Fixed problem with ensureSingleton

Need to also ensure that the singleton is stable. This makes
compound.scala pass.

Author: odersky

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

@@ -20,14 +20,14 @@ trait Skolemization {
 
   protected var skolemsOutstanding = false
 
-  def ensureSingleton(tp: Type): SingletonType = tp.stripTypeVar match {
-    case tp: SingletonType => 
+  def ensureStableSingleton(tp: Type): SingletonType = tp.stripTypeVar match {
+    case tp: SingletonType if tp.isStable => 
       tp
     case tp: ValueType => 
       skolemsOutstanding = true
       SkolemType(tp)
     case tp: TypeProxy => 
-      ensureSingleton(tp.underlying)
+      ensureStableSingleton(tp.underlying)
   }
 
   /** Approximate a type `tp` with a type that does not contain skolem types.

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

@@ -527,7 +527,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling wi
     val saved = skolemsOutstanding
     try {
       val rebindNeeded = tp2.refinementRefersToThis
-      val base = if (rebindNeeded) ensureSingleton(tp1) else tp1
+      val base = if (rebindNeeded) ensureStableSingleton(tp1) else tp1
       val rinfo2 = if (rebindNeeded) tp2.refinedInfo.substSkolem(tp2, base) else tp2.refinedInfo
       def qualifies(m: SingleDenotation) = isSubType(m.info, rinfo2)
       def memberMatches(mbr: Denotation): Boolean = mbr match { // inlined hasAltWith for performance

tests/pending/pos/compound.scala renamed to tests/pos/compound.scala

@@ -1,5 +1,3 @@
-// There's still a problem with var's here, presumably because they are not paths.
-// Needs some more investigation.
 abstract class A { type T }
 
 abstract class B { val xz: Any }
@@ -11,6 +9,6 @@ abstract class Test {
 }
 
 abstract class Test2 {
-  val yy: A with B { type T; val xz: T } = null;
+  var yy: A with B { type T; val xz: T } = null;
   val xx: A with B { type T; val xz: T } = yy
 }