Checking that a template parameter is an enum

[John Colvin via Digitalmars-d-learn]

On Friday, 2 October 2015 at 08:13:00 UTC, John Colvin wrote:
> On Thursday, 1 October 2015 at 22:26:39 UTC, Nordlöw wrote:
>> On Thursday, 1 October 2015 at 02:06:48 UTC, Fusxfaranto wrote:
>>> [...]
>>
>> Thanks!
>>
>> BTW: Is there some way to turn the recursive definition of 
>> `allSame`
>>
>> template allSame(V...)
>>     if (isExpressions!(V))
>> {
>>     static if (V.length <= 1)
>>         enum allSame = true;
>>     else
>>         enum allSame = V[0] == V[1] && allSame!(V[1..$]);
>> }
>>
>> into an iterative definition?
>
> Why? To avoid slowing down compilation with all those template 
> instantiations?
>
> How about a O(log2(N)) depth recursive version, something like 
> this:
>
> template allSame(V ...)
>     if (isExpressions!V)
> {
>     static if (V.length <= 1)
>         enum allSame = true;
>     else static if(V.length & 1)
>         enum allSame = V[$-1] == V[0]
>             && V[0 .. $/2] == V[$/2 .. $-1]
>             && allSame!(V[0 .. $/2]);
>     else
>         enum allSame = V[0..$/2] == V[$/2 .. $]
>             && allSame!(V[0 .. $/2]);
> }

Although you should consider that isExpressions is instantiating 
V.length templates anyway (it uses a binary split to avoid 
excessive template recursion depth, but it ends up checking them 
all one-per-template in the end anyway.