Fix scala#1009: Do not forget to skolemize some illegal prefixes

Author: smarter

src/dotty/tools/dotc/core/TypeOps.scala

@@ -45,8 +45,10 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
    *  The scheme is efficient in particular because we expect that unsafe situations are rare;
    *  most compiles would contain none, so no scanning would be necessary.
    */
-  final def asSeenFrom(tp: Type, pre: Type, cls: Symbol): Type =
-    asSeenFrom(tp, pre, cls, null)
+  final def asSeenFrom(tp: Type, pre: Type, cls: Symbol): Type = {
+    val m = if (isLegalPrefix(pre)) null else new AsSeenFromMap(pre, cls)
+    asSeenFrom(tp, pre, cls, m)
+  }
 
   /** Helper method, taking a map argument which is instantiated only for more
    *  complicated cases of asSeenFrom.

tests/neg/skolemize.scala

@@ -0,0 +1,13 @@
+class Contra[-T]
+
+class Foo {
+  val foo: Contra[this.type] = new Contra[this.type]
+}
+object Test {
+  def test: Unit = {
+    val e1 = new Foo
+    val f1: Contra[Foo] = e1.foo // error
+    var e2 = new Foo
+    val f2: Contra[Foo] = e2.foo // error
+  }
+}