Allow `==` on Enum values in const expressions.

[leafpetersen]

As reported in:

• Analyzer incorrectly reports "In constant expressions, operands of this operator must be of type 'bool', 'num', 'String' or 'null'" for == operator with enum operand sdk#26980
• const_eval_throws_exception error sdk#36511
• CFE produces the following error in flutter code when constant_update_2018 flag is turned on sdk#36564
• VM fails to report an error when equality comparing enums in a const context sdk#36528

The specification does not allow == to be used on enum values in const contexts. This seems reasonable to allow, customers have asked for it, and it should be an easy change to make.

Example from dart-lang/sdk#26980

enum MyEnum { a, b }

class Test {
  const Test(MyEnum arg) : value = arg == MyEnum.a ? 0 : 1;
  final int value;
}

void main() {
  Test q = const Test(MyEnum.a);
  Test r = const Test(MyEnum.a);
  print(identical(q, r));
}

[leafpetersen]

cc @Hixie @mit-mit

[mpfaff]

Is anything being done about this? It seems like a pretty basic language feature, especially considering how weak Dart's enums already are.

[mit-mit]

@Hixie looks like you opened dart-lang/sdk#29278 originally; do you still believe this would be a solid improvement?

[Hixie]

I think this is table stakes. I think we should go much, much further and allow all code in const expressions except code that ends up having side-effects (e.g. opening sockets, printing to stdout, accessing the filesystem, that kind of thing).

[rdnobrega]

@mit-mit Why wouldn't it be? Enums are already constant-like structures, and using static class members is just a lot of boilerplate code for such a simple thing...

[mit-mit]

Why wouldn't it be?

I'm not saying it isn't, just asking for input.

[rdnobrega]

@mit-mit Hi! I'd like to say what I think and am trying to do and would like your feedback on the topic. I have a list of type of working hours, being 4: hflex, hnor, hnot, henor. Today we can write with a class for const expressions:

class WorkType { final int _value; static const HFLEX = WorkType._internal(0); static const HNUCL = WorkType._internal(1); static const HENOR = WorkType._internal(2); static const HENOT = WorkType._internal(3); const WorkType._internal(this._value); }

with enums, I'd like to write like this:
enum WorkType { HFLEX, HNUCL, HENOR, HENOT }

Furthermore, I'd like enums to be even more flexible, first with value setting for ints only:
enum WorkType { HFLEX: 0, HNUCL: 1, HENOR: 2, HENOT: 3}

and being even more flexible, enumeration with another enums:
enum WorkType { HFLEX, HNUCL, HENOR, HENOT }
enum WorkGroup<WorkType> { Group1: WorkType.HNUCL + WorkType.HENOR + WorkType.HENOT }

This would allow loop in WorkType.values() without the concern of getting groups instead of single enum values.

This would help A LOT coding flags. Of course, all of this is already possible with classes, but it is a lot more code and doesn't really quite match.

[eernstg]

This feature could be expected to be available in Dart already: An instance of an enum class has primitive equality, and that is the property that we otherwise require when constant evaluation needs to evaluated ==. So it's a surprising exception that operator == in constant expressions isn't specified using the same concept. Cf. #1811 where this generalization is proposed.

[lrhn]

Closing as duplicate of #1811.