Binding rvalues to ref parameters redux

[Andrei Alexandrescu]

On 3/28/19 3:59 PM, bitwise wrote:
> I agree that the nonassignability requirement fills a major pothole, but 
> it doesn't address the issue of expressing your intent as the writer of 
> a function with ref parameters. The caller may have trouble determining 
> if a function will modify the argument passed to it. The issue could be 
> mitigated by function/parameter naming convention, comments, 
> documentation, or providing source code to the caller, but all of these 
> solutions have the potential to be spotty or absent, so I believe 
> something should be done to make the writer's intent clear to the caller.

A salient point.

Also made in the DIP as "In such cases, w.price is not assignable and 
calls such as applyDiscount(w.price) or w.price.applyDiscount will 
succeed but not perform any meaningful update. A maintainer expecting 
such calls to fail may be surprised. We consider this is an inevitable 
price to pay for the gained flexibility."

I reckon that there's implied that some sort of @rvalue attribute would 
be a better solution, a la void fun(@rvalue ref T). I'm afraid this 
would be widely protested, even more widely than doing nothing or going 
with the nonassignable requirement. You are of course encouraged to say 
what you think is right and raise the matter to other folks as well.

The nonassignable requirement is a major improvement, but won't catch 
all potential misuses. I don't know how to do better.