consider making "Enumerations must contain at least one case" a warning

[unkarjedy]

In IntelliJ Scala Plugin there is an action which creates a new Class/Object/File/Enum:

When you create class/object/file it creates a tiny template which is compilable.
However in Scala 3 enum can't have zero cases.
So the template for Scala 3 enum produces code with error "Enumerations must contain at least one case"

It's not an issue with Java enum because Java supports enum with zero items.

I wonder is there a reason to make it an error?
I could write a sealed class MyEnum without any inheritors and the code would perfectly compile.
Maybe it makes sense to remove the error?
Or at least make it a warning?

(also see SCL-20747)

[bishabosha]

I don't think there's any technical reason not to, I can trace the original check being added to #5015, so potentially there may be a repl issue.
(I did a quick check and enum Foo is not accepted as a complete statement by the repl)

this would not require a formal grammar change either.

Edit: One potential problem is that previously all enums have a scala.deriving.Mirror defined for them, this would mean some enums have no mirror.

[odersky]

I'm not against allowing empty enums.

[unkarjedy]

Would it be safe to include in the next minor release or even patch update?

[odersky]

I think there are a number of issues to work out:

• Parsing enum Foo as enum Foo {}, this was the essence of Empty enum bodies are accepted #5015, which got fixed in a different way.

• Removing the error message

• Fixing all code that depends on enumCases being non-empty. For instance, I see this in Desugar.scala

    enumCases.last.pushAttachment(DesugarEnums.DefinesEnumLookupMethods, ())

[bishabosha]

There is also the decision of if we will synthesize sum mirrors with no cases: #11764, which I think was previously decided no because code in community build assumes there will be at least one case (I guess libraries could release minor update to support zero-cases)

This can still be orthogonal to if enum will compile with zero cases