[feature request] Domain specific conversions

[klemens-morgenstern]

I mentioned that to Vinnie already, but I'd profit from a context object for json, and after some thinking I'd just go for an additional parameter getting passed by const ref and propagated to all the tag_invokes. I don't know if the current overload resolution would work with this though.

json::value_from(T, some_tag{});

void
tag_invoke(json::value_from_tag, json::value &target, const U &src, const some_tag &);

json::value_to<T>(json::value, some_tag{});

void
tag_invoke(json::value_to_tag<U>, json::value &target, const some_tag &);

but that would allow me to attach a context, e.g. to turn all the std::chrono types into ISO8601 string including proper adl for that:

namespace my_date_library
{
struct my_date_tag;

void
tag_invoke(json::value_from_tag, json::value &target, const U &src, const my_date_tag &);

void
tag_invoke(json::value_to_tag<U>, json::value &target, const my_date_tag &);

}

and then I can combine those into my own tag:

struct klemens_tag: my_date_library::my_date_tag {};