Exclude `Selectable` from unused member warning

[som-snytt]

Follow up #22729 (comment)

Discussed in #23322

^{Originally posted by spavikevik June 5, 2025}
We've been using Selectable extensively for fixtures in tests like this:

val f = new Selectable {
  val userId = Some("...")
}

But updating to Scala 3.7.x we are seeing a bunch of unused private member warnings, presumably because of #22729. Is this going to be a permanent limitation from Scala 3.7.x on? And if so, what would be the recommended way to fix the warnings?

Or can this be addressed in a subsequent release?

[som-snytt]

The existing check used an explicit refinement to "silence" the warning for a member of an anonymous class.

The PR uses the inferred refinement for a Selectable for the same purpose.