[Circe] Add a way to skip dispatch over intermediate abstract classes/traits

[vpavkin]

Reproduction: https://github.com/vpavkin/magnolia/blob/e278256e713ddf3b1aa3365898e930eff168274b/tests/src/main/scala/IntermediateTraitsTest.scala

Shapeless generic skips intermediate traits/abstract classes and forms a coproduct of only leaf types.
This makes perfect sense for many scenarios, JSON included.

Magnolia, on the other hand, dispatches through all intermediate types.
For encoding it's not that bad, but for decoding it's a showstopper.

Given

  sealed trait T
  case class A(a: Int) extends T
  case class B(b: String) extends T
  sealed trait C extends T
  case class C1(c1: Int) extends C
  case class C2(c2: String) extends C

Encoding an instance of C1 as T will produce this (which is not nice, but manageable probably):

{
   "C":{
      "C1":{
         "c1":2
      }
   }
}

But problem is that to properly decode an instance of C1 we have to scope it as C, which is very unfriendly to JSON API users. And the more simple and intuitive form won't decode:

{
   "C1":{
      "c1":2
   }
}

[propensive]

Thanks for the report and reproduction. I think it shouldn't be too difficult to fix. We currently use knownDirectSubclasses whereas we really want all subclasses, not just the direct ones. I think it should be possible to call knownDirectSubclasses recursively. I'll take a look.

[propensive]

Would you mind checking whether this branch fixes the issue, @vpavkin? I haven't had a chance to test it...

https://github.com/propensive/magnolia/tree/known-direct-and-indirect-subclasses

[vpavkin]

@propensive thanks! Will test the branch asap, hopefully by the end of the day will have some results

[vpavkin]

@propensive it helped! I rebased #86 onto #98 and now these two pass:

tests/runMain CirceRecursiveTypeTest
tests/runMain IntermediateTraitsTest

[propensive]

Great, that's progress! It's only a partial fix, so I've reopened after it auto-closed.