preview in: it seems that return type auto solves scope variable assignment to this

[Dmytro Katyukha]

On Thursday, 9 February 2023 at 17:59:39 UTC, Dennis wrote:
> On Thursday, 9 February 2023 at 17:42:00 UTC, Dmytro Katyukha 
> wrote:
>> So, the first question is: where in this code assignment to 
>> `this` happens?
>
> When calling a member function like `other.toAbsolute`, you're 
> assigning `other` to the implicit `this` parameter, as if you 
> are calling `toAbsolute(this: other)`.
>
>> So, the second question is: what happens in this case?
>
> Functions with `auto` return get attributes inferred, just like 
> template functions. The compiler infers `scope` based on the 
> implementation of `toAbsolute`.

Hi, thank you. Got it)

So, the next questions:
- Do it have sense to explicitly annotate all (most) member 
methods of a struct with `scope` attribute? Or it is better to 
use `auto` everywhere?.
- What drawbacks of annotating all (most) parameters as scope? Is 
it correct approach?
- What about `in` parameter attribute? Why it is not used wide in 
Phobos?

How to correctly call method from standard lib (for example 
[expandTilde](https://dlang.org/phobos/std_path.html#expandTilde)) that does not annotated param with `scope` attribute?

For example:

```d
@safe unittest {
     scope string p1 = "~/projects/d";
     scope string p2 = expandTilde(p1);
     assert(p1 != p2);
}
```

In this case, following error raised:

```
source/app.d(41,35): Error: scope variable `p1` assigned to 
non-scope parameter `inputPath` calling `expandTilde`

```

Is it correct, to use `.dup` method?

```diff
  @safe unittest {
      scope string p1 = "~/projects/d";
-    scope string p2 = expandTilde(p1);
+    scope string p2 = expandTilde(p1.dup);
      assert(p1 != p2);
  }
```