[SR-12881] DefaultIndices dynamic dispatch

stdlib/public/core/Indices.swift

@@ -81,6 +81,23 @@ extension DefaultIndices: Collection {
   public var indices: Indices {
     return self
   }
+
+  @_alwaysEmitIntoClient
+  public func index(_ i: Index, offsetBy distance: Int) -> Index {
+    return _elements.index(i, offsetBy: distance)
+  }
+
+  @_alwaysEmitIntoClient
+  public func index(
+    _ i: Index, offsetBy distance: Int, limitedBy limit: Index
+  ) -> Index? {
+    return _elements.index(i, offsetBy: distance, limitedBy: limit)
+  }
+
+  @_alwaysEmitIntoClient
+  public func distance(from start: Index, to end: Index) -> Int {
+    return _elements.distance(from: start, to: end)
+  }
 }
 
 extension DefaultIndices: BidirectionalCollection

test/stdlib/DefaultIndices.swift

@@ -0,0 +1,40 @@
+// RUN: %target-run-stdlib-swift
+// REQUIRES: executable_test
+
+import StdlibUnittest
+
+var suite = TestSuite("DefaultIndices")
+defer { runAllTests() }
+
+extension Collection {
+  func genericDistance(from start: Index, to end: Index) -> Int {
+    return distance(from: start, to: end)
+  }
+
+  func genericIndex(_ i: Index, offsetBy distance: Int) -> Index {
+    return index(i, offsetBy: distance)
+  }
+
+  func genericIndex(
+    _ i: Index, offsetBy distance: Int, limitedBy limit: Index
+  ) -> Index? {
+    return index(i, offsetBy: distance, limitedBy: limit)
+  }
+}
+
+suite.test("Bidirectional dispatch") {
+  var r = (0...10).indices
+  expectType(DefaultIndices<ClosedRange<Int>>.self, &r)
+
+  let d = r.distance(from: r.endIndex, to: r.startIndex)
+  let d2 = r.genericDistance(from: r.endIndex, to: r.startIndex)
+  expectEqual(d, d2)
+
+  let i = r.index(r.endIndex, offsetBy: -1)
+  let i2 = r.genericIndex(r.endIndex, offsetBy: -1)
+  expectEqual(i, i2)
+
+  let j = r.index(r.startIndex, offsetBy: -1, limitedBy: r.startIndex)
+  let j2 = r.genericIndex(r.startIndex, offsetBy: -1, limitedBy: r.startIndex)
+  expectEqual(j, j2)
+}