Kotlin 1.3 & Java 11 & enum

Language Design
7

I have the same issue.

Apache Maven 3.5.4
Java version: 13.0.1-BellSoft, vendor: BellSoft, runtime: /usr/lib/jvm/bellsoft-java13-amd64
OS name: "linux", version: "4.15.0-66-generic", arch: "amd64", family: "unix"

enum class Interval(open val duration: Duration) {
    Min1(Duration.ofMinutes(1)),
    Min5(Duration.ofMinutes(5)),
    Min15(Duration.ofMinutes(15)),
    Min30(Duration.ofMinutes(30)),
    Hour(Duration.ofHours(1)),
    Day(Duration.ofDays(1)),
    Week(Duration.ofDays(7)),

    Month(Duration.ofDays(30)) {
        override fun idx(ts: Instant): Long {
            val dateTime = ts.atZone(ZoneOffset.UTC)
            return ((dateTime.year - 1970) * 12 + dateTime.monthValue - 1).toLong()
        }

        override val duration: Duration
            get() = throw UnsupportedOperationException("Duration of month interval is ambiguous")
    };

    private val durationNs by lazy { duration.toNanos() }

    open fun idx(ts: Instant): Long = ts.asTimestampNs() / durationNs

    companion object {
        fun all(): Set<Interval> = EnumSet.allOf(Interval::class.java)
    }
}

[ERROR] Failed to execute goal org.jetbrains.kotlin:kotlin-maven-plugin:1.3.50:kapt ... on project ...: Compilation failure
[ERROR] .../target/kaptStubs/compile/.../Interval.java:28: error: invalid method declaration; return type required
[ERROR]         Interval() {
[ERROR]         ^