[pcwalton]

> So I remembered correctly, Nim does not reach UB in non-release code (or rather, code without --boundCheck:on), it throws an exception.

That's not really correct. It's undefined behavior either way; you're just getting lucky because the compiler doesn't happen to take advantage of the undefined behavior to perform optimizations at -O0.

[filwit]

I'm not sure what you're implying.. you can turn on most optimizations and still keep nil-checks on in Nim (either the whole project via --nilChecks:one, or select portions of code via {.push.}).

Unless you're claiming your example was still hiting UB, even with nil-checks on, and just happened to throw an exception by random chance, I'm not really sure how you figure UB is still happening here (since the exception will be thrown, preventing the deref). Nothing is preventing you from using nil-checks in production code.