Fix _customContainsEquatableElement implementation for Stride* types (#39903)

* Fix `_customContainsEquatableElement` implementation for `Stride*` types

* Add tests for `Stride*.contains`

Author: xwu

stdlib/public/core/Stride.swift

@@ -379,8 +379,10 @@ extension StrideTo: Sequence {
   public func _customContainsEquatableElement(
     _ element: Element
   ) -> Bool? {
-    if element < _start || _end <= element {
-      return false
+    if _stride < 0 {
+      if element <= _end || _start < element { return false }
+    } else {
+      if element < _start || _end <= element { return false }
     }
     return nil
   }
@@ -592,8 +594,10 @@ extension StrideThrough: Sequence {
   public func _customContainsEquatableElement(
     _ element: Element
   ) -> Bool? {
-    if element < _start || _end < element {
-      return false
+    if _stride < 0 {
+      if element < _end || _start < element { return false }
+    } else {
+      if element < _start || _end < element { return false }
     }
     return nil
   }

test/stdlib/Strideable.swift

@@ -255,5 +255,25 @@ StrideTestSuite.test("StrideToIterator/past end/backward") {
   strideIteratorTest(stride(from: 3, to: 0, by: -1), nonNilResults: 3)
 }
 
+StrideTestSuite.test("Contains") {
+  expectTrue(stride(from: 1, through: 5, by: 1).contains(3))
+  expectTrue(stride(from: 1, to: 5, by: 1).contains(3))
+  expectTrue(stride(from: 1, through: 5, by: 1).contains(5))
+  expectFalse(stride(from: 1, to: 5, by: 1).contains(5))
+  expectFalse(stride(from: 1, through: 5, by: -1).contains(3))
+  expectFalse(stride(from: 1, to: 5, by: -1).contains(3))
+  expectFalse(stride(from: 1, through: 5, by: -1).contains(1))
+  expectFalse(stride(from: 1, to: 5, by: -1).contains(1))
+
+  expectTrue(stride(from: 5, through: 1, by: -1).contains(3))
+  expectTrue(stride(from: 5, to: 1, by: -1).contains(3))
+  expectTrue(stride(from: 5, through: 1, by: -1).contains(1))
+  expectFalse(stride(from: 5, to: 1, by: -1).contains(1))
+  expectFalse(stride(from: 5, through: 1, by: 1).contains(3))
+  expectFalse(stride(from: 5, to: 1, by: 1).contains(3))
+  expectFalse(stride(from: 5, through: 1, by: 1).contains(5))
+  expectFalse(stride(from: 5, to: 1, by: 1).contains(5))
+}
+
 runAllTests()