[stdlib][WIP] Recursive where clause constraints

benchmark/single-source/PopFrontGeneric.swift

@@ -19,8 +19,6 @@ let arrayCount = 1024
 // being really slow).
 @_versioned
 protocol MyArrayBufferProtocol : MutableCollection, RandomAccessCollection {
-  associatedtype Element
-
   mutating func myReplace<C>(
     _ subRange: Range<Int>,
     with newValues: C

lib/Sema/CSDiag.cpp

@@ -6771,7 +6771,7 @@ bool FailureDiagnosis::visitArrayExpr(ArrayExpr *E) {
     contextualElementType = ProtocolConformanceRef::getTypeWitnessByName(
         contextualType,
         *Conformance,
-        CS->getASTContext().Id_Element,
+        CS->getASTContext().getIdentifier("ArrayLiteralElement"),
         &CS->TC)
       ->getDesugaredType();
     elementTypePurpose = CTP_ArrayElement;

stdlib/public/core/ArrayBufferProtocol.swift

@@ -18,9 +18,6 @@ internal protocol _ArrayBufferProtocol
 
   associatedtype Indices : RandomAccessCollection = CountableRange<Int>
 
-  /// The type of elements stored in the buffer.
-  associatedtype Element
-
   /// Create an empty buffer.
   init()
 
@@ -76,7 +73,7 @@ internal protocol _ArrayBufferProtocol
     _ subrange: Range<Int>,
     with newCount: Int,
     elementsOf newValues: C
-  ) where C : Collection, C.Iterator.Element == Element
+  ) where C : Collection, C.Element == Element
 
   /// Returns a `_SliceBuffer` containing the elements in `bounds`.
   subscript(bounds: Range<Int>) -> _SliceBuffer<Element> { get }
@@ -141,7 +138,7 @@ extension _ArrayBufferProtocol {
     _ subrange: Range<Int>,
     with newCount: Int,
     elementsOf newValues: C
-  ) where C : Collection, C.Iterator.Element == Element {
+  ) where C : Collection, C.Element == Element {
     _sanityCheck(startIndex == 0, "_SliceBuffer should override this function.")
     let oldCount = self.count
     let eraseCount = subrange.count

stdlib/public/core/ArrayType.swift

@@ -31,7 +31,7 @@ internal protocol _ArrayProtocol
   /// element. Otherwise, `nil`.
   var _baseAddressIfContiguous: UnsafeMutablePointer<Element>? { get }
 
-  subscript(index: Int) -> Iterator.Element { get set }
+  subscript(index: Int) -> Element { get set }
 
   //===--- basic mutations ------------------------------------------------===//
 
@@ -50,7 +50,7 @@ internal protocol _ArrayProtocol
   /// - Complexity: O(`self.count`).
   ///
   /// - Precondition: `startIndex <= i`, `i <= endIndex`.
-  mutating func insert(_ newElement: Iterator.Element, at i: Int)
+  mutating func insert(_ newElement: Element, at i: Int)
 
   /// Remove and return the element at the given index.
   ///
@@ -60,7 +60,7 @@ internal protocol _ArrayProtocol
   ///
   /// - Precondition: `count > index`.
   @discardableResult
-  mutating func remove(at index: Int) -> Iterator.Element
+  mutating func remove(at index: Int) -> Element
 
   //===--- implementation detail  -----------------------------------------===//
 

stdlib/public/core/Arrays.swift.gyb

@@ -1083,7 +1083,7 @@ extension ${Self} : RangeReplaceableCollection, _ArrayProtocol {
   /// - Parameter s: The sequence of elements to turn into an array.
   @_inlineable
   public init<S : Sequence>(_ s: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
 
     self = ${Self}(
       _buffer: _Buffer(
@@ -1436,7 +1436,7 @@ extension ${Self} : RangeReplaceableCollection, _ArrayProtocol {
   @_inlineable
   @_semantics("array.append_contentsOf")
   public mutating func append<S : Sequence>(contentsOf newElements: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
 
     let newElementsCount = newElements.underestimatedCount
     reserveCapacityForAppend(newElementsCount: newElementsCount)
@@ -1779,8 +1779,8 @@ extension ${Self} {
 
