeVariables and AllowAmbiguousTypes.

You can define a function that takes the TypeApplication after a parameter:

lol :: String -> forall a. IO ()
lol str = putStrLn str

main = lol "hello" @Int

This works! Unfortunately, it is impossible to *refer* to that a type variable in the body of the function. It is as though ScopedTypeVariables were not turned on.

A reproduction (tested on 8.2.2 and 8.4.1):

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE RankNTypes          #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Data.Typeable
import Data.Proxy

no :: String -> forall x. Typeable x => IO ()
no _ = print (typeRep (Proxy :: Proxy x))

yes :: forall x. Typeable x => String -> IO ()
yes _ = print (typeRep (Proxy :: Proxy x))

no fails to compile, saying that x type variable is not in scope. yes works just fine.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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