Make eraseInfo work for classes with EmptyScopes (#19550)

Fixes #19530

Author: noti0na1

Diff for: compiler/src/dotty/tools/dotc/core/Scopes.scala

@@ -159,19 +159,28 @@ object Scopes {
     }
 
     /** The scope that keeps only those symbols from this scope that match the
-     *  given predicates. If all symbols match, returns the scope itself, otherwise
-     *  a copy with the matching symbols.
+     *  given predicates, renamed with the given rename function.
+     *  If renaming is not needed for a symbol, the rename function should return `null`.
+     *  If all symbols match and none are renamed, returns the scope itself, otherwise
+     *  a copy with the matching and renamed symbols.
      */
-    final def filteredScope(p: Symbol => Boolean)(using Context): Scope = {
+    final def filteredScope(
+        keep: Symbol => Boolean,
+        rename: Symbol => Name | Null = _ => null)(using Context): Scope =
       var result: MutableScope | Null = null
-      for (sym <- iterator)
-        if (!p(sym)) {
-          if (result == null) result = cloneScope
+      for sym <- iterator do
+        def drop() =
+          if result == null then result = cloneScope
           result.nn.unlink(sym)
-        }
+        if keep(sym) then
+          val newName = rename(sym)
+          if newName != null then
+            drop()
+            result.nn.enter(newName, sym)
+        else
+          drop()
       // TODO: improve flow typing to handle this case
-      if (result == null) this else result.uncheckedNN
-    }
+      if result == null then this else result.uncheckedNN
 
     def implicitDecls(using Context): List[TermRef] = Nil
 

Diff for: compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -725,14 +725,14 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
                 tr1 :: trs1.filterNot(_.isAnyRef)
               case nil => nil
             }
-          var erasedDecls = decls.filteredScope(sym => !sym.isType || sym.isClass).openForMutations
-          for dcl <- erasedDecls.iterator do
-            if dcl.lastKnownDenotation.unforcedAnnotation(defn.TargetNameAnnot).isDefined
-               && dcl.targetName != dcl.name
-            then
-              if erasedDecls eq decls then erasedDecls = erasedDecls.cloneScope
-              erasedDecls.unlink(dcl)
-              erasedDecls.enter(dcl.targetName, dcl)
+          val erasedDecls = decls.filteredScope(
+              keep = sym => !sym.isType || sym.isClass,
+              rename = sym =>
+                if sym.lastKnownDenotation.unforcedAnnotation(defn.TargetNameAnnot).isDefined
+                  && sym.targetName != sym.name
+                then sym.targetName
+                else null
+            )
           val selfType1 = if cls.is(Module) then cls.sourceModule.termRef else NoType
           tp.derivedClassInfo(NoPrefix, erasedParents, erasedDecls, selfType1)
             // can't replace selftype by NoType because this would lose the sourceModule link
@@ -814,7 +814,8 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
         eraseResult(tp1.resultType) match
           case rt: MethodType => rt
           case rt => MethodType(Nil, Nil, rt)
-      case tp1 => this(tp1)
+      case tp1 =>
+        this(tp1)
 
   private def eraseDerivedValueClass(tp: Type)(using Context): Type = {
     val cls = tp.classSymbol.asClass

Diff for: tests/pos/i19530.scala

@@ -0,0 +1,3 @@
+object A {
+  def x = classOf[scala.Singleton]
+}