Fix #1009: Do not forget to skolemize some types

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

@@ -102,7 +102,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
             asSeenFrom(tp.parent, pre, cls, theMap),
             tp.refinedName,
             asSeenFrom(tp.refinedInfo, pre, cls, theMap))
-        case tp: TypeAlias if theMap == null => // if theMap exists, need to do the variance calculation
+        case tp: TypeAlias if tp.variance == 1 => // if variance != 1, need to do the variance calculation
           tp.derivedTypeAlias(asSeenFrom(tp.alias, pre, cls, theMap))
         case _ =>
           (if (theMap != null) theMap else new AsSeenFromMap(pre, cls))

test/dotc/tests.scala

@@ -172,6 +172,7 @@ class tests extends CompilerTest {
   @Test def neg_validate = compileFile(negDir, "validate", xerrors = 18)
   @Test def neg_validateParsing = compileFile(negDir, "validate-parsing", xerrors = 7)
   @Test def neg_validateRefchecks = compileFile(negDir, "validate-refchecks", xerrors = 2)
+  @Test def neg_skolemize = compileFile(negDir, "skolemize", xerrors = 2)
 
   @Test def run_all = runFiles(runDir)
 

tests/neg/skolemize.scala

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