monostate: Backport of std::monostate

The monostate sub-package provides a facility akin to that provided by the C++17 std::monostate class.

The public header is

#include "arene/base/monostate.hpp"

The Bazel target is

//:monostate

A type with only one valid value

arene::base::monostate is a type with only one valid value. It has no behaviour, and all instances compare equal.

It can be used as a replacement for void to indicate "no value": whereas void cannot be stored in a container, and cannot be passed as a function argument, arene::base::monostate can. When this substitution needs to happen automatically based on an input type, arene::base::monostate_identity<T> can be used: it is the same as T, unless T is void in which case it is arene::base::monostate.

arene::base::monostate_identity is used in arene::base::result to allow void as a value type: arene::base::result<void, E> stores arene::base::monostate as the value type.

arene::base::monostate can also be used with arene::base::variant when it is desirable to model "no value"; arene::base::variant<arene::base::monostate, std::string, int> can either be a string, an integer, or "nothing". This is the purpose for which std::monostate was introduced in C++17, and provides a better alternative to arene::base::optional<arene::base::variant<std::string, int>>, since it can be queried with the same API as std::string and int, rather than having to check the status of the arene::base::optional wrapper separately.