Special case PreDenotation ops for NoDenotation

NoDenotation is a frequent result, so we should keep post processing
it as simple as possible.

Author: odersky

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

@@ -982,7 +982,7 @@ object Denotations {
     final def first: SingleDenotation = this
     final def last: SingleDenotation = this
 
-    final def matches(other: SingleDenotation)(using Context): Boolean =
+    def matches(other: SingleDenotation)(using Context): Boolean =
       val d = signature.matchDegree(other.signature)
 
       d match
@@ -1013,13 +1013,16 @@ object Denotations {
     end matches
 
     def mapInherited(ownDenots: PreDenotation, prevDenots: PreDenotation, pre: Type)(using Context): SingleDenotation =
-      if (hasUniqueSym && prevDenots.containsSym(symbol)) NoDenotation
-      else if (isType) filterDisjoint(ownDenots).asSeenFrom(pre)
-      else asSeenFrom(pre).filterDisjoint(ownDenots)
+      if hasUniqueSym && (symbol.is(Private) || prevDenots.containsSym(symbol)) then
+        NoDenotation
+      else if isType then
+        filterDisjoint(ownDenots).asSeenFrom(pre)
+      else
+        asSeenFrom(pre).filterDisjoint(ownDenots)
 
-    final def filterWithPredicate(p: SingleDenotation => Boolean): SingleDenotation =
+    def filterWithPredicate(p: SingleDenotation => Boolean): SingleDenotation =
       if (p(this)) this else NoDenotation
-    final def filterDisjoint(denots: PreDenotation)(using Context): SingleDenotation =
+    def filterDisjoint(denots: PreDenotation)(using Context): SingleDenotation =
       if (denots.exists && denots.matches(this)) NoDenotation else this
     def filterWithFlags(required: FlagSet, excluded: FlagSet)(using Context): SingleDenotation =
       if (required.isEmpty && excluded.isEmpty || compatibleWith(required, excluded)) this else NoDenotation

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

@@ -2313,6 +2313,13 @@ object SymDenotations {
     override def owner: Symbol = throw new AssertionError("NoDenotation.owner")
     override def computeAsSeenFrom(pre: Type)(using Context): SingleDenotation = this
     override def mapInfo(f: Type => Type)(using Context): SingleDenotation = this
+
+    override def matches(other: SingleDenotation)(using Context): Boolean = false
+    override def mapInherited(ownDenots: PreDenotation, prevDenots: PreDenotation, pre: Type)(using Context): SingleDenotation = this
+    override def filterWithPredicate(p: SingleDenotation => Boolean): SingleDenotation = this
+    override def filterDisjoint(denots: PreDenotation)(using Context): SingleDenotation = this
+    override def filterWithFlags(required: FlagSet, excluded: FlagSet)(using Context): SingleDenotation = this
+
     NoSymbol.denot = this
     validFor = Period.allInRun(NoRunId)
   }