Codegen: Transient private vals in super constructor of data class

[remcomokveld]

I have a base class which has private vals defined in its constructor. and a data class subclass which passes these values to the super constructor.

abstract class MyAbstractClass(
  @Transient private val texts: Map<String, String>
  @Transient private val imageUrls: Map<String, String>
) {
  val title: String? get() = texts["title"]
  val posterImage: String? get() = images["poster"]
}

@JsonClass(generateAdapter = true)
data class MyConcreteClass(
  val texts: Map<String, String>
) : MyAbstractClass(texts)

Right now it is then required to annotate all private properties of MyAbstractClass as @Transient because the codegen will otherwise throw an exception.

Since having all primary constructor properties being a val is a requirement of a data class I think the moshi codegen module should be able to figure out that the private property in the super class should never be (de)serialized and thus there is no need to crash when the property is private.

[ZacSweers]

To make sure I'm reading this right: is the ask namely to just treat private, non-mutable, supertype properties as implicitly transient?

I'm kind of on board with that, but wondering about the implications of:

• what if it's mutable?
• what if it's not settable but not private? (Kotlin inheritance with non-settable properties #835)

[swankjesse]

I don’t think we treat data classes as special. (And I think we can also remove TargetType.isDataClass which is unused).

[swankjesse]

We conferred. It’s a good proposal in isolation, but it brings up some potentially surprising consequences...

Lets start with this class:

open class BaseServerResponse(
  var error_message: String?
)

Servers send the error_message field down sometimes when something breaks. Client developers are like “hey, why is this a val? nobody writes it!” and they flip it to a val.

open class BaseServerResponse(
  val error_message: String?
)

With this change the system appears to work normally but now Moshi ignores this field! Bad!