Remove Manifest and friends

[smarter]

It's been deprecated for 5 years, it's time to say good bye 👋 .

This PR cannot be merged until we have a STARR that contains #6742

[mkeskells]

you will mess up all of the benchmarks @retronym looks after by deleting the code to make scalac compiling faster 😆

[xeno-by]

Manifests are part of the language specification: https://www.scala-lang.org/files/archive/spec/2.12/07-implicits.html#manifests. I suggest that their removal goes through the SIP process.

[dwijnand]

It's been deprecated for 5 years, it's time to say good bye 👋 .

When were they deprecated? Last I knew (6 years ago) they were un-deprecated in #1455 (for 2.10.0).

[smarter]

Manifests are part of the language specification: https://www.scala-lang.org/files/archive/spec/2.12/07-implicits.html#manifests. I suggest that their removal goes through the SIP process.

No objection from me.

When were they deprecated? Last I knew (6 years ago) they were un-deprecated in #1455 (for 2.10.0).

Indeed. I guess I assumed they were, and in practice they pretty much are.

[dwijnand]

I guess I assumed they were, and in practice they pretty much are.

They're pretty much in use, though, as well.. I don't think we can just drop them in 2.13.0-M5.

[retronym]

The problem with removing Manifests is that the replacement (TypeTag) is extremely heavyweight and only provided in an experimental Scala module (scala-reflect).

[lrytz]

We also need to look at this in the context of Scala 3 - what's going to happen to scala.reflect? We should make that plan first before taking any actions.

[lrytz]

Closing, as this needs discussion / decission outside the PR queue.

[smarter]

Closing, as this needs discussion / decission outside the PR queue.

Can we define one place for such discussion? Otherwise it's likely to spread randomly.

[lrytz]

I really think the bigger underlying topic is scala.reflect vs Scala 3 - are there any past discussions on that topic?

[dwijnand]

@odersky mentions in https://contributors.scala-lang.org/t/what-kinds-of-macros-should-scala-3-support/1850/14:

The future Scala 3 macro design is intended to replace the existing def macros and the scala.reflect infrastructure.

But I don't think creating a new thread in https://contributors.scala-lang.org/ that's specifically about Manifest would wrong. We can then link it to or from other locations.

[smarter]

I think it's pretty clear that scala.reflect if it exists in Scala 3 won't be recognizable since the current API exposes implementation details.

[dwijnand]

Bootstrapped the thread: https://contributors.scala-lang.org/t/scala-reflect-and-scala-3/1967