Not to bash on your proposal - but why should sink parameters not be
by-value? Duplicate is default, move an optimization.

There is also auto{v} which gives you a copy if lvalue, or a moved-to if
rvalue.

G

On Wed, Oct 2, 2019, 17:03 Michael Daum via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I've become quite fond of a utility called `CopyTemp` which exists in the
> Unreal Engine code base. So much so that I think that it should be
> standardized. For lack of a better name, I'll call it `std::duplicate`.
>
> Like std::move, std::duplicate would take an argument of any value
> category and provide a return which is an rvalue. However, std::duplicate
> would accomplish this by copying when it had to. Sample Implementation
> <https://godbolt.org/z/7nIJNT>
>
> Once we have this utility, we are able to say that the best practice for
> providing a sink parameter is to declare it with rvalue reference type.
> Attempts to call the sink with lvalue arguments will cause a compiler error
> forcing to caller to either `std::move` or `std::duplicate` the arg when
> passed into the call. Compilers would be able to provide very clean,
> readable errors if they wanted to, and the resulting code would be optimal
> in terms of number of copies and moves.
> Example <https://godbolt.org/z/Q_E_J->
>
> The strengths of this proposal are:
> - It is library only
> - It is tiny, self contained, and easy to implement
> - Helps solve a problem for which we have no great best practice (sink
> parameters) in a teachable, optimal way which actually makes the calling
> code more safe and readable
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