Disallow use of toDenot and toClassDenot conversions in Symbols

We can't allow them in symbols since they would not kick in when
Symbol and ClassSymbol are made opaque type aliases, because inside
Symbols we know that these types are aliases of Denotations so no
conversion would be applied then and we would possibly end up with
a denotation at the wrong validity period.

Author: odersky

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

@@ -1600,7 +1600,7 @@ object SymDenotations {
 
     // ----- Sanity checks and debugging */
 
-    def debugString: String = toString + "#" + symbol.id // !!! DEBUG
+    def debugString: String = toString + "#" + common.id // !!! DEBUG
 
     def hasSkolems(tp: Type): Boolean = tp match {
       case tp: SkolemType => true

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

@@ -555,6 +555,12 @@ object Symbols {
   /** Makes all class denotation operations available on class symbols */
   implicit def toClassDenot(cls: ClassSymbol)(using Context): ClassDenotation = cls.classDenot
 
+  /** Blocks use of `toDenot` conversion in Symbols itself */
+  private implicit def toDenotALT(sym: Symbol)(using Context): SymDenotation = sym.denot
+
+  /** Blocks use of `toClassDenot` conversion in Symbols itself */
+  private implicit def toClassDenotALT(cls: ClassSymbol)(using Context): ClassDenotation = cls.classDenot
+
   /** The Definitions object */
   def defn(using Context): Definitions = ctx.definitions