Handle variance unsoundness in scalac

The included test

     pos-special/variances-constr.scala

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.

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/neg/variances-constr.scala b/tests/neg/variances-constr.scala
new file mode 100644
index 000000000..a5259c4da
--- /dev/null
+++ b/tests/neg/variances-constr.scala
@@ -0,0 +1,26 @@
+class C[+A] {
+
+  private[this] var y: A = _
+  def getY: A = y
+
+  class Inner(x: A) { // error A appears contravariantly
+    y = x
+  }
+  class Inner2[B <: A](x: B) { // error A appears contravariantly
+    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
+    val i2 = new y.Inner2(1)
+    val s2: String = x.getY
+    println(s)
+  }
+}
+