stdlib: fix incorrect distance measurement between UTF-16 indices

String.UTF16View.Index.distance(to:) was returning a negated result.

Fixes SR-1988.

Author: gribozavr

stdlib/public/SDK/Foundation/ExtraStringAPIs.swift

@@ -21,7 +21,7 @@ extension String.UTF16View.Index : Strideable {
   }
 
   public func distance(to other: String.UTF16View.Index) -> Int {
-    return other._offset.distance(to: _offset)
+    return _offset.distance(to: other._offset)
   }
 
   public func advanced(by n: Int) -> String.UTF16View.Index {

validation-test/stdlib/StringViews.swift

@@ -14,6 +14,7 @@
 
 import Swift
 import StdlibUnittest
+import StdlibCollectionUnittest
 
 #if _runtime(_ObjC)
 // FIXME: Foundation leaks through StdlibUnittest.  It adds some conformances
@@ -740,5 +741,112 @@ tests.test("UnicodeScalars->String") {
   }
 }
 
+struct StringViewTest {
+  var string: String
+  var utf8: [UInt8]
+  var utf16: [UInt16]
+  var unicodeScalars: [UnicodeScalar]
+
+  init(string: String, utf8: [UInt8], utf16: [UInt16], utf32: [UInt32]) {
+    self.string = string
+    self.utf8 = utf8
+    self.utf16 = utf16
+    self.unicodeScalars = utf32.map { UnicodeScalar($0)! }
+  }
+}
+
+var stringViewTests: [StringViewTest] = [
+  StringViewTest(
+    string: "",
+    utf8: [],
+    utf16: [],
+    utf32: []),
+  StringViewTest(
+    string: "\u{0000}",
+    utf8: [0x00],
+    utf16: [0x00],
+    utf32: [0x00]),
+  StringViewTest(
+    string: "a",
+    utf8: [0x61],
+    utf16: [0x61],
+    utf32: [0x61]),
+  StringViewTest(
+    string: "aa",
+    utf8: [0x61, 0x61],
+    utf16: [0x61, 0x61],
+    utf32: [0x61, 0x61]),
+  StringViewTest(
+    string: "ab",
+    utf8: [0x61, 0x62],
+    utf16: [0x61, 0x62],
+    utf32: [0x61, 0x62]),
+  StringViewTest(
+    string: "abc",
+    utf8: [0x61, 0x62, 0x63],
+    utf16: [0x61, 0x62, 0x63],
+    utf32: [0x61, 0x62, 0x63]),
+  StringViewTest(
+    string: "\u{007f}",
+    utf8: [0x7f],
+    utf16: [0x7f],
+    utf32: [0x7f]),
+  StringViewTest(
+    string: "\u{0430}",
+    utf8: [0xd0, 0xb0],
+    utf16: [0x0430],
+    utf32: [0x0430]),
+  StringViewTest(
+    string: "\u{0430}\u{0431}\u{0432}",
+    utf8: [0xd0, 0xb0, 0xd0, 0xb1, 0xd0, 0xb2],
+    utf16: [0x0430, 0x0431, 0x0432],
+    utf32: [0x0430, 0x0431, 0x0432]),
+  StringViewTest(
+    string: "\u{1f425}",
+    utf8: [0xf0, 0x9f, 0x90, 0xa5],
+    utf16: [0xd83d, 0xdc25],
+    utf32: [0x1f425]),
+]
+
+tests.test("String.UTF16View.Index/Strideable")
+  .forEach(in: stringViewTests) {
+  test in
+
+  func allIndices<C : Collection>(of c: C) -> [C.Index]
+  where C.Indices.Iterator.Element == C.Index
+  {
+    var result = Array(c.indices)
+    result.append(c.endIndex)
+    return result
+  }
+
+  checkStrideable(
+    instances: allIndices(of: test.string.utf16),
+    distances: Array(0..<test.string.utf16.count),
+    distanceOracle: { $1 - $0 })
+}
+
+tests.test("String.UTF8View/Collection")
+  .forEach(in: stringViewTests) {
+  test in
+
+  // FIXME(ABI): should be `checkBidirectionalCollection`.
+  checkForwardCollection(test.utf8, test.string.utf8) { $0 == $1 }
+}
+
+tests.test("String.UTF16View/RandomAccessCollection")
+  .forEach(in: stringViewTests) {
+  test in
+
+  checkRandomAccessCollection(test.utf16, test.string.utf16) { $0 == $1 }
+}
+
+tests.test("String.UTF32View/BidirectionalCollection")
+  .forEach(in: stringViewTests) {
+  test in
+
+  checkBidirectionalCollection(
+    test.unicodeScalars, test.string.unicodeScalars) { $0 == $1 }
+}
 
 runAllTests()