Pattern matcher unsoundness when mixing covariance and nonvariance

[scabug]

The following code will give a ClassCast exception.

package test
object patternUnsoundness extends App {

  class C[+T]

  case class D[S](_s: S) extends C[S] {
    var s: S = _s
  }

  val x = new D[String]("abc")
  val y: C[Object] = x

  y match {
    case d @ D(x) => d.s = new Integer(1)
  }

  val z: String = x.s // ClassCast exception
}

Unfortunately, it looks like a difficult fix, because the obvious fix (require that the type parameter S is determined uniquely) would break all :: patterns, because List is declared to be covariant whereas :: is declared to be non-variant. Of course :: could and should have been declared covariant so there's no unsoundness problem for Lists themselves.

(JZ: edited to s/Some/::/)

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7952?orig=1
Reporter: @odersky

[scabug]

@paulp said:
By 'Some' I take it you mean :: . It won't be as easy as making :: covariant, because :: is built on vars which are set from elsewhere (I assume this is why it's invariant.)

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@paulp said:
Also, I think it will be an interesting challenge to do in a source-compatible way. There's no "deprecatedInvariance" AFAIK, but independently of that I'm not sure it can be done.

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Stephen Compall (s11001001) said:
In 2.11.0-M7 with -Xstrict-inference:

7952.scala:13: type mismatch;
 found   : Integer
 required: S
    case d @ D(x) => d.s = new Integer(1)
                           ^

Changing new Integer(1) to d.s allows it to compile.

[SethTisue]

Scala 3.3.0:

[error] ./S.scala:14:28
[error] Found:    Integer
[error] Required: S$1
[error] 
[error] where:    S$1 is a type locally defined in object patternUnsoundness with bounds <: Object
[error] 
[error] Note that implicit conversions were not tried because the result of an implicit conversion
[error] must be more specific than S$1
[error]     case d @ D(x) => d.s = new Integer(1)
[error]                            ^^^^^^^^^^^^^^