Static indexing

[Steven Schveighoffer]

On 1/12/22 3:59 AM, JG wrote:
> Hi,
> 
> I want to make a type which has two fields x and y but can
> also be indexed with [0] and [1] checked at compile time.
> 
> Is the following reasonable / correct?
> 
>      struct Point
>      {
>        double x;
>        double y;
>        alias expand = typeof(this).tupleof;
>        alias expand this;
>      }
>      unittest
>      {
>        Point p = Point(1.2,3.4);
>        assert(p[0]==1.2);
>        assert(p[1]==3.4);
>        assert(!__traits(compiles,Point.init[3]));
>      }

I was going to reply that you can't do it this way, but it works. Very 
interesting!

I would say to go with that, and I love that technique! Seems like it 
started allowing alias to `tupleof` in 2.094.

`std.typecons.Tuple` does it much differently. It declares the tuple as 
an alias-this'd member, and then defines named accessors for each of the 
items (if you give them names).

One thing I did find is that foreach doesn't like your mechanism, 
whereas the Tuple mechanism does work:

```d
foreach(x, y; only(Point(1.0, 2.0)) {} // error
foreach(x, y; only(tuple(1.0, 2.0)) {} // ok
```

-Steve