Don't transform infos of classes that get eliminated at erasure

Some classes get mapped to other classes at erasure. There's no need to transform
their info.

Fixes #19506

Author: odersky

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

@@ -814,7 +814,10 @@ 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: ClassInfo if defn.specialErasure.contains(tp1.cls) =>
+        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]
+}