immutable bug?

[Adam D. Ruppe]

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.