Allow struct constructors with all parameters optional

[Lance Bachmeier]

On Tuesday, 27 August 2024 at 08:48:19 UTC, Ogi wrote:

> D disallows parameterless constructors for structs, and there 
> are good reasons for this. But constructors with all parameters 
> optional are prohibited too. This restriction doesn’t make much 
> sense, especially now when D supports named arguments. This 
> should be valid:
>
> ```D
> struct S {
>     this(int x = 0, int y = 0) {
>          writeln(i"S($(x), $(y))");
>     }
> }
> auto s1 = S(y:42); // S(0, 42)
> auto s2 = S(); // default initialization
> ```

I'll point out that you can already write a library that makes 
your code run. I'd like a way to do it directly without jumping 
through these hoops.

foo.d:

```
import std;
struct S {
     this(int x, int y = 0) {
          writeln(i"S($(x), $(y))");
     }
}
```

bar.d:

```
import foo;
alias _S = foo.S;
_S S() {
   return _S(0, 0);
}
```

baz.d:

```
public import foo, bar;
alias S = bar.S;
```

call.d:

```
import baz;
void main() {
   auto s2 = S();
}
```