Scala 2.13.15: warn unused false positive when pattern matching on HOF parameter

[PiotrBosak]

Reproduction steps

https://scastie.scala-lang.org/LI6ZuhHfS3CjvYepfhJjyw
Scala version: 2.13.15 with "-Ywarn-unused" flag
(Is the bug only in Scala 3.x? If so, report it at scala/scala3 instead.)

    List(("first", "second"))
      .map { case (_, _) =>
        for {
          a <- Some("a")
          b = a // b is not used warning
        } yield b
      }

Problem

When using a pattern match anonymous function in a HOF, variables assigned inside
a for-comprehension(b = a) are reported as unused.
Same snippet in 2.13.14 works fine.

Explain how the above behavior isn't what you expected.
Do not report it as unused since it clearly is used

Workaround

  List(("first", "second"))
    .map { a =>
      a match {
        case (_, _) =>
          for {
            a <- Some("a")
            b = a
          } yield b
      }
    }

[som-snytt]

Thanks for the report. This is fixed for 2.13.16 in scala/scala#10870