fix issue 9701 - allow UDAs to be attached to enum values

[ghost]

Only allows the UDA to be defined before the enum value, in such a way:

enum Enum
{
    @("value") value,
}

Not sure if the following should be allowed but I don't see why not, though would require reworking a bit of code. Not currently implemented with this commit:

enum Enum
{
    @("all"):
    hasAll,
    thisOneToo,

    @("extra"):
    hasAllAndExtra,
} 

[dlang-bot]

Fix	Bugzilla	Description
✓	9701	UDAs cannot be attached to enum values.

[schuetzm]

I don't think the : syntax needs to be supported. Elsewhere, it only works wherever a static if is allowed, which isn't the case in enum declarations.

[WalterBright]

This is an enhancement, not a bug. As such, it needs a rationale for it.

[ghost]

@WalterBright

It seems more like a bug/overlooked feature than an enhancement. An UDA can be applied to virtually anything that has a name that can be declared. Even in instances where it does nothing, eg aliases.

struct SomeStruct { }
@("some uda string") alias ThatStruct = SomeStruct; // valid code

writeln(__traits(getAttributes, ThatStruct));       // prints nothing

UDAs can be declared for an enum that is only a single value, which means you can do the following below.

enum Enum
{
    A, B, C, D
}

@("uda of A") enum Enum_A = Enum.A;
@("uda of B") enum Enum_B = Enum.B;
@("uda of C") enum Enum_C = Enum.C;
@("uda of D") enum Enum_D = Enum.D;

writeln(__traits(getAttributes, Enum_A)); // prints "uda of A"

The workaround isn't pretty, it shouldn't have to exist considering that a UDA can be attached to virtually everything else. Including anonymous enums and single value enums. It's only natural that multi value enums have UDAs as well.

[CZDanol]

Thumbs up, I also miss this feature.

[jpf91]

As such, it needs a rationale for it.

@WalterBright rationale: It's required in some serialization/deserialization libraries. The main reason are reserved identifiers or naming conventions:

Consider this json format:

{
    "type": "in" | "out"
}

vibe.D json can serialize enums as strings IIRC.

enum Type {@name("in") in_, name("out") out_}
struct MyJSON{Type type;}

auto json = parseJSON!MyJSON(jsonString);

Basically the same reason than for attributes on fields.

[JinShil]

I've been informed by @WalterBright that this change will require a DIP.

[wilzbach]

I've been informed by @WalterBright that this change will require a DIP.

@skl131313 submitted a DIP for this -> dlang/DIPs#105

[JinShil]

A DIP for this is no longer required. See https://forum.dlang.org/post/gcgtotrhuirhcvvbxrfm@forum.dlang.org

However, it appears the original contributor is no longer on GitHub. This PR will require someone to adopt it and carry it across the finish line.

[JinShil]

Revived at #8319