`-Wunused` ignores annotations extended from `unused`

[soronpo]

Works properly in Scala 2.13.10 (with -Xlint:unused)

Compiler version

v3.3.0-RC5

Minimized code

//> using scala "3.3.0-RC4"
//> using option "-Wunused:all"
import scala.annotation.unused

class extunused extends unused("extended")

def hithere: Unit = {
  @extunused
  val x = 1
  print("hi")
}

Output

[warn] .\test1.sc:9:7: unused local definition
[warn]   val x = 1

Expectation

No warning.

[som-snytt]

Same behavior in Scala 2. (Edit: "sometimes", which is probably just a bug.)

However, relatedly, Scala 2 accepts subclasses of @nowarn, but apparently Scala 3 does not. Noticed at scala/bug#12367 but I haven't looked for the dotty ticket. Edit: probably no ticket was filed. Also noted on that issue is the challenge of using the "hardcoded" constructor args from subclasses. It says Lukas had a WIP for Scala 2.

It looks like subclasses of @deprecated are handled but that is @deprecatedInheritance.