Can an existentially quantified type variable be forced to have only a single type?

Question

Consider the following code

trait Foo[T] {
  def one: Foo[_ >: T]
  def two: T
  def three(x: T)
}

def test[T](f: Foo[T]) = {
  val b = f.one
  b.three(b         


        

Answer 1

def one: Foo[_ >: T] is equivalent to

def one: Foo[U >: T] forSome {type U >: T}

this one instead works

def one: Foo[U forSome {type U >: T}]

I do not however understand why this would make a difference. It seems like it should not to me. (shrug)