[stdlib] SR-7266 Add ReverseCompatibility test

Author: danielctull

test/stdlib/ReverseCompatibility.swift

@@ -0,0 +1,34 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -o %t/a.out3 -swift-version 3 && %target-run %t/a.out3
+// RUN: %target-build-swift %s -o %t/a.out4 -swift-version 4 && %target-run %t/a.out4
+// RUN: %target-build-swift %s -o %t/a.out5 -swift-version 5 && %target-run %t/a.out5
+// REQUIRES: executable_test
+
+import StdlibUnittest
+
+#if swift(>=4.2)
+let swiftVersion = ">=4.2"
+#else
+let swiftVersion = "<4.2"
+#endif
+
+let tests = TestSuite("ReverseCompatibility")
+
+tests.test("Double reverse type/Collection/\(swiftVersion)") {
+  func reverse<C : BidirectionalCollection>(_ xs: C) {
+    var result = xs.reversed().reversed()
+#if swift(>=4.2)
+    expectType(C.self, &result)
+#else
+    expectType(ReversedCollection<ReversedCollection<C>>.self, &result)
+#endif
+  }
+  reverse(Array(0..<10))
+
+  func backwardCompatible<C : BidirectionalCollection>(_ xs: C) {
+    typealias ExpectedType = ReversedCollection<ReversedCollection<C>>
+    var result: ExpectedType = xs.reversed().reversed()
+    expectType(ExpectedType.self, &result)
+  }
+  backwardCompatible(Array(0..<10))
+}