--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/unittests/ADT/StringSetTest.cpp	Wed Aug 14 19:46:37 2019 +0900
@@ -0,0 +1,44 @@
+//===- llvm/unittest/ADT/StringSetTest.cpp - StringSet unit tests ----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/StringSet.h"
+#include "gtest/gtest.h"
+using namespace llvm;
+
+namespace {
+
+// Test fixture
+class StringSetTest : public testing::Test {};
+
+TEST_F(StringSetTest, IterSetKeys) {
+  StringSet<> Set;
+  Set.insert("A");
+  Set.insert("B");
+  Set.insert("C");
+  Set.insert("D");
+
+  auto Keys = to_vector<4>(Set.keys());
+  llvm::sort(Keys);
+
+  SmallVector<StringRef, 4> Expected = {"A", "B", "C", "D"};
+  EXPECT_EQ(Expected, Keys);
+}
+
+TEST_F(StringSetTest, InsertAndCountStringMapEntry) {
+  // Test insert(StringMapEntry) and count(StringMapEntry)
+  // which are required for set_difference(StringSet, StringSet).
+  StringSet<> Set;
+  StringMapEntry<StringRef> *Element = StringMapEntry<StringRef>::Create("A");
+  Set.insert(*Element);
+  size_t Count = Set.count(*Element);
+  size_t Expected = 1;
+  EXPECT_EQ(Expected, Count);
+  Element->Destroy();
+}
+
+} // end anonymous namespace