meaning of "auto ref const"?

[Picaud Vincent via Digitalmars-d-learn]

On Tuesday, 20 December 2016 at 20:08:32 UTC, Ali Çehreli wrote:

> If the purpose is optimization, the good news are

Yes it is :)

> * Classes are already reference types so there is no lvalue or 
> rvalue reference distinction there

Ok, this one is quite intuitive.

> import std.stdio;
> ...

Thank you for the illustrative example, I have reproduced it.

> There is a surprising difference in D:
>
> In D, non-constness of an object seems to be more important in 
> overload resolution: Notice how mutable lvalue above is passed 
> to by-copy instead of the potentially-more-optimal by-const-ref 
> above. D realizes that a mutable object is for mutation and 
> because by-const-ref cannot mutate it, D passes it to the 
> by-copy function. (This may be seen as a bug by some.)

Thank you for pointing out this. I was not aware of that, and for 
sure this is not the C++  behavior.

> Interestingly, enabling the by-mutable-ref overload above, now 
> the mutable object goes to by-ref and there is no automatic 
> copy:

Ok, that is "moral" and without surprise.

> --- rvalue ---
> constructor       1
> foo(by-copy)      1
> destructor for    1
>
> --- mutable lvalue ---
> constructor       2
> foo(by-ref)       2
> destructor for    2
>
> --- const lvalue ---
> constructor       3
> foo(by-ref-const) 3
> destructor for    3
>
> Ali
>
> [1] I have an issue with "rvalue reference" as rvalue 
> references can be references to lvalues as well. :p

Thank you for your time and these valuable explanations, I learnt 
a lot.
--Vincent