return scope ref outlives the scope of the argument

[Jonathan M Davis]

On Tuesday, June 25, 2019 1:32:58 AM MDT Eugene Wissner via Digitalmars-d-
learn wrote:
> struct Container
> {
> }
>
> static struct Inserter
> {
>      private Container* container;
>
>      private this(return scope ref Container container) @trusted
>      {
>          this.container = &container;
>      }
>
> }
>
> auto func()()
> {
>      Container container;
>      return Inserter(container);
> }
>
> void main()
> {
>      static assert(!is(typeof(func!())));
> }
>
> The code above compiles with dmd 2.085, but not 2.086 (with
> -preview=dip1000). What am I doing wrong?

Okay. I clearly looked over what you posted too quickly and assumed that the
subject was the error that you were actually getting. The @trusted there is
what's making the static asertion fail.

Inserter is able to compile with -dip1000 (or -preview=dip1000), because you
marked it as @trusted, which throws away the scope checks. If you mark it
@safe, it won't compile. Without -dip1000, I wouldn't have expected anything
to be caught, but trying it on run.dlang.io, it looks like the return
probably makes it fail, which I find surprising, since I didn't think that
return had any effect without -dip25, but I haven't done much with return on
parameters.

You'd have an easier time figuring out what's going on if you'd just not
make func a template rather than use the static assertion, because then
you'd see the compiler errors.

In any case, by using @trusted, you're getting around the scope compiler
checks, which is why Inserter is able to compile with -dip1000. Without
-dip1000, I'm not experience enough with return parameters to know what the
compiler will or won't catch, but the code is an @safety probelm regardless.
It does look like the behavior changed with 2.086 even without -dip1000
being used, which probably has something to do with how the compiler was
changed for DIP 1000, though it probably wasn't on purpose, since in theory,
the behavior shouldn't have changed without -dip1000, but I don't know.

- Jonathan M Davis