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.

Yeah, that's already a thing that ref in D doesn't protect against:

struct Point
{
    private int _x, _y;
    ref int x() { return _x; }
    ref int y() { return _y; }
}

struct Rect
{
    private Point _origin, _lengths;
    Point origin() { return _origin; }
    Point lengths() { return _lengths; }
    void origin(Point p) { _origin = p; }
    void lengths(Point p) { _lengths = p; }
}

Rect r;
r.origin = Point(1, 2);
r.lengths = Point(5, 5);
doubleMyValue(r.lengths.x);
assert(r.lengths.x == 10); // fail

-Steve