DIP1000: 'return scope' ambiguity and why you can't make opIndex work

[Dennis]

On Saturday, 19 June 2021 at 15:18:20 UTC, Dukc wrote:
> ```d
> ref float opIndex(return ref float[] vector, size_t i){
>    return vector[i];
> }
> ```
>
> So the problem is that `return` allows returning the vector 
> array itself by reference, but not any of it's elements.
>
> I think this function should compile, `return` or no. DIP1000 
> is not supposed to be transitive, so it should only check 
> returning `vector` itself by reference, but not anything that 
> it refers to.

It currently does compile. `float[] vector` is assumed to have 
infinite lifetime here, so you can return its elements by ref 
just fine. You cannot call this `opIndex` on a slice of stack 
memory, then your signature has to add the `scope` keyword:


```D
ref float opIndex(return ref scope float[] vector, size_t i){
    return vector[i];
}
```
And then the compiler needs to know that the returned float's 
lifetime is bound to the stack memory you put in, so then you 
want `return scope` semantics (which you can't get here because 
there's a `ref` return and a `ref` param.