Enhanced enums: dart throws unexpected compile error for the declaration of enum with factory.

[iarkh]

Dart SDK version: 2.17.0-28.0.dev (dev) (Tue Jan 18 12:05:00 2022 -0800) on "windows_x64"

Dart produces a compile time error for the following source code example:

// SharedOptions=--enable-experiment=enhanced-enums
enum E {
  e1,
  e2;
  factory E.f(int i) => E.values[i];
}

main() {}

Seems like there should not be an error here.

Sample output is:

$> dart --enable-experiment=enhanced-enums test.dart
test.dart:2:6: Error: Constant evaluation error:
enum E {
     ^
test.dart:2:6: Context: Expected constant 'null' to be of type 'E', but was of type 'Null'.
 - 'E' is from 'test.dart'.
enum E {
     ^
test.dart:2:6: Context: While analyzing:
enum E {
     ^

[mraleph]

/cc @chloestefantsova

[eernstg]

@lrhn, is it intended that a constructor const E(); will be generated in an enum declaration E in the case where E contains a factory constructor (redirecting or not)? This seems inconsistent with the treatment of a similar situation for classes:

class C {
  factory C.fac() = D;
}

class D implements C {
  D();
}

void main() {
  C(); // Error, there is no constructor named `C`.
}

So I would think that there should be an error in the original example, because it needs to invoke a constructor named E, and there is no such thing, but the spec seems to disagree.

[chloestefantsova]

I'm working on a fix (https://dart-review.googlesource.com/c/sdk/+/229540) according to the current spec.

[lrhn]

The design document does indeed say:

If no generative constructors were declared, and no unnamed factory constructor was added, a default generative constructor const Name._$(this.index, this._$name); is added.

So, yes, it's intended.

There might be reason for not wanting any generative constructors on normal classes. There is never a reason to not have a generative constructor for an enum class, because it must have at least one enum element that must be constructable.

Adding a factory constructor does not change that, so unless it actually uses the unnamed constructor name, we'll always add the default constructor.