Exceptions
+ Best solution for rare errors
Expected!Andrei
+ Best general error-handling mechanism

No matter which error camp you belong to, errno is universally scorned.

Before TLS support became ubiquitous, global errno was useless, with TLS it's no longer a real problem, now can we improve errno even futher?
Exception last_error; // TLS & Rich error info

LastExceptionWhichOccuredOnThisThread
+ Zero-cost in hot-path
+ Decent performance in error-path (Better than Exceptions)
- No support for fibers
- Not pure

I'm not claming this is the best general error-handling mechanism, it's not suitable for a standard library etc.

I'm just wondering, does this fill a niche for certain libraries.