Implementation of sum/product Body types is impossible

[djarek]

It is impossible to compose Bodies into sum types, e.g.:

template <typename... Ts>
struct variant_body
{
    using value_type = std::variant<typename Ts::value_type...>;
    struct reader;
    struct writer;
};

using my_body = variant_body<beast::http::empty_body, json_body, beast::http::string_body>;

due to the requirement of Body::reader and Body::writer that the message's body type is exactly the same type as Body. The same restriction applies to product types(not as useful as a variant_body). Not sure how to make this use-case possible without complicating other use-cases.

[vinniefalco]

How about if we change the constructor of reader and writer to something like this:

template<bool isRequest, class Fields>
void writer(header<isRequest, Fields> const&, value_type const& body);

[djarek]

I believe that would solve this issue.