emit unchecked warning when outer is not checked in pattern match

[scabug]

Maybe 3 different bugs in here? You decide.

1. No unchecked warnings.
2. See contents of Test3.
3. ClassCastException for Test1.

trait Outer[T] {
  sealed trait Inner
  case class C1(val x: T) extends Inner
  case class C2(val x: T) extends Inner
}

object Test1 extends Outer[Int] {
  def f(x: Any) = println(x match {
    case C1(x) => true
    case _     => false
  })

  def main(args: Array[String]): Unit = {
    f(Test1.C1(5))
    f(Test2.C1("abc"))
  }
}

object Test2 extends Outer[String] { }

object Test3 {
  type SomeInner = Outer[X]#Inner forSome { type X }

  // Clearly Test1.C1(5) is an instance of SomeInner, as seen here:
  var x: SomeInner = _
  x = Test1.C1(5)

  // And yet,
  //
  // def f0(x: SomeInner) = x match { case Test1.C1(_) => }
  //
  // ./a.scala:35: error: constructor cannot be instantiated to expected type;
  //  found   : Test1.C1
  //  required: Outer[X]#Inner where type X
  //   def f0(x: SomeInner) = x match { case Test1.C1(_) => }
  //
  // Whereas this is no problem:
  def f1(x: Any) = x match { case Test1.C1(_) => }
}

/**

% rcscalac -unchecked ./a.scala
% rcscala Test1
true
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer

**/

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6583?orig=1
Reporter: @paulp
Affected Versions: 2.10.0

[scabug]

@adriaanm said:
I'd say two bugs:

• we should either emit an unchecked warning or do the outer test (I'm reusing logic from the old pattern matcher to decide that: needsOuterTest in Types)
• existential types should be approximated (to wildcards) before checking type conformance in adaptConstrPattern

[scabug]

@adriaanm said:
in 2.10.x we cannot do better than emitting unchecked warnings, and we should decide for 2.11 whether to change the spec or do better outer tests (i'll open a separate ticket for that)

[scabug]

@adriaanm said:
on the fence about changing warning behaviour in a minor release

[scabug]

Bernhard Messerer (messi) said:
Has this been forgotten? This is really, really awkward now that no warnings are emitted anymore and just adds to the things one needs to "just know" about Scala, adding to complexity. While the "best" solution would be for these things to "just work", a warning would be better than nothing.

[SethTisue]

fyi, the largest ticket/discussion in this general area is #4440