I think one of the reasons this isn't standard is that it's too easy to make mistakes with it. For example, if `std::string readValue3()` was changed to `std::s

I think one of the reasons this isn't standard is that it's too easy to make mistakes with it. For example, if `std::string readValue3()` was changed to `std::string& readValue3()`, that reference would outlive the temporary guard, and any code that retained that reference would be broken. That's not so different from regular C++ mutex issues, but the downside here is that the convenience of synchronized_value also makes it harder to spot the mistake.