Proposal: `meta_any::try_cast` to one of a closed set of types

[globberwops]

Suppose I have an instance of meta_any that I know may hold one of a closed set of different types.

With the current interface of meta_any, I can try_cast to each of the types, one after the other, and check the returned pointer until I find the correct type.

If there was e.g. an overload for try_cast that took a parameter pack of those types and returned an std::variant, containing the result of the first succesful cast, I could then std::visit that std::variant:

template <typename Arg, typename Out>
auto try_cast_impl(const entt::meta_any &any, Out &out) -> bool {
  if (const Arg *value = any.try_cast<Arg>(); value != nullptr) {
    out = *value;
    return true;
  }
  return false;
}

template <typename... Args>
auto try_cast(const entt::meta_any &any) -> std::variant<Args...> {
  std::variant<Args...> result;
  (try_cast_impl<Args>(any, result) || ...);
  return result;
}

...

const auto any_variant = try_cast<double, std::string>(any);

std::visit(
  [](auto &&arg) {
  if constexpr (std::is_same_v<T, double>) {
    ...
  } else if constexpr (std::is_same_v<T, std::string>) {
    ...
  }
}, any_variant);

This overload could be added to meta_any, but some thought would need to be put into communicating an unsuccesful try.

[skypjack]

Not sure this is easy-ish to implement actually, at least without complicating things.
Let's leave the issue open for a few days to give me time to check and give you a more precise answer. 👍

[globberwops]

@skypjack Thanks for taking a look! Here's a working example: https://godbolt.org/z/dv34Gbjh6