Tighten flag checking rules on completion

Count a denotation as completing if it has the `Touched`
flag set. This assumption follows from the definition
of the flag `Touched` provided in its javadoc comment
in Flags.scala. Also, this commit unconditionally unsets
this flag for all denotations once they are completed. The
rationale is, again, to comply with the definition of
`Touched`.

Author: anatoliykmetyuk

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

@@ -173,7 +173,7 @@ object SymDenotations {
     private def setMyFlags(flags: FlagSet) =
       lazy val immutableFlags = if wasLoaded then AfterLoadFlags else FromStartFlags
       lazy val changedImmutableFlags = (myFlags ^ flags) & immutableFlags
-      if isCompleting && !isLoading then assert(changedImmutableFlags.isEmpty,
+      if myFlags.is(Touched) && !isLoading then assert(changedImmutableFlags.isEmpty,
         s"Illegal mutation of flags ${changedImmutableFlags.flagsString} on completion of $this")
       myFlags = flags
 
@@ -254,7 +254,7 @@ object SymDenotations {
         indent += 1
 
         if (myFlags.is(Touched)) throw CyclicReference(this)
-        setMyFlags(myFlags | Touched)
+        myFlags |= Touched
 
         // completions.println(s"completing ${this.debugString}")
         try completer.complete(this)(ctx.withPhase(validFor.firstPhaseId))
@@ -270,9 +270,10 @@ object SymDenotations {
       }
       else {
         if (myFlags.is(Touched)) throw CyclicReference(this)
-        setMyFlags(myFlags | Touched)
+        myFlags |= Touched
         completer.complete(this)(ctx.withPhase(validFor.firstPhaseId))
       }
+      myFlags &~= Touched
 
     protected[dotc] def info_=(tp: Type): Unit = {
       /* // DEBUG