Aimed at fixing compilation error https://github.com/nlohmann/json/issues/4197. It was a school project, asking me to make contributions. Sorry for the inconvenience I made.

docs/Makefile

@@ -15,8 +15,11 @@ cxx_standard = $(lastword c++11 $(filter c++%, $(subst ., ,$1)))
 %.output: %.cpp
 	@echo "standard $(call cxx_standard $(<:.cpp=))"
 	$(MAKE) $(<:.cpp=) \
-		CPPFLAGS="-I $(SRCDIR) -DJSON_USE_GLOBAL_UDLS=0" \
-		CXXFLAGS="-std=$(call cxx_standard,$(<:.cpp=)) -Wno-deprecated-declarations"
+
+		CPPFLAGS="-I ../single_include -DJSON_USE_GLOBAL_UDLS=0" \
+		CXXFLAGS="-std=c++2a -Wno-deprecated-declarations"
+		CPPFLAGS += -include <compare>
+		examples/operator_spaceship__const_reference.c++20: examples/operator_spaceship__const_reference.c++20.cpp $(CXX) $(CXXFLAGS) $(CPPFLAGS) $< -o $@
 	./$(<:.cpp=) > $@
 	rm $(<:.cpp=)
 
@@ -41,5 +44,6 @@ check_output_portable: $(filter-out examples/meta.test examples/max_size.test ex
 
 clean:
 	rm -fr $(EXAMPLES:.cpp=)
+	rm -f examples/operator_spaceship__const_reference.c++20
 	$(MAKE) clean -C docset
 	$(MAKE) clean -C mkdocs

tests/src/unit-make1.cpp

@@ -0,0 +1,92 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.11.3
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2023 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+#include <iostream>
+using nlohmann::json;
+
+TEST_SUITE("nlohmann/json test suite - Extended")
+{
+    TEST_CASE("Nested Structures")
+    {
+        SECTION("Nested Objects")
+        {
+            json nested_object = {
+                {"person", {
+                    {"name", "Bob"},
+                    {"age", 40},
+                    {"address", {
+                        {"city", "Example City"},
+                        {"zip", "12345"}
+                    }}
+                }}
+            };
+
+            CHECK(nested_object["person"]["name"] == "Bob");
+            CHECK(nested_object["person"]["address"]["city"] == "Example City");
+        }
+
+        SECTION("Nested Arrays")
+        {
+            json nested_array = {
+                {"numbers", {1, 2, {3, 4}, 5}}
+            };
+
+            CHECK(nested_array["numbers"][2][1] == 4);
+        }
+    }
+
+    TEST_CASE("Exception Handling")
+    {
+        SECTION("Parsing Invalid JSON")
+        {
+            // Expecting a parse error for invalid JSON
+            CHECK_THROWS_AS(json::parse("invalid_json_string"), json::parse_error);
+        }
+
+        SECTION("Accessing Nonexistent Key")
+        {
+            json object = {{"name", "Alice"}, {"age", 25}};
+
+            // Expecting an exception when accessing a nonexistent key
+            CHECK_THROWS_AS(object.at("nonexistent_key"), json::out_of_range);
+        }
+    }
+
+    TEST_CASE("Additional Serialization and Deserialization")
+    {
+        SECTION("Serialize and Deserialize with Custom Format")
+        {
+            json data = {{"key1", 42}, {"key2", "value"}};
+
+            // Serialize with indentation for human-readable format
+            std::string serialized = data.dump(2);
+
+            // Deserialize the serialized string
+            json parsed = json::parse(serialized);
+
+            CHECK(parsed == data);
+        }
+
+        // SECTION("Deserialize from Stream")
+        // {
+        //     std::istringstream stream(R"({"name": "Charlie", "age": 35})");
+
+        //     // Deserialize from the input stream
+        //     json parsed;
+        //     stream >> parsed;
+
+        //     CHECK(parsed["name"] == "Charlie");
+        //     CHECK(parsed["age"] == 35);
+        // }
+    }
+
+    // Add more test cases and sections as needed to cover other functionalities.
+}

tests/src/unit-make2.cpp

@@ -0,0 +1,58 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.11.3
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2023 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+#include <iostream>
+using nlohmann::json;
+
+TEST_SUITE("nlohmann/json test suite")
+{
+    TEST_CASE("Basic JSON Operations")
+    {
+        SECTION("Construction and Type Checks")
+        {
+            json number = 42;
+            json text = "Hello, JSON!";
+            json array = {1, 2, 3};
+            json object = {{"key", "value"}};
+
+            CHECK(number.is_number());
+            CHECK(text.is_string());
+            CHECK(array.is_array());
+            CHECK(object.is_object());
+        }
+
+        SECTION("Serialization and Deserialization")
+        {
+            json original = {{"name", "John"}, {"age", 30}};
+            std::string serialized = original.dump();
+            json parsed = json::parse(serialized);
+
+            CHECK(parsed == original);
+        }
+
+        SECTION("Array and Object Operations")
+        {
+            json array = {1, 2, 3};
+            array.push_back(4);
+            array.insert(array.begin() + 1, 10);
+
+            CHECK(array.size() == 5);
+            CHECK(array[1] == 10);
+
+            json object = {{"name", "Alice"}, {"age", 25}};
+            object["city"] = "Wonderland";
+
+            CHECK(object["city"] == "Wonderland");
+        }
+    }
+
+    // Add more test cases and sections as needed to cover other functionalities.
+}

tests/src/unit-make3.cpp

@@ -0,0 +1,57 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.11.3
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2023 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+#include <iostream>
+using nlohmann::json;
+
+TEST_SUITE("nlohmann/json test suite - More")
+{
+    TEST_CASE("Comparing JSON Objects")
+    {
+        SECTION("Ordering of Object Keys Matters")
+        {
+            json object1 = {
+                {"name", "Alice"},
+                {"age", 25},
+                {"city", "Wonderland"}
+            };
+
+            json object2 = {
+                {"name", "Alice"},
+                {"city", "Wonderland"},
+                {"age", 25}
+            };
+
+            // Expecting the objects to be different due to key order
+            CHECK(object1 != object2);
+        }
+
+        SECTION("Ordering of Object Keys Doesn't Matter")
+        {
+            json object1 = {
+                {"name", "Bob"},
+                {"age", 30},
+                {"city", "Example City"}
+            };
+
+            json object2 = {
+                {"age", 30},
+                {"name", "Bob"},
+                {"city", "Example City"}
+            };
+
+            // Expecting the objects to be considered equal
+            CHECK(object1 == object2);
+        }
+    }
+
+    // Add more test cases and sections as needed to cover other functionalities.
+}