(https://bugs.swift.org/browse/SR-12692) Demonstrates that the current model for protocol extensions and conditional conformances is… unreasonable.

ASimplerExample.swift

extension Sequence {
  var array: Array<Element> {
    print("preallocating", self.underestimatedCount)
    return Array(self)
  }
}

_ = Array(0..<1000).reversed().array                 // preallocating 1000
_ = repeatElement(1..<10, count: 200).joined().array // preallocating 0

ConditionalConformanceOfGenerics.swift

/// The total number of single-hop index steps made in our test collections.
var totalSteps = 0

/// A collection that tracks the number of single-hop index steps executed.
struct CountSteps0 : RandomAccessCollection {
  init(count: Int) { self.count = count }
  var count: Int

  var startIndex: Int { 0 }
  var endIndex: Int { count }
  func index(after i: Int) -> Int { 
    totalSteps += 1
    return i + 1
  }
  func index(before i: Int) -> Int { 
    totalSteps += 1
    return i - 1
  }
  subscript(i: Int) -> Int { return i }
}

/// A collection that is conditionally Random Access 
struct CountSteps1<T: Collection> : Collection {
  init(count: Int) { self.count = count }
  var count: Int

  var startIndex: Int { 0 }
  var endIndex: Int { count }
  func index(after i: Int) -> Int { 
    totalSteps += 1
    return i + 1
  }
  subscript(i: Int) -> Int { return i }
}

extension CountSteps1
  : RandomAccessCollection, BidirectionalCollection 
     where T : RandomAccessCollection 
{
  func index(before i: Int) -> Int { 
    totalSteps += 1
    return i - 1
  }
  // Even if you don't take advantage of the easy defaults where Index == Int,
  // which you typically won't get because your Index will be based on T's index
  // type, it's broken.  You can see this by uncommenting the two other
  // requirements you're supposed to implement if your collection is random
  // access.
  /*
  func index(_ i: Int, offsetBy d: Int) -> Int {
    return i + d
  }
  func distance(from i: Int, to j: Int) -> Int {
    return j - i
  }
  */
}

extension Collection {
  /// Returns the 500th index position if it exists, or `nil` otherwise.
  ///
  /// Complexity: O(1) if `Self` conforms to `RandomAccessCollection`, O(N)
  /// otherwise.
  func midPoint() -> Index { 
    index(startIndex, offsetBy: count / 2, limitedBy: endIndex)!
  }
}

// CountSteps0 picks up a number of defaults designed to make it easy to create
// `RandomAccessCollection`s.
let x0 = CountSteps0(count: 900)
_ = x0.midPoint()
print("steps:", totalSteps) // 0

/// CountSteps1 doesn't pick up any RandomAccessCollection default
/// implementations, so the complexity guarantee of the `midPoint()` algorithm
/// is broken.  This is the case even if we fill in all the requirements you're
/// supposed to have to include in a model of `RandomAccessCollection`
/// (uncomment above).
let x1 = CountSteps1<Range<Int>>(count: 900)
_ = x1.midPoint()
print("steps:", totalSteps) // 450

// Prove that x1 really is a RandomAccessCollection
extension RandomAccessCollection {
  var isRandomAccess: Bool { true }
}

extension Collection {
  /// Returns `endIndex` offset by `n` positions.
  ///
  /// - Requires: if `Self` does not conform to `BidirectionalCollection`, `n ==
  ///   0`.
  func offsetEnd(by n: Int) -> Index {
    index(endIndex, offsetBy: n)
  }
}
print(x1.isRandomAccess)
_ = x1.offsetEnd(by: -1) // TRAP

@jens-bc right.