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?

You're storing a pointer to a scope variable. That's violating the entire
point of scope. If something is scope, you can't store any kind of reference
to it. And since container is a local variable in func, and Inserter tries
to return from func with a pointer to container, you definitely have an
@safety problem, because that pointer would be invalid once func returned.

- Jonathan M Davis