Spurious non-exhaustive pattern match warning on type alias unapply

[Daenyth]

Reproduction steps

Scala version: 2.13.10

case class Foo[A, B](a: A, b: B)

type Bar[A] = Foo[Int, A]
object Bar {
  def apply[B](a: Int, b: B) = Foo(a, b)
  def unapply[B](bar: Bar[B]): Option[(Int, B)] = Foo.unapply(bar)
}

def blah(x: Bar[Int]): (Int, Int) =
 x match {
   case Bar(a, b) => (a, b)
 }

Scastie

Problem

The x match line emits a "non-exhaustive match" warning.

The code previously worked in 2.12

Workaround

Define unapply to return Some((bar.a, bar.b)) explicitly

[som-snytt]

Warns since 2.13.4. https://github.com/scala/scala/releases/tag/v2.13.4

Alias is not pertinent?

[som-snytt]

Behavior is as expected and documented.

for custom extractors: demarking irrefutable extractors as such, by defining the return type as Some

(where "demarking" is a Britishism for "marking"?) (I guess a return type is a kind of demarcation.)

[Daenyth]

The part of this that confuses me is that Foo.unapply returns Option, not Some, so you can't invoke it and have the return type of Bar.unapply be Some

[som-snytt]

Scala 3 is simpler for this case. I don't know why Scala 2 doesn't emit Some for case unapply.