[stdlib] Add Collection constraints via protocol where clauses

stdlib/public/core/ArrayBufferProtocol.swift

@@ -16,7 +16,7 @@
 internal protocol _ArrayBufferProtocol
   : MutableCollection, RandomAccessCollection {
 
-  associatedtype Indices : RandomAccessCollection = CountableRange<Int>
+  associatedtype Indices = CountableRange<Int>
 
   /// The type of elements stored in the buffer.
   associatedtype Element

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, Indices: BidirectionalCollection {
 
 // TODO: swift-3-indexing-model - replaces functionality in BidirectionalIndex
   /// Returns the position immediately before the given index.
@@ -84,17 +84,11 @@ 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.
-  associatedtype Indices : _BidirectionalIndexable, Collection
-    = DefaultBidirectionalIndices<Self>
-  // FIXME(ABI)#95 (Recursive Protocol Constraints):
-  // associatedtype Indices : BidirectionalCollection
+  associatedtype Indices = DefaultBidirectionalIndices<Self>
 
   /// The indices that are valid for subscripting the collection, in ascending
   /// order.

stdlib/public/core/Collection.swift

@@ -646,7 +646,11 @@ public struct IndexingIterator<
 /// forward or bidirectional collection must traverse the entire collection to
 /// count the number of contained elements, accessing its `count` property is
 /// an O(*n*) operation.
-public protocol Collection : _Indexable, Sequence {
+public protocol Collection : _Indexable, Sequence 
+where SubSequence: Collection, Indices: Collection,
+      SubSequence.Index == Index, 
+      SubSequence.Iterator.Element == Iterator.Element 
+{
   /// A type that represents the number of steps between a pair of
   /// indices.
   associatedtype IndexDistance = Int
@@ -672,10 +676,10 @@ public protocol Collection : _Indexable, Sequence {
   /// This associated type appears as a requirement in the `Sequence`
   /// protocol, but it is restated here with stricter constraints. In a
   /// collection, the subsequence should also conform to `Collection`.
-  associatedtype SubSequence : _IndexableBase, Sequence = Slice<Self>
-      where Self.SubSequence.Index == Index,
-            Self.Iterator.Element == Self.SubSequence.Iterator.Element,
+  associatedtype SubSequence = Slice<Self>
+      where Iterator.Element == SubSequence.Iterator.Element,
             SubSequence.SubSequence == SubSequence
+
   // FIXME(ABI)#98 (Recursive Protocol Constraints):
   // FIXME(ABI)#99 (Associated Types with where clauses):
   // associatedtype SubSequence : Collection
@@ -736,10 +740,9 @@ public protocol Collection : _Indexable, Sequence {
 
   /// A type that represents the indices that are valid for subscripting the
   /// collection, in ascending order.
-  associatedtype Indices : _Indexable, Sequence = DefaultIndices<Self>
+  associatedtype Indices = DefaultIndices<Self>
     where Indices.Iterator.Element == Index,
-          Indices.Index == Index,
-          Indices.SubSequence == Indices
+          Indices.Index == Index
 
   // FIXME(ABI)#100 (Recursive Protocol Constraints):
   // associatedtype Indices : Collection

stdlib/public/core/ExistentialCollection.swift.gyb

@@ -426,14 +426,11 @@ internal class _AnyRandomAccessCollectionBox<Element>
 @_fixed_layout
 @_versioned
 internal final class _${Kind}Box<S : ${Kind}> : _Any${Kind}Box<S.Iterator.Element>
+%  if Kind == 'Sequence':
   where
   S.SubSequence : ${Kind},
-%  if Kind == 'Sequence':
   S.SubSequence.Iterator.Element == S.Iterator.Element,
   S.SubSequence.SubSequence == S.SubSequence
-%  else:
-  S.SubSequence.Indices : ${Kind},
-  S.Indices : ${Kind}
 %  end
 {
   internal typealias Element = S.Iterator.Element
@@ -1040,10 +1037,7 @@ public struct ${Self}<Element>
     where
     // FIXME(ABI)#101 (Associated Types with where clauses): these constraints should be applied to
     // associated types of Collection.
-    C.SubSequence : ${SubProtocol},
-    C.SubSequence.Iterator.Element == Element,
-    C.SubSequence.Indices : ${SubProtocol},
-    C.Indices : ${SubProtocol}
+    C.SubSequence.Iterator.Element == Element
      {
     // Traversal: ${Traversal}
     // SubTraversal: ${SubTraversal}

stdlib/public/core/Mirror.swift

@@ -214,13 +214,7 @@ public struct Mirror {
     children: C,
     displayStyle: DisplayStyle? = nil,
     ancestorRepresentation: AncestorRepresentation = .generated
-  ) where
-    C.Iterator.Element == Child,
-    // FIXME(ABI)#47 (Associated Types with where clauses): these constraints should be applied to
-    // associated types of Collection.
-    C.SubSequence : Collection,
-    C.SubSequence.Indices : Collection,
-    C.Indices : Collection {
+  ) where C.Iterator.Element == Child {
 
     self.subjectType = Subject.self
     self._makeSuperclassMirror = Mirror._superclassIterator(
@@ -267,11 +261,7 @@ public struct Mirror {
     unlabeledChildren: C,
     displayStyle: DisplayStyle? = nil,
     ancestorRepresentation: AncestorRepresentation = .generated
-  ) where
-    // FIXME(ABI)#48 (Associated Types with where clauses): these constraints should be applied to
-    // associated types of Collection.
-    C.SubSequence : Collection,
-    C.Indices : Collection {
+  ) {
 
     self.subjectType = Subject.self
     self._makeSuperclassMirror = Mirror._superclassIterator(

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

@@ -48,20 +48,15 @@ public protocol _RandomAccessIndexable : _BidirectionalIndexable {
 /// `distance(from:to:)` methods with O(1) efficiency.
 public protocol RandomAccessCollection :
   _RandomAccessIndexable, BidirectionalCollection
+  where SubSequence: RandomAccessCollection, Indices: 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.
-  associatedtype Indices : _RandomAccessIndexable, BidirectionalCollection
-    = DefaultRandomAccessIndices<Self>
-  // FIXME(ABI)#103 (Recursive Protocol Constraints):
-  // associatedtype Indices : RandomAccessCollection
+  associatedtype Indices = DefaultRandomAccessIndices<Self>
 
   /// The indices that are valid for subscripting the collection, in ascending
   /// order.

validation-test/stdlib/CollectionDiagnostics.swift

@@ -23,7 +23,7 @@ struct CollectionWithBadSubSequence : Collection {
     fatalError("unreachable")
   }
 
-  // expected-note@+3 {{possibly intended match 'CollectionWithBadSubSequence.SubSequence' (aka 'OpaqueValue<Int8>') does not conform to 'Sequence'}}
+  // expected-note@+3 {{possibly intended match}}
   // expected-note@+2 {{possibly intended match}}
   // expected-note@+1 {{possibly intended match}}
   typealias SubSequence = OpaqueValue<Int8>