Incorrect "refinements checks for subtyping" generated in some cases

[mario-bucev]

The following snippet is said to be invalid:

import stainless.lang._

object RefnChecks {
  def fun(x: BigInt): Option[Some[BigInt]] = {
    (
      if (x == 0) Some[Some[BigInt]](Some(x))
      else None[Some[BigInt]]()
    ) match {
      case _ => None()
    }
  }
}

In particular for the VC:

[Warning ]  - Result for 'refinements checks for subtyping' VC for fun @6:7:
[Warning ] v$3306.isInstanceOf[Some]
[Warning ] RefnChecks.scala:6:7:  => INVALID
                 if (x == 0) Some[Some[BigInt]](Some(x))
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
[Warning ] Found counter-example:
[Warning ]   v$3306: Option[BigInt] -> None[BigInt]()

The AdtSpecialization introduces these refinements (this transformation seems to be correct):

def fun(x: BigInt): Option[{ v$3305: Option[BigInt] | v$3305 is Some }] = {
  val targetBound$3303: Boolean = x == 0
  // v$3306 from the VC stems from here
  val targetBound$3304: Option[{ v$3306: Option[BigInt] | v$3306 is Some }] = if (targetBound$3303) {
    Some[{ v$3307: Option[BigInt] | v$3307 is Some }](Some[BigInt](x))
  } else {
    None$325[{ v$3308: Option[BigInt] | v$3308 is Some }]()
  }
  targetBound$3304 match {
    case _ =>
      None$325[{ v$3310: Option[BigInt] | v$3310 is Some }]()
  }
}