[interop] add a testcase for std::set iteration in Swift

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

@@ -12,3 +12,8 @@ module StdMap {
   header "std-map.h"
   requires cplusplus
 }
+
+module StdSet {
+  header "std-set.h"
+  requires cplusplus
+}

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

@@ -0,0 +1,10 @@
+#ifndef TEST_INTEROP_CXX_STDLIB_INPUTS_STD_SET_H
+#define TEST_INTEROP_CXX_STDLIB_INPUTS_STD_SET_H
+
+#include <set>
+
+using SetOfCInt = std::set<int>;
+
+inline SetOfCInt initSetOfCInt() { return {1, 5, 3}; }
+
+#endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_SET_H

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

@@ -0,0 +1,29 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xfrontend -validate-tbd-against-ir=none)
+//
+// REQUIRES: executable_test
+//
+// Enable this everywhere once we have a solution for modularizing other C++ stdlibs: rdar://87654514
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+import StdlibUnittest
+import StdSet
+import CxxStdlib
+import Cxx
+
+var StdSetTestSuite = TestSuite("StdSet")
+
+extension SetOfCInt.const_iterator : UnsafeCxxInputIterator { }
+extension SetOfCInt : CxxSequence { }
+
+StdSetTestSuite.test("iterate") {
+    let s = initSetOfCInt()
+    var result = [CInt]()
+    for x in s {
+        result.append(x)
+    }
+    expectEqual(result[0], 1)
+    expectEqual(result[1], 3)
+    expectEqual(result[2], 5)
+}
+
+runAllTests()