auto ref is on the docket

[via Digitalmars-d]

On Wednesday, 24 June 2015 at 07:12:53 UTC, Iain Buclaw wrote:
> On 22 Jun 2015 08:40, "Andrei Alexandrescu via Digitalmars-d" < 
> digitalmars-d at puremagic.com> wrote:
>>
>> On 6/21/15 11:31 PM, Andrei Alexandrescu wrote:
>>>
>>> On 6/21/15 10:25 PM, Walter Bright wrote:
>>>>
>>>> The idea is that fun(5) would be lowered to:
>>>>
>>>>     auto tmp = 5;
>>>>     fun(tmp);
>>>
>>>
>>> I don't think that lowering is recommended - it prolongs the 
>>> lifetime of the temporary through the end of the caller. But 
>>> that may be actually a good thing.
>>
>>
>> On second thought - Walter's lowering, which makes the rvalue 
>> last more
> than strictly necessary, may be the most flexible of all at the 
> cost of more resource consumption (for types that define 
> destructors). -- Andrei
>>
>
> I think keeping the lifetime of objects strictly in the call 
> expression is the behaviour that will give least surprise.

Call expression, or statement containing the call expression? For 
normal temporaries, it's the latter AFAIK.

>
> int i = 42;
> struct S { ~this() { i++; } }
>
> // auto __autoreftmp = S();
> foo(S());  // __autoreftmp
> // __dtor(__autoreftmp);
> assert(i == 43);

It would make a difference for:

     auto x = foo(S()) + foo(S());

>
> As for optimisations, I think it should be possible to assert 
> that the reference never escapes, and use more aggressive 
> optimisations based on that, something that is still not 
> possible with 'scope ref' or 'in ref' parameters ...

Not possible because it's not implemented, or are there 
fundamental reasons against it?