fix: nlohmann::adl_serializer for std::optional (#9147)

This allows templates such as `NLOHMANN_DEFINE_TYPE_*` templates and other generators with things like `std::vector<std::optional<T>>`.

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

src/libutil/tests/json-utils.cc

@@ -0,0 +1,58 @@
+#include <vector>
+#include <optional>
+
+#include <gtest/gtest.h>
+
+#include "json-utils.hh"
+
+namespace nix {
+
+/* Test `to_json` and `from_json` with `std::optional` types.
+ * We are specifically interested in whether we can _nest_ optionals in STL
+ * containers so we that we can leverage existing adl_serializer templates. */
+
+TEST(to_json, optionalInt) {
+    std::optional<int> val = std::make_optional(420);
+    ASSERT_EQ(nlohmann::json(val), nlohmann::json(420));
+    val = std::nullopt;
+    ASSERT_EQ(nlohmann::json(val), nlohmann::json(nullptr));
+}
+
+TEST(to_json, vectorOfOptionalInts) {
+    std::vector<std::optional<int>> vals = {
+        std::make_optional(420),
+        std::nullopt,
+    };
+    ASSERT_EQ(nlohmann::json(vals), nlohmann::json::parse("[420,null]"));
+}
+
+TEST(to_json, optionalVectorOfInts) {
+    std::optional<std::vector<int>> val = std::make_optional(std::vector<int> {
+        -420,
+        420,
+    });
+    ASSERT_EQ(nlohmann::json(val), nlohmann::json::parse("[-420,420]"));
+    val = std::nullopt;
+    ASSERT_EQ(nlohmann::json(val), nlohmann::json(nullptr));
+}
+
+TEST(from_json, optionalInt) {
+    nlohmann::json json = 420;
+    std::optional<int> val = json;
+    ASSERT_TRUE(val.has_value());
+    ASSERT_EQ(*val, 420);
+    json = nullptr;
+    json.get_to(val);
+    ASSERT_FALSE(val.has_value());
+}
+
+TEST(from_json, vectorOfOptionalInts) {
+    nlohmann::json json = { 420, nullptr };
+    std::vector<std::optional<int>> vals = json;
+    ASSERT_EQ(vals.size(), 2);
+    ASSERT_TRUE(vals.at(0).has_value());
+    ASSERT_EQ(*vals.at(0), 420);
+    ASSERT_FALSE(vals.at(1).has_value());
+}
+
+} /* namespace nix */