CBOR serializer encodes null class instance as empty map instead of null

[Jacob-Amazon]

Traced to this line:

kotlinx.serialization/formats/cbor/commonMain/src/kotlinx/serialization/cbor/internal/Encoder.kt

Line 106 in d4d066d

if (isClass) getDestination().encodeEmptyMap()

I consider this a bug. Null objects should be encoded as the simple type null, no matter whether they are class types or other types.

In our use case, this is causing issues after deserialization, since the empty map is deserialized into a default instance of the type, rather than null.

Please consider removing this line and always encoding as null.

[sandwwraith]

Can you please provide an example of serializable class and expected/actual output?

[Jacob-Amazon]

@Serializable
data class Test(val nullableClass: Inner?, val nullableMap: Map<String, String>?)

@Serializable
data class Inner(val test: Int)

val test = Test(nullableClass = null, nullableMap = null)
Cbor.encodeToByteArray(Test.serializer(), test)
// Hex output: bf6d6e756c6c61626c65436c617373a06b6e756c6c61626c654d6170f6ff

The hex output bf6d6e756c6c61626c65436c617373a06b6e756c6c61626c654d6170f6ff looks like this (from https://cbor.me/):

{ "nullableClass": {}, "nullableMap": null}
BF                               # map(*)
   6D                            # text(13)
      6E756C6C61626C65436C617373 # "nullableClass"
   A0                            # map(0)
   6B                            # text(11)
      6E756C6C61626C654D6170     # "nullableMap"
   F6                            # primitive(22)
   FF                            # primitive(*)

Expected output would be this: bf6d6e756c6c61626c65436c617373f66b6e756c6c61626c654d6170f6ff

{ "nullableClass": null, "nullableMap": null}
BF                               # map(*)
   6D                            # text(13)
      6E756C6C61626C65436C617373 # "nullableClass"
   F6                            # primitive(22)
   6B                            # text(11)
      6E756C6C61626C654D6170     # "nullableMap"
   F6                            # primitive(22)
   FF                            # primitive(*)