The lack of support for optional<T&> is not an issue at all in my opinion. The actual issue is that std::optional is not a monadic type in the vein of Rust's Op

The lack of support for optional<T&> is not an issue at all in my opinion. The actual issue is that std::optional is not a monadic type in the vein of Rust's Option or Haskell's Maybe. So really, what does it buy you over std::pair<T, bool>? Except being unsafe by default since it allows you to access an unconstructed T. Basic monadic operations don't arrive for std::optional until C++23, which is an unforced error. They should have been there from the beginning.