False `Unreachable case except for null` warning in `inline def` with a match expression with type params

[arainko]

Compiler version

3.3.7, 3.8.1

Minimized code

//> using scala 3.8.1

case object Err
type Err = Err.type

enum Plan[+E <: Err] {
  case A()
  case B()
  case C()

  final inline def narrow[Sub <: Plan[Err]](inline fn: Sub => String): Option[String] =
    this match {
      case a: Sub => Some(fn(a))
      case other => None
    }
}

@main def main = {
  val plan = Plan.A()

  println(plan.narrow[Plan.A[Err]](_.toString)) // Some(A())
  println(plan.narrow[Plan.B[Err]](_.toString)) // None, therefore the 'other' case is actually reachable!
}

Output

$ scala run .
Compiling project (Scala 3.8.1, JVM (17))
[warn] ./Plan.scala:14:12
[warn] Unreachable case except for null (if this is intentional, consider writing case null => instead).
[warn]       case other => None
[warn]            ^^^^^
Compiled project (Scala 3.8.1, JVM (17))
Some(A())
None

Expectation

No warning

[olhotak]

Probably related to #24979 . There, it's also a case of the space engine inferring an incorrect space for an abstract type.