For information / discussion: Add support for Scala 3 enums

[DavidGregory084]

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Fixes

Fixes #1017.

However, it does so using Scala 3's Mirror.Sum. This forbids any subclasses which are not products or sums themselves.

This is why the test for knownSubclasses is failing - TestUnion.UT has a subclass UC which is a normal class.

Purpose

This PR adds support for Scala 3 enums.

Background Context

The problem with the current implementation is that it assumes that the .children of a TypeRepr will be subclasses of that TypeRepr.

Scala 3 enum cases are encoded as vals which are bound to an anonymous class. They are ascribed with the type of the parent definition.

For example:

enum Color {
  case Red, Green, Blue
}

// Color.Red is encoded as ===>

val Red: Color = $new(0, "Red")

This causes the subclasses function of the knownSubclasses helper to loop indefinitely trying to find the subclasses of Color because the tpt.tpe of the ValDef children is the same TypeRepr for Color that we started with. This is the cause for #1019.

I do not know how to get the singleton TypeRepr of the enum case from the ValDef, but I think that would be the way to solve this problem while preserving the existing behaviour of the macro.

References

Scala 3 Enumerations Implementation

[DavidGregory084]

I think that a less WIP implementation of this should probably use Implicits.search so that it could surface any implicit search error messages to the user

[DavidGregory084]

By using the termSymbol we get the fully qualified name of the enum case rather than the name of the enum as our type discriminator.

[DavidGregory084]

I don't know what to do here to get the singleton Type that the compiler-generated Mirror gives us