How can I override a class' method in a Scala 3 compiler plugin?

I want to auto-generate an overriden method in a compiler plugin (Scala 3) for a class like:

trait SpecialSerialize {
  def toJson(sb: StringBuilder, c:SJConfig): Unit = {println("wrong")}
}

case class Person(name:String, age:Int) extends SpecialSerialize

The plugin would generate:

case class Person(name:String, age:Int) extends SpecialSerialize {
  override def toJson(sb: StringBuilder, c:SJConfig): Unit = ... // code here
}

I have a phase:

class ReflectionWorkerPhase extends PluginPhase {
  import tpd._

  val phaseName = "reflectionWorker"

  override val runsAfter = Set(Pickler.name)

  override def transformTypeDef(tree: TypeDef)(implicit ctx: Context): Tree =
    if tree.isClassDef && !tree.rhs.symbol.isStatic then  // only look at classes
      
      // 0. Get a FreshContext so we can set the tree to this tree. (for '{} later)
      implicit val fresh = ctx.fresh
      fresh.setTree(tree)
      QuotesCache.init(fresh)
      implicit val quotes:Quotes = QuotesImpl.apply()  // picks up fresh
      import quotes.reflect.*

        // 1. Set up method symbol, define parameters and return type
        val toJsonSymbol = Symbol.newMethod(
          Symbol.spliceOwner,
          "toJson",
          MethodType(
            List("sb","config"))( // parameter list
              _ => List( // types of the parameters
                TypeRepr.of[StringBuilder],
                TypeRepr.of[SJConfig],
                ), 
              _ => TypeRepr.typeConstructorOf(classOf[Unit])  // return type
            ),
            Flags.Override, // Note override here
            Symbol.noSymbol
          )

        // 2. Get our class' Symbol for ownership reassignment
        val classDef = tree.asInstanceOf[ClassDef]
        val classSymbol = classDef.symbol

        // 3. Define our method definition (DefDef) using our method symbol defined above
        val toJsonMethodDef = DefDef(
          toJsonSymbol, 
          {
            case List(List(sb: Term, config: Term)) => 
              given Quotes = toJsonSymbol.asQuotes
              Some({
                // Multiple quotes here intentional...
                // 开发者_StackOverflow社区Real code will generate a list of quoted statements
                quoted.Expr.ofList(List(
                  '{ println("Hello") }, 
                  '{ println("World") }
                ))
              }.asTerm.changeOwner(toJsonSymbol))
          }
        ).changeOwner(classSymbol)

        // 4. Add toJsonMethodDef to tree and return
        val cd = ClassDef.copy(classDef)(
          name = classDef.name,
          constr = classDef.constructor,
          parents = classDef.parents,
          selfOpt = classDef.self,
          body = toJsonMethodDef +: classDef.body
        )
        cd.asInstanceOf[dotty.tools.dotc.ast.tpd.Tree] 
      else 
        tree
}

When I use the plugin on a sample Person class, I get this error on compile:

Exception in thread "sbt-bg-threads-1" java.lang.ClassFormatError: Duplicate method name "toJson" with signature "(Lscala.collection.mutable.StringBuilder;Lco.blocke.scala_reflection.SJConfig;)V" in class file com/foo/Person
    at java.base/java.lang.ClassLoader.defineClass1(Native Method)
    at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1013)
    at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
    at java.base/java.net.URLClassLoader.defineClass(URLClassLoader.java:524)
    at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:427)
    at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:421)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:712)
...

So something in the other compile phases failed to recognize my generated method as being an override and tried to copy in the "master" method from the trait, and of course the JVM lost its mind at runtime, finding 2 copies of toJson with the same signature.

How can I fix this so my generated method is recognized as a valid override such that a 2nd toJson method won't be generated in a later phase?