   @_inlineable
   public func _copyContents(
-    initializing buffer: UnsafeMutableBufferPointer<Iterator.Element>
-  ) -> (Iterator,UnsafeMutableBufferPointer<Iterator.Element>.Index) {
+    initializing buffer: UnsafeMutableBufferPointer<Element>
+  ) -> (Iterator,UnsafeMutableBufferPointer<Element>.Index) {
 
     guard !self.isEmpty else { return (makeIterator(),buffer.startIndex) }
 
@@ -1817,7 +1817,7 @@ internal struct _InitializeMemoryFromCollection<
 > : _PointerFunction {
   @_inlineable
   @_versioned
-  func call(_ rawMemory: UnsafeMutablePointer<C.Iterator.Element>, count: Int) {
+  func call(_ rawMemory: UnsafeMutablePointer<C.Element>, count: Int) {
     var p = rawMemory
     var q = newValues.startIndex
     for _ in 0..<count {
@@ -1846,7 +1846,7 @@ extension _ArrayBufferProtocol {
     _ bounds: Range<Int>,
     with newValues: C,
     count insertCount: Int
-  ) where C.Iterator.Element == Element {
+  ) where C.Element == Element {
 
     let growth = insertCount - bounds.count
     let newCount = self.count + growth
@@ -1918,7 +1918,7 @@ extension ${Self} {
   public mutating func replaceSubrange<C>(
     _ subrange: Range<Int>,
     with newElements: C
-  ) where C : Collection, C.Iterator.Element == Element {
+  ) where C : Collection, C.Element == Element {
 % if Self in ['Array', 'ContiguousArray']:
     _precondition(subrange.lowerBound >= self._buffer.startIndex,
       "${Self} replace: subrange start is negative")
@@ -2131,7 +2131,7 @@ extension _ArrayBufferProtocol {
   @_versioned
   internal mutating func _arrayAppendSequence<S : Sequence>(
     _ newItems: S
-  ) where S.Iterator.Element == Element {
+  ) where S.Element == Element {
 
     // this function is only ever called from append(contentsOf:)
     // which should always have exhausted its capacity before calling
@@ -2410,13 +2410,13 @@ extension ${Self} {
   public mutating func replaceRange<C>(
     _ subRange: Range<Int>,
     with newElements: C
-  ) where C : Collection, C.Iterator.Element == Element {
+  ) where C : Collection, C.Element == Element {
     Builtin.unreachable()
   }
 
   @available(*, unavailable, renamed: "append(contentsOf:)")
   public mutating func appendContentsOf<S : Sequence>(_ newElements: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     Builtin.unreachable()
   }
 }

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.
@@ -127,7 +124,7 @@ public protocol BidirectionalCollection
   ///     // Prints "50"
   ///     
   /// - Complexity: O(1)
-  var last: Iterator.Element? { get }
+  var last: Element? { get }
 
   /// Accesses a contiguous subrange of the collection's elements.
   ///
@@ -231,7 +228,7 @@ extension BidirectionalCollection where SubSequence == Self {
   ///
   /// - Complexity: O(1).
   /// - SeeAlso: `removeLast()`
-  public mutating func popLast() -> Iterator.Element? {
+  public mutating func popLast() -> Element? {
     guard !isEmpty else { return nil }
     let element = last!
     self = self[startIndex..<index(before: endIndex)]
@@ -248,7 +245,7 @@ extension BidirectionalCollection where SubSequence == Self {
   /// - Complexity: O(1)
   /// - SeeAlso: `popLast()`
   @discardableResult
-  public mutating func removeLast() -> Iterator.Element {
+  public mutating func removeLast() -> Element {
     let element = last!
     self = self[startIndex..<index(before: endIndex)]
     return element