PostHocDispatch.swift

// Based on https://gist.github.com/dabrahams/852dfdb0b628e68567b4d97499f196f9
struct Dispatch<Model> {
  func apply<A, R>(_ a: A, _ f: (Model) -> R) -> R {
    f(a as! Model)
  }
}

protocol RandomAccessCollectionDispatch {
  func fastCount<C: Collection>(_ x: C) -> Int?
}

extension Dispatch: RandomAccessCollectionDispatch
  where Model: RandomAccessCollection
{
  func fastCount<C: Collection>(_ x: C) -> Int? {
    apply(x) { $0.count }
  }
}

extension Collection {
  internal var randomAccessDispatch: RandomAccessCollectionDispatch? {
    Dispatch<Self>() as? RandomAccessCollectionDispatch
  }
}

struct ConditionalCollection<Base: Collection>: Collection {
  typealias Index = Base.Index
  var base: Base
  
  var startIndex: Index { base.startIndex }
  var endIndex: Index { base.endIndex }
  func index(after i: Index) -> Index {
    base.index(after: i)
  }
  subscript(position: Index) -> Base.Element {
    base[position]
  }
}

extension ConditionalCollection: BidirectionalCollection
  where Base: BidirectionalCollection
{
  func index(before i: Base.Index) -> Base.Index {
    base.index(before: i)
  }
}

extension ConditionalCollection: RandomAccessCollection
  where Base: RandomAccessCollection
{
  func distance(from start: Base.Index, to end: Base.Index) -> Int {
    base.distance(from: start, to: end)
  }
  func index(_ i: Base.Index, offsetBy distance: Int) -> Base.Index {
    base.index(i, offsetBy: distance)
  }
}

func conformanceTest<C: Collection>(_ c: C) {
  print(c.randomAccessDispatch?.fastCount(c) as Any)
}

conformanceTest(1...10) // 10
conformanceTest("asdf") // nil
conformanceTest(ConditionalCollection(base: 1...10)) // 10
conformanceTest(ConditionalCollection(base: "asdf")) // nil