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i got it to work with this:

project.tasks.withType<Checkstyle> {

                project.afterEvaluate {
                    val filteredList = source.toList().filter { *filter logic here* }
                    source = if (filteredList.isNotEmpty()) {
                        project.files(filteredList).asFileTree
                    } else {
                        project.files().asFileTree
                    }
                }
            }