How to search for an enum by values and why enum items aren't unique

[Meta via Digitalmars-d-learn]

On Wednesday, 27 July 2016 at 13:59:54 UTC, stunaep wrote:
> So how would I make a function that takes an enum and an id as 
> a parameter and returns a member in the enum? I tried for quite 
> some time to do this but it wont let me pass Test as a 
> parameter unless I use templates. I finally came up with this 
> but it wont let me return null when there's nothing found
>
>>E findEnumMember(E)(int id) if (is(E == enum)) {
>>	auto found = [EnumMembers!E].find!(a => a.id == id)();
>>	if(!found.empty)
>>		return found.front;
>>	else
>>		...What do I return? null gives error
>>}

If you're going to do it like this your only real options are to 
return a Nullable!E or throw an exception if the id isn't found.