Ambiguous type variable warning gets confused with RankN records #3380

When writing

type X a =
  { a :: forall f. f a
  , b :: forall f. a
  }

I'd expect a warning for an unused/ambiguous type variable f for the b field.

Instead I get no warning. If I then change the code to say:

type X a =
  { a :: forall g. g a
  , b :: forall f. a
  }

I get a warning about the redundant f. (I suspect a problem in one of the type collecting traversals)