@swift-ci create

> We should ensure that a conditionally unavailable conformance is never discovered by a dynamic `as?` cast, even if the witness table symbol exists on the deployment target.

I think dynamic casts ought to succeed if the conformance is available at runtime. The alternative seems far less desirable – by forcing dynamic casts to fail, we would cripple these additions with a second-rate status.

Code that does dynamic casting already needs to prepare for failure, much like the else branch of an `if #available` check.

Hm, that is probably overly optimistic. I think we'd only want dynamic casts to consider these conformances if the running code was compiled with an stdlib module that included them – but I think we'd definitely want them in that case. (These new conformances may cause regressions in existing binaries, I guess esp. for any code that included its own conformance for these.)