Is it possible to do this with a template?

[Mitacha]

On Friday, 17 December 2021 at 07:52:18 UTC, rempas wrote:
> I want to use an expression and put it in place inside the `if` 
> parentheses. The expression is: `is(typeof(val) == type)`. I 
> want to use a template called "is_same" that will take the 
> value and a type to place them to the respective places. I have 
> tried the following but it doesn't seem to work:
>
> ```
> mixin template is_same(val, type) {
>   is(typeof(val) == type)
> }
>
> void main() {
>   int val = 10;
>   static if (is_same!(val, int)) {}
> }
> ```
>
> When trying to compile, I'm taking the following error message:
>
> ```
> Error: declaration expected, not `is`
> ```
>
> Is this a limitation of templates in D or is there a way to 
> bypass this?

It isn't really about limitation of templates. You're trying to 
use mixin template and it's main purpose is to inject 
declarations. If you want to replace `is expression` with 
template you could use something like this:

```d
bool is_same(alias value, T)() {
     return is(typeof(value) == T);
     }

void main() {
     int value = 10;
     static if (is_same!(value, int)) {
         writeln("it is true!");
     } else {
         writeln("it is false!");
     }
}
```
Personally, I don't see any benefit with replacing that kind of 
`is expressions` with templates. Perhaps I'm missing something :)