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

[aliak]

On Thursday, 26 July 2018 at 06:37:41 UTC, Simen Kjærås wrote:
> On Wednesday, 25 July 2018 at 21:59:00 UTC, aliak wrote:
>> 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.
>
> Template this[0] (and CopyTypeQualifiers[1])to the rescue!
>

Ah! Genius!

I had no idea that using TemplateThisParameters would not 
necessitate qualifying the function in question either.

>
> As for assigning to Optional!(immutable int), the language 
> basically forbids this (cannot modify struct with immutable 
> members). It would, as you say, cause problems when you can get 
> a pointer to the contents.

So is this undefined behaviour?

import std.stdio;

struct S(T) {
     T value;
     void opUnary(string op)() inout {
         mixin(op ~ "cast(T)value;");
     }
}

void main() {
     immutable a = S!int(2);
     ++a;
}