Sealed failure types - flat but restrictive

ApiCallFailure.kt

package com.myapp.api.common.failure

import com.myapp.api.common.failure.differentpackage.ApiCall3Failure

// This Common errors type has to know all the other types to include it in, which isn't nice. They can't include it.
// Also, they all must be in the same module (not sooo bad) and PACKAGE (very restrictive), so this third one won't work
// as it's been put in another package.
sealed class Common() : ApiCall1Failure, ApiCall2Failure /*, ApiCall3Failure */ {
    object NetworkFailure : Common()
    object ServerFailure : Common()
}

sealed interface ApiCall1Failure {
    object Specific1 : ApiCall1Failure
}

sealed interface ApiCall2Failure {
    object Specific2 : ApiCall2Failure
}

fun test() {
    val fail: ApiCall2Failure = ApiCall2Failure.Specific2
    // This is sealed, can generate the branches, all mixed in with one another, flat.
    // Whether that's a good or a bad thing is another matter; could be useful to group the common ones to delegate handling.
    when (fail) {
        Common.NetworkFailure -> TODO()
        Common.ServerFailure -> TODO()
        ApiCall2Failure.Specific2 -> TODO()
    }
}

DifferentPackage.kt

package com.myapp.api.common.failure.differentpackage

sealed interface ApiCall3Failure {
    object Specific3 : ApiCall3Failure
}