Motivation for non-public conformances providing public API

CollectionAdapter.swift

/// A collection formed by adapting some `Base` collection, typically
/// displaying some altered form of the base collection's behavior.
///
/// Default implementations of all requirements are provided, forwarding their
/// implementations to the `Base` collection instance.
public protocol CollectionAdapter: Collection {
  /// The type of the underlying collection being adapted.
  associatedtype Base: Collection
  
  /// The instance of the underlying collection being adapted.
  var base: Base { get }

  // Redeclarations of associated types allow us to attach defaults.
  associatedtype Index = Base.Index
  associatedtype Element = Base.Element
}

/// Default implementations depending on the `Index` type.
extension CollectionAdapter where Index == Base.Index {
  public var startIndex: Index { base.startIndex }
  public var endIndex: Index { base.endIndex }
  public func index(after i: Index) -> Index { base.index(after: i) }
}

/// Default implementations depending on the `Index` and `Element` types.
extension CollectionAdapter where Index == Base.Index, Element == Base.Element {
  public subscript(i: Index) -> Element { base[i] }
}

extension BidirectionalCollection
  where Self: CollectionAdapter,
        Base: BidirectionalCollection, Index == Base.Index
{
  public func index(before i: Index) -> Index { base.index(before: i) }
}

public struct FilterCollection<Base: Collection>: CollectionAdapter {
  public typealias Index = Base.Index // https://bugs.swift.org/browse/SR-12923
  
  public var base: Base
  var isIncluded: (Base.Element)->Bool // https://bugs.swift.org/browse/SR-12923

  private func advancedTillValid(_ i: Index) -> Index {
    var j = i
    while j != endIndex && !isIncluded(base[j]) {
      j = base.index(after: j)
    }
    return j
  }
  
  public var startIndex: Index {
    return advancedTillValid(base.startIndex)
  }
  
  public func index(after i: Index) -> Index {
    return advancedTillValid(base.index(after: i))
  }
}

extension FilterCollection: BidirectionalCollection
  where Base: BidirectionalCollection
{
  public func index(before i: Index) -> Index {
    var j = base.index(before: i)
    while !isIncluded(base[j]) {
      if j == base.startIndex { return i }
      j = base.index(before: j)
    }
    return j
  }
}

public struct MapCollection<Base: Collection, Element>: CollectionAdapter, Collection {
  public typealias Index = Base.Index // https://bugs.swift.org/browse/SR-12923
  public typealias Element = Element
  public var base: Base
  var mapping: (Base.Element)->Element

  public subscript(_ i: Index) -> Element { mapping(base[i]) }
}

extension MapCollection: BidirectionalCollection
  where Base: BidirectionalCollection {}