How to refer to a type variable hidden under an alias?

scopedLens.hs

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE ScopedTypeVariables #-}

type Lens s t a b =
  forall f.
  Functor f =>
  (a -> f b)
  -> s
  -> f t


one :: forall a b x. Lens (a, x) (b, x) a b
one f (a, x) = (, x) <$> fa
  where
    -- This is the problematic line, if I delete the type signature everything works
    -- But how should I write a type signature for this?
    fa :: f b --- it complains about f1 b /= f b
    fa = f a