This feature flag, part of RFC 1023, is not intended to be stabilized as-is. But this issue tracks discussion about whether some external feature it needed. Perhaps there is a cleaner way to address the balance between negative reasoning and API evolution. See the RFC for details.

This is used for both fundamental types and fundamental traits.

Current status: #29635 (comment)

todo

• enforce types are generic (Tracking issue for fundamental feature #29635 (comment))
• Do we even want this?