[stdlib][DNM] Replace several subscript gets with reads

stdlib/public/core/DropWhile.swift

@@ -224,7 +224,9 @@ extension LazyDropWhileCollection: Collection {
 
   @inlinable // lazy-performance
   public subscript(position: Index) -> Element {
-    return _base[position.base]
+    _read {
+      yield _base[position.base]
+    }
   }
 }
 

stdlib/public/core/ExistentialCollection.swift.gyb

@@ -536,7 +536,9 @@ internal final class _${Kind}Box<S : ${Kind}> : _Any${Kind}Box<S.Element>
 
   @inlinable
   internal override subscript(position: _AnyIndexBox) -> Element {
-    return _base[_unbox(position)]
+    _read {
+      yield _base[_unbox(position)]      
+    }
   }
 
   @inlinable
@@ -1022,7 +1024,9 @@ extension ${Self}: ${SelfProtocol} {
   ///   `position != endIndex`.
   @inlinable
   public subscript(position: Index) -> Element {
-    return _box[position._box]
+    _read {
+      yield _box[position._box]      
+    }
   }
 
   @inlinable

stdlib/public/core/Filter.swift

@@ -320,7 +320,9 @@ extension LazyFilterCollection : LazyCollectionProtocol {
   /// `position != endIndex`.
   @inlinable // lazy-performance
   public subscript(position: Index) -> Element {
-    return _base[position]
+    _read {
+      yield _base[position]      
+    }
   }
 
   @inlinable // lazy-performance

stdlib/public/core/Flatten.swift

@@ -244,6 +244,7 @@ extension FlattenCollection.Index : Hashable
 }
 
 extension FlattenCollection : Sequence {
+  public typealias Element = Base.Element.Element
   public typealias Iterator = FlattenSequence<Base>.Iterator
   public typealias SubSequence = Slice<FlattenCollection>
 
@@ -461,8 +462,10 @@ extension FlattenCollection : Collection {
   /// - Precondition: `position` is a valid position in `self` and
   ///   `position != endIndex`.
   @inlinable // lazy-performance
-  public subscript(position: Index) -> Base.Element.Element {
-    return _base[position._outer][position._inner!]
+  public subscript(position: Index) -> Element {
+    _read {
+      yield _base[position._outer][position._inner!]
+    }
   }
 
   @inlinable // lazy-performance

stdlib/public/core/Indices.swift

@@ -53,7 +53,9 @@ extension DefaultIndices: Collection {
   @inlinable
   public subscript(i: Index) -> Elements.Index {
     // FIXME: swift-3-indexing-model: range check.
-    return i
+    _read {
+      yield i
+    }
   }
 
   @inlinable

stdlib/public/core/KeyValuePairs.swift

@@ -121,7 +121,9 @@ extension KeyValuePairs : RandomAccessCollection {
   /// - Returns: The key-value pair at position `position`.
   @inlinable // trivial-implementation
   public subscript(position: Index) -> Element {
-    return _elements[position]
+    _read {      
+      yield _elements[position]
+    }
   }
 }
 

stdlib/public/core/LazyCollection.swift

@@ -151,7 +151,9 @@ extension LazyCollection : Collection {
   ///   `position != endIndex`.
   @inlinable
   public subscript(position: Index) -> Element {
-    return _base[position]
+    _read {
+      yield _base[position]      
+    }
   }
 
   /// A Boolean value indicating whether the collection is empty.

stdlib/public/core/Repeat.swift

@@ -78,8 +78,10 @@ extension Repeated: RandomAccessCollection {
   ///   `endIndex` property.
   @inlinable // trivial-implementation
   public subscript(position: Int) -> Element {
-    _precondition(position >= 0 && position < count, "Index out of range")
-    return repeatedValue
+    _read {
+      _precondition(position >= 0 && position < count, "Index out of range")
+      yield repeatedValue      
+    }
   }
 }
 

stdlib/public/core/Reverse.swift

@@ -245,7 +245,9 @@ extension ReversedCollection: BidirectionalCollection {
 
   @inlinable
   public subscript(position: Index) -> Element {
-    return _base[_base.index(before: position.base)]
+    _read {
+      yield _base[_base.index(before: position.base)]      
+    }
   }
 }
 

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -267,10 +267,10 @@ extension Unsafe${Mutable}BufferPointer: ${Mutable}Collection, RandomAccessColle
   ///   range `0..<count`.
   @inlinable // unsafe-performance
   public subscript(i: Int) -> Element {
-    get {
+    _read {
       _debugPrecondition(i >= 0)
       _debugPrecondition(i < endIndex)
-      return _position![i]
+      yield _position![i]
     }
 %if Mutable:
     nonmutating _modify {