Deprecate/remove INothing?

[armanbilge]

To follow suit with typelevel/fs2#2870.

@bpholt had an excellent minimization in #52 (comment) demonstrating why we needed INothing instead of Nothing. Unfortunately, subsequent refactors made it no longer applicable and I was unable to find another one, tracked in #73.

I took a quick try at replacing INothing with Nothing. So far I encountered two issues:

• To get the Encoder[INothing] into implicit scope on Scala 3, we put it inside a companion object for INothing. We can't do this with Nothing.
  

  feral/lambda/shared/src/main/scala-3/feral/lambda/INothing.scala

  Lines 27 to 28 in 81d1997

  	type INothing <: Nothing
  	object INothing {

• This compile test no longer compiles, which may mean trouble for inference when using Kleisli-based tracing.
  

  feral/lambda/shared/src/test/scala/feral/lambda/TracedHandlerSuite.scala

  Line 39 in 81d1997

  TracedHandler(ioEntryPoint, Kleisli[IO, Span[IO], Option[INothing]](???))

  type mismatch;
   found   : cats.data.Kleisli[cats.effect.IO,natchez.Span[cats.effect.IO],Option[Nothing]]
   required: cats.data.Kleisli[[+A]cats.effect.IO[A],natchez.Span[[+A]cats.effect.IO[A]],Option[Result]]
  Note: Option[Nothing] <: Option[Result], but class Kleisli is invariant in type B.
  You may wish to define B as +B instead.
  

  Update: the broken compile test is only broken on Scala 2, Scala 3 seems okay.

[armanbilge]

So apparently there is this:

• Add codec for Nothing circe/circe#1598

Reviving that PR seems like the best way to solve our implicit scope problems in the long-term.

Edit: nice, an issue as well.

• Add Encoder/Decoder for Nothing circe/circe#1519

[armanbilge]

Hmm, tried reviving the Circe PR in armanbilge/circe@3973601 but it doesn't seem to work actually 😕

[armanbilge]

I took another try at the Circe PR this afternoon. It works in Scala 3 but some form of INothing is needed for Scala 2.13. I guess we're stuck with it for now.