[Issue 22916] [dip1000] copy of ref return still treated as scope variable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22916

Dennis <dkorpel at live.nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |---

--- Comment #6 from Dennis <dkorpel at live.nl> ---
(In reply to Walter Bright from comment #5)
> which is interpreted as `ref` and `return scope` attached to `this`. `a` is
> `this`. Since `a` is `scope`, the return value of `a.index()` is also
> `scope`.

And that's what the compiler does wrong. The `return scope` applies to the
`ref` return, not the returned `int*` value. This can be showcased by this
example:

```
ref int f(ref return scope int* x) @safe
{ 
    return *x;
}
```
Notice how the return value has no pointers, and yet the compiler will raise an
error if you don't annotate if `return scope`. dmd will also infer this as
`return scope` itself if you make it a template.

The job of `return scope` here is to prevent returning `f(stackPointer)` by ref
or escaping `&f(stackPointer)`, but this works:

```
int g() @safe
{
    int x;
    int y = f(&x); // `y` is not `scope`, how could it be?
    return y; // 'escaping' the result of f()
}
```

The only thing my original example does differently is giving the pointer
payload a different  type (`int*` instead of `int`), which shouldn't affect
this example since it's the second layer of indirection, it shouldn't be
affected by `scope` or `return scope`.

By the way, I reduced the original example to `int**` because you usually
request that, but it might be actually illuminating to see what the original
code looked like:

```
void scoot(scope Array!string a) @safe
{
    a[0] = a[1];
}
```

This obviously doesn't violate `scope` and it works with `string[]`, so it
should also work with library array types, or it's poor design of dip1000.

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