optional assignment

[Paul Backus]

On Monday, 30 December 2024 at 15:07:41 UTC, Basile B. wrote:
> A common pattern in a world where `null` exists is
>
> ```d
> if (!a)
>   a = b;
> ```
> [...]
>
> I propose the get rid of the statement layer. The two statments 
> (there are more actually) can be a single expression:
>
> ```d
> a ?= b;
> ```

```d
ref optAssign(T, U)(ref T dest, U value)
{
     if (!dest) dest = value;
     return dest;
}

unittest
{
     int n;
     int* p;

     n.optAssign(123);
     assert(n == 123);
     n.optAssign(456);
     assert(n == 123);

     p.optAssign(&n);
     assert(*p == 123);
     p.optAssign(new int(456));
     assert(*p == 123);
}
```