fix some msvc warning

1. unit-udt.cpp(248,24): warning C4189: 'large_id': local variable is initialized but not referenced
2. unit-udt.cpp(167,13): warning C4505: 'udt::operator ==': unreferenced function with internal linkage has been removed

tests/src/unit-udt.cpp

@@ -163,12 +163,13 @@ static bool operator==(const contact& lhs, const contact& rhs)
     return std::tie(lhs.m_person, lhs.m_address) ==
            std::tie(rhs.m_person, rhs.m_address);
 }
-
+#if JSON_DISABLE_ENUM_SERIALIZATION
 static bool operator==(const contact_book& lhs, const contact_book& rhs)
 {
     return std::tie(lhs.m_book_name, lhs.m_book_id, lhs.m_contacts) ==
            std::tie(rhs.m_book_name, rhs.m_book_id, rhs.m_contacts);
 }
+#endif // JSON_DISABLE_ENUM_SERIALIZATION
 } // namespace udt
 
 // from_json methods
@@ -245,7 +246,9 @@ TEST_CASE("basic usage" * doctest::test_suite("udt"))
     const udt::person senior_programmer{{42}, {"王芳"}, udt::country::china};
     const udt::address addr{"Paris"};
     const udt::contact cpp_programmer{sfinae_addict, addr};
+#if JSON_DISABLE_ENUM_SERIALIZATION
     const udt::book_id large_id{static_cast<udt::book_id>(static_cast<std::uint64_t>(1) << 63)}; // verify large unsigned enums are handled correctly
+#endif // JSON_DISABLE_ENUM_SERIALIZATION
     const udt::contact_book book{{"C++"}, static_cast<udt::book_id>(42u), {cpp_programmer, {senior_programmer, addr}}};
 
     SECTION("conversion to json via free-functions")