Normalize folly::get_optional for nested map case

Summary:
`folly::get_optional` allows the user to specify the optional type, e.g.
```
// default case - return folly::Optional
get_optional(m, 1);

// return std::optional
get_optional<std::optional>(m, 1);
```

`folly::get_optional` also supports nested maps, however it hardcodes the return type to always be `folly::Optional`. This diff relaxes the constraints and allows the user to specify optional type, defaulting to `folly::Optional`

Reviewed By: Gownta

Differential Revision: D66497266

fbshipit-source-id: bef87007956c7df8c7c07a0e70716b18e770e90d

folly/container/MapUtil.h

@@ -272,14 +272,22 @@ auto extract_default(const KeysDefault&... keysDefault) ->
  * Given a map of maps and a path of keys, return a Optional<V> if the nested
  * key exists and None if the nested keys does not exist in the map.
  */
-template <class Map, class Key1, class Key2, class... Keys>
+template <
+    template <typename> class Optional = folly::Optional,
+    class Map,
+    class Key1,
+    class Key2,
+    class... Keys>
 auto get_optional(
     const Map& map, const Key1& key1, const Key2& key2, const Keys&... keys)
-    -> folly::Optional<
+    -> Optional<
         typename detail::NestedMapType<Map, 2 + sizeof...(Keys)>::type> {
   auto pos = map.find(key1);
-  return pos != map.end() ? get_optional(pos->second, key2, keys...)
-                          : folly::none;
+  if (pos != map.end()) {
+    return get_optional<Optional>(pos->second, key2, keys...);
+  } else {
+    return {};
+  }
 }
 
 /**

folly/container/test/MapUtilTest.cpp

@@ -106,6 +106,11 @@ TEST(MapUtil, getOptionalStd) {
   EXPECT_TRUE(get_optional<std::optional>(m, 1).has_value());
   EXPECT_EQ(2, get_optional<std::optional>(m, 1).value());
   EXPECT_FALSE(get_optional<std::optional>(m, 2).has_value());
+
+  std::map<int, std::map<int, int>> m2{{1, {{2, 3}}}};
+  EXPECT_TRUE(get_optional<std::optional>(m2, 1, 2).has_value());
+  EXPECT_EQ(3, get_optional<std::optional>(m2, 1, 2).value());
+  EXPECT_FALSE(get_optional<std::optional>(m2, 1, 3).has_value());
 }
 
 TEST(MapUtil, getRefDefault) {