Skolemize unstable prefixes in asSeenFrom

Skolemize unstable prefixes in asSeenFrom provided

 - the prefix appears at least once in non-variant or contra-variant position
 - we are in phase typer.

After typer, we have already established soundness, so there's no need to
do skolemization again. We can simply do the (otherwise unsound) substitution
from this-type to prefix.

1 files changed, 27 insertions, 0 deletions

diff --git a/tests/neg/i0583-skolemize.scala b/tests/neg/i0583-skolemize.scala
new file mode 100644
index 000000000..e0bb99e5d
--- /dev/null
+++ b/tests/neg/i0583-skolemize.scala
@@ -0,0 +1,27 @@
+import scala.collection.mutable.ListBuffer
+
+object Test1 {
+
+  class Box[T](x: T)
+  def box[T](x: T) = new Box[T](x)
+
+  class C[T] { def x: T = ???; def x_=(y: T): Unit = ??? }
+
+  val c: C[Int] = ???
+  val d: C[Float] = ???
+
+  val xs: List[C[_]] = List(c, d)
+
+  xs(0).x = xs(1).x
+
+}
+object Test {
+  def main(args: Array[String]): Unit = {
+    val f: ListBuffer[Int] = ListBuffer(1,2)
+    val g: ListBuffer[Double] = ListBuffer(3.0,4.0)
+    val lb: ListBuffer[ListBuffer[_]] = ListBuffer(f, g)
+    lb(0)(0) = lb(1)(0)
+    val x: Int = f(0)
+  }
+}
+