I suspect that this is similar to the issue I asked about here: https://forum.dlang.org/post/vukxaqprjbyrdpioukwa@forum.dlang.org, but I can't figure it out.

This compiles:

---
private mixin template assign(string op_)
{
  ref auto opOpAssign(string op, this RHS)(RHS rhs) if (op == op_)
  {
    mixin("x " ~ op ~ "= rhs.x;");
    return this;
  }
}

struct V
{
  int x;

  mixin assign!"+";
  // mixin assign!"-";
}

unittest
{
  auto v = V(2);
  v += v;
  assert(v.x == 4);
}
---

However, if I uncomment the second mixin, there is an error "v is not a scalar, it is a V". I guess I somehow need to merge these overloads, but I don't know how.

On Thursday, 17 September 2020 at 21:05:59 UTC, 60rntogo wrote:
> struct V
> {
>   int x;
>
>   mixin assign!"+";
>   // mixin assign!"-";
> }
>
> However, if I uncomment the second mixin, there is an error "v is not a scalar, it is a V". I guess I somehow need to merge these overloads, but I don't know how.

Usually, that would be:

struct V {
    int x;

    mixin assign!"+" a;
    mixin assign!"-" b;
    alias opOpAssign = a.opOpAssign;
    alias opOpAssign = b.opOpAssign;
}

However, I can't seem to get that working. It seems to be an instance of this issue:
https://issues.dlang.org/show_bug.cgi?id=18118

Note that explicitly calling the methods does work:

    v.opOpAssign!"-"(v);
    v.opOpAssign!"+"(v);


I don't know any good workarounds for this, you'll probably have to write a separate opOpAssign method for V that performs the forwarding to the mixed-in methods. This is, to put it very nicely, not an optimal solution.


btw, I'm somewhat surprised by your use of a template this parameter (https://dlang.org/spec/template.html#template_this_parameter). Generally this will work, but you're probably better off with

    ref auto opOpAssign(string op)(typeof(this) rhs) if (op == op_)


--
  Simen

On Thursday, 17 September 2020 at 22:07:54 UTC, Simen Kjærås wrote:
> Usually, that would be:
>
> struct V {
>     int x;
>
>     mixin assign!"+" a;
>     mixin assign!"-" b;
>     alias opOpAssign = a.opOpAssign;
>     alias opOpAssign = b.opOpAssign;
> }
>
> However, I can't seem to get that working. It seems to be an instance of this issue:
> https://issues.dlang.org/show_bug.cgi?id=18118

Yes, I tried that. It didn't work so I assumed that it must have been wrong. Is this a bug in the compiler or an issue in the language specification?

> btw, I'm somewhat surprised by your use of a template this parameter (https://dlang.org/spec/template.html#template_this_parameter). Generally this will work, but you're probably better off with
>
>     ref auto opOpAssign(string op)(typeof(this) rhs) if (op == op_)

I learned it from here: http://ddili.org/ders/d.en/mixin.html, could you elaborate on why this is not advisable?