Allow final structs & classes serialization

[DavidKeller]

Hi,

I can't serialize a class T marked as final because the detail::has_memfn_serializer<> metafunction relies on the following trick to detect the serialize function presence.

struct derived: T {
        using T::serialize;
        no serialize(...) const;
};

I've updated the tests/base/include/json_conformance.hpp to add final to the tested class in order to experience the limitation.

This MR provides another implementation of the detail::has_memfn_serializer<> metafunction, but it ignores the return type of the T::serialize() function. I'm not comfortable with this choice because in your implementation, my understanding is that you ignore the return type as well, but it seems there is some facility to check it (i.e. return_value_check) ¯_(ツ)_/¯.

Some tests are failing as well on my Fedora 39, and there are warnings.

Do you want me to do some cleaning ?

[niXman]

hi,

sorry for the delay.
will look at this this weekend.

thank you!