[FR] operator bool

[Domagoj Šarić]

wrapper around has_value

ps. don't know if you get notifications for new comments in closed issues: #61

[Mingxin Wang]

While std::any does not have operator bool, proxy is not sufficiently motivated to do so. Besides, since smart pointer usually have operator bool, if assign a default-constructed std::unique_ptr to proxy, proxy::operator bool will become ambiguous.