immutable bug?

[Maxim Fomin]

On Saturday, 11 January 2014 at 18:52:39 UTC, Adam D. Ruppe wrote:
> On Saturday, 11 January 2014 at 18:48:15 UTC, Maxim Fomin wrote:
>> It is legal exactly because function is marked as pure. Result 
>> of pure function is implicitly convertible to immutable.
>
> It shouldn't be here though... the reason it is implicitly 
> convertable is that pure means the result is unique. But, with 
> the hidden this pointer having a reference to the data as well, 
> it obviously is not unique. I think the compiler should catch 
> this; i'd call it a bug.

Creating undefined behavior is not a sufficient reason to be a 
bug.

Changelog mentions example with pure struct constructors which 
provide value implicitly castable to immutable. So, strictly 
speaking current behavior is conforming to spec.

You can still argue that the example does not address the issue 
with 'this' parameter, so code should be rejected (which should 
happen from safity aspect). I think this case was not considered 
when 'unique expression' was introduced, so it is yet another 
hole in type system (to be more precise, bug is in spec-language, 
not compiler).