SE-0286: Forward Scan for Trailing Closures

Evolution Proposal Reviews
16

Thanks for looking into this, @Douglas_Gregor!

Another issue I have found with new rule would silently change behavior mixed with overloading:

struct Result {}
struct Either<T, U: Error> {}

struct Query {
  func execute(
    onCompletion: (Either<Result, Error>) -> Void,
    onError: ((Error) -> Void)? = nil,
    onSuccess: ((Result) -> Void)? = nil) {
    print("execute 1")
  }

  func execute(
         retries: Int = 3,
         randomized: Bool = true,
         onSuccess: ((Result) -> Void)? = nil) {
    print("execute 2")
  }
}

func test(q: Query) {
  q.execute { _ in print("success!") }
}

Backward scan currently prefers second overload (Int, Bool, (Result) -> Void where forward matching (based on the toolchain) would choice the first overload and match onCompletion:.

This is something which is going to be much harder to detect and diagnose post type-check...

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