As always, and generally speaking, if anything they show that drawing the line is difficult since they show in their paper [1] that both std::expected and boost

As always, and generally speaking, if anything they show that drawing the line is difficult since they show in their paper [1] that both std::expected and boost::LEAF are worse than std::exception. Excerpts from paper: > Single threaded the fib code using std::expected is more than four times slower than using traditional exceptions. > This has much less overhead than std::expected, but it is still not for free. For fib we see a slowdown of approx. 60% compared to traditional exceptions, which is still problematic. Now, admittedly, these workloads are too simple (sqrt and fibonacci) and only shown for the single-threaded use-cases but I guess you have to start from something that is easy enough to reason about otherwise getting to any sane conclusions might be too difficult. However, they continue to show in their blog how they significantly sped up the std::exception implementation in a very non-trivial workload - multi-threaded and JIT'ed database kernel. So, to prove the point about exceptions being slower in complex workloads, such as the one above, somebody would actually have to rewrite their whole codebase to use something else such as std::expected, return values or whatever. Non-trivial to say the least. [1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2544r0.html https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p25...