No warning for unused patvars in for comprehension assignment

[som-snytt]

Questions are not bug reports

I had a question...

Reproduction steps

Scala version: 2.13.16

class C {
  def g = {
    val t = Option((27, 42))
    for {
      ns <- t
      (i, j) = ns
    } yield 42
  }
}

Problem

Does not warn unused i, j under -Wunused.

It correctly does not warn for the unused "intermediates" during the tupling map, but should warn because they are not used after that.

The existing test only verifies that it does not yield a false positive under -Wunused:-patvars (turned off).

package <empty> {
  class C extends scala.AnyRef {
    def <init>(): C = {
      C.super.<init>();
      ()
    };
    def g: Option[Int] = {
      val t: Option[(Int, Int)] = scala.Option.apply[(Int, Int)](scala.Tuple2.apply[Int, Int](17, 42));
      t.map[((Int, Int), (Int, Int))](((ns: (Int, Int)) => {
  <synthetic> <artifact> private[this] val x$2: ((Int, Int), Int, Int) = (ns: (Int, Int) @unchecked) match {
    case (x$1 @ (_1: Int, _2: Int): (Int, Int)((i @ _), (j @ _))) => scala.Tuple3.apply[(Int, Int), Int, Int](x$1, i, j)
  };
  val x$1: (Int, Int) = x$2._1;
  val i: Int = x$2._2;
  val j: Int = x$2._3;
  scala.Tuple2.apply[(Int, Int), (Int, Int)](ns, x$1)
})).map[Int](((x$3: ((Int, Int), (Int, Int))) => (x$3: ((Int, Int), (Int, Int)) @unchecked) match {
        case (_1: (Int, Int), _2: (Int, Int)): ((Int, Int), (Int, Int))((ns @ _), (_1: Int, _2: Int): (Int, Int)((i @ _), (j @ _))) => 42
      }))
    }
  }
}

[som-snytt]

Verified it works under the port to dotty in scala/scala3#20894

-- [E198] Unused Symbol Warning: t13070.scala:7:7 ------------------------------
7 |      (i, j) = ns
  |       ^
  |       unused pattern variable
-- [E198] Unused Symbol Warning: t13070.scala:7:10 -----------------------------
7 |      (i, j) = ns
  |          ^
  |          unused pattern variable
2 warnings found