Refinements to realizability

 - Unstable types can still be realizable (i.e. if their underlying
   type is concrete with no bad bounds).
 - SkolemTypes are unrealizable if there underlying type is.

Author: odersky

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

@@ -66,22 +66,33 @@ class CheckRealizable(implicit ctx: Context) {
    */
   private def isLateInitialized(sym: Symbol) = sym.is(LateInitialized, butNot = Module)
 
+  def underlyingRealizability(tp: Type): Realizability =
+    realizability(tp).mapError(r => new ProblemInUnderlying(tp, r))
+
   /** The realizability status of given type `tp`*/
   def realizability(tp: Type): Realizability = tp.dealias match {
     case tp: TermRef =>
       val sym = tp.symbol
       if (sym.is(Stable)) realizability(tp.prefix)
       else {
         val r =
-          if (!sym.isStable) NotStable
-          else if (!isLateInitialized(sym)) Realizable
-          else if (!sym.isEffectivelyFinal) new NotFinal(sym)
-          else realizability(tp.info).mapError(r => new ProblemInUnderlying(tp.info, r))
+          if (sym.isStable && !isLateInitialized(sym))
+            // it's realizable because we know that the symbol has been created
+            Realizable
+          else if (!sym.isEffectivelyFinal)
+            // it's potentially not realizable since it might be overridden with a member of nonrealizable type
+            new NotFinal(sym)
+          else
+            // otherwise we need to look at the info to determine realizability
+            // roughly: it's realizable if the info does not have bad bounds
+            underlyingRealizability(tp.info)
         r andAlso {
           sym.setFlag(Stable)
           realizability(tp.prefix)
         }
       }
+    case tp: SkolemType =>
+      underlyingRealizability(tp.info)
     case _: SingletonType | NoPrefix =>
       Realizable
     case tp =>