Unable to find member method

[abeln]

For the explicit nulls project, we want to have an extension method that strips away nullability:

implicit class NN[T](x:T|Null) {
  def nn: T = x.asInstanceOf[T]
}

This works must of the time, but weirdly fails in the code below:

class Foo {
  val x1: String|Null = null
  x1.nn.length
  val x2: String = x1.nn
  x1.nn.length
}

-- [E008] Member Not Found Error: kk.scala:10:8 --------------------------------
10 |  x1.nn.length
   |  ^^^^^^^^^^^^
   |  value length is not a member of T
   |
   |  where:    T is a type variable with constraint >: String
one error found

Line 10 is the second x1.nn.length. However, both

class Foo {
  val x1: String|Null = null
  x1.nn.length
  // val x2: String = x1.nn <---------- commented out
  x1.nn.length
}

and

class Foo {
  val x1: String|Null = null
  x1.nn.length
  val x2 = x1.nn // <---------- the type of `x2` is inferred
  x1.nn.length
}

typecheck fine.

@liufengyun any hints about what could be going here?