Address remaining review comments

Author: odersky

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

@@ -138,7 +138,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
    *  everytime we compare components of the previous pair of types.
    *  This type is used for capture conversion in `isSubArgs`.
    */
-  private [this] var leftRoot: Type = null
+  private [this] var leftRoot: Type = _
 
   protected def isSubType(tp1: Type, tp2: Type, a: ApproxState): Boolean = {
     val savedApprox = approx

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3623,7 +3623,7 @@ object Types {
    *  and fill in their infos with types that refers to the skolem types recursively.
    *  This is used in `captureWildcards` in `Typer`.
    */
-  case class SkolemType(var info: Type) extends UncachedProxyType with ValueType with SingletonType {
+  case class SkolemType(private[dotc] var info: Type) extends UncachedProxyType with ValueType with SingletonType {
     override def underlying(implicit ctx: Context): Type = info
     def derivedSkolemType(info: Type)(implicit ctx: Context): SkolemType =
       if (info eq this.info) this else SkolemType(info)

tests/pos/capture.scala

@@ -14,7 +14,9 @@
 //    Convert `t` to `t.cast[C[$j.CAP]]` where `$j` is a skolem of type `TypeBox[Nothing, Any`]
 //    (or more generally, `TypeBox[L, U]`) wgere `L` and `U` are the bounds of the wildcard).
 //
-// skolemization-of-ewach-param is more robust since it is stable under widening.
+// skolemization-of-each-param is more robust since it is stable under widening.
+// By contrast, skolemization-as-a-whole risks losing capturing ability if the skolem
+// type is widened in some context. This leads to the difference in behavior shown below.
 class Test {
 
   abstract class Liftable[T]
@@ -31,5 +33,5 @@ class Test {
 
   runtimeClass.toExpr(ClassIsLiftable) // OK for skolemization-as-a-whole and skolemization-of-each-param
 
-  runtimeClass.toExpr // only works with skolemization-of-each-param
+  runtimeClass.toExpr // only works with skolemization-of-each-param, also works in scalac.
 }