Serialization of Set as Collection does not work

Describe the bug

I’m using Retrofit with kotlinx.serialization and the following code:

@Serializable
data class PackagesWrapper(
    val purls: Collection<String>
)

@POST("api/packages/bulk_search")
suspend fun getPackageVulnerabilities(@Body packageUrls: PackagesWrapper): List<PackageVulnerabilities>

When calling service.getPackageVulnerabilities(PackagesWrapper(packageMap.keys)) where keys is a Set, I get

java.lang.ClassCastException: class java.util.LinkedHashMap$LinkedKeySet cannot be cast to class java.util.List (java.util.LinkedHashMap$LinkedKeySet and java.util.List are in module java.base of loader 'bootstrap')

To Reproduce

Checkout https://github.com/oss-review-toolkit/ort/tree/vc2kxs-bug and run ./gradlew :advisor:test --tests org.ossreviewtoolkit.advisor.advisors.VulnerableCodeTest.

Expected behavior

As a Set is a Collection that can be serialized to a JSON array, I’d expected this to just work. However, ArrayListSerializer seems to hard-code in ListLikeSerializer<E, List<E>, ArrayList<E>>(element) that the “list like” collection needs to be a List.

Environment

• Kotlin version: 1.4.32
• Library version: 1.1.0
• Kotlin platforms: JVM
• Gradle version: 7.0
• IDE version (if bug is related to the IDE): IntellijIDEA 2021.1
• Other relevant context: Windows 10 64-bit

[sandwwraith]

Oh sorry, I think I’ve got distracted and mistyped. I meant

I wonder why you need exactly Collection static type and why you can’t replace it with val purls: Set<String>?

Use-case ’ a Set and sometimes a List, and want to avoid an additional toList() conversion’ looks reasonable

[sschuberth]

I filed a draft PR to move forward with this, in the hope that tests would be executed as part of PR checks, but unfortunately no tests seem to be run as part of the PR 😢