Add requirement StringProtocol.SubSequence : StringProtocol

Author: DougGregor

stdlib/public/core/String.swift

@@ -21,7 +21,7 @@ public protocol StringProtocol
   TextOutputStream, TextOutputStreamable,
   LosslessStringConvertible, ExpressibleByStringLiteral,
   Hashable, Comparable
-  where Iterator.Element == Character {
+  where Iterator.Element == Character, SubSequence : StringProtocol {
 
   associatedtype UTF8View : /*Bidirectional*/Collection
   where UTF8View.Element == UInt8 // Unicode.UTF8.CodeUnit

test/stdlib/StringCompatibility.swift

@@ -18,10 +18,14 @@ extension MyString : BidirectionalCollection {
   typealias Iterator = String.Iterator
   typealias Index = String.Index
   typealias IndexDistance = String.IndexDistance
+  typealias SubSequence = MyString
   func makeIterator() -> Iterator { return base.makeIterator() }
   var startIndex: String.Index { return base.startIndex }
   var endIndex: String.Index { return base.startIndex }
   subscript(i: Index) -> Character { return base[i] }
+  subscript(indices: Range<Index>) -> MyString {
+    return MyString(base: String(self.base[indices]))
+  }
   func index(after i: Index) -> Index { return base.index(after: i) }
   func index(before i: Index) -> Index { return base.index(before: i) }
   func index(_ i: Index, offsetBy n: IndexDistance) -> Index {