Test stillValid at initial phase, revive -twice

stillValid should always be tested at the phase where the initial
denotation is defined. If we do not do this we get false stale symbol errors.
Here is a scenario:

To `bringForward` `Predef$$ArrowAsscoc`, we check whether its initial denotation
has an owner which contains the denotation as one of its members. But if we do this
at a later phase (e.g. GenBCode), the owner of the initial denotation is Predef, but
Predef no longer contains ArraoAssoc - it lost the member at flatten.

With this fix we can run everything with -twice, except dotc_core. I left a comment what
goes wrong there.

1 files changed, 13 insertions, 11 deletions

diff --git a/src/dotty/tools/dotc/core/SymDenotations.scala b/src/dotty/tools/dotc/core/SymDenotations.scala
index 990c6e317..e572f129b 100644
--- a/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -41,15 +41,19 @@ trait SymDenotations { this: Context =>
 
   def stillValid(denot: SymDenotation): Boolean =
     if (denot is ValidForever) true
-    else try {
-      val owner = denot.owner.denot
-      stillValid(owner) && (
-           !owner.isClass
-        || (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol)
-        || denot.isSelfSym
-        )
-    } catch {
-      case ex: StaleSymbol => false
+    else {
+      val initial = denot.initial
+      if (initial ne denot) 
+        ctx.withPhase(initial.validFor.firstPhaseId).stillValid(initial.asSymDenotation)
+      else try {
+        val owner = denot.owner.denot
+        stillValid(owner) && (
+          !owner.isClass
+          || (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol)
+          || denot.isSelfSym)
+      } catch {
+        case ex: StaleSymbol => false
+      }
     }
 }
 
@@ -72,8 +76,6 @@ object SymDenotations {
 
     /** Debug only
     override def validFor_=(p: Period) = {
-      if (name == "Trees".toTermName && p.runId == 3)
-        assert(symbol.id != 6161)
       super.validFor_=(p)
     }
     */