https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67295
--- Comment #12 from Jeffrey A. Law <law at redhat dot com> --- WRT c#10. I've never looked at any enabling/disabling aspects of REE. But it can exploit using implicit and explicit extensions in the IL to remove later extensions in the IL. It also handles cases that combine can't (like multiple reaching definitions). So unless there's a strong need to avoid ree, enabling it seems like a win on most, if not all, architectures.
