Automatically inferring associatedtype when conforming to a refinement

[anthonylatsis]

Associated type inference is intentionally limited to a single level for performance reasons. The associated type inference engine was notorious for how unstable and bogus it could be. Not because of quality of implementation – improving the performance of type inference is itself a very complex problem fundamentally.

@Douglas_Gregor

Dave is dramatically understating the pain that this inference has caused. Because this is the only place we do global type inference, it’s put tremendous pressure on the type checker that caused a huge number of bugs, crashes, and outright incomprehensible behavior. I reimplemented the inference of associated type witnesses in April of 2015 (Reimplement inference of type witnesses with a separate non-recursive… · apple/swift@126e404 · GitHub, for reference), when the existing implementation unbearable. It got far better, but it’s still not global enough to actually be predictable, and the legacy of this mis-feature manifests in a number of weird ways (e.g., typealiases in protocol extensions cannot be used to satisfy associated type requirements, weird rules for when a defaulted associated type gets used).