More where clauses

Author: airspeedswift

stdlib/public/core/BidirectionalCollection.swift

@@ -65,8 +65,8 @@ public protocol _BidirectionalIndexable : _Indexable {
 ///   `c.index(before: c.index(after: i)) == i`.
 /// - If `i > c.startIndex && i <= c.endIndex`
 ///   `c.index(after: c.index(before: i)) == i`.
-public protocol BidirectionalCollection
-  : _BidirectionalIndexable, Collection {
+public protocol BidirectionalCollection : _BidirectionalIndexable, Collection 
+where SubSequence: BidirectionalCollection {
 
 // TODO: swift-3-indexing-model - replaces functionality in BidirectionalIndex
   /// Returns the position immediately before the given index.
@@ -84,10 +84,7 @@ public protocol BidirectionalCollection
 
   /// A sequence that can represent a contiguous subrange of the collection's
   /// elements.
-  associatedtype SubSequence : _BidirectionalIndexable, Collection
-    = BidirectionalSlice<Self>
-  // FIXME(ABI)#93 (Recursive Protocol Constraints):
-  // associatedtype SubSequence : BidirectionalCollection
+  associatedtype SubSequence = BidirectionalSlice<Self>
 
   /// A type that represents the indices that are valid for subscripting the
   /// collection, in ascending order.

stdlib/public/core/MutableCollection.swift

@@ -196,14 +196,9 @@ public protocol _MutableIndexable : _Indexable {
 ///     // Must be equivalent to:
 ///     a[i] = x
 ///     let y = x
-public protocol MutableCollection : _MutableIndexable, Collection {
-  // FIXME(ABI)#181: should be constrained to MutableCollection
-  // (<rdar://problem/20715009> Implement recursive protocol
-  // constraints)
-  /// A collection that represents a contiguous subrange of the collection's
-  /// elements.
-  associatedtype SubSequence : Collection /*: MutableCollection*/
-    = MutableSlice<Self>
+public protocol MutableCollection : _MutableIndexable, Collection
+where SubSequence: MutableCollection {
+  associatedtype SubSequence = MutableSlice<Self>
 
   /// Accesses the element at the specified position.
   ///

stdlib/public/core/RandomAccessCollection.swift

@@ -48,13 +48,11 @@ public protocol _RandomAccessIndexable : _BidirectionalIndexable {
 /// `distance(from:to:)` methods with O(1) efficiency.
 public protocol RandomAccessCollection :
   _RandomAccessIndexable, BidirectionalCollection
+  where SubSequence: RandomAccessCollection
 {
   /// A collection that represents a contiguous subrange of the collection's
   /// elements.
-  associatedtype SubSequence : _RandomAccessIndexable, BidirectionalCollection
-    = RandomAccessSlice<Self>
-  // FIXME(ABI)#102 (Recursive Protocol Constraints):
-  // associatedtype SubSequence : RandomAccessCollection
+  associatedtype SubSequence = RandomAccessSlice<Self>
 
   /// A type that represents the indices that are valid for subscripting the
   /// collection, in ascending order.