decodeElementIndex

indeed produces element index. You need to switch over it and then select proper property to decode

Could you show a tiny example please? I've tried a lot of different things since there is no documentation so far, but did not manage to make it work.

Hm, probably that part in keep should be covered more in details

Oh, I forgot to look at keep. I tried to use project documentation.

There’s an old debug branch

sourcegen

that contains test data manually generated and committed to the repo. It has similar structure to what the plugin produces: https://github.com/Kotlin/kotlinx.serialization/blob/refactoring_sourcegen/integration-tests/test-data-sourcegen/src/SimpleData.kt#L61

Thanks for example. But it seems it is impossible to use this solution in real life, it is too complicated.