"else if" for template constraints

[Enamex via Digitalmars-d]

On Friday, 4 September 2015 at 15:52:08 UTC, Enamex wrote:
> The biggest problem, I think, is that a template can has 
> multiple 'predicates' to agree to be instantiated, but only 
> some of them can mutually exclusive (the specialization syntax 
> produces mutually exclusive ones, the if-constraints don't).
> [...]
> template Bar(T) {
>     static if( is(T == int) || is(T == string) || ... ) {
>         //stuff
>     }
>     else static if( stuff ) {
>         // other stuff
>     }
>     else {
>         template return; // I know my T took whatever your type 
> was but I actually don't match, please exclude me from your 
> list for this instance...
>     }
> }
> [...]

On second thought, it wouldn't help as much as I'd thought with 
overloading problems. What we want is a 'which template has more 
'&&'ed expressions in its constraint?' which sounds pretty awful. 
I have no idea how this could work :/

Especially given that D's constraints are way more open than, 
say, Haskell's, in their checking; though ironically not their 
declaration/implementation (D is open because it checks for 
structural conformance of a struct instead of nominative & 
structural; but it's more restricted because it easily only 
checks for structure of the type as declared, with no way to 
attach recognized-to-the-constraint ad-hoc interfaces to a type 
like Haskell's with type-classes).