Help with collapsing sourceSet-s in a composite build

I tried to skin the cat another way. Instead of adding the base and generated source set outputs to the Jar directly, I added them as additional main source set outputs, and declared them also as compileOnly dependencies:

sourceSets {
    base {
    }
    generator {
    }
    generated {
        java {
            srcDirs += [new File(generatorOutputDir, "java")]
        }
        resources {
            srcDirs += [new File(generatorOutputDir, "resources")]
        }
    }
    main {
        output.dir(sourceSets.base.output)
        output.dir(sourceSets.generated.output)
    }
}
dependencies {
    generatorImplementation sourceSets.base.output
    generatedImplementation sourceSets.base.output
    compileOnly sourceSets.base.output
    compileOnly sourceSets.generated.output
}

This works ok at first, meaning that the A.A1 jar correctly contains all of base, generated, and main classes. However, I’m getting another error from A.A2 which has A.A1 as both annotationProcessor and implementation dependency:

// A.A2
dependencies {
    annotationProcessor project (":A:A1")
    implementation project(":A:A1")
}

The (wrong) compilation command line I see looks like:

... -processorpath ...;A1.jar;... -classpath ...;$buildDir\classes\java\main;....

Why are the entries different?? I want the A1.jar dependency not the path to the main source set output dir only.