DIP 1016--ref T accepts r-values--Formal Assessment

[Steven Schveighoffer]

On 1/31/19 11:04 AM, Olivier FAURE wrote:
> On Thursday, 31 January 2019 at 02:10:05 UTC, Manu wrote:
>> I still can't see a truck-sized hole.
> 
> I don't know if it's truck-sized, but here's another corner case:
> 
>      int doubleMyValue(ref int x) {
>          x *= 2;
>          return x;
>      }
> 
>      Point pt;
>      pt.x = 5;
>      pt.y = foobar();
> 
>      doubleMyValue(pt.x);
>      assert(pt.x == 10);
> 
> Question: in the above code, will the assertion pass?
> 
> Answer: it depends on Point's implementation. If x is a member variable, 
> then yes. If it's a getter, then doubleMyValue will take a rvalue and x 
> won't be mutated and the assertion will fail.
> 
> I think this is a non-trivial conceptual problem.

BTW, the DIP discusses how to annotate these rare situations:

int doubleMyValue(ref int x) { ... }
@disable int doubleMyValue(int x);

-Steve