Why this doesn't produce an error or works as expected?

[monkyyy]

On Friday, 4 July 2025 at 17:15:48 UTC, partypooper wrote:
> ```d
> struct Vector
> {
>     float x, y;
>
>     Vector opBinary(string op)(inout Vector rhs) const if (op 
> == "+")
>     {
>         return Vector(mixin("x", op, "rhs.x"), mixin("y", op, 
> "rhs.y"),);
>     }
>
>     ref Vector opOpAssign(string op)(inout Vector rhs) if (op 
> == "+" || op == "-")
>     {
>         mixin("x", op, "=rhs.x;");
>         mixin("y", op, "=rhs.y;");
>         return this;
>     }
> }
>
> struct A
> {
>     Vector _pos, _dest;
>
>     this(Vector pos)
>     {
>         this._pos = pos;
>         this._dest = pos;
>     }
>
>     Vector pos() => _pos;
>     Vector pos(Vector v)
>     {
>         _dest.x = v.x;
>         _dest.y = v.y;
>         return _pos = v;
>     }
> }
>
> void main()
> {
>     import std.stdio : writeln;
>
>     A a = A(Vector(10, 20));
>     a.pos += Vector(1, 2); // silently does nothing
>     writeln(a);
> }
> ```
>
> How I can make it to work, if it is possible? Or else how to 
> make compiler error/warning for such cases?

if you add ref to line 28 it works; there wont be a reasonable 
way to make an error happen, you just have to know when to do 
refness