Avoid spurious StaleSymbol error in <refinement> members

Refinement classes and their members could give spurious stale symbol errors if the
symbol is loaded in a different run than the classfile containing it. The problem
is that refinement classes do not form part of the scope of their owners. The fix
assumes that refinement classes are always "stillValid".

Author: odersky

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

@@ -40,7 +40,7 @@ trait SymDenotations { this: Context =>
   }
 
   def stillValid(denot: SymDenotation): Boolean =
-    if (denot is ValidForever) true
+    if (denot.is(ValidForever) || denot.isRefinementClass) true
     else {
       val initial = denot.initial
       if (initial ne denot)
@@ -49,6 +49,7 @@ trait SymDenotations { this: Context =>
         val owner = denot.owner.denot
         stillValid(owner) && (
           !owner.isClass
+          || owner.isRefinementClass
           || (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol)
           || denot.isSelfSym)
       } catch {

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

@@ -408,7 +408,7 @@ object Symbols {
     /** Subclass tests and casts */
     final def isTerm(implicit ctx: Context): Boolean =
       (if(isDefinedInCurrentRun) lastDenot else denot).isTerm
-    
+
     final def isType(implicit ctx: Context): Boolean =
       (if(isDefinedInCurrentRun) lastDenot else denot).isType