title: What is std::expected in C++?
body: <p>In one of the most respected stackoverflow answer I found an example of <code>std::expected</code> template class usages:
  <a href="https://stackoverflow.com/questions/43503656/what-are-coroutines-in-c20/44244451#44244451">What are coroutines in C++20?</a></p>
  <p>At the same time I cannot find any mentioning of this class on cppreference.com. Could you please explain what it is?</p>