Don't bring symbols forward to non-existing phases.

This was observed in TabCompleterTests. The problem in general is that
we might see symbols in runs that have fewer phases than the phase at which the
symbol was created. Previously, we updated the symbol anyway. But this
is problematic since it means the symbol has a validity period that
does not correspond to a phase in the current run.

We now treat those symbols as stale without a way to recover instead.

Author: odersky

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

@@ -343,6 +343,8 @@ object Contexts {
     final def runId = period.runId
     final def phaseId = period.phaseId
 
+    final def lastPhaseId = base.phases.length - 1
+
     /** Does current phase use an erased types interpretation? */
     final def erasedTypes = phase.erasedTypes
 

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

@@ -716,7 +716,10 @@ object Denotations {
       this match {
         case symd: SymDenotation =>
           if (stillValid(symd)) return updateValidity()
-          if (acceptStale(symd)) return symd.currentSymbol.denot.orElse(symd).updateValidity()
+          if acceptStale(symd) && symd.initial.validFor.firstPhaseId <= ctx.lastPhaseId then
+            // New run might have fewer phases than old, so symbol might no longer be
+            // visible at all. TabCompleteTests have examples where this happens.
+            return symd.currentSymbol.denot.orElse(symd).updateValidity()
         case _ =>
       }
       if (!symbol.exists) return updateValidity()
@@ -790,7 +793,7 @@ object Denotations {
           //println(s"might need new denot for $cur, valid for ${cur.validFor} at $currentPeriod")
           // not found, cur points to highest existing variant
           val nextTransformerId = ctx.base.nextDenotTransformerId(cur.validFor.lastPhaseId)
-          if (currentPeriod.lastPhaseId <= nextTransformerId)
+          if currentPeriod.lastPhaseId <= nextTransformerId then
             cur.validFor = Period(currentPeriod.runId, cur.validFor.firstPhaseId, nextTransformerId)
           else
             var startPid = nextTransformerId + 1

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

@@ -2351,8 +2351,11 @@ object SymDenotations {
     if (denot.isOneOf(ValidForeverFlags) || denot.isRefinementClass || denot.isImport) true
     else {
       val initial = denot.initial
-      val firstPhaseId = initial.validFor.firstPhaseId.max(typerPhase.id)
-      if ((initial ne denot) || ctx.phaseId != firstPhaseId)
+      val firstPhaseId =
+        initial.validFor.firstPhaseId.max(typerPhase.id)
+      if firstPhaseId > ctx.lastPhaseId then
+        false
+      else if (initial ne denot) || ctx.phaseId != firstPhaseId then
         atPhase(firstPhaseId)(stillValidInOwner(initial))
       else
         stillValidInOwner(denot)