Generic operator overloading for immutable types?

[ag0aep6g via Digitalmars-d-learn]

On 06/13/2017 09:29 PM, Gary Willoughby wrote:
> Is it possible for the `result` variable in the following code to be 
> returned as an immutable type if it's created by adding two immutable 
> types?

Qualify the return type as `inout`:

     inout(Rational) opBinary(/*...*/)(/*...*/) inout {/*...*/}

(That second `inout` is the same as the first one in your code.)

Then you get a mutable/const/immutable result when you call the method 
on a mutable/const/immutable instance.

It doesn't matter a lot, though. `Rational` is a value type, so you can 
convert the return value between qualifiers as you want, anyway. But it 
affects what you get with `auto`, of course.

[...]
> struct Rational
> {
>      public long numerator;
>      public long denominator;
> 
>      public inout Rational opBinary(string op)(inout(Rational) other) const

`inout` and `const` kinda clash here. Both apply to `this`. But 
apparently, the compiler thinks `inout const` is a thing. No idea how it 
behaves. I don't think it's a thing in the language. As far as I 
understand, you should only put one of const/immutable/inout.