Add unit test

tests/src/unit-udl.cpp

@@ -0,0 +1,51 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.10.5
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2022 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#undef JSON_USE_GLOBAL_UDLS
+#define JSON_USE_GLOBAL_UDLS 0
+#include <nlohmann/json.hpp>
+
+TEST_CASE("user-defined string literals")
+{
+    auto j_expected = nlohmann::json::parse(R"({"foo": "bar", "baz": 42})");
+    auto ptr_expected = nlohmann::json::json_pointer("/foo/bar");
+
+    SECTION("using namespace nlohmann::literals::json_literals")
+    {
+        using namespace nlohmann::literals::json_literals; // NOLINT(google-build-using-namespace)
+
+        CHECK(R"({"foo": "bar", "baz": 42})"_json == j_expected);
+        CHECK("/foo/bar"_json_pointer == ptr_expected);
+    }
+
+    SECTION("using namespace nlohmann::json_literals")
+    {
+        using namespace nlohmann::json_literals; // NOLINT(google-build-using-namespace)
+
+        CHECK(R"({"foo": "bar", "baz": 42})"_json == j_expected);
+        CHECK("/foo/bar"_json_pointer == ptr_expected);
+    }
+
+    SECTION("using namespace nlohmann::literals")
+    {
+        using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+
+        CHECK(R"({"foo": "bar", "baz": 42})"_json == j_expected);
+        CHECK("/foo/bar"_json_pointer == ptr_expected);
+    }
+
+    SECTION("using namespace nlohmann")
+    {
+        using namespace nlohmann; // NOLINT(google-build-using-namespace)
+
+        CHECK(R"({"foo": "bar", "baz": 42})"_json == j_expected);
+        CHECK("/foo/bar"_json_pointer == ptr_expected);
+    }
+}