Parsing error with $_ and $_id in interpolated strings

[liufengyun]

Dotty cannot parse $_ and $_id, while Scalac only supports $_.

object Test {
  implicit class Foo(sc: StringContext) {
    object q {
      def unapply(arg: Any): Option[(Any, Any)] =
        Some((sc.parts(0), sc.parts(1)))
    }
  }

  def main(args: Array[String]): Unit = {
    val q"class $_ extends $_parent" = new Object
    println(_parent)
  }
}

[liufengyun]

Forward a comment by @xeno-by :

In scalac, this only works for interpolation strings in patterns:

06:29 ~/Projects/2121/sandbox (HEAD)$ parse 'q"foo${_}"'
/var/folders/81/zf4z2j9s4kl7qy76tgnhk2mh0000gn/T/tmp1M2lFu:2: error: unbound placeholder parameter
  q"foo${_}"
         ^

06:29 ~/Projects/2121/sandbox (HEAD)$ parse 'val q"foo${_}" = ???'
[[syntax trees at end of parser]]// Scala source: tmpbmFgw5
package <empty> {
  class wrapper extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    <synthetic> <artifact> private[this] val x$1 = $qmark$qmark$qmark: @scala.unchecked match {
      case StringContext("foo", "").q(_) => ()
    }
  }
}