Allow Conditional Compilation Inside Enum Declaration

[Basile B.]

On Tuesday, 2 April 2024 at 17:10:07 UTC, Basile B. wrote:
> On Saturday, 30 March 2024 at 14:57:00 UTC, IchorDev wrote:
>> To declare an enum type that may or may not have one or more 
>> members depending on conditional compilation statements 
>> requires duplicating the entire enum:
>> ```d
>> static if(cond){
>> 	enum A{
>> 		x,y,z,w,
>> 	}
>> }else{
>> 	enum A{
>> 		x,y,z,
>> 	}
>> }
>> ```
>>
>> For an enum type with many members—or many conditionals—this 
>> quickly becomes an insane amount of repetition.
>>
>> The logical solution is to just allow conditional compilation 
>> statements inside enums:
>> ```d
>> enum A{
>> 	x,y,z,
>>     static if(cond){
>> 		w,
>> 	}
>> }
>> ```
>
> I agree and the idea is not novel, [for example that], from 
> memory it's been seen several times in the NG too. Risk for a 
> DIP on this is that it could get rejected with a rationale such 
> as "you can do that with metaprog, which is the D way". That is 
> more or less what what Paul has replied, if you read under the 
> lines.
>
> [for example that]: 
> https://issues.dlang.org/show_bug.cgi?id=9761

forgot to say you can use a struct filled with anonymous enum 
members too.

```d
struct A
{
     enum T x = 0;
     enum T y = 1;
     enum T z = 2;
static if (cond)
     enum T w = 3;
}
```

However RN I cant confirm that the usage is 100% equivalent. Use 
of the members is likely but use of the container may not, e.g in 
std.traits (esp. the EnumMembers template ...).