Missing adaptation from by-name to result type

[gsps]

When using a valdef to unapply a by-name parameter, we don't insert the required adaptation, leading to a runtime exception:

object Foo {
  def f(z: => (Int, String)) = {
    val (x, y) = z
  }
}

object HelloWorld { def main(args: Array[String]): Unit = { Foo.f { (1, "a") } } }

results in

java.lang.ClassCastException: scala.Tuple2 cannot be cast to scala.Function0

in the body of f.