strange behavior List#corresponds

[xuwei-k]

version 0.2

trait Equal[A] {
  def equal(x: A, y: A): Boolean
}

object Equal {
  implicit def int: Equal[Int] =
    new Equal[Int] {
      def equal(x: Int, y: Int): Boolean = x == y
    }

  implicit def string: Equal[String] =
    new Equal[String] {
      def equal(x: String, y: String): Boolean = x == y
    }

  implicit def list[A](implicit A: Equal[A]): Equal[List[A]] =
    new Equal[List[A]] {
      def equal(x: List[A], y: List[A]): Boolean =
        (x corresponds y)(A.equal(_, _))
    }
}

object Main {
  def main(args: Array[String]): Unit = {
    println(implicitly[Equal[Int]].equal(42, 42)) // true

    println(implicitly[Equal[String]].equal("a", "a")) // true

    val a = 42 :: Nil
    // false in scala-native!? true in JVM.
    println(implicitly[Equal[List[Int]]].equal(a, a))

    val b = "xyz" :: Nil
    println(implicitly[Equal[List[String]]].equal(b, b)) // false !?
  }
}

[densh]

Definitely a bug, thanks for reporting @xuwei-k !

[xuwei-k]

more simple example

object Main {
  def main(args: Array[String]): Unit = {
    val a = List(1, 2, 3)

    // false in scala-native, true in JVM
    println(a.corresponds(a)(_ == _))
  }
}

[xuwei-k]

https://github.com/scala/scala/blob/v2.11.11/src/library/scala/collection/LinearSeqLike.scala#L65-L68

object Main {
  def main(args: Array[String]): Unit = {
    val a = List(1, 2, 3)

    val result = a.corresponds(a){ (x, y) =>
      println((x, y))
      true
    }

    println(result)
  }
}

(1,1)
(1,2)
(1,3)
false

😱

[densh]

@xuwei-k Thanks for the minimized example!

[densh]

I was able to find the root cause for this one: front-end incorrectly resolves symbols to SSA names for methods that were transformed by tailrec phase. Instead of using the internal loop's version of this variable, it always uses the original this, never advancing to the tail. Should be easy to fix in 0.3.

[sjrd]

See https://github.com/scala-js/scala-js/blob/f21e7f5ea652fe4a186f1ecd8c0070fbb80edc80/compiler/src/main/scala/org/scalajs/core/compiler/GenJSCode.scala#L1966-L1981, and more generally references to thisLocalVarIdent in that file, notably in this case.