Lots of proposals:
by Barry Revzin
From the article:
C++17 gave us std::optional which is, in the words of a friend of mine, one of those really simple, ultra complex types — in the sense that it’s very easy to understand and use properly, even for relatively inexperienced programmers… but extremely difficult to implement correctly, even for experts (another such is std::pair). Today, it’s well over a thousand lines of code, most of which is critical to support even its most basic functionality. optional<T> is the simplest sum type, and it appears in lots of different languages (and even has special syntax in Swift) under various related names — Maybe, Option, etc. — but in the languages I’m even nominally familiar with, it’s about as simple to implement as it is to use.
But that’s the state of affairs today. What does tomorrow bring?