Type variable escapes when encoding existential types

Consider the following program.

{-# LANGUAGE RankNTypes #-}

data Showable = Showable (forall b. (forall a. Show a => a -> b) -> b)

showable x = Showable (\f -> f x)

f :: Showable -> String
f showable = case showable of
               Showable f -> let k x = show x in f k

While this program compiles in GHC, our frontend rejects it with the following error message.

Ambiguous type variable _17
 in type Prelude.Show _17 => Showable -> [Prelude.Char]
 inferred for equation
 f showable =
   case showable of
     Showable f -> let k x = show x in f k