Add unit test for leaking memory when throwing inside of an adl serializer

(problem described in issue #3881)

tests/src/unit-no-mem-leak-on-adl-serialize.cpp

@@ -0,0 +1,82 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.11.2
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2022 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+#include <exception>
+
+struct Foo
+{
+    int a;
+    int b;
+};
+
+namespace nlohmann
+{
+template <>
+struct adl_serializer<Foo>
+{
+    static void to_json(json& j, Foo const& f)
+    {
+        switch (f.b)
+        {
+            case 0:
+                j["a"] = f.a;
+                break;
+            case 1:
+                j[0] = f.a;
+                break;
+            default:
+                j = "test";
+        }
+        if (f.a == 1)
+        {
+            throw std::runtime_error("b is invalid");
+        }
+    }
+};
+}
+
+TEST_CASE("check_for_mem_leak_on_adl_to_json-1")
+{
+    try
+    {
+        nlohmann::json j = Foo {1, 0};
+    }
+    catch (...)
+    {
+        // just ignore the exception in this POC
+    }
+}
+
+TEST_CASE("check_for_mem_leak_on_adl_to_json-2")
+{
+    try
+    {
+        nlohmann::json j = Foo {1, 1};
+    }
+    catch (...)
+    {
+        // just ignore the exception in this POC
+    }
+}
+
+TEST_CASE("check_for_mem_leak_on_adl_to_json-2")
+{
+    try
+    {
+        nlohmann::json j = Foo {1, 2};
+    }
+    catch (...)
+    {
+        // just ignore the exception in this POC
+    }
+}
+
+