Associated types constrained by `AnyObject` can't be matched?

[zoul (Tomáš Znamenáček)]

@Max_Desiatov

Do you folks think that separating simple existentials from protocols syntactically would bring more clarity to the language and would help avoiding this sort of confusion? Thus we would stop conflating the protocol and the existential type created with the protocol?

This has already popped up a few times I think, like here:

@Joe_Groff

If it weren't too late to break source compatibility, I would wager that opaque concrete types are the more commonly useful feature than existentials, and in hindsight, maybe we should have saved the protocol-in-type-position syntax for an opaque type rather than using that syntax for existentials, and decorated existentials in some way instead. Rust is making that pivot even despite having source compatibility.