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

[partypooper via Digitalmars-d-learn]

```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?