Function observable 

pub fn observable() -> &'static Conformance

An Observable conformance on Foundation.JSONDecoder.

Observation.Observable has no requirements at all — the macro that usually applies it supplies everything through extensions — so the witness table is nothing but its own descriptor. What is scarce is not the conformance but the class: focusedValue wants AnyObject & Observable, and a parity reference has to be able to name the same type. JSONDecoder is the only public Swift class reachable here that exports a nominal type descriptor, which is what registering a conformance needs.