how is this array subtyping inside struct (bug?) possible?

[mw]

On Monday, 10 August 2020 at 02:38:55 UTC, RazvanN wrote:
> On Sunday, 9 August 2020 at 21:12:58 UTC, mw wrote:
>
>> I'm *directly* access the underlying array, so why its length 
>> changed to 0 after writeln?
>>
> You are accessing the underlying array after it was consumed. 
> The line writeln(s0.fns) passes class, if you want to pass the 
> underlying array you should type `writeln(so.fns.array)` and 
> then it will not consume the array.

I know that; and that's exactly I call it a subtyping bug: i.e. 
directly access & access via subtyping behave differently:

```
   foreach (i; 0..3) {
     S* s1 = new S();
     *s1 = s0;  //  copy the value from s0 to *s1
     writeln(s0.fns.array);
     writeln(s0.fns.array.length);  //*directly* access the 
underlying array
   }
```

output:
```
["abc"]
1
["abc"]
1
["abc"]
1
```

via subtyping:

```
   foreach (i; 0..3) {
     S* s1 = new S();
     *s1 = s0;  //  copy the value from s0 to *s1
     writeln(s0.fns);
     writeln(s0.fns.length);
   }
```

output:
```
["abc"]
0
[]
0
[]
0
```

And the direct access is the expected behavior that the subtyping 
mechanism want to provide a convenience for.

Since it introduce a surprise to the user, I'd call that a bug.


>> So a plain array *is* also a range? I've thought (an array's) 
>> range is a *separate* struct (which wrap the original array). 
>> The wrapper range can be consumed, but the underlying array 
>> should stay the same.
>>
>> I never seen it's mentioned in the doc:
>>
>> https://dlang.org/spec/arrays.html
>
> You had a type in your code.

What do you mean? can you elaborate?