Allow Conditional Compilation Inside Enum Declaration

[Basile B.]

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,
> 	}
> }
> ```

Yesterday I was thinking to add that feature to another language 
(understand "one where there's no implication", you can 
experiment and "nobody will loose money") and realized a problem 
that's not been raised before:

That would make the grammar significantly more complex. For now 
the body of a "EnumDeclaration" can only contain "EnumMembers", 
whereas if you permit "StaticIfDecl" or "VersionDecl" then you 
also have to special case those in order to be sure that they can 
only contain EnumMembers.