[BUG] @Superbuilder causes compile error when used from scala code due to missing type for return type.

[kurellajunior]

Describe the bug
A class (hierarchy) annotated with @SuperBuilder cannot be instantiated with the builder pattern if any of the super building methods are used from Scala code. (The class (hierarchy in question is built in a Java module before in the same maven project)

To Reproduce
In a java module:

package fun;
import lombok.experimental.SuperBuilder;

public interface Example{
    @SuperBuilder
    abstract class Parent{
        private Double mod;
    }
    @SuperBuilder
    class Child extends Parent {
        private String group;
    }
}

in the scala module

package fun
import com.rhenus.fun.coder.backend.model.sub.PrincipalDiagnosisPrediction
import fun.Example.Child

object ExampleUse {
  def main(args: Array[String]): Unit = {
    val c = Child.builder().build() // compiles fine
    val c1 = Child.builder().mod(2.4).build() // compiles, but wrong type
    val c2: Child = Child.builder().mod(2.4).build() // compile error
    val c3: Child = Child.builder().mod(2.4).build().asInstanceOf[Child] // cast to correct type compiles
  }
}

⇒ leads to compile error

Found: fun.Example#Child#ChildBuilder[?, ?]#C
Required: fun.Example.Child

Expected behavior
compiles fine

Version info (please complete the following information):

• Lombok version : <lombok.version>1.18.34</lombok.version> (from spring boot 3.3.4)
• Platform: IntelliJ, maven clean package

Additional context

The generated class file:

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package fun;

import lombok.Generated;

public interface Example {
  public abstract static class Parent {
    private Double mod;

    @Generated
    protected Parent(final ParentBuilder<?, ?> b) {
      this.mod = b.mod;
    }

    @Generated
    public abstract static class ParentBuilder<C extends Parent, B extends ParentBuilder<C, B>> {
      @Generated
      private Double mod;

      public ParentBuilder() {
      }

      @Generated
      public B mod(final Double mod) {
        this.mod = mod;
        return (B)this.self();
      }

      @Generated
      protected abstract B self();

      @Generated
      public abstract C build();

      @Generated
      public String toString() {
        return "Example.Parent.ParentBuilder(mod=" + this.mod + ")";
      }
    }
  }

  public static class Child extends Parent {
    private String group;

    @Generated
    protected Child(final ChildBuilder<?, ?> b) {
      super(b);
      this.group = b.group;
    }

    @Generated
    public static ChildBuilder<?, ?> builder() {
      return new ChildBuilderImpl();
    }

    @Generated
    public abstract static class ChildBuilder<C extends Child, B extends ChildBuilder<C, B>> extends Parent.ParentBuilder<C, B> {
      @Generated
      private String group;

      public ChildBuilder() {
      }

      @Generated
      public B group(final String group) {
        this.group = group;
        return (B)this.self();
      }

      @Generated
      protected abstract B self();

      @Generated
      public abstract C build();

      @Generated
      public String toString() {
        String var10000 = super.toString();
        return "Example.Child.ChildBuilder(super=" + var10000 + ", group=" + this.group + ")";
      }
    }

    @Generated
    private static final class ChildBuilderImpl extends ChildBuilder<Child, ChildBuilderImpl> {
      @Generated
      private ChildBuilderImpl() {
      }

      @Generated
      protected ChildBuilderImpl self() {
        return this;
      }

      @Generated
      public Child build() {
        return new Child(this);
      }
    }
  }
}

The fix would be in the buidler() method of the Child:

    @Generated
    protected Child(final ChildBuilder<Child, ?> b) {
      super(b);
      this.group = b.group;
    }

because we do know, that we are going to return a Child here, this type would be propagated to all super builders and scala would know the correct type.

[janrieke]

I have never used a @SuperBuilder from Scala, but this looks like a type-inference problem in the Scala compiler. If the same logic implemented as Java code compiles (which I assume), there is probably nothing Lombok can do to mitigate the problem.

[kurellajunior]

As scalac is way more strict with types than javac it might still be a type problem. I will investigate further by building the generated classes by Hand in Java and see what the problem is (and how a fix could be. Maybe you can do a similar approach too?

[kurellajunior]

I reduced it to the maximum possible minimum I found and detected the problem. The builder()-method is too weakly typed.
Workaround til fix: cast to correct type in scala code