Type constraint

[IchorDev]

On Wednesday, 4 October 2023 at 01:46:42 UTC, Joel wrote:
> I think the if without static is still static, since it's part 
> of the function name part, or so (outside of the curly bracket 
> scope).

You can't have regular if-statements inside templates. Regular 
if-statements are checked at runtime but templates only exist at 
compile-time.

The way to write a *template constraint*—which is what you want, 
and uses the `if` keyword—is thus:

```d
struct List(T)
if(__traits(isIntegral, T)){
   auto addUp()
     //(Add numbers)
   }
}
```
Notice that the curly brace for the template comes after the 
template constraint, and that there are only 2 curly braces 
rather than the 3 from your example.

If the statement in the template constraint 
(`__traits(isIntegral, T)`) evaluates false at compile-time, then 
you will get a compiler error saying that the instantiation 
doesn't match the template constraints.