Make eraseInfo work for classes with empty scopes

Fixes scala#19530

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

@@ -175,7 +175,7 @@ object Scopes {
 
     def implicitDecls(using Context): List[TermRef] = Nil
 
-    def openForMutations: MutableScope = unsupported("openForMutations")
+    def openForMutations: MutableScope = unsupported(s"openForMutations $this")
 
     final def toText(printer: Printer): Text = printer.toText(this)
 

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

@@ -725,14 +725,17 @@ 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)
+          var erasedDecls = decls.filteredScope(sym => !sym.isType || sym.isClass)
+          if !erasedDecls.isEmpty then
+            var eds = erasedDecls.openForMutations
+            for dcl <- eds.iterator do
+              if dcl.lastKnownDenotation.unforcedAnnotation(defn.TargetNameAnnot).isDefined
+                 && dcl.targetName != dcl.name
+              then
+                if eds eq decls then eds = eds.cloneScope
+                eds.unlink(dcl)
+                eds.enter(dcl.targetName, dcl)
+            erasedDecls = eds
           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 +817,8 @@ 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

tests/neg/i19506.scala

@@ -1,4 +1,4 @@
-//> using options "-source:3.4-migration",
+//> using options -source 3.4-migration
 
 trait Reader[T]
 def read[T: Reader](s: String, trace: Boolean = false): T = ???

tests/pos/i19530.scala

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