Unsoundness of variance checking with regard to constructors.

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Consider:

    class C[+A] {

      private[this] var y: A = _
      def getY: A = y

      class Inner(x: A) {
        y = x
      }
    }

    object Test {

      def main(args: Array[String]) = {
        val x = new C[String]
        val y: C[Any] = x
        val i = new y.Inner(1)
        val s: String = x.getY
        println(s)
      }
    }

This demonstrates an unsoundness in the variance checking of scalac.
Scalac excludes symbols owned by constructors from the
checking. This is unsound, as can be demonstrated by compiling the test
and observing output of the program run:

Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
at Test$.main(variances-constr.scala:17)
at Test.main(variances-constr.scala)

Dotty allows this code only under -language:Scala2 and issues a migration warning.

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Imported From: https://issues.scala-lang.org/browse/SI-9549?orig=1
Reporter: @odersky
See #8737

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@retronym said:
I guess the "ignore usages in constructors" rule was intended to allow a constructor to accept parameters of the type a covariant type parameter of a directly enclosing class.

class C[+A] { def this(a: A) = this() }

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@retronym said:
Merging with your previous reports of the same issue, #8737