Derivation documentation: use inline def derived so that derives is employed

[sideeffffect]

This PR complements the the issue

• Derivation Works Without Derives Clause #16916

[nicolasstucki]

I believe there is a test case with this example that also needs to be updated.

[bishabosha]

a cursory search of "given derived" reveals quite a lot of instances, including in the docs/_docs/reference/contextual/derivation-macro.md document, which is derivation using macro quotes and splices instead of inline match

[bishabosha]

I think the macro version of this document should also be updated:
https://github.com/lampepfl/dotty/blob/1a2615b03f7f8507cf5926120beb187cb09c14ac/docs/_docs/reference/contextual/derivation-macro.md?plain=1#L34

[sideeffffect]

I have some sad news to share. The version with inline def fails to compile with

No given instance of type Playground.Eq[Playground.Opt.Sm[T]] was found

https://scastie.scala-lang.org/aT00re3sQ3WArpD8OCQtqw

[bishabosha]

There are many more examples of given derived on both of these documentation pages, with these changes now the content is inconsistent

[bishabosha]

I have some sad news to share. The version with inline def fails to compile with

No given instance of type Playground.Eq[Playground.Opt.Sm[T]] was found

https://scastie.scala-lang.org/aT00re3sQ3WArpD8OCQtqw

oops :O, would you like to try and fix the example?

[sideeffffect]

I tried but failed :(
I first suspected that the derivation only works on regular types, but break on Opt because it has a type parameter and the derivation mechanism can't deal with that.
But it turns out that the derivation can't deal with plain enums either... maybe I'm doing something totally wrong?

enum Color derives Eq:
  case Black
  case White

No given instance of type Playground.Eq[(Playground.Color.Black : Playground.Color)] was found

https://scastie.scala-lang.org/DfysP6t9QHOs4EZV4YepwA

[bishabosha]

I tried and fixed the example:
https://scastie.scala-lang.org/M0XuMdp9TAaxKFADtcHHxA

the issue is that because Eq[T] is invariant, when it comes to deriving elemInstances, there will not exist given Eq[Color.Black.type] so in that case you have to explicitly invoke the derived method again. So you need to do another type test on the element to see if it is the recursive case.

the example also protects against infinite types such as this:

scala> case class Stream[+T](t: T, ts: Stream[T]) derives Eq
-- Error: ----------------------------------------------------------------------
1 |case class Stream[+T](t: T, ts: Stream[T]) derives Eq
  |                                                   ^
  |                                             infinite recursive derivation

I guess if someone wants to support this case they could modify it to pass down the root instance for reuse

[sideeffffect]

@bishabosha you seem to know what you're talking about. You're probably much more qualified than me. Feel free to take over this Pull Request.

About the infinitely recursive type, it could still be useful, Eq can return false upon first different elements. So there could be utility in that and it would be great if the documentation explained how to implement derivation for types like this.

[bishabosha]

I can take over

[sideeffffect]

Hello @nicolasstucki @bishabosha , are there any news regarding this issue?

[bishabosha]

Hey yeah I'll work on this today

[bishabosha]

closed in favour of #17414