Returning a reference to be manipulated

[kdevel]

On Friday, 14 April 2023 at 09:42:14 UTC, Bastiaan Veelo wrote:
>> [...]
>> Up to dmd v2.100.2 I am warned/get an error during compilation:
>>
>> ```
>> $ dmd returnref2.d
>> returnref2.d(3): Deprecation: returning `i` escapes a 
>> reference to parameter `i`
>> returnref2.d(1):        perhaps annotate the parameter with 
>> `return`
>> $ dmd -dip1000 returnref2.d
>> returnref2.d(3): Error: returning `i` escapes a reference to 
>> parameter `i`
>> returnref2.d(1):        perhaps annotate the parameter with 
>> `return`
>> ```
>>
>> With later dmd versions (up to including v2.102.2) the code 
>> compiles without complaints. Is this intended?
>
> I think this is intended. Adding `@safe:` on top makes the 
> complaint come back (in dmd  2.102 it is deprecated, in 2.103 
> it is an error).

What irritates me is that only with `return ref int`

```
ref int foo (return ref int i)
{
    return i;
}
[...]
```

dmd complains:

```
returnref2.d(9): Error: returning `foo(i)` escapes a reference to 
local variable `i`
```

The documentation [1] says that a `ref` parameter may not be 
returned unless it is `return ref`. But in fact it is returned 
unless it is `return ref`. Probably i get/got the meaning of the 
English modal verb "may" wrong.

The documentation also says that `return ref` parameters are used 
to ensure that the returned reference will not outlive the 
matching argument's lifetime. I don't get it! Is there any 
legitimate use of returning a ref such that it outlives the 
matching argument's lifetime? If not: Isn't this `return ref` 
completely redundant?

[1] https://dlang.org/spec/function.html