Do not copy annotations from a class to its synthetic companion object.

Normally, annotations applied to a class have no business being
replicated on a synthetic companion object. One exception was the
`@alpha` method, which is supposed to apply as well. Instead of
trying to identify those annotations without the symbols during
`Desugar`, we change `erasedName` to go look on the companion class
of synthetic module classes.

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -365,6 +365,7 @@ object desugar {
     val companionMods = mods
         .withFlags((mods.flags & (AccessFlags | Final)).toCommonFlags)
         .withMods(Nil)
+        .withAnnotations(Nil)
 
     var defaultGetters: List[Tree] = Nil
 

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

@@ -491,21 +491,26 @@ object SymDenotations {
 
     /** The name given in an `@alpha` annotation if one is present, `name` otherwise */
     final def erasedName(using Context): Name =
-      getAnnotation(defn.AlphaAnnot) match {
-        case Some(ann) =>
-          ann.arguments match {
-            case Literal(Constant(str: String)) :: Nil =>
-              if (isType)
-                if (is(ModuleClass))
-                  str.toTypeName.moduleClassName
-                else
-                  str.toTypeName
+      def fromAnnot(ann: Annotation): Name =
+        ann.arguments match
+          case Literal(Constant(str: String)) :: Nil =>
+            if (isType)
+              if (is(ModuleClass))
+                str.toTypeName.moduleClassName
               else
-                str.toTermName
-            case _ => name
-          }
-        case _ => name
-      }
+                str.toTypeName
+            else
+              str.toTermName
+          case _ => name
+      getAnnotation(defn.AlphaAnnot) match
+        case Some(ann) => fromAnnot(ann)
+        case _ =>
+          if isAllOf(ModuleClass | Synthetic) then
+            companionClass.getAnnotation(defn.AlphaAnnot) match
+              case Some(ann) => fromAnnot(ann)
+              case _ => name
+          else
+            name
 
     // ----- Tests -------------------------------------------------