Determine the builder class for a FreeBuilder type without the need of a static method in the type

Expected behavior

When I use a FreeBuilder interface/abstract class as a target of a mapper, I expect mapstruct to use the builder to construct the instance of that interface/abstract class.

Actual behavior

At compilation, I get the message

Ambiguous factory methods found for creating Target: Target Builder.build(), Target Builder.buildPartial(). See https://mapstruct.org/faq/#ambiguous for more info.

even though I use @Builder to specify the build method in @Mapper or in @BeanMapping.

Steps to reproduce the problem

I define my source and target class

public class Source {

    private String value1;

    private int value2;

    public String getValue1() {
        return value1;
    }
    public void setValue1(String value1) {
        this.value1 = value1;
    }
    public int getValue2() {
        return value2;
    }
    public void setValue2(int value2) {
        this.value2 = value2;
    }
}

@FreeBuilder
public interface Target {

    String getValue1();

    int getValue2();

    class Builder extends Target_Builder {
    }

}

and the mapper

import org.mapstruct.Mapper;

//@Mapper(uses = Target.Builder.class, builder = @Builder(buildMethod = "buildPartial"))
//@Mapper(uses = Target.Builder.class, builder = @Builder(buildMethod = "build"))
//@Mapper(uses = Target.Builder.class, builder = @Builder)
@Mapper(uses = Target.Builder.class)
public interface ExampleMapper {

    //    @BeanMapping(builder = @Builder(buildMethod = "buildPartial"))
//    @BeanMapping(builder = @Builder(buildMethod = "build"))
//    @BeanMapping(builder = @Builder)
    Target apply(Source source);

}

with POM containing

    <properties>
        <freebuilder.version>2.8.0</freebuilder.version>
        <mapstruct.version>1.5.5.Final</mapstruct.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.mapstruct</groupId>
            <artifactId>mapstruct</artifactId>
            <version>${mapstruct.version}</version>
        </dependency>
        <dependency>
            <groupId>org.inferred</groupId>
            <artifactId>freebuilder</artifactId>
            <version>${freebuilder.version}</version>
            <optional>true</optional>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                    <annotationProcessorPaths>
                        <path>
                            <groupId>org.mapstruct</groupId>
                            <artifactId>mapstruct-processor</artifactId>
                            <version>${mapstruct.version}</version>
                        </path>
                        <path>
                            <groupId>org.inferred</groupId>
                            <artifactId>freebuilder</artifactId>
                            <version>${freebuilder.version}</version>
                        </path>
                    </annotationProcessorPaths>
                </configuration>
            </plugin>
        </plugins>
    </build>

Then run maven compile.

I attached a minimal project:
mapstruct-freebuilder.zip

MapStruct Version

MapStruct 1.5.5.Final

Comienza con el proyecto mínimo adjunto, ExampleMapper, Target.Builder y la configuración de @Builder; después ejecuta Maven compile para reproducir el error de método de fábrica ambiguo. Sigue cómo el procesador de anotaciones descubre los builders para la interfaz Target de FreeBuilder. Se considera terminado cuando un mapper puede seleccionar el método build o buildPartial configurado sin requerir un método estático en el tipo Target.