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kotlinlang

Why doesn't the compiler complain when something implementing a serializable interface isn't itself serializable?

Are you referring to something like this?
```@Serializable
sealed interface Person {
    val name: String
}

data class PersonWithAge(
    override val name: String,
    val age: Int,
) : Person```
A compiler _error_ here wouldn't really be appropriate, since the above code represents a case where you specifically _don't_ want a particular subclass to be serializable.

Perhaps a warning would be more appropriate, forcing the developer to explicity annotate their intention to have this subclass remain unserializable in order to suppress the warning.

Yes. If I do Json.encodeToString(Person.serializer(), PersonWithAge()) it will crash

If you have a sealed class instead of an interface the compiler will fail. 

This is a reoccurring common mistake our developers make.