optional assignment

[Quirin Schroll]

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;
> ```
>
> that is semantically equivalent, if `a` is a pointer (or if it 
> implements opCast!bool), to D
>
> ```d
> if (a is null)
>   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;
> ```
>
> As a new binary assign operator.

C# has this as the `??` and `??=` operator. I use it sometimes 
and it’s handy. I don’t care much about its exact syntax (some 
will suggest `?:` for the binary one, but `??=` looks a lot 
better than `?:=` IMO).

It should be overloadable, too: `opBinary!"??"` and 
`opBinaryRight!"??"` and `opOpAssign!"??"` should worl as 
intended.

It’s also an easy way to chicken out of null sutff:
```d
class C;

C f(int x);
C g(int x);

int n = 42;
C c = f(n) ?? g(n) ?? throw new InvalidOperationException();
```