pattern matcher still allows unsound case class reconstruction

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Note that without the second parameter to Unravel, this is correctly failed in 2.10 (but not in 2.9.)

a.scala:11: error: type mismatch;
 found   : Contra[Nothing]
 required: Contra[Any]
    case Unravel(m) => g(m)
                         ^
one error found

But with it:

trait Covariant[+A]
trait Contra[-A] { def accept(p: A): Unit }
trait Invariant[A] extends Covariant[A] with Contra[A]

case class Unravel[A](m: Contra[A], msg: A)

object Test extends Covariant[Any] {
  def g(m: Contra[Any]): Unit = m accept 5
  def f(x: Any): Unit = x match {
    case Unravel(m, msg) => g(m)
    case _               =>
  }
  def main(args: Array[String]) {
    f(Unravel[String](new Contra[String] { def accept(x: String) = x.length }, ""))
  }
}
// java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
//   at Test$$anon$1.accept(a.scala:18)
//   at Test$.g(a.scala:13)
//   at Test$.f(a.scala:15)
//   at Test$.main(a.scala:18)
//   at Test.main(a.scala)

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Imported From: https://issues.scala-lang.org/browse/SI-7886?orig=1
Reporter: @paulp

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@paulp said:
scala/scala#3005

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@retronym said:
I'm assuming we forgot to close this ticket after merging. Please open a new ticket if there were residuals I've missed.

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@retronym said (edited on Nov 29, 2013 9:15:34 AM UTC):
Reopening. Special casing Any doesn't seem to be the right course of action.

git diff /Users/jason/code/scala/test/files/neg/t7886.scala
diff --git a/test/files/neg/t7886.scala b/test/files/neg/t7886.scala
index 55d80a0..1db8be9 100644
--- a/test/files/neg/t7886.scala
+++ b/test/files/neg/t7886.scala
@@ -2,11 +2,12 @@ trait Covariant[+A]
 trait Contra[-A] { def accept(p: A): Unit }
 trait Invariant[A] extends Covariant[A] with Contra[A]

-case class Unravel[A](m: Contra[A], msg: A)
+trait T
+case class Unravel[A](m: Contra[A], msg: A) extends T

 object Test extends Covariant[Any] {
   def g(m: Contra[Any]): Unit = m accept 5
-  def f(x: Any): Unit = x match {
+  def f(x: T): Unit = x match {
     case Unravel(m, msg) => g(m)
     case _               =>
   }
ticket/5900-2 ~/code/scala scalac-hash v2.11.0-M7 test/files/neg/t7886.scala && scala-hash v2.11.0-M7 Test
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
	at Test$$anon$1.accept(t7886.scala:15)

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@retronym said:
Here's the best way I've found to look at this:

https://gist.github.com/retronym/7704153

In that example, I've added the parent type T to circumvent the bandaid fix for pt=Any

The pattern matching inference unsoundly casts Unravel[_] to Unravel[Any].

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@retronym said:
The residual test is added in scala/scala#3514 which also reverts the fix for the originally reported case, as it was responsible for the regression noted in the comments of #5900

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@retronym said:
Another example from #8563.

object Test extends App {
  sealed trait Node[+A]
  case class L[C,D](f: C => D) extends Node[C => D]
 
  def test[A,B](n: Node[A => B]): A => B = n match {
    case l: L[c,d] => l.f
  }
 
  println {
    test(new L[Int,Int](identity) with Node[Nothing]: Node[Int => String])(3): String // java.lang.Integer cannot be cast to java.lang.String
  }
}

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@retronym said:
Maybe that's a different bug, as it also is unsound with a manually expaneded unapply.

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@adriaanm said:
See also #8241

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@retronym said:
Another example from Noel Welsh + @djspiewak

  {
    sealed trait Foo[A] {
      def run: A =
        this match {
          case Map(elt, f) => f("Can I have some type safety please?x")
        }
    }
    final case class Map[A,B](elt: A, f: A => B) extends Foo[B]
    // expand the case class and a type error is issued, as per examples above.
    // final class Map[A,B](val elt: A, val f: A => B) extends Foo[B]
    // object Map {
    //  def unapply[A, B](m: Map[A, B]): Option[(A, A => B)] = Some((m.elt, m.f))
    //}
  }