Retroactive Conformances vs. Swift-in-the-OS

[jrose (Jordan Rose)]

There's some discussion of this in the (long) threads on Scoped Conformances - #2 by jrose and An Implementation Model for Rational Protocol Conformance Behavior, but regardless we'd still need to handle all the concerns raised in the "Co-existing conformances" section:

@jrose

A few other Apple people have pointed out that it would actually be possible to support conflicting conformances, except for in dynamic casts and features that use them (like print). This is because when a conformance is used at compile time, the compiler knows exactly where to find it, and it can be sure to continue using that implementation even if another one appears at run time in another module. However, it does complicate the language and runtime a little to support these "compile-time-only" conformances, and it still doesn't solve the problem when you do want to make the conformance available for dynamic casting, like the CustomStringConvertible example above.