dip1000 and preview in combine to cause extra safety errors

[Dennis]

On Thursday, 9 June 2022 at 13:00:01 UTC, Timon Gehr wrote:
> Well, not a big fan. This is the wrong way around.

It was needed to avoid breaking existing code, which is sometimes 
annotated incorrectly because of compiler bugs and the `return 
ref scope` ambiguity issue. Once the dip1000 by default 
transition has progressed and people corrected the `return ref` / 
`return scope` annotations in their code, I think it can become 
an error in `@system` code again.