Cannot dyn upcast to target w/ auto traits implied by source's principal's supertraits

[compiler-errors]

Originally posted by @Ekleog in #65991 (comment)

I just tried using this feature to upcast to Any + Send + Sync, and it seems to me like this is currently not allowed:

trait DynSized: 'static + Any + Send + Sync + DeepSizeOf {}
impl<T: 'static + Any + Send + Sync + DeepSizeOf> DynSized for T {}

fn main() {
    let ptr = Arc::new(()) as Arc<dyn DynSized>;
    let ptr_any = ptr as Arc<dyn Any + Send + Sync>;
    // my goal:
    let res = Arc::<dyn Any + Send + Sync>::downcast::<()>(ptr);
}

(playground link)

With references, the situation does not seem better than with Arc: this does not compile either

fn main() {
    let ptr = &() as &'static dyn DynSized;
    let ptr_any = ptr as &'static (dyn Any + Send + Sync);
}

(playground link)

Is this expected? I can't find any reference to this limitation, neither explicitly in the RFC text nor in this tracking issue. The RFC text does mention not supporting upcasting to multiple traits, but I'd have expected lifetimes as well as auto traits to not be included in the limitation.

If it is expected that this feature is stabilized without this support, does anyone know where I should watch, to know of the news on that front?

---

Minimized:

trait Source: Send + Target {}

fn upcast(x: &dyn Source) -> &(dyn Target + Send) { x }

We should be able to upcast to a dyn type w/ auto-traits that show up in the principal's supertraits too.

[compiler-errors]

I'll fix it.

@rustbot claim

[Ekleog]

This would be awesome, thank you so much for looking into it! ❤️

[crlf0710]

This includes the previously unsupported dyn Source => dyn Source + Send coercion, right? I think it's a good enhancement.

[compiler-errors]

@crlf0710: Yes, if Source has Send as one of its supertraits.