All of `build/generated` is added to main sourceSet.

The plugin in it's default configuration adds the directory build/generated to the main sourceSet.

However, this is problematic, since this directory is already used as the output directory for other methods of code generation, like annotation processors.

I ran into this problem in a project where an annotation processor is used for the test sourceSet for which the generated code is placed in build/generated/sources/annotationProcessor.java/test.
Since these generated classes have dependencies which are only available in the test classpath, adding build/generated to the main sourceSet breaks the build process.

I'm able to work around this by changing the output directory for this plugin and manipulating the main sourceSet like so:

tasks.withType(com.netflix.graphql.dgs.codegen.gradle.GenerateJavaTask) {
    generatedSourcesDir = "${project.buildDir.absolutePath}/dgs-codegen"
}

sourceSets {
    main {
        java {
            def dirs = srcDirs
            dirs.remove(file("${project.buildDir.absolutePath}/generated"))
            srcDirs = dirs
            srcDir "${project.buildDir.absolutePath}/dgs-codegen/generated"
        }
    }
}

But I believe that it would be better if this plugin would put its generated sources in some sub directory of build/generated instead, where they don't interfere with other generated sources.

Start by locating the plugin's default source-set configuration and the GenerateJavaTask mentioned in the issue. Check how generatedSourcesDir and build/generated are used, then verify that the plugin's generated sources no longer include unrelated annotation-processor output and that the reported test-source-set build can complete.