- Impl is largely ready
- Blocked on deciding the question of upcast safety
- Hold design meeting about upcast safety and reach a conclusion
It was discovered in #89190 that extending "unsizing coercions" to include upcasts can cause deref coercions not to trigger, if those deref coercions "deref" to a
dyn type. Current plan is to issue future compatibility warnings in #89461, since the only known use of this is emulating the upcasting coercion (and hence the code will continue to work afterwards, but the Deref impl is not invoked).