Add tests to check that the YAML parser supports merge keys

This is useful as it's an optional addition to the YAML spec, and
LOOT's masterlists heavily rely on it being supported.

src/tests/api/internals/main.cpp

@@ -66,6 +66,12 @@ TEST(ModuloOperator, shouldConformToTheCpp11Standard) {
   EXPECT_EQ(-2, -9 % -7);
 }
 
+TEST(YamlCpp, shouldSupportMergeKeys) {
+  YAML::Node node = YAML::Load("{<<: {a: 1}}");
+  ASSERT_TRUE(node["a"]);
+  EXPECT_EQ(1, node["a"].as<int>());
+}
+
 int main(int argc, char **argv) {
   // Set the locale to get encoding conversions working correctly.
   std::locale::global(boost::locale::generator().generate(""));

src/tests/api/internals/metadata_list_test.h

@@ -142,6 +142,33 @@ TEST_P(MetadataListTest, loadShouldLoadGroups) {
             groups.find(Group("group2"))->GetAfterGroups());
 }
 
+TEST_P(MetadataListTest, loadYamlParsingShouldSupportMergeKeys) {
+  using std::endl;
+
+  boost::filesystem::ofstream out(metadataPath);
+  out << "common:" << endl
+    << "  - &earlier" << endl
+    << "    name: earlier" << endl
+    << "    after:" << endl
+    << "      - earliest" << endl
+    << "groups:" << endl
+    << "  - name: default" << endl
+    << "    <<: *earlier" << endl;
+
+  out.close();
+
+  MetadataList metadataList;
+  ASSERT_NO_THROW(metadataList.Load(metadataPath));
+
+  auto groups = metadataList.Groups();
+
+  EXPECT_EQ(1, groups.size());
+
+  EXPECT_EQ(1, groups.count(Group("default")));
+  EXPECT_EQ(std::unordered_set<std::string>({ "earliest" }),
+    groups.find(Group("default"))->GetAfterGroups());
+}
+
 TEST_P(MetadataListTest, loadShouldThrowIfAnInvalidMetadataFileIsGiven) {
   MetadataList ml;
   for (const auto& path : invalidMetadataPaths) {