Scala 3 Java interop: method on base class called instead of method on derived class

[christian-mibex]

Not sure if it's a bug or to be expected. We ran into runtime issues after upgrading to Scala 3.

//> using scala 3

//> using repository "https://maven.atlassian.com/repository/public"

//> using lib "com.atlassian.webhooks:atlassian-webhooks-api:6.1.6"
//> using lib "com.atlassian.bitbucket.server:bitbucket-webhooks-api:7.14.0"

import com.atlassian.bitbucket.webhook.WebhookScopeAdapter
import com.atlassian.webhooks._

val createRequest = WebhookCreateRequest.builder
    .name("name")
    .url("url")
    .build()

println("Runs with scala 2")

Basically the class hierarchy of WebhookCreateRequest looks like (simplified):

public class CreateRequest extends AbstractRequest {
    
    private CreateRequest(Builder builder) { super(builder); }

    public static Builder builder() { return new Builder();}

    public static class Builder extends AbstractRequest.AbstractBuilder<Builder> {

        protected Builder self() { return this; }

        public build() { return new CreateRequest(this);}

    }
}

public abstract class AbstractRequest {

    abstract static class AbstractBuilder<B extends AbstractBuilder<B>> {
        private String name;
        private String url;

        AbstractBuilder() {}
        protected abstract B self();


        public B name(String value) {
            this.name = value;
            return this.self();
        }

        public B url(String value) {
            this.url = value;
            return this.self();
        }

    }
}

Now the inner class AbstractBuilder is not public, and when the Scala compiler calls the method on it instead of CreateRequest an IllegalAccessException is called.

The bytecode output of Scala 2/ Java compilation

//       10: invokestatic  #30                 // Method .../WebhookCreateRequest.builder:()L.../WebhookCreateRequest$Builder;
//       13: ldc           #32                 // String name
//       15: invokevirtual #35                 // Method .../WebhookCreateRequest$Builder.name:(Ljava/lang/String;)L.../AbstractWebhookRequest$AbstractBuilder;
//       18: checkcast     #12                 // class .../WebhookCreateRequest$Builder
//       21: ldc           #37                 // String url
//       23: invokevirtual #39                 // Method .../WebhookCreateRequest$Builder.url:(Ljava/lang/String;)L.../AbstractWebhookRequest$AbstractBuilder;

Compared to Scala 3 (issue on last line)

//       10: invokestatic  #34                 // Method .../WebhookCreateRequest.builder:()L.../WebhookCreateRequest$Builder;
//       13: ldc           #36                 // String name
//       15: invokevirtual #39                 // Method .../WebhookCreateRequest$Builder.name:(Ljava/lang/String;)L.../AbstractWebhookRequest$AbstractBuilder;
//       18: ldc           #41                 // String url
//       20: invokevirtual #43                 // Method .../AbstractWebhookRequest$AbstractBuilder.url:(Ljava/lang/String;)L.../AbstractWebhookRequest$AbstractBuilder;

Is there a reason the second and all further chained builder calls (here the url()) call) are compiled to call the base class method?