First, it surprised me that I can't index a struct like that. So:

struct A(T...) {
    alias S = T;
    alias S this;
}

alias B = A!(int, double);
B[0] x; // Actually an array

Then, it surprised me again, that I actually can index it, sometimes

static if (!is(B[0] == B[1]))
    pragma(msg, "Works!");

Why is this language like this :(

On Friday, 27 July 2018 at 10:17:21 UTC, Yuxuan Shui wrote:
> First, it surprised me that I can't index a struct like that. So:
>
> struct A(T...) {
>     alias S = T;
>     alias S this;
> }
>
> alias B = A!(int, double);
> B[0] x; // Actually an array
>
> Then, it surprised me again, that I actually can index it, sometimes
>
> static if (!is(B[0] == B[1]))
>     pragma(msg, "Works!");
>
> Why is this language like this :(

Oh no, is it just defining arrays in the is() statement, though?

But wait, this works:

alias C = A!(1,2,3);
static if (C[0] < C[1])
   pragma(msg, "Ha!");

On 07/27/2018 12:19 PM, Yuxuan Shui wrote:
> On Friday, 27 July 2018 at 10:17:21 UTC, Yuxuan Shui wrote:
>> First, it surprised me that I can't index a struct like that. So:
>>
>> struct A(T...) {
>>     alias S = T;
>>     alias S this;
>> }
>>
>> alias B = A!(int, double);
>> B[0] x; // Actually an array
>>
>> Then, it surprised me again, that I actually can index it, sometimes
>>
>> static if (!is(B[0] == B[1]))
>>     pragma(msg, "Works!");
>>
>> Why is this language like this :(
> 
> Oh no, is it just defining arrays in the is() statement, though?

Yup.

> But wait, this works:
> 
> alias C = A!(1,2,3);
> static if (C[0] < C[1])
>     pragma(msg, "Ha!");

Looks like DMD decides that `C[0]` and `C[1]` can't be types in that situation, so it tries the alias this. That's in line with how alias this is supposed to work: only kick in when the code wouldn't compile otherwise.

On Friday, 27 July 2018 at 10:48:08 UTC, ag0aep6g wrote:
> On 07/27/2018 12:19 PM, Yuxuan Shui wrote:
>> On Friday, 27 July 2018 at 10:17:21 UTC, Yuxuan Shui wrote:
>>> [...]
>> 
>> Oh no, is it just defining arrays in the is() statement, though?
>
> Yup.
>
>> But wait, this works:
>> 
>> alias C = A!(1,2,3);
>> static if (C[0] < C[1])
>>     pragma(msg, "Ha!");
>
> Looks like DMD decides that `C[0]` and `C[1]` can't be types in that situation, so it tries the alias this. That's in line with how alias this is supposed to work: only kick in when the code wouldn't compile otherwise.

Let's just say this is confusing as hell.