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.

Author: sjrd

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

@@ -491,7 +491,10 @@ 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 {
+      val alphaAnnot =
+        if isAllOf(ModuleClass | Synthetic) then companionClass.getAnnotation(defn.AlphaAnnot)
+        else getAnnotation(defn.AlphaAnnot)
+      alphaAnnot match {
         case Some(ann) =>
           ann.arguments match {
             case Literal(Constant(str: String)) :: Nil =>