Lazy template arguments?

[comco]

On Saturday, 5 January 2013 at 10:37:31 UTC, bearophile wrote:
> In some case I'd like a hypothetical static ternary operator 
> usable on types:
>
> alias T = (U.sizeof <= 16) ?? ushort : size_t;
>
> That is equivalent to:
>
> static if (U.sizeof <= 16) {
>     alias T = ushort;
> } else {
>     alias T = size_t;
> }
>
>
> The syntax of a possible library implementation is acceptable:
>
> alias T = Ternary!(U.sizeof <= 16, ushort, size_t);
>
>
> But I think to implement it well in library code you need a 
> kind of "lazy" for types (it means the T2 type is not computed 
> if b is false):
>
>
> template Ternary(bool b, T1, lazy T2) {
>     static if (b)
>         alias Ternary = T1;
>     else
>         alias Ternary = T2;
> }
>
>
> Are such lazy type arguments generally useful for other 
> purposes?
>
> Bye,
> bearophile

Isn't the whole ct metaprogramming with types kind of functional 
and immutable? So there is no perceivable difference between the 
results using different evaluation strategies? I thought that the 
template instantiation is already implemented as being lazy?