stdlib: Range.contains: fix coding style and make the function public

... so that it actually has a chance to get referenced in the witness
table.

But since the function is in a constrained extension, it can't be.

Author: gribozavr

stdlib/public/core/Range.swift

@@ -154,15 +154,16 @@ extension Range : CustomReflectable {
 }
 
 /// O(1) implementation of `contains()` for ranges of comparable elements.
-extension Range where Element: Comparable {
+extension Range where Element : Comparable {
   @warn_unused_result
-  func _customContainsEquatableElement(element: Generator.Element) -> Bool? {
+  public func _customContainsEquatableElement(element: Element) -> Bool? {
     return element >= self.startIndex && element < self.endIndex
   }
-    
-  // FIXME: copied from SequenceAlgorithms as a workaround for https://bugs.swift.org/browse/SR-435
+
+  // FIXME: copied from SequenceAlgorithms as a workaround for
+  // https://bugs.swift.org/browse/SR-435
   @warn_unused_result
-  public func contains(element: Generator.Element) -> Bool {
+  public func contains(element: Element) -> Bool {
     if let result = _customContainsEquatableElement(element) {
       return result
     }

validation-test/stdlib/SequenceType.swift.gyb

@@ -608,18 +608,40 @@ SequenceTypeTests.test("Set<T>.contains/CustomImplementation/${dispatch}") {
 
 % end
 
-SequenceTypeTests.test("Range<Element>.contains/WhereElementIsComparable/dispatch") {
-  let start = 0
-  let end = 10
-  let range = Range(start: MinimalComparableIndexValue(start), end: MinimalComparableIndexValue(end))
-  let count = 20
+SequenceTypeTests.test("Range<Element>.contains/WhereElementIsComparable/staticDispatch") {
+  let start = 10
+  let end = 20
+  let range: Range<MinimalComparableIndexValue> =
+    MinimalComparableIndexValue(start)..<MinimalComparableIndexValue(end)
+  expectEqual(
+    1, MinimalComparableIndexValue.timesLessWasCalled.value)
+  let count = 30
   for test in 0..<count {
     expectEqual(
       test >= start && test < end,
       range.contains(MinimalComparableIndexValue(test)))
   }
   expectEqual(
-    count * 2, MinimalComparableIndexValue.timesLessWasCalled.value)
+    51, MinimalComparableIndexValue.timesLessWasCalled.value)
+}
+
+SequenceTypeTests.test("Range<Element>.contains/WhereElementIsComparable/dispatch") {
+  let start = 10
+  let end = 20
+  let range: Range<MinimalComparableIndexValue> =
+    MinimalComparableIndexValue(start)..<MinimalComparableIndexValue(end)
+  expectEqual(
+    1, MinimalComparableIndexValue.timesLessWasCalled.value)
+  let count = 30
+  for test in 0..<count {
+    expectEqual(
+      test >= start && test < end,
+      LoggingCollection(range).contains(MinimalComparableIndexValue(test)))
+  }
+  // Dispatch through a protocol does not get the optimization because the
+  // custom contains method is not getting into the witness table.
+  expectEqual(
+    1, MinimalComparableIndexValue.timesLessWasCalled.value)
 }
 
 SequenceTypeTests.test("contains/Predicate") {