Add getDeclaringClass implementation to enums

Author: anatoliykmetyuk

compiler/src/dotty/tools/dotc/transform/SyntheticMethods.scala

@@ -110,7 +110,7 @@ class SyntheticMethods(thisPhase: DenotTransformer) {
         case nme.name => _ => Literal(Constant("Hello World"))
         case nme.ordinal => _ => Literal(Constant(0))
         case nme.compareTo => vrefss => compareToBody(vrefss.head.head)
-        case nme.getDeclaringClass => _ => Literal(Constant(null))
+        case nme.getDeclaringClass => _ => getDeclaringClassBody
       }
       ctx.log(s"adding $synthetic to $clazz at ${ctx.phase}")
       DefDef(synthetic, syntheticRHS(ctx.withOwner(synthetic))).withSpan(ctx.owner.span.focus)
@@ -138,6 +138,21 @@ class SyntheticMethods(thisPhase: DenotTransformer) {
       Apply(thisOrdinalCompareTo, otherOrdinalBoxed :: Nil)
     }
 
+    /** An enum case `X` in
+     *
+     * ```
+     * enum Foo { case X }
+     * ```
+     *
+     * gets the `getDeclaringClass` method:
+     *
+     * ```
+     * def getDeclaringClass: Class[Foo] = Predef.classOf[Foo]
+     * ```
+     */
+    def getDeclaringClassBody: Tree =
+      Ident(defn.Predef_classOfR).appliedToType(clazz.info.parents.head)
+
     /** The class
      *
      *  ```