[FR] SBO-only targets

[Domagoj Šarić]

Add a trait to specify that only targets that fit into the SBO are allowed (to be assigned).
(or is specifying noexcept copyiablity enough, i.e. it implies that?)

[Mingxin Wang]

For SBO, the implementation of proxy is different from mainstream implementation of std::function or std::any. proxy is a runtime binding of "pointers" rather than "values". SBO is only in the scope of function template make_proxy (it will select details::deep_ptr or details::sbo_ptr at compile-time), but out of the scope of class template proxy. If we need make_proxy to reject value types that uses details::deep_ptr, it might be a good idea to wrap make_proxy in another function template with more constraints, e.g.,

template <class F, class T, class... Args> requires(pro::proxiable<pro::details::sbo_ptr<T>, F>)
pro::proxy<F> make_proxy_with_no_heap_allocation(Args&&... args)
    { return pro::make_proxy<F, T>(std::forward<Args>(args)...); }

Unfortunately, pro::details::sbo_ptr is not public, and therefore you may need to copy the implementation of pro::details::sbo_ptr<T> from proxy.h to somewhere else to make the code work, in case it changes in the future. Like std::any, there is currently no plan to make SBO information public, but you can always implement your own make_proxy().

[Tian L.]

Close this issue since there's no more update.