-Ywarn-unused:patvars false positive in for-comprehension

[oleg-py]

Tested on Scala 2.12.8 and 2.13.0-M5.

Compiling this object with -Ywarn-unused:patvars

object Warnings {
  case class Foo(bar: Int, buzz: String)

  def test = for {
    foo @ Foo(bar, buzz) <- List(Foo(1, "x"))
    x = s"$bar $buzz $foo"
  } yield x
  
  def main(args: Array[String]) = println(test)
}

results in a warning:

pattern var foo in value $anonfun is never used; `foo@_' suppresses this warning

This is probably caused by desugaring of the above for-comprehension, being roughly:

List(Foo(1, "x")).map({
  case (foo @ Foo((bar @ _), (buzz @ _))) =>
    val x = StringContext("", " ", " ", "").s(bar, buzz, foo);
    (foo, x)
}).map({
  case ((foo @ Foo((bar @ _), (buzz @ _))), (x @ _)) => x
}));

Note that exact same pattern used in first map is carried over to the second map, even though now its variables aren't used.

[som-snytt]

This duplicates #10287 where, as you note, the desugaring makes it too much of a bother. Leaving open to track use cases (test cases).

[SethTisue]

consolidating at #10287