"Error: The macro can only be applied to enums or enum values." - What interface to use instead?

[Lootwig]

The message from the title is occuring when trying to apply below macro to this declaration, which looks a lot like an enum to my naive eyes:

@X()
enum LetsTry {
  one;
}

import 'dart:async';

import 'package:macros/macros.dart';

macro class X
    implements
        EnumDefinitionMacro,
        EnumValueTypesMacro,
        EnumDeclarationsMacro,
        EnumValueDeclarationsMacro,
        EnumValueDefinitionMacro,
        EnumTypesMacro {
  const X();

  @override
  FutureOr<void> buildDeclarationsForEnum(EnumDeclaration enuum, EnumDeclarationBuilder builder) {
    builder.report(Diagnostic(DiagnosticMessage('I ran'), Severity.warning));
  }

  @override
  FutureOr<void> buildDeclarationsForEnumValue(EnumValueDeclaration entry, EnumDeclarationBuilder builder) {
    builder.report(Diagnostic(DiagnosticMessage('I ran'), Severity.warning));
  }

  @override
  FutureOr<void> buildDefinitionForEnum(EnumDeclaration enuum, EnumDefinitionBuilder builder) {
    builder.report(Diagnostic(DiagnosticMessage('I ran'), Severity.warning));
  }

  @override
  FutureOr<void> buildDefinitionForEnumValue(EnumValueDeclaration entry, EnumValueDefinitionBuilder builder) {
    builder.report(Diagnostic(DiagnosticMessage('I ran'), Severity.warning));
  }

  @override
  FutureOr<void> buildTypesForEnum(EnumDeclaration enuum, EnumTypeBuilder builder) {
    builder.report(Diagnostic(DiagnosticMessage('I ran'), Severity.warning));
  }

  @override
  FutureOr<void> buildTypesForEnumValue(EnumValueDeclaration entry, TypeBuilder builder) {
    builder.report(Diagnostic(DiagnosticMessage('I ran'), Severity.warning));
  }
}

[dart-github-bot]

Summary: The user is attempting to apply a macro to a class that resembles an enum, but the macro only supports enums. They are seeking an alternative interface to use for their macro.

[mit-mit]

cc @jakemac53

[jakemac53]

That just looks like a bug

[jakemac53]

Looks like this is an error produced by the CFE so that is probably where the issue is. There is test coverage for the shared code for this use case so it should generally work.

[Lootwig]

@jakemac53 anything I can do to help?

[jakemac53]

@jakemac53 anything I can do to help?

The repro you provided is already great, that is probably all we need to investigate (and thanks for that!)

[albinpk]

Is there any update on this issue?

[jakemac53]

No meaningful updates, and it will still be quite some time before the underlying support needed for reading annotations is there.

Although, this package does support any type with a toJson method and fromJson constructor - so you could try using enhanced enums to provide those members on your enum type, and then it might just work. Not sure if enums allow factory constructors though, or may be this package could allow a static method called fromJson instead of a constructor.