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

[Dennis]

On Monday, 21 June 2021 at 16:10:53 UTC, Dukc wrote:
> With your solution, DIP1000 will prevent the return value of 
> `*customPtr` outliving `customPtr`. We would ideally want only 
> to prevent `*customPtr` outliving whatever `customPtr.ptr` 
> points to, which is what happens now if I understood the table 
> right.

That was my first reservation when considering it, but so far I 
failed to find a scenario where the `scope` outlives its `ref`. 
Take for example this:

```D
@safe:
struct CustomPtr {
    private ubyte* ptr;
    ubyte* get() return scope {return ptr;}
}

void f(scope ubyte* ptr0) {
     scope ubyte* ptr1;
     ptr1 = ptr0; // fine
     {
         CustomPtr c;
         c.ptr = ptr0;
         ptr1 = c.get(); // scope variable `c` assigned to `ptr1` 
with longer lifetime
     }
}
```

While ptr0 can be assigned to ptr1, when going through a 
`CustomPtr c`, dmd still sees it as assigning "scope variable 
`c`".