Variadic grouping

[bearophile]

> A possible static ternary operator syntax:
>
> enum foo = ct_cond !? Foo!5 : Bar!6;
>
> But in my opinion the need for it is not strong enough, better 
> to keep the language simpler.

A static ternary operator is sometimes handy, this is working 
code:

import std.typetuple: TypeTuple;

template Iota(int n) {
     static if (n <= 0)
         alias TypeTuple!() Iota;
     else
         alias TypeTuple!(Iota!(n-1), n-1) Iota;
}

void main() {
     int[3] a, b;
     foreach (i; Iota!3)
         a[i] = b[i];
}


With the recently introduced syntax for enum and templates you 
could assume this works:

enum Iota(int n) = (n <= 0) ?
                    TypeTuple!() :
                    TypeTuple!(Iota!(n-1), n-1);

But that's a regular ternary operator, so despite only one branch 
is computed, both are verified for type, because they have to 
return the same type, so it gives:

Error: template instance test.Iota!-497 recursive expansion

A static ternary operator is allowed to return two different 
types, so no type is computed for the other branch, and this 
works:

enum Iota(int n) = (n <= 0) !?
                    TypeTuple!() :
                    TypeTuple!(Iota!(n-1), n-1);

Bye,
bearophile