Type parameter variance not checked for coherence: ClassCastException

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This exploits a hole similar to the examples in #7278, but all of those use type members. Maybe it seems more serious if it can easily arise with the more frequently used type parameters.

class Covariant[+A](val x: A)
trait Invariant[A] extends Covariant[A]
object Foo extends Covariant[Any]("abc") with Invariant[Int] { def apply(x: Int): Int = x * 7 }
object Bar { def main(args: Array[String]) { Foo(Foo.x) } }
// java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
//   at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:105)
//   at Bar$.main(a.scala:4)
//   at Bar.main(a.scala)

The base type sequence of Foo.type is

Foo.type
Invariant[Int]
Covariant[Int] with Covariant[Any]
Object
Any

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8241?orig=1
Reporter: @paulp
See #7278

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@paulp said:
Damn, I guess this is #7886.