[cxx-interop] Add a test for std::optional usage from Swift

Author: egorzhdan

test/Interop/Cxx/stdlib/Inputs/module.modulemap

@@ -13,6 +13,11 @@ module StdMap {
   requires cplusplus
 }
 
+module StdOptional {
+  header "std-optional.h"
+  requires cplusplus
+}
+
 module StdSet {
   header "std-set.h"
   requires cplusplus

test/Interop/Cxx/stdlib/Inputs/std-optional.h

@@ -0,0 +1,12 @@
+#ifndef TEST_INTEROP_CXX_STDLIB_INPUTS_STD_OPTIONAL_H
+#define TEST_INTEROP_CXX_STDLIB_INPUTS_STD_OPTIONAL_H
+
+#include <optional>
+
+using CxxOptional = std::optional<int>;
+
+inline CxxOptional getNonNilOptional() { return {123}; }
+
+inline CxxOptional getNilOptional() { return {std::nullopt}; }
+
+#endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_OPTIONAL_H

test/Interop/Cxx/stdlib/use-std-optional.swift

@@ -0,0 +1,18 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xcc -std=c++17)
+//
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx
+
+import StdlibUnittest
+import StdOptional
+import CxxStdlib
+
+var StdOptionalTestSuite = TestSuite("StdOptional")
+
+StdOptionalTestSuite.test("pointee") {
+  let nonNilOpt = getNonNilOptional()
+  let pointee = nonNilOpt.pointee
+  expectEqual(123, pointee)
+}
+
+runAllTests()