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

[partypooper]

```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?