Make ADTs subtypes of Product and Serializable

When writing an ADT, always have the root type extend Product and Serializable.

Reason

It allows the compiler to infer better types than it would otherwise.

Given the following simple ADT:

sealed abstract class Status
object Status {
  final case object Ok extends Status
  final case object Nok extends Status
}

The compiler will sometimes infer some wonky types:

List(Status.Ok, Status.Nok)
// res0: List[Product with Serializable with Status] = List(Ok, Nok)

This happens because inferring types is done by looking at the most specific supertype of Status.Ok and Status.Nok.

Status is a supertype of both, but it’s not the most specific one: Status.Ok and Status.Nok both also extend Product and Serializable by virtue of being case objects.

As far as the compiler is concerned, the most precise supertype it can find for both Status.Ok and Status.Nok is thus Product with Serializable with Status.

Now, consider the following implementation - the only difference is in the types that Status extend:

sealed abstract class Status extends Product with Serializable
object Status {
  final case object Ok extends Status
  final case object Nok extends Status
}

Here, the most specific supertype of both Status.Ok and Status.Nok is Status, which leads to the correct types being inferred:

List(Status.Ok, Status.Nok)
// res2: List[Status] = List(Ok, Nok)

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