deserialization: IllegalStateException when null occurs in JSON input in place of an expected non-null object

[AcidApeStudios]

SerializationException should be thrown instead.

Example: try to parse this JSON:

{"foo":null}

for deserializing this class:

class MyClass(val foo: SomeObject)

This exception is thrown:

    java.lang.IllegalStateException: Expected class kotlinx.serialization.json.JsonObject but found class kotlinx.serialization.json.JsonNull
        at kotlinx.serialization.json.internal.AbstractJsonTreeInput.beginStructure(TreeJsonInput.kt:224)

[qwwdfsad]

Please update to kotlinx.serialization 0.20.0.
On 0.20.0 JsonDecodingException is properly thrown: Unexpected JSON token at offset 7: Expected string literal with quotes. Use 'JsonConfiguration.isLenient = true' to accept non-compliant JSON. JSON input: {"foo":null}

[AcidApeStudios]

This is 0.20.0. My above example was too simple, sorry. The bug only manifests itself during polymorphic deserialization:

    @Serializable
    abstract class MessageBase {
        var name = ""
    }

    @Serializable
    @SerialName("derived")
    class DerivedMessage : MessageBase() {
        @Serializable
        class CompoundObject {
            var a = ""
        }
        var c = CompoundObject()
    }

    @Test
    fun scratch() {
        val serialModule = SerializersModule {
            polymorphic(MessageBase::class) {
                DerivedMessage::class with DerivedMessage.serializer()
            }
        }

        Json(context = serialModule).parse(MessageBase.serializer(), """{"name":"foo","type":"derived","c":null}""")
    }

The problem is in AbstractTreeJsonInput.beginStructure(): there are cast() calls, those use check() and hence throw IllegalStateException if the check fails.