An Optional!T and the implementation of the underlying type's opUnary

[aliak]

On Wednesday, 25 July 2018 at 18:01:54 UTC, Atila Neves wrote:
>
> This works for me:
>
> struct Optional(T) {
>
>     private T _value;
>     private bool empty = true;
>
>     this(T value) {
>         _value = value;
>         empty = false;
>     }
>
>     auto opUnary(string op)() {
>         if(!empty) mixin(op ~ "_value;");
>         return this;
>     }
>
>     string toString() const {
>         import std.conv: text;
>         return empty
>             ? "None!" ~ T.stringof
>             : text("Some!", T.stringof, "(", _value.text, ")");
>     }
> }
>
>
> void main() {
>     import std.stdio;
>
>     Optional!int nope;
>     writeln(nope);
>
>     auto mut = Optional!int(3);
>     ++mut; // compiles
>     writeln(mut);
>
>     immutable imut = Optional!int(7);
>     // ++imut; // error
> }

It needs to work with const as well and immutable too.

immutable a = 3;
auto b = -a; // is ok, should be ok with the optional as well.

Plus T can be a custom type as well with "some" definition of 
opUnary. I can't seem to find any implementation guidelines 
either so I assume opUnary or any of the ops implementation 
details is implementation defined.