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

[Paul Backus]

On Monday, 10 August 2020 at 13:44:17 UTC, Avrina wrote:
>
> This really doesn't excuse why this bug is happening. Why is 
> writeln() using front() and popFront() that modifies the range 
> rather than simply using foreach()?

foreach over a range also uses front() and popFront().

> Why is it modifying something it knows to be a class? This is 
> definitely a bug that should be fixed.

The behavior of writeln in this case is documented under 
`std.format.formatValue` [1], which is used by `writeln` 
internally:

> For the class objects which have input range interface,
>
>    * If the instance toString has overridden Object.toString, 
> it is used.
>    * Otherwise, the objects are formatted as input range.

So everything is working as intended. However, there is certainly 
a documentation issue here, since it is not at all obvious from 
the `writeln` docs that it uses `formatValue`.

[1] 
http://dpldocs.info/experimental-docs/std.format.formatValue.html