Replace SingleDenotation by SymDenotation after erasure

This is an alternative fix for scala#11303, the downside is that when
time-travelling from post-erasure to pre-erasure, we need to recompute
the SingleDenotation. This commit passes all tests except
tests/pos/t2399.scala which fails compilation from tasty. This could
probably be fixed by changing the logic in computeDenot to only run
`memberDenot` for opaque types, for other infos we can just use
`asSeenFrom` instead.

Author: smarter

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

@@ -851,7 +851,10 @@ object Denotations {
       else if valid.runId != currentPeriod.runId then
         toNewRun
       else if currentPeriod.code > valid.code then
-        goForward
+        if ctx.erasedTypes && this.isInstanceOf[NonSymSingleDenotation] then
+          symbol.denot.current
+        else
+          goForward
       else
         goBack
     end current

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

@@ -2156,11 +2156,20 @@ object Types {
       lastDenotation match {
         case lastd0: SingleDenotation =>
           val lastd = lastd0.skipRemoved
-          if (lastd.validFor.runId == ctx.runId && (checkedPeriod != Nowhere)) finish(lastd.current)
+
+          val needsRecompute =
+            prefix != NoPrefix
+            && lastd.isInstanceOf[SymDenotation]
+            && checkedPeriod.containsPhaseId(erasurePhase.next.id)
+            && !ctx.erasedTypes
+            && ctx.phase.id < checkedPeriod.firstPhaseId
+            && infoDependsOnPrefix(lastd.symbol.denot, prefix)
+
+          if (lastd.validFor.runId == ctx.runId && (checkedPeriod != Nowhere) && !needsRecompute) finish(lastd.current)
           else lastd match {
             case lastd: SymDenotation =>
-              if (stillValid(lastd) && (checkedPeriod != Nowhere)) finish(lastd.current)
-              else finish(memberDenot(lastd.initial.name, allowPrivate = false))
+              if (stillValid(lastd) && (checkedPeriod != Nowhere) && !needsRecompute) finish(lastd.current)
+              else memberDenot(lastd.initial.name, allowPrivate = needsRecompute)
             case _ =>
               fromDesignator
           }
@@ -2299,6 +2308,7 @@ object Types {
      */
     private def infoDependsOnPrefix(symd: SymDenotation, prefix: Type)(using Context): Boolean =
       symd.maybeOwner.membersNeedAsSeenFrom(prefix) && !symd.is(NonMember)
+      || prefix.isInstanceOf[Types.ThisType] && symd.is(Opaque)
 
     /** Is this a reference to a class or object member? */
     def isMemberRef(using Context): Boolean = designator match {

tests/pos/t6231c.scala

@@ -0,0 +1,9 @@
+object Bug {
+  def bar(ev: Any) = {
+    trait X(val x: Int) {
+      def qux: () => x.type = { () => println(ev); x }
+    }
+    (new X(1) {}).qux()
+  }
+}
+