Maybe I wasn't clear enough.

Kotlin checks exhaustivity of when only if it is used as an expression, not as a statement.

https://kotlinlang.org/docs/reference/control-flow.html#when...

Kotlin doesn't allow destructuring in when:

https://discuss.kotlinlang.org/t/destructuring-in-when/2391

This is just straight up weak coming from any reasonable ML derived language with real pattern matching.