Dependent function loses information about singleton type

[OlivierBlanvillain]

object Test {
  def id: (x: 1 | 0) => x.type = x => x
  id(0): 0
}

-- [E007] Type Mismatch Error: /test.scala:3:4 
3 |  id(0): 0
  |  ^^^^^
  |  Found:    (1 : Int) | (0 : Int)
  |  Required: (0 : Int)
1 error found