Is there a way to disable copying of an alias this'd member? - trying to make a NotNull type

[Simen Kjærås]

On Friday, 24 May 2019 at 10:16:50 UTC, aliak wrote:
> Basically, I want this to fail:
>
> auto notNull(T, Args...)(Args args) {
>     return NotNull!T(new T(args));
> }
>
> struct NotNull(T) {
>   private T _value;
>   @property ref inout(T) value() inout { return this._value; }
>   alias value this;
>   //disable opAssign to null as well
> }
>
> class C {}
> void func(ref C t) {
>   t = null;
> }
>
> auto a = notNull!C;
> func(a); // i want a compile error here
>
> Any ideas that don't involve disabling copying or making the 
> property non-ref?

Pretty sure that can't be done. On the other hand, why is the 
property ref if you're explicitly not going to use its ref-ness? 
Alternatively, can you show me how you use its ref-ness?

And just for completeness, you are aware that alias this takes an 
overload set, so that this works?

    struct NotNull(T) {
        private T _value;
        @property inout(T) value() inout { return _value; }
        @property void value(T val) { _value = val; } // new
        alias value this;
        // disable opAssign to null as well
    }

    class C {}
    void func(ref C t) { t = null; }

    unittest {
        NotNull n;
        n = new C(); // Look ma, I'm assigning without ref!
        func(n); // Does not compile - value() doesn't return by 
ref
    }

--
   Simen