Restrict type of function parameter to a defined list of types?

[Paul Backus]

On Sunday, 12 December 2021 at 13:42:08 UTC, Martin B wrote:
> On Sunday, 12 December 2021 at 13:21:06 UTC, Adam D Ruppe wrote:
>> On Sunday, 12 December 2021 at 13:11:58 UTC, Martin B wrote:
>> Just add a forwarding overload:
> Hi Adam,
>
> i am wondering if there is another possibility without having 
> to create overloads for each parameter type - something like 
> this pseudocode:
> ```
> Nullable!string str(T : {string, Nullable!string}) (T setter) {
>     return this._str = setter;
> }
> ```

You can use a [template constraint][1]:

```d
Nullable!string str(T)(T setter)
     if (is(T == string) || is(T == Nullable!string))
{
     return this._str = setter;
}
```

[1]: https://dlang.org/spec/template.html#template_constraints